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Fuel 156 (2015) 87–95 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel The dual effect of sodium halides on the formation of methane gas hydrate Ngoc N Nguyen, Anh V Nguyen ⇑ School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia h i g h l i g h t s g r a p h i c a l a b s t r a c t Sodium halides are methane hydrate promoters and inhibitors Promoter capability decreases with decreasing the ion size Our findings are explained by hydrophobic hydration Gas hydrate promotion by halides is owing to their hydrophobic nature Salt recovery into gas hydrates is significant a r t i c l e i n f o Article history: Received 15 September 2014 Received in revised form 10 April 2015 Accepted 14 April 2015 Available online 23 April 2015 Keywords: Methane Gas hydrate Salt effect Hydrophobic hydration a b s t r a c t Inorganic salts are known to inhibit the formation of gas hydrates Here we show the duality of sodium halides of submolar concentration in affecting the formation of methane gas hydrates Sodium halides, especially NaI, at low concentration effectively promote methane hydrate formation while they all turn to be an inhibitor at high concentration Maximum gas consumption, growth rate and induction time were experimentally determined as a function of salt type and concentration We explain the dual effect of salts by the hydrophobic hydration The promoting effect of dilute sodium halides is due to the fact that large and polarizable anions (e.g iodide) behave as hydrophobic entities and interact with surrounding water molecules to form hydrophobic hydration shells whose water structure is similar to that of hydrophobic hydration shells of methane Since hydrophobic hydration of methane in neat water is thermodynamically unfavourable because it associates with a negative entropy change and a partial loss in the hydrogen-bonded network, the structurally similar shells of halide ions facilitate the process of entropy change and, therefore, facilitate gas hydrate nucleation Our proposal also explains the decrease in the promoting capability of salts in the order from iodide to fluoride because of the decrease in hydrophobicity of the halide ions The inhibition effect of salts at high concentration is explained by the advantageous competition of the halide ions with methane gas molecules to gain water for hydration as well as their radical effect on distorting the water structure Our hypothesis is experimentally supported by the difference in the salt recovery into hydrates and the hydrophobicity (measured by contact angle) of halide ions Further research is required to obtain a fuller insight of the influence of salts and additives on gas hydrate formation Ó 2015 Elsevier Ltd All rights reserved ⇑ Corresponding author Tel.: +61 336 53665; fax: +61 336 54199 E-mail address: anh.nguyen@eng.uq.edu.au (A.V Nguyen) http://dx.doi.org/10.1016/j.fuel.2015.04.022 0016-2361/Ó 2015 Elsevier Ltd All rights reserved 88 N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 Introduction Gas clathrate hydrates, commonly referred to as gas hydrates, are of enduring interest because of their fascinating science and huge potential applications [1] They are ice-like crystalline solids comprising water (the host molecules) and a suitable gas (the guest molecules) The water molecules form a cage-like structure which traps the gas molecules inside and the gas molecules, in turn, stabilize the water solid-like structure [2], explaining why gas hydrates can form at temperature fairly beyond the freezing point Since their first discovery in 1810 by Sir Humphrey Davy, gas hydrates have still been a hot topic of research owing to their scientific mystery Although the intensive research activities have revealed interesting insights of this simply composed but sophisticatedly behaving material, many important questions still remain a big challenge to scientists and engineers for the future application of methane hydrates [3] Regarding applications, gas hydrates have enormous potentials in energy supply to climate change and industrial processes For example, occurring massively in numerous oceanic sediments worldwide and containing an energy estimated to double the energy of total fossil fuels available [4], methane (natural gas) hydrate poses both an excellent opportunity for future energy supply and an unignorable risk to the environment if released to the atmosphere Hence, establishing an environment-friendly approach to exploit natural gas hydrate is not only important for energy security but also crucial for environment protection In another fashion, artificial gas hydrate is considered as a promising approach for oceanic sequestration of carbon dioxide [5–7], gas separation [8–13] and desalination [14,15] For example, carbon dioxide gas hydrate was reported to form in the marine environment at the depth of 3700 m [7], supporting the idea of storage of carbon dioxide in form of gas hydrate in ocean sediments Gas hydrate formation is a sensitive process which is influenced by the aqueous solution Both thermodynamic and kinetic properties are radically affected by additives or impurities such as surfactants [8,16–18], salts and fine solid particles [19,20] Depending on the type and the concentration of additives, the influence can either promote or inhibit the hydrate formation This fact has given rise to a tireless effort to establish the influence of additives, with the aim to control the formation and dissociation of gas hydrates Of the influencing factors, the presence of salts is critical because of its relevance to the potential application of gas hydrate processes As a result, a great deal of research has been focused on examining the influence of salts on gas hydrate formation and dissociation The previous research, however, mostly focused on concentrated saline solutions, resulting in the conclusion that salts are a thermodynamic inhibitor [21–25] It was not until the very recent work by Faezeh et al who investigated the influence of sodium halides, of low concentrations of between and 500 mM, on the formation of carbon dioxide gas hydrate, showing that sodium halides at low concentrations promote the formation of carbon dioxide gas hydrate [26] Therefore, salts are not only an inhibitor as widely known but can also be a promoter as in case of low concentrations Besides carbon dioxide, methane is also the most common gas hydrate former However, methane is different from carbon dioxide in that methane does not dissociate in water whereas carbon dioxide can partially dissociate in water, lowering the solution pH slightly Hence, it is important to know whether the gas hydrates of these two guests share the same salt-dependent patterns More importantly, previous research mostly focused on experimental measurement of macroscopic kinetic parameters and left the microscopic mechanism unanswered [3] While the inhibition of gas hydrate formation by concentrated saline solutions is attributed to the reduction in gas solubility, increase in viscosity, water-gaining competition between ions and guests as well as the perturbation of water structure by ions, the promotion by dilute sodium halides solutions has not been satisfactorily explained This work aims to investigate the influence of sodium halides, of submolar concentrations (0–1000 mM), on the formation of methane gas hydrate and provide an explanation for the experimental observation Indeed, the research outcome helps to draw a more comprehensible conclusion about the influence of sodium halides on methane gas hydrate formation Experimental procedure 2.1 Materials Methane used in this work was of 99.995% purity and bottled in a G-size cylinder, supplied by Coregas (Brisbane, Australia) Salts used were sodium iodide (99.99% purity, Merck), sodium bromide (99% purity, Sigma Aldrich), sodium chloride (99.9% purity, Ajax FineChem) and sodium fluoride (99% purity, Mallinckrodt) Water used was deionized (DI) water produced by a Milli-Q system (Milipore, USA) Saline solutions of different concentrations were prepared by dissolving an accurate amount (weighted using a Mettler Toledo digital balance with 0.0001 g sensitivity) of the desired salt in an accurate volume of DI water using a volumetric flask The aqueous solutions were kept for h to be stable and homogenized before applied to experiments In the cases of sensibly oxidisable salts such as sodium iodide and sodium bromide, the solutions were stored in a cold and dark environment in a fridge 2.2 Experimental setup and procedure for methane gas hydrate formation The system used to study the influence of sodium halides on kinetics of methane gas hydrate formation is schematically depicted in Fig The main component is a stainless steel reactor (8) (Parr Instruments, USA) which has a volume of 450 mL and can withstand for a pressure up to 2900 psi (20 MPa) It is assembled with a stirrer (M) the speed of which is adjusted and controlled by a speed controller (9) A set of pipes, valves and data acquisition system are also assembled to the reactor Of these components, valve (2) and valve (4) are used for controlling the pressurization of reactor, valve (3) is for depressurization and ventilation, and valve (5) is a relief valve which can automatically activate to release gas if the vessel is over-pressurized The pressure and temperature inside reactor are simultaneously recorded by a Wika S-11 pressure transducer (PT) (Wika, Germany) and a T-type thermocouple (TT) (Parr Instruments, USA), respectively The time-dependent readouts are processed by a National Instruments NI-DAQ 9174 data acquisition device before being displayed on the screen and stored in a PC by a Labview VI developed by our team This data acquisition system records the instantaneous pressure and temperature every second and produces average values for every 30 s The outputs are shown and saved in both graphical and numerical forms Each experiment for methane gas hydrate formation was performed in the following procedure Reactor was initially cleaned three times with DI water and dried by compressed air, then partially filled with 80 mL of saline solution of desired concentration before being properly sealed In order to eliminate contamination to gas hydrate system, the air initially inside the reactor was discharged before starting the measurement It was achieved by recharging the reactor with methane gas to 500 psi and then completely venting it three times Subsequently, the reactor was pressurized to 1465 psi (10 MPa) by compressing methane from the N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 89 Fig Schematic diagram of the experimental setup gas cylinder This high pressure, well above equilibrium pressure of 2.7 MPa for CH4 hydrate formation at 1.5 °C, suitably generated in high driving force and, therefore, better kinetics of gas hydrate formation for our studies It assisted in reducing the experiment time and mitigating the stochasticity of gas hydrate formation (gas hydrate formation is less stochastic at high driving force [1]) Once the target pressure was reached, reactor was disconnected from the gas cylinder In the meantime, data acquisition system was assembled The reactor was then submerged into cooling bath with temperature being pre-set to 0.5 °C The stirrer was switched on to rotate at a speed of 120 rpm Then the data acquisition system started recording and displaying the change of temperature and pressure versus time Once gas hydrate formation had finished, indicated by the stabilisation of both temperature and pressure, data acquisition system and stirrer were stopped and taken off, the reactor was then quickly released and opened Gas hydrate crystal was quickly separated and weighted using a Mettler Toledo digital balance with 0.0001 g sensitivity The mass of the residual solution in the reactor was also determined The entire duration of one experiment lasted for about h 2.3 Calculation of gas uptake and hydrate formation rate As the reactor was enclosed and no chemical reaction occurred, the total amount of methane inside the reactor remained constant during the experiments and was equal to the amount of gaseous methane at the beginning of each experiment which is described by n0 (moles) During an experiment, methane transferred from the gas phase into the hydrate phase and was consumed by gas hydrate formation Mass transfer led to a decrease in the amount of gaseous methane in the reactor, and visually indicated by a decrease in pressure Gas consumption at time t is calculated using equation of state of real gas as follows [26]: DnðtÞ ¼ n0 À nt ¼ PV ZRT À t¼0 PV ZRT VR Thus, for a relative comparison the effect of salts, the change in V (less than 1% as per our calculation) can be neglected Z is the methane compressibility factor which is calculated as a function of T and P using the Brill–Beggs equation [27] The growth rate of hydrate, r(t), was calculated using the gas consumption versus time as follows: rðtÞ ¼ dDn Dnðt þ DtÞ À DnðtÞ % dt Dt ð2Þ where the typical time step is D(t) = 0.5 Moving averaging with time steps was successfully applied to the numerical calculation of derivatives remove the physically unreal pikes occurring by the numerical errors 2.4 Determination of salt concentration and hydrophobicity The concentrations of sodium halides in the feed solutions, gas hydrates (product) and residual solutions (waste) were determined by inductively coupled plasma (ICP) technique The hydrophobicity of halide salts was determined by measuring the contact angle between a droplet of the saturated salt solution placed on its crystal surface The saturated solutions were used instead of pure water to avoid the dissolution of crystal surfaces The large salt crystals were prepared by crystallization from their saturated solution under controlled humidity and temperature The volume of each droplet was lL The contact angle was measured using a camera to capture the images of the droplet on the crystal surface The images were then digitized and processed to calculate the contact angle using a Matlab code developed by our team The images were taken at the frequency of 15 frames per second Experimental results 3.1 Gas pressure and temperature versus time: P–T graphs ð1Þ t where nt are the molar amount of methane at time t, T and P are the instantaneous absolute temperature and pressure, R is the universal gas constant, V is the gas volume in the reactor The volume V is equal to the total volume of reactor VR less the volume of fluid Vf As specific volume of methane gas hydrate is slightly larger than that of water, Vf increases slightly during gas hydrate formation, causing a small decrease in V However, the volume of the resulting gas hydrate is very small, compared to the total volume of reactor Fig shows the typical results obtained with NaI solutions for the change in gas pressure and temperature versus time For the first ten minutes, there was a sudden drop in both the temperature and pressure due to the cooling of the system being immerged into the cooling bath The drop in pressure was induced by the gas contraction and dissolution into the liquid phase The induction period was then followed During this period, the temperature of system remained fairly constant at approximately 1.5 °C while the gas pressure continued decreasing, indicating that methane continued 90 N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 concentrations over it and another group of curves for high salt concentrations below it The transition salt concentration was 50–75 mM The dilute salt solutions promote hydrate formation while concentration higher than the transition concentration inhibits hydrate formation For example, 50 mM NaI solution increased the gas consumption by about 20% compared to neat water However, concentrated salt solutions such as 250 mM NaF solution and 1000 mM solutions of NaI, NaBr or NaCl significantly reduced the gas consumption Furthermore, no significant rise in temperature was observed for these cases because the associating slow kinetics produced no significant amount of heat exceeding the heat transfer efficiency of the system It noted that as a 250 mM NaF solution was found to virtually inhibit gas enclathration and, therefore, no higher concentrations of NaF was investigated 3.3 Growth rate and induction time Fig Pressure (solid lines) and temperature (dashed lines) versus time during methane gas hydrate formation in sodium iodide solutions of different concentrations The pressure and temperature at zero time were 1465 psi (10 MPa) and 24– 25 °C, respectively dissolving into liquid phase to create a supersaturated solution needed for the initiation of gas hydrate formation The induction period lasted for a few minutes to hours, depending on the salt concentration Subsequently, the pressure still continued decreasing but the temperature started rising rapidly to about 5–7 °C and stayed at this temperature for about one hour even though the cooling bath kept cooling the system at the required temperature This increase in temperature is assigned to the exothermal formation of gas hydrates and indicates the onset of gas hydrate formation After some time, the gas hydrate formation diminished and completed as evidenced by the sudden drop in temperature to a constant value and the stabilization of pressure at a constant level The changes in gas pressure and temperature with NaBr, NaCl and NaF solutions share the same trend Therefore, we only show here the T–P graphs of the experiments with NaI solutions 3.2 Methane gas consumption versus time The change in gas pressure and temperature versus time was converted into the gas consumption using Eq (1) Fig shows the gas consumption versus time and the corresponding temperature of methane hydrate formation in sodium halides solutions of different concentrations For the concentrations examined the change in gas consumption with time follows a similar increasing trend The rapid increase for the first ten minutes is due to the dissolution of gas because of cooling as shown in Fig This increase in gas consumption for this period is independent of salt type, possibly because of the low salt concentrations used in the experiments, creating no discernible effect in gas solubility [28,29] After the first ten minutes, the gas consumption started increasing due to gas hydrate formation and became salt-dependent The curve for neat water (blue) divides the gas consumption curves into two groups, one group of curves for very dilute salt Growth rate provides explicit information on gas hydrate kinetics and, therefore, is indicative to the influence of salts The rate of hydrate growth in different salt solutions at different concentrations was calculated using Eq (2) Fig shows the calculated rate versus time The growth rate for the first 10–20 is not related to gas hydrate formation and is not shown in Fig As shown in the left graph of Fig 4, the rate of hydrate growth in individual solutions of low salt concentration (75 mM) reaches a peak at a specific time which corresponds to the sudden rise in temperature as exemplified in Fig for NaI Evidently, this significant hydrate growth indicates the onset of gas hydrate formation Approximately, the next one hour is the gas hydrate formation period as evidenced by both the temperature rise and moderate growth rate Then, the growth rate eventually decreased to zero (and the temperature approached a constant value – see Fig 2), indicating that gas hydrate formation finished The peak of the growth in water did not occur sooner than the peaks of the growth in the dilute salt solutions The right graph of Fig shows the hydrate growth rate in high salt concentration solutions Evidently, the growth rate significantly decreased in the concentrated salt solutions The peaks of the growth rate in all solutions are below the peak of the growth rate in water The peaks of the growth rate in the 1000 mM NaI and NaBr solutions are shifted to the end, indicating not only slow growth rate but also long induction time as discussed in the next paragraph In the 1000 mM NaCl and 250 mM NaF solutions, no apparent peaks on the growth rate (and temperature, not shown) curves are observed, explicitly indicating that methane gas hydrate formation in these two solutions was virtually inhibited A reduction in gas consumption by approximately 50% in comparison with water was also identified with the two solutions (Fig 3) We define the induction time as the time interval between the beginning of each experiment and the point of time at which growth rate peaked Theoretically, induction time is the period of time between the creation of supersaturated solution and the occurrence of first crystals of gas hydrate It is, however, practically difficult to quantify induction time defined in this way On contrary, our definition is practically applicable and provides a good approximation for comparing the effect of salts on the kinetics of gas hydrate formation As shown in Fig 4, the induction time of methane gas hydrate formation in 75 mM salt solutions was shorter than that in water and 75 mM NaI solution significantly reduced the induction time of methane hydrate formation Fig shows the experimental results for the maximum gas consumption and induction time Evidently, with increasing salt concentration, the gas consumption by the hydrate formation first increased, reaching a maximum at 50–75 mM and then decreased The effect of the dilute NaI solution on the gas consumption was the most significant, while the effect of NaF solutions on increasing N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 91 Fig Effect on sodium halides and their concentration on methane consumption versus time The water line divides the curves into two groups, showing the dual effect of sodium halides on the formation of methane gas hydrate: sodium halides promote gas hydrate formation at low concentration whereas they become an inhibitor at higher concentration Fig Effect of sodium halides and concentration on growth rate of methane hydrate versus time The blue line for water is shown as a reference (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) the gas consumption was the weakest The trend of the change in induction time with increasing salt concentration was reverse to the trend of the gas consumption In case of no peak appearing on the growth rate curve, the corresponding induction time was extremely long and, therefore, was described as infinity Discussion about the effect of sodium halides 4.1 The dual effect of sodium halides on gas hydrate formation Our experimental results for both the growth rate and induction time show that the halide salts display the dual effect on methane hydrate formation At low concentration, the halides promote the hydrate formation, while at high concentration they act as inhibitors The transition concentration is around 50–75 mM The results also show that not only salt concentration is critical to the hydrate formation but also the type of halide anions can significantly impact the hydrate formation The promoting capability decreases in the order NaI, NaCl % NaBr, NaF Thus, the halide radius, polarizability and charge density are the determining factors of promoting capability of sodium halides The larger and more polarizable halide ions are the greater promoting capability they can display The dual effect of salts on hydrate formation is of both scientific and practical importance It contributes to a comprehensive understanding of the effect of salts on the formation of gas hydrates since the conventional standpoint considers salts as a gas hydrate inhibitor only A misunderstanding of the promoting behaviour of dilute salt solutions may be serious since we may thus underestimate the correspondingly potential risks For example, fluids inside submarine pipelines often contain small content of salts (sodium chloride and magnesium chloride) and underestimating the effect of their presence may cause catastrophic problems For example, the explosion of BP’s rig in the Gulf of Mexico in 2010 was attributed to the blockage of pipelines caused by gas hydrate formation [30] Understanding the microscopic conceptual picture of the salt effect is a challenging but fascinating task The extraordinary properties of sub-molar concentration salt solutions have increasingly attracted research interests in several fields and have been intensively investigated thanks to the advancement in instrumental techniques and theoretical approaches Many ongoing efforts reveal anomalous behaviours of dilute sodium halide solutions on microscopic scale and, thereafter, linking them to many macroscopic observations is rewarding However, there still remain many mysteries about sub-molar solutions and motivates researchers in 92 N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 accomplishing the structural conformation of clathrate We visually simplify this process as depicted on Fig As the transformation progresses, the size and the structural conformation of hydration shells change by establishing and breaking hydrogen bonds, and water clusters undergo through different high-energy states such activation barriers [4] Consequently, hydrophobic hydration shells can only form and then transform to gas clathrate cavities if the thermodynamic driving force of formation and transformation surpasses activation barriers This argument explains why gas hydrate can nucleate at a certain region of temperature and pressure Once clathrate cavities have formed in solution, they develop to construct unit cells by vertex-linking or face-sharing and then further grow into gas hydrate crystals 4.3 Possible effects of halide ions on gas hydrate nucleation Fig Dependence of maximum gas consumption (top) and induction time (bottom) on sodium halide salts and their concentration The induction times at high concentrations of NaCl and NaF are extremely (infinitely) long many disciplines For instance, as the salts influence gas hydrate kinetics in many aspects such as changing the induction time, the growth rate and the gas consumption, they must change the fashion of both gas hydrate nucleation and growth In the following sections, we attempt to provide and argue possible explanations for our result via linking some extraordinary properties of halide ions, water structure and microscopic picture of gas hydrate formation 4.2 Microscopic conceptual picture of gas hydrate formation To discuss the possible mechanisms of the dual effect of sodium halides on gas hydrate kinetics, we should first briefly describe the pathway of gas hydrate formation So far, several hypotheses of gas hydrate formation have been proposed but all of them have been under controversy and criticism [4] According to a widely accepted hypothesis developed by Sloan et al [4] (as originally suggested by Frank and Evans in their ‘‘iceberg’’ model), gas hydrate nucleation originates from the formation of water clusters around dissolved guest molecules in such process known as hydrophobic hydration The water at hydrophobic hydration shells is proposed to be in ‘‘pre-hydrate’’ structure This theory has been intensively tested by both experimental measurements (e.g [31–33]) and computer simulations (e.g [34–36]) to determine water structure on hydration shells and the coordination number Most of the outcomes appeared to support the hypothesis since they proved the existence of hydration shells around hydrophobic molecules, in particular, methane molecules However, both computer simulation and experimental measurements have shown that the number of water molecules in the hydration shell of methane in aqueous solution (so-called coordination number) is around 19 [33,37] which is smaller than the expected values of 20 and 24 for sI small (512) and large (51262) cages, respectively Hence, the hydration shells have been proposed to undergo transformations, through various intermediate states, before The enclathration of gas in neat water, as described in Section 4.2, is thermodynamically unfavourable due to the negative entropy change and partial loss in the number of hydrogen bonds associating with the process of enclathration Consequently, the formation of first hydrophobic hydration shells of methane in neat water is thermodynamically difficult Apparently, this thermodynamic barrier is observed as the existence of metastability of gas hydrate system In sodium halide solutions, an ion-specific effect causing the extraordinary promotion of gas hydrate kinetics is possibly the hydrophobic nature of halide ions It has been suggested via both experimental (e.g [38–40]) and simulation (e.g [41]) studies that large and polarizable halide ions such as iodides display a hydrophobic nature and, therefore, function as hydrophobic entities whereas small and charge-dense ions such as fluorides cannot play the same role Discussion on the physics behind hydrophobic nature of ions is complicated and beyond the scope of this paper This extraordinary concept, however, is useful for approaching a reasonable explanation of the promoting behaviour of sodium halides As an ion, iodide is easily hydrated forming a solvation shell, compared to methane Also considered as a hydrophobic entity, the hydration of iodide is hydrophobic hydration and its solvation shell, therefore, is a hydrophobic hydration shell In contrast to the case of multivalent ions or small and charge-dense ions, iodide– water interaction is weaker than water–water interaction [42] We, therefore, propose that even being too weak to fully collapse local water structure, the weak iodide–water interaction is still sufficiently strong to structurally rearrange adjacent water molecules into a fashion similar to water structure on the hydration shell of methane Therefore, the existence of these similar hydrophobic hydration shells is proposed to facilitate the occurrence of the hydrophobic hydration of methane In the other words, the hydrophobic hydration shells of iodide ions play the important role of the seeding for gas enclathration Obviously, the hydrophobic nature of halides is the central basis for this hypothesis The idea that polarizable ions have their hydrophobic behaviours has been suggested via computer simulation [41] and the inference from several spectra interpretations [38–40] The experimental results shown in Table further prove the hydrophobic nature of halides The contact angle between the saturated salt solution and the salt crystal surface is a measure of the hydrophobicity of the ions For example, the zero contact angle for NaF shows that fluoride likes water and is strong hydrated by water, whereas the contact angle of 12.7° for NaI shows that iodide does like water as fluoride and is not hydrated by water as much as fluoride; indeed, of the halides investigated, iodide is the most hydrophobic halide The results for the contact angles in Table evidently show NaI is the most hydrophobic, 93 N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 Fig Conceptual diagram of the formation of clathrate cages via hydrophobic hydration Table Contact angle (CA) between salt crystal surfaces and their saturated solutions Example image Surface CA (°) 0s 1/15 s 5/15 s Equilibrium (after s) NaF 0.0 ± 0.0 NaCl 0.0 ± 0.0 NaBr 6.7 ± 0.2 NaI followed by NaBr whereas NaCl and NaF are hydrophilic As these salts share the same sodium cation, the difference in hydrophobicity is logically due to the halide anions It is evident from these results for contact angle that as fluoride is very hydrophilic [39] it cannot play the function as effectively as iodide does On contrary, fluoride ion strongly interacts with adjacent water molecules electrostatically and can collapse the local water structure radically, a phenomenon known as the perturbation of water structure by ions which is reported in the literature [43,44] Consequently, gas hydrate formation is hindered since the initiation of gas hydrate nucleation associating with tetrahedrally ordered water structure is hindered Furthermore, the existence of transition concentrations is possible because at these concentrations, the number of ions and, therefore, their hydration shells is adequate for the seeding for nucleation If ion density is higher than transition concentration, there occurs the competition between ions and gas molecules to gain water for hydration As ions bind water more strongly than gas molecules do, gas molecules lose their ability to constrain water to establish hydrophobic hydration shells (which are 12.7 ± 1.5 pre-hydrate cages) Another consequence is also a reduction in gas solubility in concentrated salt solutions This argument explains the inhibiting effect of salts at high concentrations The final point worth discussing is the salt collection by gas hydrates Gas hydrate is conventionally believed to be salt-free and, therefore, expected to be a novel method for desalination However, our hypothesis of considering hydration shells of polarizable ions as seeds for gas hydrate nucleation should lead to the consequence that gas hydrate crystals must contain ions, i.e these ions must be encapsulated in initial seeds of gas hydrate Indeed, the results of the chemical assaying of the products and mass balance, as shown in Table 2, support our hypothesis Evidently, the synthesized methane gas hydrate was not salt-free These results are consistent with our results from an experimental study on CO2 gas hydrate [26] and computer simulation by Qi et al [45] Furthermore, the recovery of NaI with hydrate is also higher than recovery of NaF A possible reason is that as NaI is more hydrophobic, it involves more actively in the nucleation of gas hydrate, as per our hypothesis Consequently, a larger number of iodide ions are encapsulated in gas hydrate crystals, as the seeds for gas 94 N.N Nguyen, A.V Nguyen / Fuel 156 (2015) 87–95 Table Mass balance showing the salt distribution in methane hydrates Salt a Mass (g) Salt recoverya (%) Sodium concentration (g/g) Feed Hydrate Waste Feed Hydrate Waste NaF 80.018 80.087 80.061 32.540 34.762 37.999 47.372 45.282 42.024 0.001024 0.002351 0.002048 0.000799 0.001787 0.001575 0.001160 0.003235 0.002463 33.74 ± 1.42 NaI 80.188 80.228 80.212 80.227 41.150 40.142 45.440 39.522 38.786 40.015 34.772 40.402 0.001058 0.001188 0.001188 0.001587 0.000842 0.001090 0.001082 0.001784 0.001352 0.001500 0.001481 0.001400 48.43 ± 3.18 Mass of salt in the hydrate phase divided by the total mass of salt in the feed hydrate nucleation However, it is not known whether or not the ions are within the crystal lattice or they are just trapped between the hydrate crystals Further studies are required to answer this question Conclusion A series of experiments were successfully conducted to investigate hydration of methane in water and sodium halide solutions The changes in pressure and temperature during the methane hydrate formation were measured as a function of time and concentration of NaF, NaCl, NaBr and NaI The gas consumption, growth rate, induction time and maximum gas consumption were determined The experimental results show the dual effect of the salts on methane hydrate formation While it has been widely reported that salts are an inhibitor of gas hydrate formation, the outcome of our research evidently proves that sodium halides can be either a promoter at low concentration or an inhibitor at high concentration Furthermore, large and soft (polarizable) ions like iodide were shown to be more effective promoters whereas small and hard (high-charge density) ions like fluoride were observed to be an effective inhibitor We propose that the difference in hydrophobicity (as measured by contact angle) of halides gives rise to this experimental observation Our hypothesis is supported by the results of the contact angle measurements and the salt recovery by the methane hydrate Further research is needed to better understand the role of hydrophobic hydration and hydrophobic entities in gas hydrate formation, and establish a fuller understanding of the salt effect on the formation of gas hydrates Acknowledgements The authors would like to thank to Mr Tuan D Nguyen for his technical assistance in the data acquisition system and Ms Faezeh Farhang for her technical assistance in the experimental setup Ngoc N Nguyen acknowledges the Australian Government for the Australian Awards Scholarships (AAS) References [1] Sloan ED Fundamental principles and applications of natural gas hydrates Nature 2003;426:353–9 [2] Thakur NK, Rajput S Exploration of gas hydrates: geophysical techniques Berlin: Springer-Verlag; 2011 [3] Sloan ED Clathrate hydrate 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