Aqueous phase behaviour of surfactant and its application in cloud point extraction

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AQUEOUS PHASE BEHAVIOR OF SURFACTANT AND ITS APPLICATION IN CLOUD-POINT EXTRACTION MAR MAR SWE NATIONAL UNIVERSITY OF SINGAPORE 2003 AQUEOUS PHASE BEHAVIOR OF SURFACTANT AND ITS APPLICATION IN CLOUD-POINT EXTRACTION MAR MAR SWE (B.Sc, Yangon University; B.E, Yangon Technological University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 ACKNOWLEDGEMENT Firstly, I would like to express my gratitude and deep thanks to my supervisors Dr. Liya E. Yu and Dr. Chen Bing Hung for their patience, enormous support and encouragement throughout this research project. Secondly, I would like to thank all my lab-mates, my friends and the laboratory officers in-charge in the Department of the Chemical and Environmental Engineering (ChEE) for their kind support during my study. Finally, I would like to express my sincere gratitude to the ChEE department for the full permission to use the research facilities and the National University of Singapore for providing the scholarship to pursue the Master of Engineering degree. i Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS ii SUMMARY vi NOMENCLATURE viii LIST OF FIGURES xi LIST OF TABLES xiii Chapter 1 Introduction 1 1.1 General Introduction 1 1.2 Objectives and Scope 5 1.3 Organization 7 Chapter 2 Literature Review 2.1 Solubilization by Nonionic Surfactants 8 8 2.1.1 Locus of Solubilization 8 2.1.2 Factors Affecting Solubilization 10 2.1.3 Quantitative Study on Solubilization 11 2.2 Aqueous Phase Behavior of Nonionic Surfactants 13 2.2.1 Mechanism of Clouding Phenomenon 13 2.2.2 Factors Affecting Cloud Point 13 2.2.3 Application of Clouding Phenomenon 15 2.3 Cloud-Point Extraction 15 ii Table of Contents 2.4 Properties and Applications of Selected Nonionic 18 Surfactants Chapter 3 2.4.1 Tergitol 15-S Series Surfactants 18 2.4.2 Neodol 25-7 Surfactant 19 Materials and Methods 21 3.1 Materials 21 3.2 Apparatus 22 3.2.1 HPLC 22 3.2.2 Laser Light Scattering 24 3.2.3 Water Bath 25 3.2.4 Centrifuge 25 3.3 Experimental Procedures 25 3.3.1 Equilibrium Solubilization 25 3.3.2 Micelle Size and Aggregation Number Measurement 26 3.3.3 Measurement of Cloud Point and Preconcentration Factor 28 3.3.4 Cloud-Point Extraction from Aqueous Solutions Chapter 4 Solubilization by Nonionic Surfactants 29 31 4.1 Introduction 31 4.2 Results and Discussion 31 4.2.1 Equilibrium Solubilization of Hydrophobic Organic 31 Compounds by Selected Nonionic Surfactants 4.2.2 Determination of Micelle Size and Aggregation Number 41 of Selected Nonionic Surfactants iii Table of Contents 4.3 Conclusions Chapter 5 Aqueous Phase Behavior of Selected Nonionic 46 47 Surfactants 5.1 Introduction 47 5.2 Results and Discussion 47 5.2.1 Aqueous Phase Behavior of Selected Nonionic 47 Surfactants 5.2.1.1 Tergitol 15-S-7 – Water system 48 5.2.1.2 Neodol 25-7 – Water system 48 5.2.1.3 Tergitol 15-S-9 – Water system 48 5.2.2 Effect of Added Electrolytes on Cloud Points of Selected 51 Nonionic Surfactants 5.2.3 Preconcentration Factor 5.3 Conclusions Chapter 6 Cloud-Point Extraction and Recovery Efficiency 55 59 60 6.1 Introduction 60 6.2 Results and Discussion 60 6.2.1 Extraction by Tergitol 15-S-7 60 6.2.1.1 Recovery as a function of surfactant concentration 62 6.2.1.2 Recovery as a function of initial analyte concentration 64 6.2.2 Extraction by Neodol 25-7 65 6.2.2.1 Recovery as a function of surfactant concentration 67 6.2.2.2 Recovery as a function of initial analyte concentration 68 iv Table of Contents 6.2.3 Extraction by Tergitol 15-S-9 69 6.2.3.1 Recovery as a function of surfactant concentration 70 6.2.3.2 Recovery as a function of initial analyte concentration 71 6.2.4 Comparison of Recovery Efficiency 6.2.4.1 Effect of molecular structure of surfactant on 72 72 recovery 6.2.4.2 Effect of different HLB values of surfactants on 74 recovery 6.2.5 Effect of Salts on Recovery Efficiency 6.3 Conclusions Chapter 7 Conclusions and Recommendations 76 77 80 7.1 Conclusions 80 7.2 Recommendations 81 REFERENCES 83 APPENDX A Estimation of log Kow value of 9-chloroanthracene 95 v Summary SUMMARY Solubilization of hydrophobic organic compounds (HOCs) by the readily biodegradable nonionic surfactants, Tergitol 15-S-7 and Tergitol 15-S-9, mixtures of secondary ethoxylated alcohols, and Neodol 25-7, a mixture of primary ethoxylated alcohols, was investigated. The effects of the molecular structure and the HLB values of the surfactants on the solubilization capacities of the HOCs were studied. The results showed that the surfactant with a linear chain has a larger core volume and a higher solubilization capacity compared to that of the branched surfactant. For the surfactants of the same homolog, the HLB number could be used as a good indicator for the solubilization capacity, because the surfactant with a lower HLB value has a higher solubilization capacity. Micelle-water partition coefficients of HOCs were correlated to their octanol-water partition coefficients. The correlation revealed that the hydrophobicity of surfactants as well as the properties of solutes might also have a profound influence on the micelle-water partitioning. The changes in the hydrodynamic radii and the aggregation numbers of the micelles with temperature were measured by the dynamic and static laser light scattering techniques. It is clearly demonstrated that the solubilization capacity of HOCs was mainly governed by the aggregation numbers and the core volume of the micelles of the selected nonionic surfactants. Cloud point temperatures of selected nonionic surfactants were studied along with the effect of added electrolytes on their cloud points. Sodium iodide could increase the cloud points of selected nonionic surfactants, i.e., the salt-in effect, whereas calcium chloride, sodium chloride, sodium sulphate and sodium phosphate could decrease the cloud point, i.e., the salt-out effect. Changing the concentrations of the surfactant and vi Summary the added electrolytes could optimize the preconcentration factor. It was found that a higher preconcentration factor could be achieved in the solution having a lower surfactant concentration, but a higher salt concentration. Cloud-point extraction (CPE) was facilitated at room temperature (22 ºC) by adding either sodium sulphate or sodium phosphate to the micelle solutions of the selected nonionic surfactants. The effects of the molecular structure of surfactants and the HLB values of the surfactants on the recovery efficiency of HOCs were studied as well. Recovery efficiency was governed by the preconcentration factor. A recovery was achieved at a higher preconcentration factor. Sodium phosphate gives a better recovery of acenaphthene than sodium sulphate either in Tergitol 15-S-7 or Tergitol 15-S-9. The greatest advantage of using Tergitol surfactants and Neodol surfactant as an extractant in the CPE technique lies in the fact that these surfactants do not render any fluorometric signals in the UV region and, hence, no complicated clean-up procedure and any undesirable masking of chromatographic peaks of HOCs in the effluent is required. In addition, the low volatility and toxicity and the high biodegradability of the surfactant is noted. Moreover, a shorter time to reach equilibrium phase separation is another added advantage. vii Nomenclature NOMENCLATURE a, A constant A2 osmotic second virial coefficient b, B constant c solution concentration C apparent solubility of HOC in a micellar solution Ccmc HOC solubility at CMC C0 initial HOC concentration in the bulk phase, mg / l Cs HOC concentration in the surfactant-rich phase, mg / l Csurf surfactant concentration D diffusion coefficient of surfactant molecules g(td) autocorrelation function as a function of delay time, td kB Boltzmann constant K optical constant for vertical polarized incident light Km micelle-water partition coefficient Kow octanol-water partition coefficient Mw molecular weight n refractive index of the solvent NA Avogadro’s number Nag aggregation number of a micelle Nc number of carbons in the hydrophobic group of surfactant molecules Nh number of hydrophilic groups in surfactant molecules NEO number of EO groups in surfactant molecules PL the Laplace pressure acting across the curved micelle-water viii Nomenclature interface q magnitude of a scattering wave vector r radius of micelle R the universal gas constant Rg radius of gyration Rh hydrodynamic radius of a micelle Rθ excess Rayleigh ratio T absolute temperature td delay time Xm mole fraction of HOC in the micellar phase Xa mole fraction of HOC in the aqueous phase Va, mol molar volume of water at the experimental temperature, 22 ºC Vc core volume of the micelle Vm micellar volume Vo the volume of bulk solution, milliliter Vs molecular volume of surfactant Vsr the volume of surfactant-rich, milliliter Vw molecular volume of water ABBREVIATION Ace acenaphthene 9-ChAn 9-chloroanthracene CMC critical micelle concentration DBz dibenzofuran ix Nomenclature EO ethylene oxide Fluo fluoranthene HLB hydrophilic-lipophilic balance number of surfactant L1 surfactant-rich phase POE polyoxyethylene R recovery efficiency of HOC MSR molar solubilization ratio W water phase WSR mass solubilization ratio GREEK LETTERS ηo solvent viscosity λo wavelength of incident light in vacuum γ interfacial tension across the micelle-water interface x List of Figures LIST OF FIGURES Figure Title Page Figure 1.1 Schematic description of phase equilibrium in CPE 2 Figure 2.1 Loci of solubilization of material in a surfactant micelle 9 Figure 3.1 Shimadzu HPLC system 23 Figure 3.2 Laser Light Scattering apparatus 24 Figure 4.1 Solubilization of HOCs by Tergitol 15-S-7 at 22 ºC 32 Figure 4.2 Solubilization of HOCs by Neodol 25-7 at 22 ºC 32 Figure 4.3 Solubilization of HOCs by Tergitol 15-S-9 at 22 ºC 33 Figure 4.4 Correlation of log Km and log Kow for HOCs in selected nonionic surfactants 40 Figure 5.1 A phase diagram of selected nonionic surfactant micellar solutions 50 Figure 5.2 Effect of added electrolytes on the cloud point of a Neodol 25-7 micellar solutions 53 Figure 5.3 Effect of added electrolytes on the cloud point of a Tergitol 15-S-9 micellar solutions 54 Figure 5.4 Preconcentration factors at 3 wt % surfactant concentration and different added sodium sulphate concentrations 56 Figure 5.5 Preconcentration factors at different surfactant concentrations 57 Figure 6.1 The effect of Tergitol 15-S-7 surfactant concentration on HOC 62 xi List of Figures recovery Figure 6.2 The effect of initial HOC concentration on its recovery using 3 wt % Tergitol 15-S-7 and 0.6 M sodium sulphate 64 Figure 6.3 The effect of Neodol 25-7 surfactant concentration on HOC recovery 67 Figure 6.4 The effect of initial HOC concentration on its recovery using 3 wt % Neodol 25-7 and 0.65 M sodium sulphate 68 Figure 6.5 The effect of Tergitol 15-S-9 surfactant concentration on HOC recovery 70 Figure 6.6 The effect of initial HOC concentration on its recovery using 3 wt % Tergitol 15-S-9 and 0.7 M sodium sulphate 71 Figure 6.7 Comparison of recoveries of HOCs by Tergitol 15-S-7 and Neodol 25-7 73 Figure 6.8 Comparison of recoveries of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9 74 Figure 6.9 Recovery efficiency of acenaphthene as a function of surfactant concentration 76 xii List of Tables LIST OF TABLES Table Title Page Table 3.1 The properties of selected nonionic surfactants 21 Table 3.2 The selected physical properties of HOCs 22 Table 3.3 Fluorescence characteristics of HOCs 23 Table 4.1 Comparison of solubilization of HOCs by Tergitol 15-S-7 and Neodol 25-7 surfactants at 22 ºC 34 Table 4.2 Comparison of solubilization of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9 surfactants at 22 ºC 35 Table 4.3 A comparison of properties of micelles of Tergitol 15-S-7 and Neodol 25-7 surfactants obtained from Laser Light Scattering 44 Table 4.4 A comparison of properties of micelles of Tergitol 15-S-7 and Tergitol 15-S-9 surfactants obtained from Laser Light Scattering 45 xiii Chapter 1 Introduction Chapter 1 Introduction 1.1 General Introduction Hydrophobic organic compounds (HOC), such as polycyclic aromatic hydrocarbons (PAH) and dibenzofuran, are ubiquitous environmental organic pollutants formed by a number of industrial and combustion processes. The great concern of their impacts on environment arises from their potential carcinogenic and mutagenic properties (Neff, 1985; Kiceniuk, 1994; Mizesko et al., 2001). Moreover, they have low aqueous solubility and highly affinity to the sediment. Due to their high toxicity, selective analytical methods are required for analyses and assessments on their persistence in the environment. An extraction technique based on the clouding phenomenon of nonionic surfactants has become very attractive in recent years (Li et al., 2002). Clouding phenomenon is one of the common properties of the nonionic surfactants. A micellar solution of a suitable nonionic surfactant becomes cloudy at a well-defined temperature. As the temperature increases, micellar growth resulting from the dehydration of the polyoxyethylene chain of the hydrophilic group and increased intermicellar attraction causes the formation of large particles and the solution becomes visibly turbid. Above the cloud point, the homogeneous surfactant solution separates into two immiscible phases; one that contains most of the surfactant, called surfactant-rich phase (L1), while the other, called excess water phase (W), is almost free of the 1 Chapter 1 Introduction Micelle Surfactant-rich Phase Aqueous Phase Monomer HOC Figure 1.1 A schematic description of a phase equilibrium in CPE 2 Chapter 1 Introduction surfactant and the surfactant concentration is only near its critical micelle concentration (CMC) (See Figure 1.1). Phase separation occurs due to the difference in density of micelle-rich phase (surfactant-rich phase) and micelle poor phase (Nakagawa, 1963). The surfactant-rich phase is not necessary to be in the top, as it depends on the densities of these two phases. The phase separation is reversible; when the mixture is cooled to the temperature below the cloud point, these two phases merge to form a clear phase again. The hydrophobic organic compounds initially present in the solution and bound to the micelles will be favorably extracted to the surfactant-rich phase (L1), while leaves only a very small portion in the aqueous phase. The cloud-point extraction (CPE) by nonionic surfactant was firstly utilized for the extraction of metal ions from aqueous sample (Watanabe, 1978). The scope of CPE technique was extended to protein separations (Bordier, 1981) and separation of biomaterials (Saitoh, 1991). Moreover, it has been successfully demonstrated in extraction of selected organic compounds of great environmental concerns (Böckelen et al., 1993; Hinze, et al., 1989; Fernándz et al., 1998; Bai et al., 2001; Materna et al., 2001; Li et al., 2002). There are several advantages of using the CPE technique compared with the traditional solvent extraction: a possibility of combining preconcentration and extraction in one step; water is utilized as main solvent so that it is less toxic and cost effective; and the presence of surfactant can minimize the loss of analytes due to their adsorption to the container. 3 Chapter 1 Introduction However, some surfactants reported in the literature have caused problems because they contain aromatic rings, which, due to their resistance to biodegradation, not only raise environmental concerns, but also disturb the analysis of analytes using HPLC owing to their large UV absorbance and fluorometric signal. The search for the proper surfactants and the development of a simple extraction process become as two key issues for the successful application of CPE. Micelle-enhanced solubilization of nonpolar compounds is one of the more significant applications of surfactants. The solubility of predominantly hydrophobic molecules in aqueous solutions is enhanced by the addition of surfactants to the solution. More explicitly, solubilization may be defined as the spontaneous dissolution of a substance by the reversible interaction with the micelles of a surfactant in a solvent to form the thermodynamically stable isotropic solution with reduced thermodynamic activity of the solubilized material (Rosen, 1989). The examples of the solubilization involve the detergency, microemulsion polymerization, micellar catalysis, and extraction. It is also important in enhanced oil recovery. In recent years, solubilization of organic compounds of environmental interest by micellar solution of surfactants has been studied (Kile et al., 1989; Edwards et al., 1991; Diallo et al., 1994; Kim et al., 2000; Li et al., 2002). Several kinds of nonionic surfactants are widely used in the studies due to their low critical micelle concentrations (CMC) and possibly high molecular weights of micelles compared to ionic surfactants. 4 Chapter 1 Introduction Based on the research progress on these two areas, i.e., the cloud-point extraction and solubilization of hydrophobic organic compounds (HOCs); the objectives of this study are to develop a simple and practical cloud-point extraction technique, and to study solubilization behavior of HOCs by selected nonionic surfactants. 1.2 Objectives and Scope The overall objective of this study is to develop a simple but practical cloud-point extraction (CPE) technique and extraction of HOCs from aqueous samples as well as the solubilization behavior of hydrophobic organic compounds (HOCs) by selected nonionic surfactants. Nonionic surfactants, Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 were chosen in this study. The choice of these surfactants is based on the following factors. The first is their environmentally benign nature. Tergitol 15-S surfactants such as Tergitol 15-S-7 and Tergitol 15-S-9 are mixtures of secondary alcohol ethoxylates, and are developed as an alternative to traditionally used surfactants such as nonyl phenol ethoxylates due to their biodegradable nature. These two surfactants have the average ethylene oxides 7.3 for Tergitol 15-S-7 and 8.9 for Tergitol 15-S-9 so that their HLB values are 12.4 and 13.3 respectively. Neodol 25-7, a mixture of linear primary alcohol ethoxylates, has been widely utilized in the high-performance biodegradable detergent formulations. In addition, Neodol 25-7 has similar molecular weight and HLB value as Tergitol 15-S-7, so that the results can be possibly compared in terms of different molecular structures. The second reason is that these surfactants cause no disturbance in the sample analysis that uses UV spectroscopy (Bai et al., 2001 and Li and Chen, 2002). Because they contain no double or π bond in their molecules, so it renders no 5 Chapter 1 Introduction fluorometric signals in the UV range. Additionally, choice of Tergitol surfactants, especially Tergitol 15-S-7, is also based on the known high solubilization power for large triglyceride oils and fatty alcohols (Chen et al., 1997, 1998) and PAHs (Li and Chen, 2002), and its high extraction efficiency for PAHs (Bai et al., 2001). The scope encompasses the following aspects: 1) Study the solubilization capacity of selected HOCs by the nonionic surfactants, Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9, and the correlation between the hydrophobicity of these nonionic surfactants and the micelle-water partition coefficients as well as the octanol-water partition coefficients of these HOCs. 2) Measure the cloud point temperature of micellar solutions of selected nonionic surfactants. 3) Investigate the temperature effect on the size and aggregation number of the micelles of these nonionic surfactants below their cloud points. 4) Examine the effect of added electrolytes on the cloud points of the micellar solutions of these nonionic surfactants and optimization of the preconcentration factor. 5) Develop a simple, but practical cloud point extraction technique to extract HOCs from aqueous samples. The recovery efficiency of HOCs will be correlated with the molecular structure and the HLB values of the surfactants of the same homolog. 6 Chapter 1 Introduction 1.3 Organization Chapter one is the introduction of the thesis. It gives the brief introductions on the cloud-point extraction and solubilization of HOCs. Chapter two is the background section, which includes the literature and theoretical reviews. Chapter three describes the materials and methods. It also outlines the experimental procedures. Chapter four presents the equilibrium solubilization data of HOCs by selected nonionic surfactants as well as selected properties of micelles at different temperatures. Chapter five focuses on the aqueous phase behavior of selected nonionic surfactants, such as clouding phenomena and effect of electrolytes in cloud point temperature as well as the optimization of preconcentration factor, which governed on the recovery efficiency. Chapter six gives the experimental results of cloud-point extraction and recovery efficiency of HOCs by selected nonionic surfactants as well as the effect of salts on recovery efficiency along with discussion in details. Chapter seven is the conclusion section of the thesis. 7 Chapter 2 Literature Review Chapter 2 Literature Review 2.1 Solubilization by Nonionic Surfactants A surfactant molecule is amphiphilic, having two distinct structure moieties, a hydrophilic head and one or two hydrophobic tails. The tail, usually a long hydrocarbon or fluorocarbon chain, acts to reduce solubility in water while the polar head, often ionizable, has the opposite effect. These unique amphiphilic structures and properties give surfactants many applications. In addition, if the surfactant concentration exceeds a certain threshold, called the critical micelle concentration (CMC), at a temperature higher than its Krafft temperature, surfactant monomers in aqueous solution will tend to aggregate to form micelles in colloidal-size to achieve segregation of their lipophilic parts from water. The major types of micelles appear to be small spherical, elongated cylindrical (rod-like), lamellar (disk-like), and vesicles. Under such conditions, the hydrophobic organic compounds are incorporated in the hydrophobic cores of the micelles, which is often referred to as solubilization (Rosen, 1989). 2.1.1 Locus of Solubilization The exact location in the micelle, at which solubilization occurs i.e., the locus of solubilization varies with the nature of the material solubilized and the type of the interaction occurring between surfactant and solubilizates (Rosen, 1989). Data on sites of solubilization are mainly obtained from studies on the solubilizates before and after solubilization by using X-ray diffraction, UV Spectroscopy and NMR spectrometry. 8 Chapter 2 Literature Review Based on these studies, solubilization is believed to take place at a number of different sites in the micelle shown in Figure 2.1: (1) on the surface of the micelle, at the micelle-water interface; (2) between the hydrophilic head groups (e.g., in polyoxyethylated materials); (3) in the palisade layer of the micelle between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprise the outer core of the micelle interior; (4) more deeply in the palisade layer; and (5) in the inner core of the micelle. 4 1 2 1 5 3 Figure 2.1 Loci of solubilization of material in a surfactant micelle Saturated aliphatic and alicyclic hydrocarbons and other types of molecules that are not polarized or not easily polarizable are solubilized in aqueous media in the inner core of micelle between the ends of hydrophobic groups of the surfactant molecules. Large polar molecules, such as long chain alcohols or polar dyestuffs, are believed to be solubilized, in aqueous media, mainly between the individual molecules of the 9 Chapter 2 Literature Review surfactant in the palisade layer. In that type of solubilization, the polar group of solubilizate oriented toward the polar group of the surfactants and the nonpolar portions oriented toward the interior of the micelle. Small polar molecules in aqueous medium are generally solubilized close to the surface in the palisade layer or by adsorption at the micelle-water interface. In concentrated aqueous surfactant solutions, the loci of solubilization for a particular type of solubilizate with high polarity are solubilized mainly in the outer region of the micellar structures, whereas nonpolar solubilizates are contained in the inner portions. 2.1.2 Factors Affecting Solubilization There are many factors affecting the extent of solubilization. They may include the structure and nature of surfactant or the solubilizate, addition of electrolyte, effect of polymeric organic additives, temperature, formation of mesophases, etc. (Rosen, 1989). The molecular structure and hydrophilic-lipophilic balance number (HLB) are widely used to predict the solubilization power of the hydrophobic solubilizates (Edwards et al., 1991; Diallo et al., 1994; Li et al., 2002). The HLB number, firstly introduced by Griffin (Myers, 1988), is one of the common indicators of surfactant suitability for a given application. For an ethoxylated nonionic surfactant, the HLB value may be expressed as: HLB = (degree of ethoxylation in %)/ 5 (2.1) The HLB value defined by Equation 2.1 ranges from 0 to 20. The lower HLB value the surfactant has, more hydrophobic it is; and vice versa. 10 Chapter 2 Literature Review The addition of electrolyte could increase solubilization of hydrophobic solutes, resulting from salting out effect of the surfactant, which is often manifested as a lowering the cloud point and an increase in the aggregation number (Pennell et al., 1997, Li and Chen, 2002). Generally, an increase in temperature will result in an increase in the extent of solubilization for both polar and nonpolar solubilizates (Rosen, 1989). With an increase in temperature, nonionic surfactant solutions tend to increase the aggregation numbers and/or the size of micelles, leading to increasing solubilization capacity (Pennell et al., 1997, Li and Chen, 2002). 2.1.3 Quantitative Study on Solubilization To describe solubilization phenomena in general, a weight solubilization ratio (WSR) is commonly adopted and defined as the weight of organic compound solubilized by a unit mass of surfactant, and the molar solubilization ratio (MSR) is defined as the number of moles of organic compound solubilized by per mole of surfactant added to the solution (Attwood and Florence, 1985; Edwards et al., 1991). In the presence of hydrophobic organic compounds, WSR as well as MSR can be obtained from the slope of the solubilization curve. The MSR for solubilization of HOCs can be calculated as follow: MSR = C − Ccmc Csurf − CMC (2.2) where C is the apparent solubility of HOC in a micellar solution at the particular surfactant concentration equal to Csurf; and Ccmc is the saturation concentration of HOC at CMC. 11 Chapter 2 Literature Review The micelle-water partition coefficient, Km, is a parameter that indicates the distribution of organic molecules between the micellar phase and the aqueous phase. Km = Xm Xa (2.3) where Xm is the mole fraction of HOC in the micellar pseudophase and Xa is the mole fraction of HOC in the micelle-free aqueous phase. The mole fraction of HOC in the micellar pseudophase, Xm, can be calculated in terms of the MSR (Edwards et al., 1991). Xm = MSR 1 + MSR (2.4) The mole fraction of HOC in aqueous phase is approximated for dilute solutions by Xa = CcmcVa, mol (2.5) where Va, mol is the molar volume of water at the experimental temperature. In this study, a comparison of solubilization capacity of hydrophobic organic compounds (HOCs) by selected nonionic surfactants is investigated. The correlation between micelle-water partition coefficients with octanol-water partition coefficients of HOCs is also studied to find out the hydrophobicity of these nonionic surfactants. Additionally, the temperature effects on the micelle properties, such as the hydrodynamic radius and aggregation number are measured as well. 12 Chapter 2 Literature Review 2.2 Aqueous Phase Behavior of Nonionic Surfactants 2.2.1 Mechanism of Clouding Phenomenon When a micellar solution of a weakly polar surfactant, such as nonionic or zwitterionic surfactant is heated above a certain temperature, a clear solution becomes turbid, which is called the cloud point temperature or lower consolute temperature (LCST). Above the cloud point, the homogeneous micellar solution will separate into two immiscible phases; surfactant-rich phase which contains most of the surfactant, and excess water phase that is almost micelle-free and has surfactant only around its CMC at that temperature. The phase separation is reversible. The two phases merge to form a clear phase again, when the mixture is cooled to a temperature below the cloud point. The mechanism of clouding phenomena has been extensively studied (Clint, 1991). The studies utilizing classical light scattering have indicated that a rapid increase in micellar aggregation number when the isotropic micellar solution approaching the twophase boundary accounts for the noticeable turbidity change in the solution. However, a more recent explanation based on the small-angle neutron-scattering data suggests that only a modest growth in micelle size occurs, but that intermicellar interaction increases markedly as the two-phase boundary is approached. 2.2.2 Factors Affecting Cloud Point The solubility of nonionic surfactant in water decreases with increasing temperature by the dehydration of ethylene oxide (EO) chains. Below the cloud point, surfactant dissolves in water, and above it, water dissolves in surfactant. Schott (1969) suggested that at constant EO content, the cloud point could be lowered (1) by decreasing the molecular weight of the surfactant; (2) with a broader distribution of POE chain 13 Chapter 2 Literature Review lengths; (3) with branching of the hydrophobic groups; (4) with a more central portion of the POE hydrophilic group in the surfactant molecule; and (5) with the replacement of the terminal hydroxyl group by a methoxyl and with that of ether linkage between the hydrophilic and hydrophobic groups by an ester linkage. For a particular class of nonionic surfactant, the number of ethylene oxide units in the molecule has dramatic effect on the cloud point. As the number of ethylene oxide units increases, the cloud point becomes higher. Additionally, cloud point of nonionic surfactant has also been shown as a function of its own concentration (Sadaghiania et al., 1990; Gu and Galera-Gómez, 1995; Bai et al., 2001; Li and Chen, 2002). The cloud point of nonionic surfactants is very sensitive to some additives, such as hydrocarbons, polymers, alcohols, electrolytes and second surfactants. The effects of various additives on the cloud point of nonionic surfactants have been studied. It is important to understand the magnitude and nature of these additive effects as well as the mechanisms involved, so that surfactant systems can be suitably tailored to exhibit the clouding behavior at the desired temperature (Goel, 1998). The most common additives are electrolytes and secondary surfactants, such as anionic or cationic surfactants. The electrolytes, which can increase the cloud point, cause a “salt-in” effect. On the other hand, those depressing the cloud point are called “saltout” effect. Bai et al. (2001) studied the effect of several kinds of electrolytes including NaCl, NaI, Na2SO4, MgCl2 and CaCl2 on the cloud points of the Tergitol 15-S-7 micellar solutions. The results showed that NaI could increase the cloud point temperature, whereas the rest of the salts can lower the cloud point temperature. In addition, among the salts lowering the cloud point of Tergitol 15-S-7, Na2SO4 acts 14 Chapter 2 Literature Review most effectively to decrease the cloud point. Furthermore, Li et al. (2002) concluded that sodium phosphate could depress more sufficiently the cloud point of both Tergitol 15-S-7 and Neodol 25-7 micellar solutions than sodium sulphate. Mixed ionic-nonionic surfactant systems are considerably more important in many applications (Gu and Galera-Gómez, 1995). For example, in the field application of enhanced oil recovery, mixed surfactant systems show advantageous solubilization behavior; and exhibit cloud points higher than those of pure nonionic surfactants and Kraft points lower than those of pure ionic surfactants. For a particular class of nonionic surfactants, the cloud point of a dilute mixed solution generally lies somewhere intermediate between the cloud points of individual surfactants present in the mixtures (Gu, T. and Galera-Gómez, 1995). In addition, Schott (2003) proposed a linear relation between the cloud point and the number of oxyethylene units of watersoluble polyoxyethylated nonionic surfactants from their experimental observations. 2.2.3 Application of Clouding Phenomenon By using the clouding phenomenon of nonionic surfactants, the most significant application is the cloud-point extraction technique. These include the extraction of metal ions, the separation and purification of biomolecules and the extraction of environmental pollutants. As an alternative to the traditional solvent extraction, cloudpoint extraction technique has a promising future. 2.3 Cloud-Point Extraction Recently, an extraction technique based on the clouding phenomenon of nonionic surfactants is becoming more and more attractive. As the temperature of a nonionic 15 Chapter 2 Literature Review surfactant solution is increased, the solubility of surfactant in water diminishes due to the dehydration of the polyoxyethylene chain of hydrophilic group in surfactant molecules. Above the cloud point, the homogeneous micellar solution will separate into two immiscible phases. The hydrophobic organic compounds originally present in the solution will be extracted into the oil-like surfactant-rich phase, while leaves only a very small part in the micelle-free aqueous solution phase. As a new separation technique, CPE offers some advantages over traditional solvent extraction. A small volume of surfactant-rich phase allows not only to preconcentrate and extract analyte in one step, but also to achieve higher preconcentration factor and higher recovery. Water is used as the main solvent, which is benign to environment so that the extraction process is less toxic and cost-effective. The other advantage lies in the fact that the presence of surfactant can minimize losses due to the adsorption of HOC onto container. Additionally, the benefit of CPE arises from the good compatibility between surfactant-rich phase and hydroorganic mobile phase in the HPLC analysis, which offers great convenience to the analysis of trace quantities of hydrophobic materials. The clouding phenomenon is firstly utilized for the extraction of metal ions from aqueous solutions after the addition of a nonionic surfactant, such as polyoxyethylene7.5-nonylphenyl ether (PONPE-7.5), and an appropriate chelating agent (Watanabe and Tanaka, 1978). The scope of the CPE technique was then extended to the protein separation by using a nonionic surfactant Triton X-114 (Bordier, 1981; Saitoh and Hinze, 1991). More recently, studies were initiated on the extraction of organic compounds of environmental concern (Böckelen and Niessner, 1993; Hinze, et al., 16 Chapter 2 Literature Review 1989; Pinto et al., 1994; Ferrer et al., 1996; Fernándz et al., 1998; Bai et al., 2001;Pino et al., 2001; Materna et al., 2001; Li and Chen, 2002). The typical surfactants commonly used in the CPE processes are Triton series ( Union Carbide), Igepal Series (Rhodia) and PONPE series (polyetylenegycol nonylphenyl ethers). However, these surfactants often disturb the HPLC analysis of the PAHs using a fluorescence detector, as the π-bonds in these surfactant molecules render large UV absorbance and fluorometric signals (Pinto et al., 1994; Ferrer et al., 1996). To resolve this conflict, sophisticated clean-up steps to separate surfactants from the analytes before HPLC analysis have been proposed (Ferrer et al., 1996). But it leads to the lower recovery of analytes and makes the analyzing procedure much more complicated. To avoid the disturbance, the use of other surfactants like Brij series (primary ethoxylated alcohols), Genapol series and anionic surfactants, such as sodium dodecyl sulphate (SDS) have been suggested as well. By using Brij series and Genapol series as extractants, the extraction has to take place at higher temperatures due to their high cloud points (Fernández et al., 1998; Pino et al., 2001, 2002). In addition, the use of anionic surfactants as an effective extractant in CPE often requires the addition of salts and the adjustment of pH, usually to a very low value (Casero et al., 1999). Bai et al. (2001) and Li et al. (2002) firstly introduced two new kinds of nonionic surfactants, such as Tergitol series and Neodol series for CPE technique. These surfactants have cloud points slightly higher than room temperature. However, small amounts of added electrolytes could reduce their cloud point sufficiently below the room temperature so that CPE can be performed under ambient conditions (22 ºC). As 17 Chapter 2 Literature Review primary and secondary alcohol ethoxylates surfactants, there is no disturbance in HPLC analysis. Moreover, these surfactants are biodegradable. There are some important parameters such as the surfactant concentration, ionic strength, equilibration temperature and time, pH, pressure, initial analyte concentration and so on can influence the recovery efficiency of the CPE process (Fernándz et al., 1998; Quina and Hinze, 1999). In the case of HOC, some factors, such as pH, have only slight or almost no influence on the recovery efficiency. Bai et al. (2001) reported there is no influence on the recovery efficiency by equilibration time. The purpose of this study is to develop a simple, but practical cloud point extraction technique to extract HOC from aqueous solutions with an optimized preconcentration factor. The extraction efficiency of HOC by surfactants with different molecular structures as well as different HLB values is compared. In addition, the effect of salts on acenaphthene recovery is investigated. 2.4 Properties and Applications of Selected Nonionic Surfactants 2.4.1 Tergitol 15-S Series Surfactants Tergitol 15-S surfactants, such as Tergitol 15-S-7 and Tergitol 15-S-9 are biodegradable and fluidic. Tergitol 15-S surfactants are mixtures of linear secondary alcohols react with ethylene oxide. Tergitol 15-S surfactants give excellent detergency, outstanding wetting properties, excellent rinse ability, low foam stabilities, versatile solubility characteristics, low pour point, low neat and aqueous viscosities, narrow aqueous gel range and rapid 18 Chapter 2 Literature Review dissolution rates. Tergitol 15-S surfactants are chemically stable in the presence of dilute acids, bases, and salts, and are compatible with anionic, cationic and other nonionic surfactants. They are also soluble in water, chlorinated solvents and most organic solvents (Union Carbide Corp., 1993). Specific examples of the applications of Tergitol 15-S surfactants include: household and industrial laundry detergents, hard-surface cleaners and degreasers, industrial and institutional cleaners, hydrocarbons and water-based laundry prespotters, car care products, paper deinking, rewetting, pulping and deresinating, oil-in-water emulsions, textile wet processing, dye assist and leveling agents for carpets and textiles, wetting agents, coupling agents, and emulsifiers for fiber lubricants, emulsifier for polyethylene textile softeners, dispersant and wetting agents, metal cleaners and acidcleaning compounds, low-temperature soak-tank cleaning systems, oil field chemicals, water treatment operations, circuit board cleaners and leather hide soaking, tanning, and dyeing operations. 2.4.2 Neodol 25-7 Surfactant Neodol 25-7 surfactant is a high purity and biodegradable clear liquid surfactant, which is widely utilized in high-performance detergent formulations. It is a mixture of primary alcohol ethoxylates. Neodol 25-7 surfactant gives the superior detergency for particular soils, good grease cutting ability, outstanding dishwashing foam performance, spray tower pumping characteristics and suitable skin mildness. Neodol 25-7 is compatible with enzymes, cationic, anionic and other nonionic surfactants. In addition, Neodol 25-7 can be 19 Chapter 2 Literature Review formulated in combination with other ingredients, such as alkyl benzene sulphonate, alpha olefin sulphonate and fatty acid diethanol amide. Neodol 25-7 is soluble in water and most organic solvents. 20 Chapter 3 Materials and Methods Chapter 3 Materials and Methods 3.1 Materials The nonionic surfactants used in this study include Tergitol 15-S-7, Tergitol 15-S-9 (Union Carbide, USA) and Neodol 25-7 (Shell Chemicals). HPLC-grade methanol and acetone were obtained from Fisher Chemical. Reagent grade of acenaphthene, 9chloroanthracene, dibenzofuran, fluoranthene and phenanthrene were purchased from Aldrich. The selected properties of these nonionic surfactants are shown in Table 3.1 and the selected physical properties of HOCs are given in Table 3.2. Analytical grade calcium chloride, sodium chloride, sodium iodide, sodium phosphate and sodium sulphate were obtained from Merck. Deionized water from a Milli-Q purification system (Millipore, USA) having resistivity greater than 18.2 MΩ-cm was used in preparing samples. All chemicals were used as received without further purification. Table 3.1 The properties of selected nonionic surfactants Molecular Formula Molecular Weight, Da HLBa CMCb, mg/l Tergitol 15-S-7 C11-12 H23-31O(CH2CH2O)7..3H 515 12.4 39 Neodol 25-7 C12-15 H23-31O(CH2CH2O)7..3H 515 12.4 9 Tergitol 15-S-9 C11-12 H23-31O(CH2CH2O)8..9H 584 13.3 56 Surfactant a b Calculated values using HLB= degree of ethoxylation in % / 5 Provided by supplier 21 Chapter 3 Materials and Methods Table 3.2 The selected physical properties of HOCs Molecular Weight Aqueous Solubilitya, (ppm) Purity ( %) 154.21 2.92 98 168.00 4.037 98 C14H9Cl 212.68 0.083 98 Fluoranthene C16H10 202.26 0.051 98 Phenanthrene C14H10 178.23 1.00 98 Name Molecular Formula Acenaphthene C12H10 Dibenzofuran C12H8O Molecular Structure O Cl 9-Chloroanthracene a measured 3.2 Apparatus 3.2.1 HPLC The separation and purification of the HOC analytes in the surfactant micellar solutions were carried out by using the Shimadzu HPLC system consisting of one LC10ATVP pump, two DGU-14A degassers, a SIL-10ADVP auto injector, a CTO10ASVP column oven, an SCL-10AVP system controller, and an RF-10AXL fluorescence detector (Figure 3.1). HOC concentrations were obtained from data processed with the Shimadzu software Class-VP 5.03. The stationary phase column was an Agilent PAH column (250×4.6 mm i.d.) packed with 5 µm particles and connected to a Guard cartridge (Agilent 79918PH-534) and the Guard cartridge holder (Agilent 79918PH-100). At least triplicate samples from experiments under the same conditions were drawn to determine the HOC concentration in micellar solutions. The 22 Chapter 3 Materials and Methods mobile phase consisted of 85/15 methanol/water by volume, and its flow rate was 1 ml/min. The methanol was degasified with a helium stream by removing the bubble from the mobile phase to avoid the unexpected high backpressure in the column. Figure 3.1 Shimadzu HPLC system Table 3.3 Fluorescence Characteristics of HOCs λex, nm λem, nm Acenaphthene 215 345 Dibenzofuran 278 316 9-Chloroanthracene 260 390 Fluoranthene 285 441 Phenanthrene 248 395 Compound 23 Chapter 3 Materials and Methods Table 3.3 lists the fluorescence characteristics of HOCs detected by the HPLC fluorescence detector. The detection limit of fluorescence detector for all HOC used in this study is ~ one ppb according to the information supplied by the manufacturer. 3.2.2 Laser Light Scattering Figure 3.2 Laser Light Scattering apparatus The dynamic and static laser light scattering experiments were performed with the apparatus from Brookhaven Instrument Corporation (NY, USA). As shown in Figure 3.2, the system consists of a BI-200SM motor-driven Goniometer, an advanced BI9000AT digital autocorrelator and an Argon-ion laser at 514.5nm (Model 95/2, Lexel). The BIC-Zimm software was used to obtain the molecular weights of micelles, from which the aggregation number could be estimated through the Zimm plot. The 24 Chapter 3 Materials and Methods hydrodynamic radii of micelles were obtained with the BI-DLSW software and the DLS experiments in this study were carried out at a scattered angle equal to 90°. 3.2.3 Water Bath The determination of cloud point temperature was carried out in a temperaturecontrolled water-bath (Polyscience) with a good temperature control within 0.1ºC, internal circulation and digital temperature display. The heating/cooling rate of the water bath is 1ºC/min. 3.2.4 Centrifuge A centrifuge Eppendorf 5810R was used to accelerate the phase separation in cloudpoint extraction. 3.3 Experimental Procedure 3.3.1 Equilibrium Solubilization The solubilization experiments of hydrophobic organic compounds (HOCs) in surfactant micellar solutions were carried out using screw-capped culture tubes of 15ml. HOC was firstly dissolved in HPLC-grade methanol, and then the vials were wetted with such HOC-methanol solutions in the temperature-regulated water bath to dry out the methanol. After being coated with the HOC film, the vials were filled with 10-ml of surfactant solutions having concentrations above the CMC. Precaution was exercised. If methanol was not completely evaporated, it will affect the solubility of HOC in the micellar surfactant solution. The vials were then agitated on an orbital 25 Chapter 3 Materials and Methods shaker maintained at 100 rpm in an air-conditioned room of 22°C over a period up to 7 days before HPLC analysis. After shaking, the vials were allowed to settle for at least 2 hours. Aliquots of the micellar solutions containing HOC were filtered through 0.2 µm cellulose membrane syringe filters to remove fine particles. The syringe filter was first presaturated by filtering 2-ml of the same solution. At least triplicate samples were analyzed by the HPLC and their average value was taken as the HOC solubility. 3.3.2 Micelle Size and Aggregation Number Measurement The surfactant micellar solutions were prepared by dissolving the surfactant in deionized water. Subsequently, the micellar solutions were filtered through the 0.2 µm cellulose membrane syringe filters to remove the trace impurities. The Zimm plot was employed to determine the weight-averaged molecular weight of the surfactant micelle from experimental data based on the static light scattering. The molecular weight and the second virial coefficient can be obtained from the following equation on the Zimm plot: 2 2 Kc 1 ⎛⎜ q R g 1+ = Rθ M w ⎜⎝ 3 ⎞ ⎟ + 2A c 2 ⎟ ⎠ (3.1) where Rθ is the Rayleigh ratio; q is the magnitude of the scattering wave vector; Rg is the radius of gyration; Mw is the weight-averaged molecular weight; c is the surfactant concentration; A2 is the second virial coefficient; and K is the optical constant, which is given from the following equation for vertical polarized incident light: 26 Chapter 3 Materials and Methods 4π 2 no2 ⎛ dn ⎞ ⋅ K= ⎟ 4 ⎜ N A λ0 ⎝ dc ⎠ 2 (3.2) where no is the refractive index of the solvent, the continuous media; λo is the wavelength of incident light in vacuum; NA is Avogadro’s number; and dn/dc is the specific refractive index increment of the micellar solutions. The hydrodynamic radius, Rh, of the micelle was obtained from the dynamic light scattering (DLS), also commonly referred as quasi-elastic light scattering (QELS). In contrast to the static light scattering experiments that focus on the time-averaged intensities at any given scattered angle, the dynamic light scattering experiments use the variation of intensity with time, which contains the information on the random motion of the particles and therefore can be used to measure the diffusion coefficient of the particles. The fluctuating signal in the time-dependence intensity of the scattered light due to the random motion of the particle can be processed by forming the autocorrelation function, g(td), where td is the delay time. For a monodisperse suspension of rigid and globular particles, the autocorrelation function is given by g(td) = A exp( – 2q2 D td) + B (3.3) where D is the translational diffusion coefficient, principle quantity measured by DLS; A is the optical constant determined by the instrument design; and B is a constant background term. 27 Chapter 3 Materials and Methods The measured diffusion coefficient, D, can then be used to determine the particle size using the following Stokes-Einstein equation: Rh = k BT 6πη 0 D (3.4) where kB is the Boltzmann constant; T is the absolute temperature of the solution; and ηo is the viscosity of the fluid. It should be noted that the DLS experiments in this study were all carried out at a scattered angle of 90º. 3.3.3 Measurement of Cloud Point and Preconcentration Factor The surfactant solutions were prepared on the basis of weight percentage in deionized water. The required amount of surfactant was weighed and added into a 100-ml beaker along with deionized water to get the desired concentration. The beaker was then sealed with paper film to prevent the evaporative losses of water during mixing. Total weights of beaker with surfactant micellar solution before and after mixing were measured and the weight losses adjusted by deionized water. In the case of added electrolytes, the required amount of surfactant and electrolyte were firstly balanced and poured into the 100-ml beaker along with deionized water to get the desired concentration. A similar procedure was then carried out as the preparation of single surfactant solution. All the surfactant micellar solutions and solutions containing the mixed electrolytes and surfactants were equilibrated at room temperature for one hour before measuring the cloud point temperature. The cloud point of a micellar solution of each surfactant was determined by visual observation of the temperature, at which the clear solution turns turbid upon being 28 Chapter 3 Materials and Methods heated up and vice versus on cooling. Heating and cooling were regulated around the cloud point. The cloud point temperatures were reproducible within 0.2 ºC. The preconcentration factor is the volume ratio of the bulk solution before phase separation to that of the surfactant-rich phase after phase separation. It was determined with calibrated glass tubes. 3.3.4 Cloud-Point Extraction from Aqueous Solution The cloud-point extraction (CPE) was carried out in 30-ml graduated centrifuge tubes. Electrolytic stock solutions, either sodium sulphate or sodium phosphate, were prepared in deionized water. An HOC stock solution was prepared by dissolving a known amount of HOC into 50-ml of HPLC-grade acetone. The required amount of surfactant was weighed and added into a centrifuge tube along with an aliquot of HOC stock solution and electrolyte stock solution. The final solution is made sure at 25-ml. After being equilibrated statically for 15 minutes, the solutions were centrifuged at 3,500 rpm for 10 minutes to enhance the phase separation. The complete phase separation could be ensured under these conditions (Bai et al., 2001). After phase separation, a 50-µL aliquot from the surfactant rich-phase was withdrawn and transferred into HPLC auto sampling vials, and diluted with 450 µL of deionized water to reduce the viscosity. Triplicate samples were prepared for the HPLC analysis of HOC concentrations in the surfactant-rich phase. To determine the concentration of HOC in the aqueous phase, the surfactant-rich phase was carefully removed and triplicate samples from the aqueous phase were withdrawn for HPLC analysis. The 29 Chapter 3 Materials and Methods recovery efficiency of HOC can be calculated as the percentage of HOC extracted from the bulk solution into the surfactant-rich phase by the following equation: R= Vsr Cs ×100 % Vo Co (3.5) where Vsr and Vo are the volumes of the surfactant-rich phase and the bulk solution respectively; Cs and Co the concentrations of HOC in the surfactant-rich phase and the initial HOC concentration in the bulk solution, respectively. Since surfactant-rich phase is compatible with the hydroorganic mobile phase, no special washing is required to remove the surfactant from the HPLC column. 30 Chapter 4 Solubilization by Nonionic Surfactants Chapter 4 Solubilization by Nonionic Surfactants 4.1 Introduction Solubilization of hydrophobic organic compounds (HOCs) including 9- chloroanthracene, dibenzofuran and three polycyclic aromatic hydrocarbons (PAH) such as acenaphthene, fluoranthene and phenanthrene by selected nonionic surfactants, Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9, was studied. The solubilization capacity of HOC was studied with respect to the effect of molecular structure, as well as the effect of HLB values of surfactants of the same homolog. The correlation between micelle-water partition coefficients of HOCs and their octanol-water partition coefficients were studied to find out the hydrophobicity of selected nonionic surfactants. The change in hydrodynamic radius and aggregation number of surfactant micelles with temperature was measured by the dynamic and static laser light scattering techniques. 4.2 Results and Discussion 4.2.1 Equilibrium Solubilization of HOC by Selected Nonionic Surfactants Solubilization of HOCs by Tergitol 15-S-7 as well as Neodol 25-7 and Tergitol 15-S-9 at 22ºC are shown in Figures 4.1, 4.2 and 4.3, respectively. Their aqueous solubility was taken as their saturated concentration in the surfactant solution, i.e., the concentration not varying with time. The linear enhancement in equilibrium solubility 31 Chapter 4 Solubilization by Nonionic Surfactants 60 Dibenzofuran Acenaphthene 50 Solubility, [mg/l] 9-Chloroanthracene Fluoranthene 40 30 20 10 0 0 200 400 600 800 1000 1200 Surfactant concentration, [mg/l] Figure 4.1 Solubilization of HOC by Tergitol 15-S-7 at 22 ºC 60 Dibenzofuran Solubility, [mg/l] 50 Phenanthrene Fluoranthene 40 Acenaphthene 9-Chloroanthracene 30 20 10 0 0 200 400 600 800 1000 Surfactant concentration, [mg/l] 1200 Figure 4.2 Solubilization of HOC by Neodol 25-7 at 22 ºC 32 Chapter 4 Solubilization by Nonionic Surfactants 60 Dibenzofuran Phenanthrene 50 Solubility, [mg/l] Acenaphthene Fluoranthene 40 9-Chloroanthracene 30 20 10 0 0 200 400 600 800 1000 1200 Surfactant concentration, [mg/l] Figure 4.3 Solubilization of HOC by Tergitol 15-S-9 at 22 ºC above the CMC is consistent with the solubilization data reported for other hydrophobic organic compounds of environmental concern (Kile and Chiou, 1989; Valsaraj and Thibodeaux, 1989; Edwards et. al., 1991; Li et al., 2002). The slope of solubilization curve is the weight solubilization ratio (WSR), which is a dimensionless quantity, equal to the mass ratio of the HOC solubilized to that of surfactant. The corresponding MSR values could be easily calculated from the aforementioned WSR values with information on the molecular weights of the surfactants and the solubilizates. Tables 4.1 and 4.2 list the values of WSR, MSR as well as log Km of HOCs. Among three nonionic surfactants, Neodol 25-7 has higher solubilization capacity of HOCs on the WSR basis. 33 Yalkowsky, 1999 Calculated, refer to Appendix A c d 0.011 5.22c Fluoranthene Howard, 1997 0.020 4.35d 9-Chloroanthracene b 0.022 3.92b Acenaphthene Li et al., 2002 0.031a 4.57c Phenanthrene a 0.044 4.12b Dibenzofuran 0.029 0.049 0.072 0.089a 0.136 MSR (mol / mol) WSR (mol / mol) log Kow Name Tergitol 15-S-7 6.34 5.5 5.19 5.73a 5.3 log Km WSR 0.028 0.018 0.019 0.032 0.056 (mol / mol) 0.071 0.043 0.065 0.093 0.17 (mol / mol) MSR Neodol 25-7 Table 4.1 Comparison of solubilization of HOCs by Tergitol 15-S-7 and Neodol 25-7 surfactants at 22 ºC 6.49 5.71 5.22 5.71 5.47 log Km Chapter 4 Solubilization by Nonionic Surfactants 34 Yalkowsky, 1999 Calculated, refer to Appendix A c d 0.011 5.22c Fluoranthene Howard, 1997 0.020 4.35d 9-Chloroanthracene b 0.022 3.92b Acenaphthene Li et al., 2002 0.031a 4.57c Phenanthrene a 0.044 4.12b Dibenzofuran 0.029 0.049 0.072 0.089a 0.136 MSR (mol / mol) WSR (mol / mol) log Kow Name Tergitol 15-S-7 6.34 5.5 5.19 5.73a 5.3 log Km WSR 0.012 0.011 0.013 0.020 0.060 (mol / mol) 0.035 0.031 0.049 0.065 0.209 (mol / mol) MSR Tergitol 15-S-9 Table 4.2 Comparison of solubilization of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9 surfactants at 22 ºC 6.15 5.56 5.12 5.87 5.47 log Km Chapter 4 Solubilization by Nonionic Surfactants 35 Chapter 4 Solubilization by Nonionic Surfactants Despite of having the same molecular weights and the same HLB values, Tergitol 15S-7 and Neodol 25-7 have different molecular structures. Neodol 25-7 is a mixture of primary alcohol ethoxylates and Tergitol 15-S-7 is a mixture of secondary alcohol ethoxylates. The longer the aliphatic chain of the surfactant, the larger the hydrocarbon (core and palisade) region of the micelle (Yalkowsky, 1999). Because of the longer aliphatic chain length, Neodol 25-7 has a larger micellar hydrophobic core volume and that favors to solubilize more hydrophobic organic compounds. It can be clearly seen by comparing WSR values of both surfactants in Table 4.1. By comparing the solubilization capacity of Tergitol 15-S-7 and Tergitol 15-S-9, the effect of HLB value of surfactants on the solubilization capacity of HOCs is clearly shown (Table 4.2). Tergitol 15-S-7 has a lower HLB value, but higher solubilization capacity expressed in WSR. The HLB number of the surfactant is one of the most widely used indicators of its suitability for a given application (Rosen, 1989). It is a measure of surfactant hydrophobicity; the lower HLB value the surfactant has, the more hydrophobic it is. In aqueous solution, surfactant with lower, but not too small HLB values will tend to form micelles that contain more hydrophobic environment in the core or the palisade shells of the micelles, where hydrocarbons tend to reside. That is, the solubility of the hydrocarbon solubilizates increases accordingly (Rosen, 1989). The HLB value is a good indicator to judge the solubilization capacity of surfactant of the same homolog. Another explanation is that the surfactants with lower HLB values, if able to form micelles, will form larger micelles. That is, the core or the shell of the micelles has a larger volume, compared to that from the surfactant of the same series, but having a 36 Chapter 4 Solubilization by Nonionic Surfactants larger HLB value, which could accommodate more hydrocarbon molecules. This effect can also be observed from the cloud point temperature of the surfactant. In general, the surfactants with lower HLB values have lower cloud points. As approaching the cloud point, the surfactant will tend to dehydrate and the micelles will aggregate and grow. This will be discussed later in the next section. Moreover, Valsaraj and Thibodeaux (1989) concluded that the solubilization site for the same solute in different micelles might change depending on various factors, such as the type of surfactant (ionic or nonionic), chain length of the surfactant, the ionic groups of the surfactant molecule and the type of micelle formed (e.g., spherical, rod-shape or bilayered). Li and Chen (2002) suggested that the HLB value cannot be used as the sole factor to account for the solubilization capacity of the surfactant having a different structure, but the HLB value can be used as an indicator of solubilization capacity when a surfactant of the same homolog is employed. It is consistent with our experimental observations on the solubilization capacities of HOCs, except dibenzofuran, by Tergitol 15-S-7 and Tergitol 15-S-9. The molecular structure and the nature of solubilizates profoundly influence the solubilization capacity. Fluoranthene is the most hydrophobic based on its log Kow value among the five HOCs (Table 4.1 or Table 4.2), so that the solubilization locus of the fluoranthene might be in the deeper core of each surfactant micelles. In addition, Neodol 25-7 micelles have a larger hydrophobic core volume compared to Tergitol 15S-7 micelles and Tergitol 15-S-9 micelles, so that the fluoranthene molecule will reside more in the inner core of Neodol 25-7 micelles. This can be clearly seen by comparing WSR values of fluoranthene by all selected surfactants. A slightly more polarizable 37 Chapter 4 Solubilization by Nonionic Surfactants compound, such as dibenzofuran, the locus of solubilization is believed to be mainly on the surface of micelle and perhaps between the hydrophilic head groups and the shallower palisade layer of the micelle. Hence, the WSR value of dibenzofuran should be much larger in the more hydrophilic surfactant, Tergitol 15-S-9, compared to Tergitol 15-S-7 and Neodol 25-7. The locus of solubilization of acenaphthene, 9chloroanthracene and phenanthrene might be either in the inner core of micelle or more deeply in the palisade layer. As a matter of comparison, the micelle-water partition coefficients of these HOCs by selected nonionic surfactants are calculated and plotted with respect to their octanolwater partition coefficients (Figure 4.4). The correlation can be expressed as Tergitol 15-S-7: log Km = 0.90 log Kow + 1.64 (4.1) Neodol 25-7: log Km = 0.94 log Kow + 1.58 (4.2) Tergitol 15-S-9: log Km = 0.86 log Kow + 1.86 (4.3) The findings are in accord with the results of Li and Chen (2002), who investigated the solubilization of four PAH compounds, such as fluorene, naphthalene, phenanthrene and pyrene by nonionic surfactant, Tergitol 15-S-7, and reported a good linear relationship between log Km and log Kow. They also obtained a slope of 0.85 and an intercept of 1.87 on the log Km- log Kow curve. Additionally, Valsaraj and Thibodeaux (1989) studied the solubilization of eleven hydrophobic organic compounds by the anionic surfactant, sodium dodecyl sulfate, 38 Chapter 4 Solubilization by Nonionic Surfactants and presented as well a good linear relationship between log Km and log Kow. They obtained a slope of 0.847 an intercept of 1.09 on the log Km–log Kow curve. Furthermore, Edwards et al., (1991) studied the solubilization of five hydrophobic compounds, including three different PAHs, in Triton X-100 solutions. A good linear relationship between the logarithms of Km and Kow was shown as well, which indicated a slope of about 0.81 and an intercept of 1.85 on the log Km–log Kow curve. This indicates that among these three nonionic surfactants, Neodol 25-7 has the greatest solubilization capacity for HOCs due to the larger core volume of micelles. Between Tergitol 15-S-7 and Tergitol 15-S-9, the former has a greater capacity to solubilize more HOCs due to its larger hydrophobic core volume. 39 Chapter 4 Solubilization by Nonionic Surfactants Tergitol 15-S-7 Neodol 25-7 6.5 Tergitol 15-S-9 y = 0.94x + 1.58 2 R = 0.99 log Km 6 y = 0.86x + 1.86 2 R = 0.96 5.5 y = 0.90x + 1.64 2 R = 0.99 5 3.5 4 4.5 5 5.5 log Kow Figure 4.4 Correlation of log Km and log Kow for HOCs in selected nonionic surfactants 40 Chapter 4 Solubilization by Nonionic Surfactants 4.2.2 Determination of Micelle Size and Aggregation Number of Selected Nonionic Surfactants It has been known that surfactant molecules in aqueous solution aggregate at low concentrations to form micelles. Table 4.3 provides the properties of micelles obtained from the laser light scattering measurement. It is worth mentioning that with the light scattering technique, it still cannot obtain direct information regarding the micellar volume or the core volume of the micelles. However, information on the hydrodynamic radius of the micelle can usually be employed as an indicator of the micellar size. The aggregation number of micelles at different temperatures was determined by measuring the weight-averaged molecular weight of the micelles using the static light scattering and the Zimm plot. The growth of a micelle is more rapid when the temperature increases from 22°C to 30 °C for Tergitol 15-S-7, from 30 °C to 40 °C for Neodol 25-7 and from 40 °C to 55 °C for Tergitol 15-S-9. This is very common for nonionic surfactants near the cloud points (Rosen, 1989). The cloud point temperatures of these surfactants at 1 wt% are 38 °C, 46.2 °C and 62 °C for Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9, respectively. Increasing the temperature from 15 to 30 ºC, the aggregation numbers in Tergitol 15S-7 micelles increase from 276 to 777. Likewise, increasing the temperature from 30 to 40 ºC, the aggregation numbers in Neodol 25-7 micelles increases from 965 to 1330. Also increasing the temperature from 40 to 50 ºC, the aggregation numbers in Tergitol 15-S-9 micelles increase from 120 to 154. Similarly, hydrodynamic radii of the micelles grow from 9 to 31 nm for Tergitol 15-S-7 micelles, 22 to 31.2 nm for Neodol 25-7 micelles and 10.5 to 19 nm for Tergitol 15-S-9 micelles at the aforementioned respective temperature ranges. 41 Chapter 4 Solubilization by Nonionic Surfactants Due to difficulties present in the direct measurement, Tanford (1980) estimated the core volume of a micelle, Vc in Å3, by the following equation: Vc = Nag [27.4 + 26.9 (Nc – 1)] (4.4) where Nag is the aggregation number and Nc is the number of carbon atoms in the surfactant lipophile. Subsequently, Diallo et al. (1994) modified Equation (4.4) and proposed the following approximate equation to quantify the micellar core volume, Vm, of dodecyl alcohol ethoxylates (Witco) that were employed in their study to solubilize the BTX solubilizates (i.e., benzene, toluene and xylene): Vm = Nag (Vs + 4 NEO . Vw) (4.5) where Vs is the surfactant molecular volume; NEO is the number of ethylene oxide groups; and Vw is the molecular volume of water. Equation (4.5) was derived based on the assumptions that (i) the total volume of a micelle is equal to the volume of its core and hydrated polyoxyethylene shell volumes; (ii) in average four water molecules are bound to each ethylene oxide monomer; and (iii) the micellar aggregation number is not affected by solubilization. As mentioned before, the first assumption is generally true for small nonpolar, but polarizable hydrocarbons, such as benzene, which can be solubilized either in the palisade layer or in the inner hydrophobic core of the micelle (Rosen, 1989). For the large nonpolar, but slightly polarizable molecules like phenanthrene, they will be mainly solubilized in the inner core of the micelle and perhaps in the deep palisade layer near the core. For the slightly more polarizable molecules like dibenzofuran, which is solubilized mainly on the surface of the micelle 42 Chapter 4 Solubilization by Nonionic Surfactants and between the hydrophilic head groups and the palisade layer of the micelle. Therefore, the core volume of the Tergitol surfactants and Neodol surfactant micelles estimated by Equation 4.5 may still be able to render useful information on the qualitative estimation of solubilization capacity. The calculated values of the core volume Vc and the micellar volume, Vm, are given in Tables 4.3 and 4.4, with Vs and Vw obtained from the density data and the average molecular weight of surfactant and water at the experimental temperature. Diallo et al. (1994) pointed out that the capacity of ethoxylated nonionic surfactants to solubilize alkanes is governed primarily by the volume of the micelles. As mentioned before, the larger core volume of Neodol 25-7 solubilized more hydrophobic compounds in the hydrophobic core of micelles; therefore contributing to the enhancement in solubility. Pennell et al. (1997) reported that a decrease in micellar core volume could reduce the solubilization capacity, which is again confirmed in this study on the solubilization capacities of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9. 43 Calculated value using Equation 4.5 c 477 nm3 Micellar core volumec, nm3 Calculated value using Equation 4.4 93 Core volume of a micelleb, b 9 Hydrodynamic radius, nm Li et al., 2002 276 Aggregation number a 2.13 x 105 1.42 x 105 Molecular weight, Da 716 139 11 444 22 ºC 30 ºC 1344 286 31 777 4.00 x 105 Tergitol 15-S-7a 15 ºC Properties 938 174 12.7 522 2.69 x 105 15 ºC 1163 216 19.2 647 3.49 x 105 22 ºC 1734 322 22 965 4.97 x 105 30 ºC Neodol 25-7 2390 444 31.2 1330 6.85 x 105 40 ºC Table 4.3 A comparison of properties of micelles of Tergitol 15-S-7 and Neodol 25-7 surfactants obtained from Laser Light Scattering Chapter 4 Solubilization by Nonionic Surfactants 44 Calculated value using Equation 4.5 c 477 nm3 Micellar core volumec, nm3 Calculated value using Equation 4.4 93 Core volume of a micelleb, b 9 Hydrodynamic radius, nm Li et al., 2002 276 Aggregation number a 2.13 x 105 1.42 x 105 Molecular weight, Da 716 139 11 444 22 ºC 30 ºC 1344 286 31 777 4.00 x 105 Tergitol 15-S-7a 15 ºC Properties 191 33 6.6 94 5.50 x 104 15 ºC 195 34 7.8 95 5.55 x 104 22 ºC 209 37 8.6 103 5.95 x 104 30 ºC Tergitol 15-S-9 243 43 10.5 120 6.71 x 104 40 ºC 312 55 19 154 9.01 x 104 55 ºC Table 4.4 A comparison of properties of micelles of Tergitol 15-S-7 and Tergitol 15-S-9 surfactants obtained from Laser Light Scattering Chapter 4 Solubilization by Nonionic Surfactants 45 Chapter 4 Solubilization by Nonionic Surfactants 4.3 Conclusions The apparent solubilities of HOCs were measured in solutions of ethoxylated nonionic surfactants. The solubility of HOCs increases linearly with the surfactant concentration above the CMC. The slope of the log Km–log Kow curve appears to indicate that Neodol 25-7 has the highest solubilization capacity followed by Tergitol 15-S-7, and then Tergitol 15-S-9 has the lowest solubilization capacity. Additionally, an HLB value of surfactants significantly influences the solubilization capacity, as lower HLB values of surfactants have the larger hydrophobic core volume that favors to solubilize more hydrophobic compounds. Static and dynamic laser light scattering results indicated that the solubilization capacity of HOCs was probably governed by different aggregation numbers and the micellar core volume of selected nonionic surfactants as well. 46 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 5.1 Introduction Aqueous phase behavior of micellar solutions of selected nonionic surfactants at different temperatures was studied. Effect of added electrolytes on cloud points of these nonionic surfactants was investigated along with the optimization of preconcentration factor that dominates the recovery efficiency of the CPE technique. 5.2 Results and Discussion 5.2.1 Aqueous Phase Behavior of Selected Nonionic surfactants The phase diagrams of the micellar solution of the nonionic surfactants are shown in Figure 5.1. L refers to the single isotropic phase region, while 2L refers to the twophase region. The clouding phenomenon arises from the distinct change in the interaction between micelles and water with temperature. When the temperature approaches the cloud point, the interactions that are repulsive at lower temperatures apart from cloud point are becoming attractive. The water that dehydrates the POE chains is more structured (i.e., lower enthalpy and entropy) than bulk water (Clint, 1991). When the hydration layers of two approaching chains overlap, some water molecules are partially excluded from contact zone, this causes an increase in enthalpy and entropy of the system. At the cloud point, the entropy gain in the exclusion of water exceeds the repulsive 47 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants enthalpy contribution and the loss in entropy due to increased concentration and, thus, phase separation occurs. 5.2.1.1 Tergitol 15-S-7 – Water system It is an isotropic solution phase at room temperature. The cloud-point of the system is determined by visual observation of the temperature, at which the isotropic solution turns turbid upon being heated up. The cloud-point is found to decrease sharply with concentration for the very dilute solutions, going through a minimum at about 1 wt %. Above 1 wt %, the cloud-point increases slowly with increasing concentration. This phenomenon is similar to the micellar solutions, comprising nonionic surfactants of primary alcohols, such as C12E5 and C12E6 (Mitchell et al., 1983; Strey et al., 1990; Strey and Ber, 1996) as well as the micellar solution of Triton X-100 (Sadaghiania and Khan, 1990). 5.2.1.2 Neodol 25-7 – Water System The micellar solution of Neodol 25-7 yields an isotropic solution at room temperature. The cloud-point is again found to decrease sharply with concentration for the very dilute solutions, passing through a minimum at about 1 wt %. Above 1 wt %, the cloud-point increases slowly with increasing concentration. This phenomenon is similar to that of the micellar solution of Tergitol 15-S-7. 5.2.1.3 Tergitol 15-S-9 – Water System The micellar solution of Tergitol 15-S-9 yields an isotropic solution at room temperature. The cloud-point is found to decrease sharply with concentration for the 48 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants very dilute solutions until a 1 wt % solution, and then slowly decrease again until a 7 wt % solution. Above 10 wt %, there is s monotonous increase in the cloud point. The cloud point temperatures of the 1 wt % Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 micellar solutions are 38, 46.2 and 63 ºC respectively. Although having the same molecular weight and the HLB value, the cloud points of Tergitol 15-S-7 and Neodol 25-7 are still different. This may be attributed to different molecular structure of these two surfactants. This has been clearly seen by comparing the cloud point temperatures of Tergitol 15-S-7 and Tergitol 15-S-9. Tergitol 15-S-7 has an average ethylene oxide number of 7.3 and Tergitol 15-S-9 has an average ethylene oxide number of 8.9, although they share the same structure of the hydrophobic moiety. 49 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants Tergitol 15-S-9 Cloud Point, [OC] 95 90 Neodol 25-7 Tergitol 15-S-7 75 55 35 0 0.5 1 1.5 2 Cloud Point temperature [oC] Surfactant Concentration, [%] 2L 70 50 L 30 0 10 20 30 40 Surfactant concentration [%] Figure 5.1 A phase diagram of selected nonionic surfatant micellar solutions 50 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 5.2.2 Effect of Added Electrolytes on Cloud Points of Selected Nonionic Surfactants Figures 5.2 and 5.3 show the effects of electrolytes on the cloud point temperatures of Neodol 25-7 and Tergitol 15-S-9 micellar solutions. The surfactant concentrations were maintained at 1 wt % by changing the concentration of added electrolytes. It is clearly demonstrated that the addition of salts, except sodium iodide, lowers the cloud point. The effect of electrolytes on raising the cloud point is called “salting-in”, and, on the other hand, that on lowering the cloud point is called “salting-out”. These two opposite effects can be accounted for by the structure-breaking and structure-making nature of water molecules. According to ionic effects on water structure, they either disrupt or enhance the association of water molecules by hydrogen bonds into flickering clusters, shifting the equilibrium toward the left or the right, respectively (Schott, 1997). n H2O ⇔ (H2O)n (5.1) According to various measures of the ion effects on the structure of water, cations such as Li+, Na+, NH4+, Ca2+, Mg2+, etc., and anions, such as F-, SO42-, CO32-, PO43-, CH3COO-, etc., are structure-making ions, while K+, Cl-, Br-, I-, SCN-, NO3-, ClO4-, etc. are structure-breaking ions (Zaslavsky, 1995). Structure-making anions usually have multiple negative charges and generate strong electrostatic fields that not only polarize and immobilize as well as electrostrict the adjacent water molecules, but also induce additional order (entropy loss) beyond the first water layer (Kavanau, 1964). Such an effect will result in the enhanced 51 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants association of water molecules, leading to the reduced extent of hydrogen bond formation between water molecules and the ether group in nonionic surfactants. Hence, the cloud point is decreased. Water-breaking ions have the opposite effect. In general, anions have relatively stronger effects on “salting-in” or “salting-out” than cations (Schott, 1997). For the same cation, Na+, a comparison of anions indicates that the order of salting out effect is PO43- > SO42- > Cl- for both surfactants. It is because of the trivalence of PO43 that its effect on lowering cloud point is greater than other bivalent and monovalent anions. Hence, sodium phosphate is the most efficient cloud point depressor among the Na+ cations in this study. The orders of salting out effects for Neodol 25-7 and Tergitol 15-S-9 in this study are the same as that of Tergitol 15-S-7 reported by Li and Chen (2002). 52 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 70 60 Cloud Point Temperature, [oC] 50 40 30 NaCl 20 NaI CaCl2 Na2SO4 Na3PO4 10 0 0 0.2 0.4 0.6 0.8 1 1.2 Salt Concentration, [M] Figure 5.2 Effect of added electrolytes on the cloud point of a Neodol 25-7 micellar solution 53 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 80 70 Cloud Point Tenperatute [oC)] 60 50 40 30 NaCl 20 NaI CaCl2 Na2SO4 Na3PO4 10 0 0 0.2 0.4 0.6 0.8 1 1.2 Salt Concentration [M] Figure 5.3 Effect of added electrolytes on the cloud point of a Tergitol 15-S-9 micellar solution 54 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 5.2.3 Preconcentration Factor Preconcentration factor is a direct measure of the effectiveness of a preconcentration technique, which is the volume ratio of the bulk solution before phase separation to that of surfactant-rich phase after phase separation. There are different parameters that can alter the extraction process and, accordingly, the preconcentration factor. To obtain the relationship between the preconcentration factor and these parameters, a study of such factors, for instance, different surfactant and salt concentrations, on the extraction process was carried out. As mentioned above, sodium sulphate and sodium phosphate could depress the cloud point temperatures of nonionic surfactants sufficiently. The effect of added sodium sulphate and sodium phosphate on the preconcentration factor was extensively studied. Figure 5.4 gives the preconcentration factors of 3 wt % surfactant at different sodium sulphate concentrations. It is clearly demonstrated that a lower salt concentration gives a smaller preconcentration factor, due to the larger volume in the surfactant-rich phase at lower salt concentrations. Additionally, owing to the different cloud point temperature of each surfactant, the required amount of salt to preconcentrate the PAH and to dehydrate the hydrophilic part of surfactant molecule at 22 ºC is also different. For example, the cloud point of the Tergitol 15-S-7 micellar solution with an addition of 0.4 M sodium sulphate is 19.2, leading to a small preconcentration factor of 5.2. In contrast, the micellar solutions of Neodol 25-7 and Tergitol 15-S-9 do not even show the clouding behavior at the same amount of added salt. It is, thus, imperative to optimize the preconcentration factor for a plausible CPE technique. From the viewpoint of concentrating the analytes present in aqueous solutions, the larger preconcentration factor, e.g., the smaller phase volume in the surfactant-rich phase is 55 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants desired. A lower surfactant concentration gives a higher preconcentration factor. However, it becomes very difficult for sampling and accurate analysis with a very small volume of the surfactant-rich phase. On the contrary, excessive amount of added salt of “salting-out” effect can give the higher preconcentration factor, but it is likely forming the very viscous liquid crystalline phase, instead of the fluidic L1 phase, in the system, making it difficult to separate the surfactant-rich phase. Therefore, optimization of the preconcentration factor is very critical in a feasible CPE technique. Therefore, surfactant concentrations above 1.5 wt % were chosen to conduct CPE experiments in this research. 30 Tergitol 15-S-7 Preconcentration Factor 25 Neodol 25-7 Tergitol 15-S-9 20 15 10 5 0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Salt concentration, [M] Figure 5.4 Precocentration factors at 3 wt % surfactant concentration and different added sodium sulphate concentrations 56 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants The amount of salt required in Neodol 25-7 and Tergitol 15-S-9 to depress the cloud points below the ambient is more than that of Tergitol 15-S-7 due to the higher cloud points of the former than that of Tergitol 15-S-7. Moreover, there is larger amount of water content in the surfactant-rich phase at lower salt concentrations. The analytes could not be preconcentrated in the surfactant-rich phase sufficiently, if enough electrolyte is not added. Consequently, 0.6 M, 0.65 M and 0.7 M of sodium sulphate are adopted for Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 surfactants, respectively. It is not added only to achieve the optimum preconcentration factor, but also to conduct the cloud point extraction (CPE) at room temperature (22 ºC). 30 0.6M Na2SO4, Tergitol 15-S-7 Preconcentration factor 25 0.7M Na2SO4, Tergitol 15-S-9 0.65M Na2SO4, Neodol 25-7 20 0.4M Na3PO4, Tergitol 15-S-7 0.45M Na3PO4, Tergitol 15-S-9 15 10 5 0 1 1.5 2 2.5 3 3.5 4 4.5 Surfactant Concentration, [%] Figure 5.5 Preconcentration factors at different surfactant concentrations Figure 5.5 shows the preconcentration factors of selected nonionic surfactants with additional sodium sulphate and sodium phosphate as a function of the surfactant concentration. As sodium phosphate can depress the cloud point more than sodium 57 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants sulphate, the required amount of it to dehydrate the surfactant molecule is less (Figure 5.5). The preconcentration factors of 1.5 wt % Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 at designated amount of sodium sulphate concentration are found to be 27.2, 21.6 and 23.8, respectively. That is, the PAH initially present in the bulk solution can be concentrated by 20-30 fold prior to sample analysis. Furthermore, it has been known that surfactants are able to improve the mass-transfer of hydrophobic pollutants from solid or non-aqueous phase by decreasing the interfacial tension and by accumulating the hydrophobic compounds in the micelles (Volkering et al., 1995). The aqueous solubility of acenaphthene is 2.92 ppm at 22 ºC and increases to 23.2 ppm, 22.3 ppm and 14.9 ppm at the same temperature in the presence of 1000 ppm of Tergitol 15-S-7, Neodol 25-7and Tergitol 15-S-9, respectively. Similarly, the aqueous solubility of each HOCs, such as 9chloroanthracene, dibenzofuran and fluoranthene increases in the presence of either Tergitol 15-S-7 or Neodol 25-7 or Tergitol 15-S-9 surfactant micellar solution at 22 ºC. It is found that 0.4 M and 0.45 M of sodium phosphate is sufficient in Tergitol 15-S-7 and Tergitol 15-S-9, respectively based on the optimal conditions for the preconcentration factor and phase separation of the bulk solution. 58 Chapter 5 Aqueous Phase Behavior of Selected Nonionic Surfactants 5.3 Conclusion The cloud-point temperatures of 1 wt % Tergitol 15-S-7, Neodol 25-7 and Tergitol 15S-9 are 38, 46.2 and 63 ºC, respectively. The molecular structure of surfactants can influence the physiochemical properties of the surfactants significantly, such as the solubilization property, the CMC value, the cloud point, etc. It is worth mentioning that a smaller HLB value of the surfactant gives a lower cloud point and a smaller CMC value compared to that of higher one in the same surfactant series. The cloud point temperature of selected nonionic surfactants can be sufficiently lowered by adding enough either sodium sulphate or sodium phosphate so that the extraction can be carried out at an ambient temperature of 22 ºC. The preconcentration factor can be changed either by changing surfactant concentration or added salt concentration. A higher preconcentration factor can be achieved either at low surfactant concentration or high salt concentration. 59 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Chapter 6 Cloud-Point Extraction and Recovery Efficiency 6.1 Introduction Cloud-point extraction (CPE) of hydrophobic organic compounds (HOCs) was conducted at a room temperature (22 ºC) by adding sufficient sodium sulphate or sodium phosphate into the nonionic micellar solutions. Recovery efficiency was studied as a function of different surfactant concentrations and initial analyte concentrations. The comparison of recovery efficiency was presented with regard to the different molecular structure of surfactants having the same molecular weights and HLB values, as well as the surfactants of the same series, but with different HLB values. In addition, the effect of salts on acenaphthene recovery was investigated. 6.2 Results and Discussion 6.2.1 Extraction by Tergitol 15-S-7 The greatest advantage of using Tergitol 15-S-7 as an extractant is that Tergitol 15-S-7 does not yield any large fluorometric signals in the UV range (Bai et al., 2001). Hence, the complicated clean-up procedures and undesirable masking of the UV chromatographic peaks of HOCs in the effluent could be avoided by using such a surfactant. Based on the study of cloud point and preconcentration factor, the cloud-point extraction is carried out using 0.6 M sodium sulphate, an appropriate amount of Tergitol 15-S-7 and 10 minutes of centrifugation at 3,500 rpm. It is sufficient to wait 10 minutes for samples reaching an equilibrium before centrifugation (Bai et al., 60 Chapter 6 Cloud-Point Extraction and Recovery Efficiency 2001). It is noteworthy that the use of an anionic surfactant, sodium dodecyl sulphate (SDS), with the addition of HCl in preconcentrating PAHs required waiting for 24 hours before centrifugation for the surfactant-rich phase to be formed (Sicilia et al., 1999). In contrast, the use of Tergitol 15-S-7 is able to shorten the equilibration time to only 10 minutes. The addition of more sodium sulphate will yield a higher preconcentration factor, but the formation of a viscous crystalline phase floating on top of the aqueous phase occurs. It makes accurate separation of the surfactant-rich phase from the bulk phase much more difficult. Additionally, a lower surfactant concentration will yield a higher preconcentration factor, due to the smaller phase volume in the surfactant-rich phase. Again, it is very difficult for sampling and accurate analysis. Consequently, surfactant concentrations above 1.5 wt % were chosen to conduct CPE experiments. After phase separation, 20 µl of the surfactant-rich phase is directly injected to HPLC for sample analysis. The recovery of HOC in the surfactant-rich phase is calculated from the measured concentrations of HOC by HPLC and from the phase volume of the preconcentration factor measurement. The recovery percentage reported in this study is reproducible within 7 %. Indeed, the presence of very low concentration of HOC will not alter the cloud point significantly and, accordingly, the preconcentration factor. That is, the preconcentration factors are not much different from those obtained from the HOC-free micellar solutions. 61 Chapter 6 Cloud-Point Extraction and Recovery Efficiency 6.2.1.1 Recovery as a function of surfactant concentration Figure 6.1 displays the recovery efficiency of HOCs obtained by injection of the HOCcontaining surfactant-rich phase after cloud-point extraction using 10 ppm of initial analyte concentration, 0.6 M sodium sulphate, Tergitol 15-S-7 surfactant and 10 minutes of centrifugation at 3,500 rpm. Although the preconcentration factor decreases with increasing surfactant concentration, opposite trends were observed in analyte recovery. 120 Recovery, [%] 110 100 90 Acenaphthene 9-Chloroanthracene 80 Dibenzofuran Fluoranthene 70 1 2 3 4 Surfactant Concentration, [%] Figure 6.1 The effect of Tergitol 15-S-7 surfactant concentrations on HOC recovery Recovery exceeds 100 %, especially at lower HOC and higher surfactant concentrations. It was speculated that (i) it might be owing to the difficulties in the sample manipulation of the surfactant-rich phase, due to its high viscosity (Fernandez et al., 1998), or (ii) the modification in the microenvironment of the analyte due to the presence of the surfactant might alter spectroscopic properties (absorbance or 62 Chapter 6 Cloud-Point Extraction and Recovery Efficiency luminescence intensity) when employing UV-visible or fluorescence detection (Wu et al., 1998; Pelizzetti E., 1990), although Tergitol 15-S-7 does not have any absorbance in the UV range. Acenaphthene and fluoranthene are found to have relatively high recovery efficiencies of 91 and 92 %, respectively, for Tergitol 15-S-7 surfactant even at low surfactant concentration, 1.5 wt %. The recovery is slightly independent of the surfactant concentration. The recovery of dibenzofuran is very low at lower surfactant concentrations, but interestingly, the recovery efficiency increases monotonously with increasing surfactant concentrations. To achieve higher recovery, higher surfactant concentrations, at least 3 wt %, should be employed for this kind of organic compound. In contrast, the recovery efficiency of 9-chloroanthracene is only around 85 % at 1.5 wt % surfactant concentration. It is interesting to note that the recoveries of the most polar phenolic compounds increased significantly with increasing Genapol X-080 surfactant concentration from 0.5 to 3 % (v/v), and the increase in recovery becomes small in solutions containing Genapol X-080 at levels higher than 3 vol % (Santana et al., 2002). In contrast, the recoveries of the most hydrophobic phenolic compounds have only very small gains in the same concentration range of Genapol X-080 (0.5 to 3 vol %), and level off (> 3 vol %). Similarly, the recoveries of dibenzo-p-dioxins increased with increasing POLE surfactant concentrations from 1 to 3 % (w/v) and remained practically constant at concentrations greater than 3 % (w/v) (Santz et al., 2002). 63 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Based on our findings regarding recovery efficiency and surfactant concentration, a 3 wt % surfactant solution is preferably chosen for the investigation on the effect of initial analyte concentration on the recovery efficiency. 6.2.1.2 Recovery as a function of initial analyte concentration 120 Recovery, [%] 110 100 90 Acenaphthene 9-Chloroanthracene 80 Dibenzofuran Fluoranthene 70 0 5 10 15 20 25 Initial analyte concentration, [mg/l] Figure 6.2 The effect of initial HOC concentration on its recovery using 3 wt % Tergitol 15-S-7 and 0.6 M sodium sulphate Figure 6.2 illustrates the recovery efficiency of HOC by Tergitol 15-S-7 as a function of initial analyte concentration. It is interesting to note that the recovery of each HOC is found to depend on the initial concentration of HOC. Increasing the initial concentration from 5 to 20 ppm, the recovery efficiency diminishes from 114 to 96 %, from 100 to 89 %, from 106 to 81 % and from 100 to 95 % for acenaphthene, 9chloroanthracene, dibenzofuran and flouoranthene, respectively. Interestingly, Casero et al. (1999) reported that the recovery efficiency of pyrene decreases from 95 to 83 %, 64 Chapter 6 Cloud-Point Extraction and Recovery Efficiency when the initial analyte concentration increases from 0.1 to 2.5 ppm. It is possibly owing to the adsorption of HOC onto the container walls. The recoveries of acenaphthene and 9-chloroanthracene are high at low initial analyte concentrations, but their recoveries decrease with increasing initial analyte concentrations. Dibenzofuran gives a higher recovery at low initial analyte concentrations, but recovery decreases too much at higher initial analyte concentrations this may be attributed to adsorption of analyte onto containers. The recovery of fluoranthene is mostly independent of initial analyte concentrations. Though it is still not clear on the effect of the initial analyte concentration on the recovery efficiency, a hypothesis is attempted to explain the experimental observations. With a very low initial concentration of the analyte, it is expected to have more difficulties on the accurate determination of the actual analyte concentration. That is, the experimental error may lead to the overestimation of the analyte concentration in the surfactant-rich phase. Moreover, it has been observed that the synergistic molecular solubilization of hydrophobic substances, like PAHs, has been reported as well (Kile & Chiou, 1989: Li & Chen, 2002). It is also possible that such synergism takes place again in the systems of our study, so that more portions of analytes will stay in the water phase until they reach their optimal concentration. Thus, the recover efficiency is decreasing upon increasing the initial analyte concentration. 6.2.2 Extraction by Neodol 25-7 Similar to Tergitol 15-S-7, Neodol 25-7 does not give any large fluorometric signals in the UV range (Li et al., 2002). Therefore, the complicated clean-up procedures and 65 Chapter 6 Cloud-Point Extraction and Recovery Efficiency undesirable masking of chromatographic peaks of HOCs in the UV range by HPLC analysis can be avoided. Based on the preliminary results from the cloud-point and preconcentration factor investigations, the cloud-point extraction (CPE) is carried out using 0.65 M sodium sulphate, an appropriate amount of Neodol 25-7, and 10 minutes of centrifugation at 3,500 rpm. It is sufficient to wait 10 minutes for samples to reach an equilibrium before centrifugation by using Neodol 25-7 as an extractant. More sodium sulphate is required for Neodol 25-7 than for Tergitol 15-S-7, if the CPE experiments are to be carried out at ambient temperatures due to the higher cloud point of Neodol 25-7. Addition of large amount of salt will usually yield a higher preconcentration factor. However and likewise as Tergitol 15-S-7, the formation of very viscous crystalline phase floating on top of the aqueous phase is observed, which makes it very difficult to separate the surfactant-rich phase accurately. Hence, 0.65 M sodium sulphate was enough for conducting the CPE experiment at, 22ºC. Additionally; lower surfactant concentration with appropriate amount of salt can give a higher preconcentration factor, owing to the very small volume of the surfactant-rich phase. Nonetheless, the sample handling has to be sacrificed as it becomes very difficult to collect it. Therefore, the concentration of Neodol 25-7 used in this study was taken greater than 1.5 wt %. After phase separation, 20 µl of surfactant-rich phase diluted by water is directly injected to HPLC for sample analysis. The calculation procedure of the recovery 66 Chapter 6 Cloud-Point Extraction and Recovery Efficiency efficiency is the same as that of Tergitol 15-S-7. The recovery percentage reported in this study is reproducible within 18 %. 6.2.2.1 Recovery as a function of surfactant concentration Figure 6.3 displays the recovery efficiency of HOCs obtained by injection of the surfactant-rich phase of HOC after cloud-point extraction using 10 ppm initial analyte concentration, 0.65 M sodium sulphate, Neodol 25-7 and 10 minutes for centrifugation at 3,500 rpm. 120 Acenaphthene 9-Chloroanthracene 110 Dibenzofuran Recovery, [%] Fluoranthene 100 90 80 70 60 1 2 3 4 Surfactant concentration, [%] Figure 6.3 The effect of Neodol 25-7 surfactant concentrations on HOC recovery Although the preconcentration factor decreases with increasing surfactant concentration, again an opposite trend was observed in the analyte recovery. All HOCs, acenaphthene, 9-chloroanthracene, dibenzofuran and fluoranthene are found to 67 Chapter 6 Cloud-Point Extraction and Recovery Efficiency have smaller recoveries at lower surfactant concentrations, but the recoveries increase with increasing surfactant concentrations. For example, recoveries increase from 86 to 105% for acenaphthene, from 79 to 93% for 9-chloroanthracene, from 88 to 100% for dibenzofuran and from 77% to 87% for fluoranthene, when the surfactant concentrations increase from 1.5 to 4 wt %. 6.2.2.2 Recovery as a function of initial analyte concentration Figure 6.4 shows the recovery efficiency of HOC by Neodol 25-7 as a function of the initial analyte concentration. The recovery is obtained by the injection of a surfactantrich phase of each HOC after cloud-point extraction using 3 wt % of Neodol 25-7 surfactant, 0.65 M sodium sulphate, and 10 minutes of centrifugation at 3,500 rpm. 110 Recovery, [%] 100 90 80 Acenaphthene 70 9-Chloroanthracene Dibenzofuran Fluoranthene 60 0 5 10 15 20 25 Initial analyte concentration, [mg/l] Figure 6.4 The effect of initial HOC concentration on its recovery using 3 wt% Neodol 25-7 and 0.65 M sodium sulphate 68 Chapter 6 Cloud-Point Extraction and Recovery Efficiency The recoveries of HOC decrease with increasing initial analyte concentrations. The recoveries of HOCs by Neodol 25-7 are mostly independent of initial analyte concentration. For example, the recovery efficiency decreases from 93 % to 87 % and to 84 %, respectively, for 9-chloroanthracene and dibenzofuran, when the initial analyte concentration increases from 5 to 20 ppm. 6.2.3 Extraction by Tergitol 15-S-9 Tergitol 15-S-9 is a mixture of secondary alcohol ethoxylates and has the same hydrophobic moiety as Tergitol 15-S-7. The only difference between Tergitol 15-S-7 and 15-S-9 is found in the averaged chain length of the oxyethylene units (Section 3.1). Similar to Tergitol 15-S-7, it does not yield any fluorometric signal in the UV range. Based on the preliminary study on cloud point and preconcentration factor, the cloudpoint extraction (CPE) is carried out at a condition including 0.7 M sodium sulphate, appropriate amount of Tergitol 15-S-9 and 10 minutes for centrifugation at 3,500 rpm. Again, to facilitate the extraction at ambient temperatures, the required amount of added sodium sulphate is slightly more than that of Tergitol 15-S-7 and Neodol 25-7 due to its even higher cloud point. Similar phenomenon, the formation of viscous crystalline phase floating on top of the aqueous phase appears, if the sodium sulphate is overdosed. Additionally, at lower surfactant concentration with an appropriate amount of salt, the higher preconcentration factor is achieved at the expense of very small phase volume of surfactant-rich phase, which, subsequently, leads to the uneasy sample manipulation and unacceptably large experimental errors. 69 Chapter 6 Cloud-Point Extraction and Recovery Efficiency After phase separation, 20 µl of surfactant-rich phase is directly injected to HPLC for sample analysis. The calculation method of the recovery efficiency is the same as that of Tergitol 15-S-7 and Neodol 25-7. The recovery percentage reported in this study is reproducible within 8 %. 6.2.3.1 Recovery as a function of surfactant concentration Figure 6.5 displays the recovery efficiency of HOCs obtained by injection of surfactant-rich phase of HOC after cloud-point extraction using 0.7 M sodium sulphate, Tergitol 15-S-9 surfactant, 10 ppm initial analyte concentration and 10 minutes for centrifugation at 3,500 rpm. Although the preconcentration factor decreases with increasing surfactant concentration, the opposite trend was also observed in the analyte recovery. 110 Recovery, [%] 100 90 80 Acenaphthene 9-Chloroanthracene 70 Dibenzofuran Fluoranthene 60 1 2 3 4 Surfactant concentration, [mg/l] Figure 6.5 The effect of Tergitol 15-S-9 surfactant concentrations on HOC recovery 70 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Acenaphthene was found to have the largest recovery efficiency, ca. 90 %, for Tergitol 15-S-9 surfactant even at 1.5 wt %. The recovery of dibenzofuran is very low at lower surfactant concentrations, similarly as seen in Tergitol 15-S-7, but the recovery increases significantly with increasing surfactant concentrations. Consequently, a higher surfactant concentration of Tergitol 15-S-9, e.g., 3 wt %, would be desirable for recovery of this kind of organic compound from an aqueous sample to achieve higher recovery. In contrast, the recoveries of 9-chloroanthracene and fluoranthene are mostly independent of surfactant concentration. 6.2.3.2 Recovery as a function of initial analyte concentration 110 Acenaphthene 9-Chloroanthracene Dibenzofuran Recovery,[%] Fluoranthene 100 90 80 0 5 10 15 20 25 Initial analyte concentration, [mg/l] Figure 6.6 The effect of initial HOC concentration on its recovery using 3 wt% Tergitol 15-S-9 and 0.7 M sodium sulphate 71 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Figure 6.6 illustrates the recovery efficiency of HOC by Tergitol 15-S-9 as a function of initial analyte concentration. The recovery is obtained by injection of the surfactantrich phase of each HOC after cloud-point extraction using a 3 wt % surfactant solution, 0.7 M sodium sulphate and 10 minutes for centrifugation at 3,500 rpm. It is interesting to note that the recovery of each HOC is found to be a function of its initial concentration. In general, the recoveries are higher at lower initial analyte concentrations and smaller at greater initial analyte concentrations. This phenomenon has been observed as well in the recoveries of HOCs by Tergitol 15-S-7. 9-Chloroanthracene and dibenzofuran give higher recoveries at low initial analyte concentrations, but recoveries decrease too much at higher initial analyte concentration, this may be attributed to sorption of analyte onto containers. The recovery of acenaphthene and fluoranthene are very high at low initial analyte concentrations, but the changes of recoveries become slightly smaller when initial concentration exceeds 10 ppm. 6.2.4 Comparison of Recovery Efficiencies 6.2.4.1 Effect of different molecular structure of surfactants on recovery Tergitol 15-S-7 and Neodol 25-7 have the same molecular weights and HLB values, but different molecular structures. The former one is a secondary alcohol ethoxylate, while the latter is a primary one. Because of different molecular structures, all the physicochemical properties of these two surfactants are totally different. In this section, the recovery efficiencies are compared. 72 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Figure 6.7 shows the comparison on recoveries of HOCs by Tergitol 15-S-7 and Neodol 25-7 at room temperature, 22 ºC, as a function of surfactant concentration. The recovery efficiency of HOCs is obtained by the injection of surfactant-rich phase of HOC after cloud-point extraction using 10 ppm initial analyte concentration, 0.6 M sodium sulphate for Tergitol 15-S-7 and 0.65 M sodium sulphate for Neodol 25-7 and 10 minutes for centrifugation at 3,500 rpm. Filled symbols stand for the recovery of HOCs by Tergitol 15-S-7, whereas empty symbols for that of Neodol 25-7. By comparing the recovery of each HOC, Tergitol 15-S-7 yields the relatively higher recoveries than Neodol 25-7. The possibility on the lower recovery of HOCs by Neodol 25-7 might be attributed to the smaller preconcentration factor. For example, the preconcentration factor of 1.5 wt % Tergitol 15-S-7 is 27, in contrast to 22 for that of 1.5 wt % Neodol 25-7. 120 Ace, Tergitol 15-S-7 9-ChAn, Tergitol 15-S-7 DiBz, Tergitol 15-S-7 Fluo, Tergitol 15-S-7 Recovery, [%] 110 Ace, Neodol 25-7 9 ChAn, Neodol 25-7 DiBz, Neodol 25-7 Fluo, Neodol 25-7 100 90 80 70 1 2 3 4 Surfactant Concentration, [%] Figure 6.7 Comparison of recoveries of HOCs by Tergitol 15-S-7 and Neodol 25-7 73 Chapter 6 Cloud-Point Extraction and Recovery Efficiency The recovery is found to be low in dilute surfactant solutions for both surfactants, but increased with increasing surfactant concentration for acenaphthene, 9- chloroanthracene and fluoranthene. The only exceptional case is the recovery of dibenzofuran at 1.5 wt % surfactant concentration. The recovery by 1.5 wt % Tergitol 15-S-7 is inferior to that by 1.5 wt % Neodol 25-7. 6.2.4.2 Effect of different HLB values of surfactants on recovery Tergitol 15-S-7 and Tergitol 15-S-9 are of the same homolog, but having different HLB values at 12.4 and 13.3, respectively. For a particular class of nonionic surfactant, surfactant with a lower HLB value is generally more hydrophobic. In this case, Tergitol 15-S-7 is more hydrophobic than Tergitol 15-S-9. 120 Ace, Tergitol 15-S-7 9-ChAn, Tergitol 15-S-7 DiBz, Tergitol 15-S-7 Fluo, Tergitol 15-S-7 Recovery, [%] 110 Ace, Tergitol 15-S-9 9-ChAn, Tergitol 15-S-9 DiBz, Tergitol 15-S-9 Fluo, Tergitol,15-S-9 100 90 80 70 1 1.5 2 2.5 3 3.5 4 4.5 Surfactant Concentration, [%] Figure 6.8 Comparison of recoveries of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9 74 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Figure 6.8 illustrates the recoveries of HOCs by Tergitol 15-S-7 and Tergitol 15-S-9 at room temperature, 22 ºC, as a function of surfactant concentration. The recovery efficiency of HOCs is obtained by injection of the surfactant-rich phase having HOC after cloud-point extraction using 10 ppm initial analyte concentration, 0.6 M sodium sulphate for Tergitol 15-S-7 and 0.7 M sodium sulphate for Tergitol 15-S-9 and 10 minutes for centrifugation at 3,500 rpm. Filled symbols refer to the recovery of HOCs by Tergitol 15-S-7 and empty symbols for that of Tergitol 15-S-9. Again, Tergitol 15-S-7 yields a relatively higher recovery than Tergitol 15-S-9. Frankewich et al. (1994) suggested that the degree of partitioning of neutral organic molecule to a nonionic micelle might be expected to decrease as the numbers of ethylene oxide units of surfactants increases. Tergitol 15-S-7 has the ethylene oxide units of 7.3 in average, whereas Tergitol 15-S-9 has 8.9. Increasing the average numbers of the oxyethylene units in the surfactant molecule might also attribute to the decrease in the recovery efficiency. Another possibility is that Tergitol 15-S-7 has a slightly higher preconcentration factor compared to Tergitol 15-S-9, which may be reflected in the recovery efficiency. Interestingly, the recovery efficiency of acenaphthene in all surfactants is higher than that of 9-chloroanthracene, dibenzofuran and fluoranthene; it may be the slightly polar nature of acenaphthene molecules, favorably adsorbing more in the surfactant richphase. Additionally, among the three selected nonionic surfactants, Tergitol 15-S-7 gives the highest recovery for all HOCs. 75 Chapter 6 Cloud-Point Extraction and Recovery Efficiency 6.2.5 Effect of salts on recovery efficiency Figure 6.9 gives the comparison recovery efficiency of acenaphthene by different electrolytes, Na2SO4 and Na3PO4, as a function of surfactant concentration. The recovery efficiency of acenaphthene is obtained by injection of the surfactant-rich phase of acenaphthene after cloud-point extraction using 10 ppm initial concentration, appropriate amount of either Na2SO4 or Na3PO4 to be added in each surfactant micellar solution and 10 minutes for centrifugation at 3,500 rpm. The cloud point temperature of 1.5 wt % Tergitol 15-S-7 with additional 0.4 M sodium phosphate is 2.8 ºC and that of Tergitol 15-S-9 with additional 0.45 M sodium phosphate is 3 ºC. In order to maintain the same cloud point temperatures in the Tergitol 15-S-7 and 15-S-9 micellar solutions, different but a proper amount of salt has to be added, accordingly. 120 Recovery, % 110 100 90 0.6M Na2SO4, Tergitol 15-S-7 0.4M Na3PO4, Teritol 15-S-7 0.7M Na2SO4, Tergitol 15-S-9 0.45M Na3PO4, Tergitol 15-S-9 80 1 2 3 4 Surfactant Concentration, % Figure 6.9 Recovery efficiency of acenaphthene as a function of surfactant concentration 76 Chapter 6 Cloud-Point Extraction and Recovery Efficiency The recovery efficiencies are 91 and 95 % in 1.5 wt % Tergitol 15-S-7 solutions with addition of sodium sulphate and sodium phosphate, respectively. The recovery of acenaphthene by sodium sulphate is increased with increasing surfactant concentrations, but the recovery increment by sodium phosphate is not so obvious. Likewise, in 1.5 wt % Tergitol 15-S-9 solutions, additions of sodium sulphate and sodium phosphate to the micellar solutions lead to the 90 and 95 % in recovery, respectively. Sodium phosphate in general gives the higher recovery at lower surfactant concentrations and the recovery increment of acenaphthene is not so much different with increasing surfactant concentrations. Moreover, Tergitol 15-S-7 gives a higher recovery of acenaphthene compared to Tergitol 15-S-9 in either sodium sulphate or sodium phosphate. 6.3 Conclusions A simple and practical cloud-point extraction (CPE) technique is developed to preconcentrate selected HOCs at room temperature (22 ºC) by adding either sodium sulphate or sodium phosphate to surfactant the micellar solution. The greatest advantage was achieved by using the secondary alcohol ethoxylates, Tergitol 15-S-7 and Tergitol 15-S-9, and a primary alcohol ethoxylate, Neodol 25-7, as the extractants, because these surfactants do not yield any significant fluorometric signal in the UV range. Hence, the complicated clean-up procedures and undesirable masking of chromatographic peaks of HOCs in the effluent could be avoided by using these surfactants. In addition, the low volatility, toxicity and high biodegradability of these 77 Chapter 6 Cloud-Point Extraction and Recovery Efficiency surfactants are also noted advantages. Moreover, the shorter time to reach equilibrium phase separation is another added advantage. The recovery efficiency of HOCs by each surfactant was studied as a function of surfactant concentration and the initial analyte concentration. In general, the recoveries of all HOCs are low at lower surfactant concentrations, whereas recoveries increase with increasing surfactant concentrations for all surfactants studied. On the contrary, recoveries of all HOCs are higher at lower initial analyte concentrations, while recoveries decrease with increasing initial analyte concentrations due to the possible sorption of analyte to container walls at higher initial analyte concentrations. For surfactants having different molecular structures, but nearly the same molecular weights and HLB values, such as Tergitol 15-S-7 and Neodol 25-7, in this study, Tergitol 15-S-7 gives the larger recoveries of all HOCs than Neodol 25-7. It may be due to the higher preconcentration factor of Tergitol 15-S-7 surfactant. For surfactants with the same homolog, but having different HLB values, such as Tergitol 15-S-7 and Tergitol 15-S-9 in this study, again Tergitol 15-S-7 gives the higher recoveries of all HOCs as compared to Tergitol 15-S-9. This could be attributed to the more hydrophobic nature of the Tergitol 15-S-7 surfactant, which leads to more partitioning of neutral organic solutes favorably to nonionic micelles. In addition, the slightly greater preconcentration factor of Tergitol 15-S-7 surfactant may attribute to the higher recovery. For the overall comparison, it is interesting to note that the recovery efficiency of acenaphthene is higher than that of 9-chloroanthracene, dibenzofuran and fluoranthene in all surfactants. 78 Chapter 6 Cloud-Point Extraction and Recovery Efficiency Moreover, effects of added electrolytes, Na2SO4 and Na3PO4, to the micellar solutions, Tergitol 15-S-7 and Tergitol 15-S-9, on the recovery efficiency of acenaphthene were studied. Sodium phosphate yields a higher recovery even at low surfactant concentrations, but sodium sulphate does not. In general, Tergitol 15-S-7 gives a higher recovery of acenaphthene compared to Tergitol 15-S-9 with either sodium sulphate or sodium phosphate. 79 Chapter 7 Conclusions and Recommendations Chapter 7 Conclusions and Recommendations 7.1 Conclusions Surfactant enhanced solubilization of hydrophobic organic compounds (HOCs) has been experimentally examined by employing three nonionic surfactants, namely, Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9. The straight-chained molecular structure of nonionic surfactant could form micelles having a larger hydrophobic core volume, in which more hydrophobic organic compound could be solubilized than that of branch-chained surfactant. More hydrophobic surfactant could solubilize more hydrophobic organic compounds than less hydrophobic surfactant due to the larger core volume in micelles. Static and dynamic light scattering indicated that the solubilization capacity of HOCs could be predicted by the aggregation number and the core volume of the micelles of the nonionic surfactants. The correlation between the logarithms of the micelle-water partition coefficients and those of their octanol-water partition coefficients revealed the relative hydrophobicity of the nonionic surfactants. It was found that the number of EO units in the surfactant molecules has a dramatic effect on the cloud point for a particular class of nonionic surfactant. It is valuable to note that the significant effect of added electrolytes on cloud point was observed. Sodium iodide could increase the cloud point, whereas most of the salts could decrease the cloud point. Sodium phosphate was found to be the most effective electrolyte in decreasing the cloud point. The preconcentration factor could be increased either by decreasing the surfactant concentration or increasing the added salt concentration. 80 Chapter 7 Conclusions and Recommendations The simple and practical cloud-point extraction (CPE) technique was developed by using readily biodegradable surfactants, such as Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 as an extractant. Moreover, these nonionic surfactants do not render any fluorometric signals in the UV range. Hence, the complicated clean-up procedures and undesirable masking of chromatographic peaks of HOCs by the UV detector in the effluent could be avoided. Recovery efficiency could be achieved from 79 % to 114 % for Tergitol 15-S-7, from 77 % to 105 % for Neodol 25-7 and from 72 % to 102 % for Tergitol 15-S-9. It was found that Tergitol 15-S-7 could give the highest recovery than the other surfactants due to the higher preconcentration factor. Among the HOCs studied, acenaphthene is the easiest one to be recovered by each surfactant. In addition, Sodium phosphate could have a higher recovery of acenaphthene than sodium sulphate. However, in CPE process, experimental recovery exceeding 100 % is commonly observed for some organic compounds (PCBs, PAHs, amino and hydroxy aromatics) in the open literature (Fröschl et al., 1997; Wu and Huang, 1998; Wu and Huang, 1998). But, no experimental evidence was observed. 7.2 Recommendations The surfactants have been shown to enhance solubility and recovery of HOCs in aqueous systems. The solubilization study was based on the single surfactant system, so that the effect of mixed surfactants system on solubilization of HOCs could be planned for a future study. The CPE technique appears to possess a great advantage over the recovery of hydrophobic organic compounds with high efficiency. 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However, the log Kow of 9-chloroanthracene can be estimated from the log Kow - log Km graph of the selected HOCs by different nonionic surfactants. The correlation equations are: Tergitol 15-S-7: y = 0.89x + 1.67 R2 = 0.99 (A1.1) Neodol 25-7: y = 0.94x + 1.53 R2 = 0.99 (A1.2) Tergitol 15-S-9: y = 0.85x + 1.88 R2 = 0.96 (A1.3) where y stands for the log Kow and x represents the log Km. The values of the log Kow values of the 9-chloroanthracene are then calculated from the above equations by using experimental log Km values of 9-chloroanthracene by the corresponding nonionic surfactants. The calculated log Kow values of 9chloroanthracene are 4.3, 4.43 and 4.33 for Tergitol 15-S-7, Neodol 25-7 and Tergitol 15-S-9 respectively. The average value, 4.35, is used in this study. By knowing log Kow value of 9-chloroanthracene, other physical properties, such as molar volume and density of 9-chloroanthracene could be estimated from the following equation which correlates well the molar volume V and log Kow values of the PCBs (Miller et al., 1985). log Kow = 0.49 + 0.0200 V (A1.4) 95 Appendix A That is, the molar volume, V = 193 cm3 / mole, and the density, ρ = 0.7347 g / cm3. 96 [...]... lower than those of pure ionic surfactants For a particular class of nonionic surfactants, the cloud point of a dilute mixed solution generally lies somewhere intermediate between the cloud points of individual surfactants present in the mixtures (Gu, T and Galera-Gómez, 1995) In addition, Schott (2003) proposed a linear relation between the cloud point and the number of oxyethylene units of watersoluble... coefficients of these HOCs 2) Measure the cloud point temperature of micellar solutions of selected nonionic surfactants 3) Investigate the temperature effect on the size and aggregation number of the micelles of these nonionic surfactants below their cloud points 4) Examine the effect of added electrolytes on the cloud points of the micellar solutions of these nonionic surfactants and optimization of the... solvent extraction, cloudpoint extraction technique has a promising future 2.3 Cloud- Point Extraction Recently, an extraction technique based on the clouding phenomenon of nonionic surfactants is becoming more and more attractive As the temperature of a nonionic 15 Chapter 2 Literature Review surfactant solution is increased, the solubility of surfactant in water diminishes due to the dehydration of the... examples of the applications of Tergitol 15-S surfactants include: household and industrial laundry detergents, hard-surface cleaners and degreasers, industrial and institutional cleaners, hydrocarbons and water-based laundry prespotters, car care products, paper deinking, rewetting, pulping and deresinating, oil -in- water emulsions, textile wet processing, dye assist and leveling agents for carpets and. .. watersoluble polyoxyethylated nonionic surfactants from their experimental observations 2.2.3 Application of Clouding Phenomenon By using the clouding phenomenon of nonionic surfactants, the most significant application is the cloud- point extraction technique These include the extraction of metal ions, the separation and purification of biomolecules and the extraction of environmental pollutants As an alternative... additives are electrolytes and secondary surfactants, such as anionic or cationic surfactants The electrolytes, which can increase the cloud point, cause a “salt -in effect On the other hand, those depressing the cloud point are called “saltout” effect Bai et al (2001) studied the effect of several kinds of electrolytes including NaCl, NaI, Na2SO4, MgCl2 and CaCl2 on the cloud points of the Tergitol 15-S-7... in the molecule has dramatic effect on the cloud point As the number of ethylene oxide units increases, the cloud point becomes higher Additionally, cloud point of nonionic surfactant has also been shown as a function of its own concentration (Sadaghiania et al., 1990; Gu and Galera-Gómez, 1995; Bai et al., 2001; Li and Chen, 2002) The cloud point of nonionic surfactants is very sensitive to some additives,... free of the 1 Chapter 1 Introduction Micelle Surfactant- rich Phase Aqueous Phase Monomer HOC Figure 1.1 A schematic description of a phase equilibrium in CPE 2 Chapter 1 Introduction surfactant and the surfactant concentration is only near its critical micelle concentration (CMC) (See Figure 1.1) Phase separation occurs due to the difference in density of micelle-rich phase (surfactant- rich phase) and. .. that NaI could increase the cloud point temperature, whereas the rest of the salts can lower the cloud point temperature In addition, among the salts lowering the cloud point of Tergitol 15-S-7, Na2SO4 acts 14 Chapter 2 Literature Review most effectively to decrease the cloud point Furthermore, Li et al (2002) concluded that sodium phosphate could depress more sufficiently the cloud point of both Tergitol... in water decreases with increasing temperature by the dehydration of ethylene oxide (EO) chains Below the cloud point, surfactant dissolves in water, and above it, water dissolves in surfactant Schott (1969) suggested that at constant EO content, the cloud point could be lowered (1) by decreasing the molecular weight of the surfactant; (2) with a broader distribution of POE chain 13 Chapter 2 Literature