Solvation forces in confined molecular liquids

SOLVATION FORCES IN CONFINED MOLECULAR LIQUIDS RODERICK LIM YU HIN (B.Sc. Applied Science (Physics), University of North Carolina-Chapel Hill) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2002 Acknowledgements I would like to express my appreciation to many of the staff and students at the Institute of Materials Research and Engineering (IMRE) for their assistance and support. The three years I have spent in IMRE could not have been made more enjoyable or comfortable. To all (both past and present) who have passed through Biosensor Lab #05-02 (IMRE), I express my gratitude for the friendship and the tremendous amount of help I received throughout the course of my studentship. A special acknowledgement goes out to Dr. Isabel Rodriguez, Mr. Dai Chang Chun and Mr. Tan Yee Yuan for interesting and enlightening discussions over various technical matters. To my friends Peter Moran and Sharon Oh, I am grateful for our weekly mealtime discussions and for putting up with my pseudo-scientific babble. Also special thanks goes out to Bhaskar for the beautiful tunes and wonderful words that are a constant reminder of what’s truly important and what’s not. I am most grateful to my parents for their unconditional support and great patience with my impatience. A big thank you goes out to my brother, Randall Lim, for helping me with computer related issues. And not forgetting Cindy Chew, for just listening and for bringing the much needed calm back into my life. I wish to express my sincere thanks to Prof. Sam Li for the advice and support and for giving me the fantastic opportunity of pursuing further studies. Finally, I am eternally grateful to Dr. Sean O’Shea for his never yielding patience, guidance and generosity. Under his mentorship and beer-fueled discourse, I have not only learnt many lessons in science, but also about life in general, which include humility, honesty and most importantly, kindness. i Contents 1. Introduction 1.1 Motivation 1.2 Forces in Liquids 1.3 Experimental: SFA, AFM and STM 1.4 Thesis Layout 1 14 2. Literature Survey 2.1 Solvation Force 2.2 Surface Force Apparatus Measurements 2.3 Atomic Force Microscope Measurements 2.4 Computer Simulations 2.4.1 Simulations of Liquids at Isolated Surfaces 2.4.2 Simulations of Confined Liquids 2.4.3 Surface Induced In-Plane Ordering 15 15 18 23 27 28 29 33 3. Experimental Methods 3.1 The AFM Setup 3.1.1 Cantilever Characterization 3.1.2 Tip Modification 3.1.3 Probe Characterization 3.1.4 Piezo Calibration 3.2 Force Measurements 3.2.1 Applied Force Measurements 3.2.2 Sample-Modulation Force Spectroscopy 3.2.3 Friction Force Measurements 3.3 Materials 3.3.1 Liquids 3.3.2 Highly Oriented Pyrolytic Graphite (HOPG) 35 35 37 41 47 50 51 54 58 61 65 65 68 4. Solvation Forces by Atomic Force Microscopy 4.1 Solvation Forces by Sample-Modulation Force Spectroscopy (General) 4.2 Solvation Forces vs. Molecular Structure 4.2.1 Spherical Molecules: OMCTS 4.2.2 Branched Molecules: Squalane 4.2.3 Linear Molecules: Hexadecane 4.2.4 Other Liquids: Phenyloctane 4.2.5 Effects of Liquid Structure I. Periodicity of force oscillations II. Stiffness and normalized force measurements III. Comparisons with SFA 4.3 Tip Effects in Solvation Force Measurements 4.3.1 AFM Tip Effects I. Contamination 71 71 77 78 81 86 90 91 92 93 95 97 97 98 ii II. Surface Roughness, Curvature and Geometry 4.3.2 Solvation Forces Measured with Bead Modified Tips I. Roughness measurements II. Force measurements Summary 99 101 102 104 106 5. Surface Induced Molecular Ordering 5.1 Preferentially Adsorbed Systems 5.1.1 The Formation of a Two-Dimensional Supramolecular Chiral Lamellae by Diamide Molecules at the Solution/Graphite Interface: A STM Study I. Proposed Structure of 2-D lamellae II. Domain Structure on the Surface III. Comparison between 2-D and 3-D Structures IV. Dynamic Motion of the Molecules in 2-D Lamellae 5.1.2 AFM of Octadecanol on HOPG (Solvent: Phenyloctane) 5.1.3 AFM of Octadecanol on HOPG (Solvent: OMCTS) 5.1.4 AFM of Hexadecane on HOPG (Solvent: OMCTS) 5.2 Pure Liquid Systems 5.2.1 AFM of Hexadecane on HOPG 5.2.2 Other Liquids 5.3 Summary 108 108 109 114 117 122 124 127 136 139 141 141 149 150 6. Conclusion and Outlook 152 Bibliography 155 List of Publications 172 4.4 iii Summary Solvation forces in confined liquids have been studied using the atomic force microscope (AFM), and in particular using sample modulation techniques. Measurements involving liquids of differing molecular structure reveal force oscillations, which agree with computer simulations but can differ markedly from surface force apparatus (i.e. branched liquids) observations due to the smaller confinement area and the different chemical nature of the surfaces in AFM. Results show that surface roughness and liquid molecular structure can affect the magnitude of force measurements. Force measurements in solutions and liquid mixtures show that discrete co-existent molecular layers with one molecular species being preferentially adsorbed can form at the solid-liquid interface. High-resolution imaging showing in-plane ordering of the adsorbed layer is possible by controlling the force to within the measured force range of the first solvation layer. iv Chapter 1. Introduction Chapter One Introduction 1.1 Motivation The understanding of force interactions between two surfaces is crucial when considering many diverse issues in science [1]. Regardless of the environment surrounding any particular physical system, different material systems first begin to interact with each other at their surfaces – that is at the interface between the materials. Inevitably, physical processes that occur at the interface related to the atomic scale mechanisms, energetics, structure, and dynamics become important to basic science and applied technological problems. The desire to understand interfacial processes has motivated much experimental and theoretical work in areas such as adhesion, contact formation, surface deformations, elastic and plastic response characteristics of materials, hardness, micro- and nanoindentation, friction, lubrication and wear, fracture, modifications and manipulation of materials surfaces. Typically, the presence of a liquid “trapped” between two or more interacting surfaces is common to all these processes. It is well established that the physical properties of liquids can change drastically as the distance between the two confining surfaces approaches the molecular scale, greatly altering the force interactions between the two surfaces. The way the solid-liquid-solid cavity behaves can ultimately determine the overall properties of molecular and atomic self-assembly [2], biological and colloidal interactions [3], nanotribology (i.e. friction, lubrication and wear of surfaces in contact) [4] and nanorheology (i.e. material deformation) [5]. Chapter 1. Introduction The present emphasis on the miniaturization of electronic devices and the emergence of nanoscale science and nanotechnology [6] following the development of microelectromechanical systems (MEMs) and nanoelectromechanical systems (NEMs) [7] has intensified the need to understand interfacial phenomenon and their related forces at the atomic level. Hence, it is clear that understanding the nature and behavior of liquids at the solid-liquid interface is necessary. Furthermore, emphasis must be placed on elucidating the effects of liquids trapped or confined at the solid-liquid-solid cavity in order to provide an understanding as to how such liquids can alter the forces interacting between two solid surfaces at the nanometer length scale. The impetus to observe and understand force interactions with high sensitivity and spatial resolution has led to the development of experimental techniques such as the surface force apparatus (SFA), as well as proximal probe techniques such as the scanning tunneling microscope (STM) and the atomic force microscope (AFM). The latter two techniques are collectively known as scanning probe microscopes (SPM) and have been widely used to explore the properties of liquids close to solid surfaces and/or to measure the forces of surface confined liquids. The modern SFA was developed in the 1970’s [8] and is commonly employed to study both static and dynamic properties of molecularly thin films sandwiched between two molecularly smooth mica surfaces [1]. The invention of the STM [9] in 1981, broke new ground as it enabled direct atomic scale measurements between a conducting tip and substrate thereby allowing scientists to view the behavior of molecules on surfaces for the first time [10]. The invention of the Atomic Force Microscope (AFM) quickly followed in 1986, providing a further method for measuring ultra-small forces between a probe-tip and either an electrically conducting or insulating substrate [11]. Meanwhile, significant theoretical breakthroughs have led to a clearer Chapter 1. Introduction understanding of the fundamental nature of bonding and interaction in materials. Advances in computer-based modeling and molecular dynamics (MD) simulation methods, has allowed for a more comprehensive theoretical approach to elucidating complex interfacial phenomenon with atomic resolution [5, 12]. The combination of both experimental evidence and theoretical predictions show that liquids confined at small separations (a few molecular diameters) behave differently from the bulk liquid. These studies indicate that forces between two surfaces mediated by nanoconfined liquids can be oscillatory and can no longer be described by simplistic continuum theories [1]. The oscillatory force, also known as the “solvation” force, is brought about by the discrete ordering or structuring of liquid molecules under nanoconfinement and has been able to explain many interactions not predicted by continuum approaches [13]. The main objective of this dissertation is to report on experimental findings pertaining to the solvation forces of confined molecular liquids obtained using a novel sample modulation AFM technique. The measurements resulting from this work will show to be comparable to existing SFA data, which validates the method’s utility in the measurement of forces in liquids. In addition, two new experimental observations not observed in SFA studies will be presented, namely i) the observation of an oscillatory force profile for squalane, a branched alkane, and ii) the observation of discrete co-existent molecular layers with one molecular species being preferentially adsorbed at the solid-liquid interface in solutions and liquid mixtures. It will be shown that in general the magnitude and quality of solvation force measurements result from structural commensurability between the liquid molecules and the underlying substrate lattice. AFM is capable of attaining high-resolution topographic images of the solid-liquid interface by controlling Chapter 1. Introduction the force to within the measured force range of the first solvation layer. The images show that in-plane ordering occurs for molecules which are commensurate with the underlying substrate lattice. These new observations highlight some advantages of using AFM in solvation force measurements compared to the SFA. 1.2 Forces in Liquids For many years, it was believed that two principle forces operated between two surfaces in a liquid [14, 15] – the monotonically attractive van der Waals (vdW) force and electrostatic (“double-layer”) forces. For example, interactions between particles and surfaces were based on varying strengths of the two forces in which the adhesion between two particles or surfaces would be brought about if the van der Waals force were dominant while a repulsive double-layer force would keep them apart. These two forces acting together form the basis of the well-known Derjaguin-Landau-Verwey-Overbeek, or DLVO theory [1]. The DLVO theory has provided the main theoretical framework for analyzing the dispersion properties of colloids and biomolecular systems since the 1950’s. Both the van der Waals force and the double-layer force are long range interactions well described by continuum theories (the “Lifshitz theory” for the van der Waals force [16] and the Poisson-Boltzmann equation for the double-layer force [1]) beyond separations of about ten molecular diameters. More recently, experimentations with new techniques such as the SFA have revealed that other types of more complex forces can also arise in liquids in the short-range i.e. at surface separations of a few nanometers or a few molecular diameters. These forces can be monotonically attractive, monotonically repulsive, or oscillate between varying degrees of Chapter 1. Introduction attraction and repulsion. This variety of force behavior arises partly because liquids undergoing increasing confinement cease to behave as a structureless continuum with properties determined solely by the bulk properties. In the short range, structural properties such as the size and shape of the liquid molecules begin to play an important role in determining the overall force interaction. The confining surfaces themselves can no longer be treated as inert, structureless walls. Instead, the physical and chemical properties of the surfaces at the atomic scale have to be taken into account. Thus, the force laws may include surface effects such as whether the surface lattices are commensurate, whether the surfaces are amorphous or crystalline, rough or smooth, rigid or soft or fluid-like. The steric or fluctuation force [1] arises from the thermal motions of protruding head groups or the thermal fluctuations of flexible fluid-like interfaces (e.g. surfactant or lipid bilayers). The fluctuation force is short range, usually repulsive, and very effective at stabilizing the attractive van der Waals force at some small but finite separation by reducing the adhesion energy or force. Fluctuation forces occur typically at surface structures such as micelles, vesicles, lipid bilayers, microemulsion droplets, surfactantcoated colloidal particles, and biological membranes in aqueous solutions [1]. It is mainly due to the presence of the fluctuation force that fluid-like micelles and bilayers, biological membranes, emulsion droplets (in salad dressings) or gas bubbles (in beer) adhere to each other only very weakly [1]. The solvation force is a short-range force associated with the structuring of liquid molecules confined within a solid-liquid-solid cavity. Liquid molecules confined in a tight space cease to behave as a structureless medium. 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The Formation of Two-Dimensional Supramolecular Chiral Lamellae by Diamide Molecules at the Solution/ Graphite Interface: A Scanning Tunneling Microscopy Study R. Lim, J. Li, S. F. Y. Li, Z. Feng and S. Valiyaveettil, Langmuir 2000, 16, 7023 2. Atomic Hydrogen Beam Etching of Carbon Superstructures on 6H-SiC(0001) Studied by Reflection High-Energy Electron Diffraction Xie XN, Lim R, Li J, Li SFY and Loh KP, Diamond and Related Materials 2001, 10 (3-7), 1218 3. Electronic and Vibronic Properties of Mg-doped GaN: The Influence of Etching and Annealing S. Tripathy, S.J. Chua, A. Ramam, E.K. Sia, J.S. Pan, R. Lim, G. Yu and Z.X. Shen, Journal of Applied Physics 2002, 91 (5), 3398 4. Surface Oxygenation Studies on (100)-Oriented Diamond Using an Atom Beam Source and Local Anodic Oxidation K. P. Loh, X. N. Xie, Y. H. Lim, E. J. Teo, J. C. Zheng and T. Ando, Surface Science, 2002, 505 (1-3), 93 5. Solvation Forces Using Sample-Modulation Atomic Force Microscopy R. Lim, S.F.Y. Li and S.J. O'Shea, Langmuir, 2002, 18, 6116 6. Solvation Forces in Branched Molecular Liquids R. Lim and S.J. O'Shea, Physical Review Letters, 2002, 88, 246101 172 [...]... issues has remained relatively sparse despite implications in atomic resolution AFM imaging in liquids As discussed by O’Shea et al, the ability of AFM to measure oscillatory solvation forces suggests that such structural forces may be influential in non-contact imaging on the atomic scale [23, 24] AFM imaging in liquids differs from that in UHV because of additional effects imposed by the confined liquid... the utility of AFM solvation force measurements in revealing interesting properties of the underlying liquid layers, the AFM technique has not yet been fully exploited By combining AFM imaging and force spectroscopy, the experiments involved in this dissertation seek to elucidate the following questions: (i) Can AFM measure solvation forces of magnitude similar to SFA data (implying a molecularly smooth... 2.1 The following literature survey of computer simulations is meant to provide a review of work done during the last two decades These simulations cover a broad spectrum of issues such as molecular freezing or layering (with or without confinement), shear induced ordering, molecular adsorption, solvation forces in liquids of different molecular structure, the effects of roughness in solvation force... down in the short range Other reports indicated that attractive interfacial interactions with geometric constraining effects would be imposed on liquid molecules in the presence of a hard wall [68, 74] effectively bringing about density oscillations extending seven or more molecular diameters from the solid-liquid interface The coupling of all these results made the argument for structural effects in liquids. .. similar to OMCTS in that oscillatory solvation forces are observed Quantitatively, the peak-to-peak amplitudes of the force oscillations are observed to increase with increasing chain length [80] The force law of branched liquids is in marked contrast to spherical and linear chain liquids and strong oscillatory-type solvation forces are rarely observed Experiments show that generally branched liquids exhibit... as this defines the ordering of the liquid molecules confined between the surfaces Oscillatory forces may be weak if the surfaces are rough or if the molecules are irregularly shaped due to the inability of the molecules to pack into coherent layers These findings are reinforced by the weakening of force oscillations observed in SFA measurements carried out in a binary mixture of non-polar liquids [88],... section introduces computer simulations investigating the lateral extent of surface induced molecular ordering exclusive of confinement in pure liquids, solutions, and liquid mixtures at isolated surfaces 2.4.1 Simulations of Liquids at Isolated Surfaces Structural information of liquids normal to the substrate surface attained from force distance experiments in SFA and AFM can be considered indirect... structuring in liquids and solvation forces 2.1 Solvation Force Oscillatory force behavior of a liquid at an interface was first predicted by Hardy [61] in 1912 and experimentally verified sixty-nine years later by Horn and Israelachvili [17] in 15 Chapter 2 Literature Survey 1981 In the years following 1912, results of much experimental work done in diverse fields hinted at the dependence of particle interactions... [56, 119, 122] In comparing the local tip velocity with the adsorbate mobility, one concludes that only dense or strongly bound adsorbates can produce clear images Similarly, only limited information is obtained during imaging on adsorbate structure and dynamics along the direction normal to the interface The pioneering work on the oscillatory behavior of confined liquids has been outlined in section 2.1... There is one report [19] showing oscillatory 19 Chapter 2 Literature Survey behavior in the force measurements of a molecule having a single side chain methyl group (3-methylundecane, C12H26) Hence, although it is generally agreed that molecular asymmetry (in these examples, branching in alkanes) can disrupt oscillatory forces, the quantitative influence of branching on the solvation force is not clear . explain many interactions not predicted by continuum approaches [13]. The main objective of this dissertation is to report on experimental findings pertaining to the solvation forces of confined. SOLVATION FORCES IN CONFINED MOLECULAR LIQUIDS RODERICK LIM YU HIN (B.Sc. Applied Science (Physics), University of North Carolina-Chapel Hill) . molecules begin to play an important role in determining the overall force interaction. The confining surfaces themselves can no longer be treated as inert, structureless walls. Instead, the