Molecular Dynamics Simulation of Methane Hydrate Formation and

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Molecular Dynamics Simulation of Methane Hydrate Formation and Dissociation in the Clay Pores With Fatty Acids Haoqing Ji, Daoyi Chen, Chen Zhao, and Guozhong Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08808 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

Molecular Dynamics Simulation of Methane Hydrate Formation and Dissociation in the Clay Pores with Fatty Acids

Haoqing Ji†,‡, Daoyi Chen†,‡, Chen Zhao†,‡, Guozhong Wu*,†,‡

†

Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

‡

School of Environment, Tsinghua University, Beijing 100084, China

ABSTRACT

Natural gas hydrate is a promising energy resource, but it is challenging to recover methane from the clay pores rich with sediment organic matters due to the inadequate information on hydrate evolution in the organo-clay complex. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite interlayer (Na-MMT) with fatty acids by characterizing the four-body structural order parameter, radial distribution functions, and cage types. Results demonstrated the slight inhibition of fatty acids types (butyric and isovaleric acid) and molar concentrations (0.6% and 1.5%) on the methane hydrate formation in the context of this study. This finding was a result of several main processes including the formation of “quasi-hydrate” water structures

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around the hydrophobic carbon chains of fatty acids, the disruption of hydrogen bond networks around hydrate structure by the hydrophilic carboxyl of fatty acids, and the restriction on the thermal motion of the sodium ions by coordination with the carboxyl of fatty acids. Results also demonstrated the promotion of fatty acids on the methane hydrate dissociation in the Na-MMT, which highlighted the role of fatty acids accumulation in the accelerated breakdown of hydrates at relatively low decomposing temperature. Overall results provided theoretical supports for better understanding the formation mechanisms and decomposition strategies of methane hydrates in the heterogeneous sediment environments since fatty acids are microbial intermediates of special importance in the hydrate-bearing sediment.

INTRODUCTION Natural gas hydrates are non-stoichiometric compounds formed by encapsulating small gas molecules such as methane in the hydrogen-bonded water networks, which are potential energy sources distributed in the marine sediments and terrestrial permafrost1. During the past decades, considerable efforts have been made to understand the gas hydrates formation mechanisms and to develop gas recovery technologies. Most recently, a pilot test of gas production from marine hydrates was successfully completed in the Shenhu area of the South China Sea from May to July 20172. Compared with previous field trials carried out in other sites3, this pilot test achieved technological breakthrough that resulted in continuous gas production for longer time (60 days) and larger accumulative gas yield (highest instant output: 3.5


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×104 m3 · day-1, methane purity: up to 99.5%)4. Nevertheless, this is only one step on the long journey towards the realistic commercial exploitation of gas hydrates due to the complexity of the sediment environments. For example, the confirmed gas hydrates in the Shenhu area are predominantly in the clay-rich “silty” sediments5. Gas recovery from clay sediments is more challenging due to the much less permeability compared with sandy sediments, which therefore becomes a subject of increasing interests. To date, there have been controversial results about whether the clay minerals such as montmorillonite promote or inhibit the thermodynamic phase equilibrium of gas hydrate6. Because of the limited natural sediment samples, molecular dynamics (MD) simulation have been frequently used to obtain relevant information such as the thermodynamic effect of montmorillonite surface on the methane hydrates formation7, the structure and intercalation behavior of methane hydrates in the montmorillonite8-9, and the crystal growth and molecular diffusion of methane hydrates in the montmorillonite nanopores10. These studies were more likely focused on the physicochemical properties of clay minerals such as the pore size, metal ions and swelling capacity. Less information was documented about the sediment organic matters (SOM) especially when they formed organo-clays after complexation with the clay minerals. There was a knowledge gap between our understandings of the hydrate evolution in presence of SOM and the improvement of gas recovery efficiency from the SOM-rich sediments. To date, there are only a few works on the effects of SOM-mineral complex on the



gas hydrates formation. For instance, Lee et al. reported the induction time11-12, phase equilibrium13-14 and nucleation kinetics15 of CO2 hydrates in the sediment suspensions with SOM. These studies delivered new insights into the offshore CO2 sequestration process in terms of the selection of CO2 sequestration sites and the storage conditions. It requires further works to clarify how the interactions between SOM and minerals were associated with methane hydrates evolution in the organoclays, which is supposed to provide basic knowledge for successful recovery of methane from the heterogeneous systems. Moreover, SOM represents diverse organic molecules as a result of microbial activities in the gas hydrates deposits, which have different functional groups and chemical configurations. Among the microbial intermediates, fatty acids are of special importance, because they can be further degraded to methane16 and they are considered biomarkers for seafloor methane seepage overlying the hydrate-bearing sediment17. However, no significant research has been conducted to verify the effects of fatty acids on the kinetics of methane hydrate formation and dissociation. The SOM selected in current studies were strongly hydrophilic such as glucose, glycine and urea13, 15, it remains unclear how the fatty acids with hydrophobic carbon chains and hydrophilic carboxyl interact with the water cages in the clay pores. Previous studies on the oil-water systems suggested that the hydrophobic tails in the molecular structure tend to penetrate or even become embedded in the open cavities on the hydrate surface18. However, the contribution of this finding to the clay pore water was not well-understood so far.


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Accordingly, MD simulation was performed to investigate the methane hydrates evolution in the sodium montmorillonite interlayer (Na-MMT) in the present study. The butyric acid and isovaleric acid were selected as representative fatty acids. Specific objectives of this study were to clarify the (i) overall effects of fatty acids on the kinetics of methane hydrates formation and dissociation in the clay nanopore, (ii) contribution of different functional groups, carbon positions and molar concentration of fatty acids to the above effects, and (iii) methane hydrates dissociation mechanisms at different temperatures in the organo-clay complex. METHODOLOGY Modeling and Simulation: MD simulations were performed with GROMACS (version 5.0.5)19. The MMT unit cell used in this study had a stoichiometry of [Al3Mg1][Si8O20][OH]4, which possessed one negative charge. It was replicated along the xy-plane to form a supercell (10 × 6 × 1). A liquid slab (initial thickness: 3.9 ~ 4.3 nm) was intercalated in the MMT interlayer (interlayer space: 4.9 ~ 5.3 nm), which consisted of 60 sodium ions, 3265 water and 300 methane. The sodium ions were introduced to compensate for the negative charges on the MMT, while the molar ratio of water to methane was chosen to ensure the hydration of all interlayer methane and sodium ions. Organo-clay models were constructed by loading butyric or isovaleric acids into the Na-MMT interlayer. The molecular number of each fatty acid was 20 and 50, respectively, which corresponded to a molar fraction of 0.6% or 1.5%. The concentration selection was a result of our preliminary test, which allowed a clear



observation of the water cages and the fatty acids accumulation phenomenon which was quite normal in the nature. Although this concentration was about two-order of magnitude higher than that previously measured in the marine sediments20, it was difficult to use the real concentration during MD simulation due to the high computational cost. For example, it took about 25 days to complete a 3 µs simulation with 120 CPU-cores in this study. It was estimated to take several years for this work if we increased the number of water molecules by two-order of magnitude, which was beyond the computational capacity at current stage. The Na-MMT and fatty acids were modeled by the CLAYFF force field21 and the CHARMM general force field22, respectively. Methane and water were modeled using the united-atom Lennard-Jones model23 and the TIP4P/Ice model24, respectively. Standard Lorentz-Berthelot mixing rules were employed to calculate the cross interactions between different species25. Short range interactions were truncated at 1.2 nm, while the long-range Coulombic interactions were calculated by the particle mesh ewald method with a Fourier spacing of 0.12 nm26. The motion equations were integrated by the leap-frog algorithm with a time step of 1fs27. The initial configurations were energy minimized with the steepest descent algorithm, followed by 200 ps NPT equilibration under constant temperature (250 K) and pressure (500 bar). Subsequently, the methane hydrates formation process was simulated by 3 µs MD simulation at NPT ensemble (250 K, 500 bar). The resulted configurations were used as the initial configurations for the simulation of hydrates dissociation. The pressure for the hydrates dissociation simulations (NPT, 50 ns) was


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controlled at 50 bar, while the temperature was set at 293 and 303K, respectively. The Nose−Hoover thermostat28 and Parrinello−Rahman barostat29 were used to control the temperature and pressure, respectively. Semi-isotropic pressure coupling was employed so that the z direction of the simulation box could fluctuate independently from the x and y dimensions. Three-dimensional periodic boundary conditions were applied throughout simulations.

Data Analysis: The four-body structural order parameter F4φ30 was employed to quantify the hydrate-like arrangements of water molecules, which was calculated as follows: ‫ܨ‬ସఝ

௡

1 = ෍ cos 3߮௜ ݊ ௜ୀଵ

where n is the total number of water pairs with the oxygen atoms within 3.5 Å, and φi is the torsion angle formed by the oxygen atoms and two outermost hydrogen atoms in the ith water pair. For ice, liquid water and hydrate, the corresponding F4φ values are -0.4, -0.04 and 0.7, respectively31. The face-saturated incomplete cage analysis (FSICA)32 was used to quantify the cage water percentage, the number of various cage types, and the number of cage links. A cage link was counted if two cages shared one cage face. Face-saturated cages (FSCs) were characterized as those with each edge shared with two faces. A cage was identified as edge-saturated if each vertex of this cage was shared with at least three edges. On this basis, FSCs were further classified into complete cages (CCs, edge-saturated) and face-saturated incomplete cages (FSICs, not edge-saturated).



The CCs consisted of the typical cages in hydrate structures such as the 512, 51262, 51264, 51268, 435663 cages, while the FSICs would shortly occur as precursors of hydrate formation.

RESULTS AND DISCUSSIONS Effects of fatty acids on the methane hydrate formation The number density profiles of species in the MMT interlayer during hydrate formation is shown in Fig. S1 in the supporting information. Distinct peaks were observed for water and Na+ near the negatively charged MMT surface due to the electrostatic attraction. The hydration of the abundant Na+ near the MMT surface excluded the methane molecules away from the MMT surface due to the salting-out effect, which referred to the decrease in the solubility of hydrocarbon solutes caused by ion hydration.33 The fatty acids tended to get close to the MMT surface especially when loaded at high concentration, making it possible to interact with the Na+ near the MMT surface. Results showed that about 50% of the free water was transformed to water cages at the end of simulation, while the influence of the type and concentration of fatty acids was slight (Fig. 1A). Among these cages, 70 - 80% was complete cages, which were dominated by the 512, 51262, 51263, 51264 , 4151062, 4151063 and 4151064 cages (Fig. S2). These seven types of cages accounted for 83 - 93% of the complete cages, which were able to transform between each other.34-35 Some selected snapshots of cage configurations during methane hydrate formation are shown in Fig. S3. Some of


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these cages were empty while up to 97% of them were filled with methane (Fig. S2). In order to characterize the effects of MMT surface on the hydrates formation, the Na-MMT interlayer space was divided into slices with equal thickness of 0.5 nm according to the distance to the MMT surface. The F4φ values in the first four slices were calculated (only half of the interlayer space was investigated due to the geometric symmetry), which demonstrated rare formation of hydrates near the MMT surface (slice 1 in Fig. S4). This was attributed to the concentrated distribution of Na+ in this region (Fig. S1), because the thermal fluctuation of Na+ would disordered the water structure and inhibited the hydrates formation.15 The region away from the MMT surface was preferable for the readily formation of methane hydrates (slices 2 ~ 4 in Fig. S4). The addition of fatty acids was adverse for the water cage formation especially when they were loaded at high concentration. For example, the total number of the seven main types of cages decreased by 17% when 1.5% isovaleric acids were present in the Na-MMT interlayer (Fig. S2). The observed inhibition effects were attributed to the steric constraints and the hydrogen bonds with water induced by the fatty acids. The fatty acids were unlikely to be enclathrated into a hydrate cage because of the hydrophilic carboxyl group in the molecular structure, therefore, the presence of fatty acids at high concentration separated the Na-MMT interlayer space into small compartments. The limited space was unfavorable for the formation and crystallization of hydrates, which was evidenced by the obviously decrease in the links per cage (Fig. 2A). Additionally, the hydrophilic carboxyl group in the fatty



acids made it easy to form hydrogen bonds with water, which would disrupt the local hydrogen bond networks and impede the formation of hydrate structure. As shown in Fig. 2B, the hydrogen bonds between fatty acids and water doubled when its molar concentration increased from 0.6 % to 1.5 %. However, it should be noted that the overall inhibition effects of fatty acids on the rate of hydrate formation was not remarkable (Fig. 1A). It was speculated that the inhibition resulted from the aforementioned mechanisms was counteracted by the hydrophobic entities in the fatty acid structure. As shown in Fig. 3, “quasi-hydrate” water structures were clearly observed around the carbon chains of fatty acids. They were mainly consisted of five-membered water rings with adsorbed methane molecules. According to the “cage adsorption” theory36, the adsorption of methane would in turn stabilize the quasi-hydrate structure and promote it to form complete hydrate cages by further hydration. In other words, the five-membered water rings around the fatty acids would serve a template for the fast formation of 512 cages during the initial stage of simulation. For example, the number of 512 cages during the first 300 ns was up to 50% higher when the butyric acid or isovaleric acids other than the pure water was present in the Na-MMT interlayer (Fig. 3A). These findings suggested that the carbon atoms in the fatty acids resembled methane molecules in terms of rearranging the surrounding water molecules to a hydrate-like structure. It was inferred that the number of such hydrate-like structure would increase with the concentration of fatty acids, which was confirmed by the observed increase in the F4φ values following the addition of fatty acids (Fig. 1B). It was further supported by


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the radial distribution functions (RDF) of the oxygen in water around the carbons in the methane or in the fatty acids. For example, the first three characterized peaks located at 0.4 nm, 0.65 nm and 0.8 nm, respectively, in Fig. 4A were also founded in Fig. 4B. Moreover, it was noticed that the first peak in the RDF of the oxygen in water around the carbons in the butyric acid was split into two small peaks (Fig. 4B), suggesting the diverse positions of the different carbon atoms in the butyric acid. This phenomenon was more pronounced for the isovaleric acids. To clarify the effect of carbon positions on water distribution, the RDF of oxygen in water around each carbon in the fatty acids was plotted in Figs. 4C and 4D. It demonstrated that the water profile around the carbon atom farther from the carboxyl group in the fatty acid was more similar to that around the methane molecules. Another potential reason for alleviating the inhibition effects of fatty acids on hydrate formation was the restricted thermal fluctuation of Na+ as aforementioned after introducing fatty acids in the Na-MMT. As shown in Fig. 5A, the first sharp peak at around 0.23 nm in the RDF of Na+ around the oxygen atoms in the fatty acids indicated a strong coordination of Na+ with the –COO- group in the fatty acids (Fig. 5A). The number of Na+ effectively immobilized by the coordination interactions, which was counted by those within 0.3 nm from the oxygen atoms of fatty acids, increased with the concentration of fatty acids (Fig. 5B). When the butyric and isovaleric acids were added at high concentration, the diffusion coefficients of sodium ions decreased by 77% and 48%, respectively (Fig. 5C).



Effects of fatty acid on the methane hydrate dissociation Results showed that the hydrate dissociation was less dependent on the Na-MMT surface, because the difference in the decrease of F4φ in different slices was not pronounced at given temperature and pressure (Figs. S5 and S6). It further demonstrated that the presence of fatty acids was preferable for methane hydrate dissociation. As shown in Fig. 1C, the methane hydrate did not dissociate at 293 K in the Na-MMT interlayer filled with pure water. Although little changes were noted in the F4φ value at 293 K when 0.6% fatty acids were present, the “quasi-hydrate” water structures aforementioned had the tendency to break up, which was evidenced by the slight decrease in the first peak in the RDF of oxygen in water around the carbon chains of fatty acids during the first 10 ns (Figs. 6A and C). This tendency became stronger when the concentration of fatty acids increased to 1.5%, because the intensity of the corresponding RDF peaks kept on decreasing with time (Figs. 6B and D). It resulted in significant decrease in the F4φ values (Fig. 1C). It was inferred that the hydrate dissociation started from the vicinity of fatty acids towards the region away from fatty acids. This was supported by the observation that the intensity of the first peak around 0.4 nm started to decline at the beginning of simulation, but obvious decline for the second peak around 0.65 nm was not observed until 15 ns for the butyric acid (Fig. 6B). The time for the decline of the second peak was 20 ns when the isovaleric acids was present (Fig. 6D). The time difference was attributed to the higher diffusion coefficient of butyric acid than isovaleric acids (Fig. 7), because a higher diffusion coefficient indicated more


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frequent thermal motion that was supposed to induce faster break-up of the surrounding water networks. Two critical steps were recognized for the dissociation processes at 293 K when the fatty acids were present at high concentration. One was the fast collapse of the “quasi-hydrate” structure that resulted in the accumulation of methane molecules near the fatty acids, which was clearly evidenced by the increase in the intensity of the first peak of the RDF of methane around the carbon chains of fatty acids (Figs. S8A and C). This step improved the flexibility of fatty acids by reducing the bound from water cages and therefore doubled their diffusion coefficient (Fig. 7B). This triggered the second step that the fatty acids tend to move together, which was evidenced by the increase in the intensity of the RDF peaks of fatty acids around fatty acids during the initial 25 ~ 30 ns (Figs. 8B and 8D). The accumulation of fatty acids promoted the surrounding methane molecules to merge into a large gas bubble (Figs. 9A and 9B). It demonstrated that the alkyl groups in the fatty acids pointed towards the methane molecules (Figs. S8C and D), which was beneficial for the assembling and stabilization of methane bubbles37. The formation of sufficient large gas bubbles eventually led to the fast dissociation of methane hydrates because it would disorder the water near the gas-water interface and destabilize the hydrate nuclei38. These findings highlighted the role of the fatty acids accumulation and the subsequent formation of gas bubbles in the promoted dissociation of methane hydrate at relatively low temperature (293 K). This was particularly remarkable at



high concentration. By contrast, neither of these two processes took place when fatty acids were present at low concentration. For example, there was not tendency for the fatty acid molecules to move together (Figs. 8A and 8C) and they remained dispersed with few scattered methane molecules around them at the end of simulation (Figs. 9C and 9D). Therefore, the promotion effects of fatty acids on the hydrate dissociation was not observed in this case (Fig. 1C). When the temperature increased to 303 K, the methane hydrates dissociated in all simulations which was characterized by an initially slow dissociation followed by an accelerated dissociation (Fig. 1D). The fatty acids promoted the dissociation kinetics even at low concentration. It was inferred that the fast dissociation of methane hydrates in this case was not triggered by the accumulation of fatty acids aforementioned. When 0.6% butyric acid was present, the accelerated dissociation stage started at around 12 ns (Fig. 1D), but the accumulation of butyric acids and the formation of a noticeable methane gas bubble was not observed until 15 ns (Figs. 10A and 11B). Similar phenomenon was observed for the isovaleric acids that the methane hydrate transferred to the fast decline stage at 9 ns but the accumulation of isovaleric acids was not found until 12 ns (Figs. 10C and 11D). It suggested that a temperature of 303 K was high enough to directly induce the collapse of hydrate structure around a single molecule of fatty acid without relying on their accumulation. The continuous decline in the first peak at 0.4 nm of the RDF of water oxygen around the fatty acids was followed by the decline in the second peak at around 0.65 nm (Figs. 12A and C), which was quite different from that observed


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when fatty acids were present at low concentration at 293 K (Figs. 6A and C). As expected, the slow dissociation stage was shortened and the rate of the fast dissociation stage was obviously increased by increasing the concentration of fatty acids at 303 K (Fig. 1D). It should be mentioned that the hydrate nucleation was a stochastic process in terms of the hydrate induction time and the hydrate cage types. It was computationally expensive for microseconds simulation as aforementioned, which made it difficult to run several repeat simulations. Nevertheless, we thought it would not influence the main conclusions from this study because (i) the induction time was not expected to change much in context of this study, because the hydrate nucleated immediately as evidenced by the sharp increase in the number of cage water and the F4φ values (Figs. 1A and B). This observation was not random because the methane concentration in this study was beyond the critical nucleation concentration39, which meant a very short induction time with less stochasticity. Moreover, this study was focused on the hydrate growth kinetics instead of nucleation, therefore, the main conclusions would not change with the stochasticity of induction time. (ii) the hydrate cage types were expected to change when repeating several simulations as previously reported35, which might influence the hydrate stability and the corresponding decomposition process. It remains unclear about the correlation of cages types with the fatty acids. Despite this fact, the hydrate dissociation mechanisms revealed from this study would not be affected by these changes. For example, the hydrate dissociation at relatively low temperature was demonstrated to be mainly contributed by the fatty



acids accumulation which was a process independent of the cage types.

CONCLUSIONS This study demonstrated that the presence of fatty acids was adverse for water cages formation in the Na-MMT interlayer especially when they were loaded at high concentration. It also indicated that the overall effect of fatty acids on the kinetics of methane hydrate formation was an integrated result from the antagonistic effects of different functional groups in the fatty acids. The carboxyl group made it easy to form hydrogen bonds with water and thus inhibit hydrate formation, but its coordination with the sodium ions in the clay pore was favorable for the hydrate formation. Meanwhile, the carbon chain resembled methane molecules in terms of rearranging the surrounding water to a hydrate-like structure. The farther from the carboxyl group, the more obvious phenomenon of the carbon atom was. Results also demonstrated that the presence of fatty acids facilitated the hydrate dissociation in the Na-MMT interlayer. The higher concentration, the stronger promotion. This study also provided evidences on the different mechanisms for the hydrate dissociation at different temperatures. At 293 K, the hydrate decomposition followed two critical processes such as the fast collapse of the “quasi-hydrate” structure near the carbon chains of fatty acids and the formation of gas bubbles due to the accumulation of fatty acids. The latter was not observed when the temperature increased to 303 K, which was high enough to directly breakdown water cages. This implied a possibility to implement different temperature programming strategies for gas recovery from the sediments rich with fatty acids, but it awaits confirmation


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using real clay sediments which are more heterogeneous with abundance of different SOM coexistence.

SUPPORTING INFORMATION Number density profiles of species during the last 100 ns of methane hydrate formation (Figure S1), evolution of water cages during methane hydrate formation (Figure S2), snapshots of cage configurations during methane hydrate formation (Figure S3), local F4φ during methane hydrate formation (Figure S4), Local F4φ during methane hydrate dissociation at 293 K (Figure S5) and 303 K (Figure S6), distribution of methane around the carbons of fatty acids during methane hydrate dissociation at 293 K (Figs. S7 and S8) and 303 K (Figs. S9 and S10)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone/Fax: +86-0755-26030544 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully thank Professor Valeria Molinero (University of Utah) for kindly sharing with us their codes for cages visualization. This study was financially supported

by

the

Fundamental

Research

Project


of

Shenzhen,

China


(JCYJ20160513103756736),

the

Shenzhen

Peacock

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Plan

Research

Grant

(KQJSCX20170330151956264), the Economy, Trade and Information Commission of Shenzhen Municipality (HYCYPT20140507010002), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501.

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Behaviors of Methane Hydrate in Montmorillonite. J. Molecul. Modeling 2014, 20, 1-11. 10. Yan, K.-F.; Li, X.-S.; Chen, Z.-Y.; Xia, Z.-M.; Xu, C.-G.; Zhang, Z., Molecular Dynamics Simulation of the Crystal Nucleation and Growth Behavior of Methane Hydrate in the Presence of the Surface and Nanopores of Porous Sediment. Langmuir 2016, 32, 7975-7984. 11. Lamorena, R. B.; Kyung, D.; Lee, W., Effect of Organic Matters on Co2 Hydrate Formation in Ulleung Basin Sediment Suspensions. Environ. Sci. & Tech. 2011, 45, 6196-6203. 12. Lee, K.; Lee, S.-H.; Lee, W., Stochastic Nature of Carbon Dioxide Hydrate Induction Times in Na-Montmorillonite and Marine Sediment Suspensions. Intl. J. Greenhouse Gas Control 2013, 14, 15-24. 13. Park, T.; Kyung, D.; Lee, W., Effect of Organic Matter on Co2 Hydrate Phase Equilibrium in Phyllosilicate Suspensions. Environ. Sci. & Tech. 2014, 48, 6597-6603. 14. Kyung, D.; Lee, K.; Kim, H.; Lee, W., Effect of Marine Environmental Factors on the Phase


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Equilibrium of Co2 Hydrate. Intl. J.Greenhouse Gas Control 2014, 20, 285-292. 15. Kyung, D.; Lim, H.-K.; Kim, H.; Lee, W., Co2 Hydrate Nucleation Kinetics Enhanced by an Organo-Mineral Complex Formed at the Montmorillonite–Water Interface. Environ. Sci. & Tech. 2015, 49, 1197-1205. 16. Chang, H. N.; Kim, N.-J.; Kang, J.; Jeong, C. M., Biomass-Derived Volatile Fatty Acid Platform for Fuels and Chemicals. Biotech. & Bioprocess Eng. 2010, 15, 1-10. 17. Kanekiyo, A.; Takasugi, H.; Ogawa, M.; Naganuma, T., Sediment Fatty Acids Associated with Seafloor Methane Seepage in the Nankai and Sagami Troughs, Off Central Japan. Aquatic Ecosystem Health & Management 2005, 8, 73-80. 18. Kelland, M. A., History of the Development of Low Dosage Hydrate Inhibitors. Energy & Fuels 2006, 20, 825-847. 19. Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J., Gromacs: Fast, Flexible, and Free. J. Comp. Chem. 2005, 26, 1701-1718. 20. Mueller-Harvey, I.; John Parkes, R., Measurement of Volatile Fatty Acids in Pore Water from Marine Sediments by Hplc. Estuarine, Coastal & Shelf Sci. 1987, 25, 567-579. 21. Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G., Molecular Models of Hydroxide, Oxyhydroxide, and Clay Phases and the Development of a General Force Field. J. Phys. Chem. B 2004, 108, 1255-1266. 22. Vanommeslaeghe, K., et al., Charmm General Force Field: A Force Field for Drug-Like Molecules Compatible with the Charmm All-Atom Additive Biological Force Fields. J. Comp. Chem. 2010, 31, 671-690. 23. Jorgensen, W. L.; Madura, J. D.; Swenson, C. J., Optimized Intermolecular Potential Functions for Liquid Hydrocarbons. JACS 1984, 106, 6638-6646. 24. Abascal, J.; Sanz, E.; Fernández, R. G.; Vega, C., A Potential Model for the Study of Ices and Amorphous Water: Tip4p/Ice. J. Chem. Phys. 2005, 122, 234511. 25. Allen, M. P.; Tildesley, D. J., Computer Simulation of Liquids; Clarendon Press: Oxford, 1987. 26. Darden, T.; York, D.; Pedersen, L., Particle Mesh Ewald: An N⋅Log (N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089-10092. 27. Van Gunsteren, W.; Berendsen, H., A Leap-Frog Algorithm for Stochastic Dynamics. Molecular Simulation 1988, 1, 173-185. 28. Hoover, W. G., Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695-1697. 29. Parrinello, M.; Rahman, A., Polymorphic Transitions in Single Crystals: A New Molecular Dynamics Method. J. Appl. Phys. 1981, 52, 7182-7190. 30. Rodger, P.; Forester, T.; Smith, W., Simulations of the Methane Hydrate/Methane Gas Interface near Hydrate Forming Conditions Conditions. Fluid Phase Equilibria 1996, 116, 326-332. 31. Moon, C.; Hawtin, R.; Rodger, P. M., Nucleation and Control of Clathrate Hydrates: Insights from Simulation. Faraday Discussions 2007, 136, 367-382. 32. Guo, G.-J.; Zhang, Y.-G.; Liu, C.-J.; Li, K.-H., Using the Face-Saturated Incomplete Cage Analysis to Quantify the Cage Compositions and Cage Linking Structures of Amorphous Phase Hydrates. Phys. Chem. Chem. Phys. 2011, 13, 12048-12057. 33. Breslow, R., Hydrophobic Effects on Simple Organic Reactions in Water. Accounts Chem. Res. 1991, 24, 159-164. 34. Walsh, M. R., et al., The Cages, Dynamics, and Structuring of Incipient Methane Clathrate Hydrates. Phys. Chem. Chem. Phys. 2011, 13, 19951-19959.



35. Zhang, Z.; Walsh, M. R.; Guo, G.-J., Microcanonical Molecular Simulations of Methane Hydrate Nucleation and Growth: Evidence That Direct Nucleation to Si Hydrate Is among the Multiple Nucleation Pathways. Phys. Chem. Chem. Phys. 2015, 17, 8870-8876. 36. Guo, G.-J.; Li, M.; Zhang, Y.-G.; Wu, C.-H., Why Can Water Cages Adsorb Aqueous Methane? A Potential of Mean Force Calculation on Hydrate Nucleation Mechanisms. Phys. Chem. Chem. Phys. 2009, 11, 10427-10437. 37. Yagasaki, T.; Matsumoto, M.; Tanaka, H., Effects of Thermodynamic Inhibitors on the Dissociation of Methane Hydrate: A Molecular Dynamics Study. Phys. Chem. Chem. Phys. 2015. 38. Walsh, M. R.; Beckham, G. T.; Koh, C. A.; Sloan, E. D.; Wu, D. T.; Sum, A. K., Methane Hydrate Nucleation Rates from Molecular Dynamics Simulations: Effects of Aqueous Methane Concentration, Interfacial Curvature, and System Size. J. Phys. Chem. C 2011, 115, 21241-21248. 39. Guo, G.-J.; Rodger, P. M., Solubility of Aqueous Methane under Metastable Conditions: Implications for Gas Hydrate Nucleation. J. Phys. Chem. B 2013, 117, 6498-6504.


Page 20 of 33

Page 21 of 33

60

(a)

250 K, 500 bar

(b)

0.4

250 K, 500 bar

0.3

40

F4ϕ

Cage water (%)

50

30 0% fatty acid 0.6% butyric acid 1.5% butyric acid 0.6% isovaleric acid 1.5% isovaleric acid

20 10 0

0

500

1000

1500

2000

2500

0.2 0% fatty acid 0.6% butyric acid 1.5% butyric acid 0.6% isovaleric acid 1.5% isovaleric acid

0.1 0.0 0

3000

500

0.4

0.4

(c)

1000

1500

(d)

293 K, 50 bar 0.3

0.2

0.2

0.0 0

10

20

3000

0% fatty acid 0.6% butyric acid 1.5% butyric acid 0.6% isovaleric acid 1.5% isovaleric acid

0.1


2500

303 K, 50 bar

F4ϕ

0.3

0.1

2000

Time (ns)

Time (ns)

F4ϕ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


0.0

30

40

50

0

10

20

30

Time (ns)

40

50

Time (ns) Fig. 1 Changes in the percentage of cage water and F4φ parameters during simulations. Curves are smoothed using the 30-point adjacent-averaging method.



5

160

(a) Number of H-bonds

4

Links per cage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

3 2


1 0

0

500

1000 1500 2000 2500 3000

Page 22 of 33

(b)

140 120 100 80 60 40 20

0

500

Time (ns)

1000 1500 2000 2500 3000

Time (ns)

Fig. 2 (a) Links per cage, and (b) number of hydrogen-bonds between water and fatty acids during methane hydrate formation (250 K, 500 bar).


Page 23 of 33

60

(b)

(a)

50

Number of 512 cages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


40 30 0% fatty acid 0.6% butyric acid 1.5% butyric acid 0.6% isovaleric acid 1.5% isovaleric acid

20 10 0

0

500

(c)

1000 1500 2000 2500 3000

Time (ns)

Fig. 3 (a) Number of 512 cages during the methane hydrate formation (250 K, 500 bar). Snapshots of the “quasi-hydrate” structure are shown around one randomly selected (b) butyric acid and (c) isovaleric acid. The water and methane molecules within 6 Å and 7 Å from the fatty acid are shown in the snapshots, respectively. Curves in panel (a) are smoothed using the 30-point adjacent-averaging method.



2.5

(a)

0.6% butyric acid 1.5% butyric acid 0.6% isovaleric acid 1.5% isovaleric acid

g(CH4-OW)

4.0 3.0 2.0 1.0 0.0

1.0

0.4

0.8

1.2

(c)

1.6

2.0

0.0

1.5

4

C2

C3

2

0.0

0.4

0.4

0.8

0.5

1.2

1

(d)

1.6

2.0

0.6% isovaleric acid C2

3

g(C - OW)

C1

C1 - OW C2 - OW C3 - OW C4 - OW CH4 - OW

C4

1.0 0.3

r (nm)

0.6% butyric acid

3

0

2.0

0.5 0.0


1.5

r (nm) 4

(b)

2.0

g(C-OW)

5.0

g(C- OW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 24 of 33

C1

C3

C5 C4

2

C 1 - OW C 2 - OW C 3 - OW C 4 - OW C 5 - OW CH4 - OW

1

0.0

0.4

0.8

1.2

1.6

2.0

0

0.0

0.4

0.8

1.2

1.6

2.0

r (nm)

r (nm)

Fig. 4 Radial distribution functions of the oxygen in water (OW) around the (a) methane, (b) carbon chains of fatty acids, and (c, d) individual carbons at different positions on the carbon chain of fatty acids during methane hydrate formation (250 K, 500 bar).


25

(a)


20 15 10 5 0

0.2

0.4

0.6

r (nm)

0.8

1.0

Number of molecules

25

g(O-Na+)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


(b)


20 15 10 5 0

500

1000 1500 2000 2500 3000

Time (ns)

6

Diffusion coefficient

Page 25 of 33

5 4

(c)


3 2 1 0

Concentration of fatty acids

Fig. 5 Characteristics of Na+ identified by (a) radial distribution functions around the oxygen in fatty acids, (b) molecular numbers within 0.3 nm from the oxygen of fatty acids, and (c) diffusion coefficient (unit: 10-8 cm2·s-1) during methane hydrate formation (250 K, 500 bar). Curves in panel (b) are smoothed using the 30-point adjacent-averaging method.



2.0

0-5 ns 10-15 ns 20-25 ns 30-35 ns

(a)

1.0 1.8 1.7

0.5

0.0

2.0

5-10 ns 15-20 ns 25-30 ns 35-40 ns

1.5 0.36

0.4

1.0

0.5

1.6

0.0

(b)

1.5

g(C-OW)

g(C-OW)

1.5

0.40

0.8

1.2

0.44

1.6

0.0

2.0

0.0

0.4

r (nm) 2.0

0.8

1.2

1.6

2.0

1.6

2.0

r (nm) 2.0

(c)

(d)

1.5

g(C-OW)

1.5

g(C-OW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

1.0 1.6 1.5

0.5

1.0

0.5

1.4

0.0

0.35 0.40 0.45 0.50 0.55

0.0

0.4

0.8

1.2

1.6

2.0

0.0

0.0

r (nm)

0.4

0.8

1.2

r (nm)

Fig. 6 Radial distribution functions of the oxygen in water (OW) around the carbon chains of (a) 0.6% butyric acid, (b) 1.5% butyric acid, (c) 0.6% isovaleric acid, and (d) 1.5% isovaleric acid at different times during methane hydrate dissociation (293 K, 50 bar).


Page 27 of 33

(a)

20 0.6% butyric acid 0.6% isovaleric acid


2.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


1.5

1.0

0.5

0.0

0-5

5-10 10-15 15-20 20-25 25-30

(b)

1.5% butyric acid 1.5% isovaleric acid

15

10

5

0

0-5

5-10

10-15 15-20 20-25 25-30

Time (ns)

Time (ns)

Fig. 7 Diffusion coefficient (unit: 10-7 cm2·s-1) of fatty acids at different times during methane hydrate dissociation (293 K, 50 bar).



12

0-5 ns 10-15 ns 20-25 ns 30-35 ns

5-10 ns 15-20 ns 25-30 ns 35-40 ns

9

6

8

4

7

2

6

0

0.2

0.4

(b) 1.5% butyric acid 6

6

RDF

8

RDF

8

(a) 0.6% butyric acid

10

5 4

4

3

0.36

0.38

0.40

0.42

2 0.36

0.6

0.38

0.8

0.40

0.42

1.0

0

1.2

r (nm)

0.2

0.4

0.6

0.8

1.0

1.2

r (nm) 8

12 (c) 0.6% isovaleric acid

(d) 1.5% isovaleric acid 7

10

6

12

8

RDF

RDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

10

6

6 5

4

4

0.36

0.38

0.40

8

4

0.36

0.38

2

0.40

2 0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

r (nm)

0.4

0.6

0.8

1.0

r (nm)

Fig. 8 Radial distribution functions of (a, b) butyric acids around butyric acids, and (c, d) isovaleric acids around isovaleric acids during methane hydrate dissociation (293 K, 50 bar).


1.2

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) 1.5% butyric acid t = 25 ns

(b) 1.5% isovaleric acid t = 27 ns

(c) 0.6% butyric acid t = 50 ns

(d) 0.6% isovaleric acid t = 50 ns

Fig. 9 Snapshots of methane (green) within 0.5 nm from the carbon chains of fatty acids (red) during methane hydrate dissociation (293 K, 50 bar). Hydrate cages (512, 51262, 51263 and 51264) are shown in gray. Na+ and the water not belonging to any of the above cages are not shown for clarity.



12 11 10 9 8 7 6

RDF

10 8 6 4

0.36

0.38

0.40

8

0-1 ns 1-2 ns 2-3 ns 3-4 ns 4-5 ns 5-6 ns 6-9 ns 9-12 ns 12-15 ns

(b) 1.5% butyric acid 6

6

5

RDF

0-3 ns 3-6 ns 6-9 ns 9-12 ns 12-15 ns 15-18 ns 18-21 ns 21-24 ns

14 (a) 0.6% butyric acid

4

4

3

0.36

0.38

0.40

0.42

0.42

2

2 0

0.2

0.4

0.6

0.8

1.0

0

1.2

r (nm)

14 12

8

10 8

6

0.36

0.38

8

0.4

0.6

0.8

1.0

(d) 1.5% isovaleric acid 7

6

RDF

0-3 ns 3-6 ns 6-9 ns 9-12 ns 12-15 ns 15-18 ns 18-21 ns 21-24 ns

12 10

0.2

1.2

r (nm)

14 (c) 0.6% isvaleric acid

RDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

6 5

4

4

0.36

0.38

0.40

0-1 ns 1-2 ns 2-3 ns 3-4 ns 4-5 ns 5-6 ns 6-9 ns 9-12 ns 12-15 ns

0.40

4

2

2 0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

r (nm)

0.6

0.8

1.0

r (nm)

Fig. 10 Radial distribution functions of (a, b) butyric acids around butyric acids, and (c, d) isovaleric acids around isovaleric acids during methane hydrate dissociation (303 K, 50 bar).


1.2

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) 0.6% butyric acid t = 12 ns

(b) 0.6% butyric acid t = 15 ns

(c) 0.6% isovaleric acid t = 9 ns

(d) 0.6% isovaleric acid t = 12 ns

(e) 1.5% butyric acid t = 6 ns

(f) 1.5% isovaleric acid t = 3 ns

Fig. 11 Snapshots of methane (green) within 0.5 nm from the carbon chains of fatty acids (red) during methane hydrate dissociation (303 K, 50 bar). Hydrate cages (512, 51262, 51263 and 51264) are shown in gray. Na+ and the water not belonging to any of the above cages are not shown for



clarity.

2.0

(a)

0-3 ns 6-9 ns 12-15 ns 18-21 ns

2.5

3-6 ns 9-12 ns 15-18 ns 21-24 ns

1.15

(b)

1.10

2.0

1.05 1.00

g(C-OW)

g(C-OW)

1.5

1.0

0.5

0.0

1.5

0.95

0.5

0.0

0.4

0.8

1.2

1.6

0.0

2.0

2.0

(c)

0-3 ns 6-9 ns 12-15 ns 18-21 ns

2.5

3-6 ns 9-12 ns 15-18 ns 21-24 ns

1.0

0.5

0.0

0-1 ns 3-4 ns 6-9 ns

0.0

0.4

0.8

0.4

0.8

1.2

1-2 ns 4-5 ns 9-12 ns

1.2

2-3 ns 5-6 ns 12-15 ns

1.6

2.0

1.15

(d)

1.10

2.0

1.05 1.00 0.95

1.5

0.65

1.6

2.0

0.70

1.0 0.5

0.0

0.70

r (nm)

g(C-OW)

1.5

0.65

1.0

r (nm)

g(C-OW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

0.0

0-1 ns 3-4 ns 6-9 ns

0.0

0.4

0.8

1-2 ns 4-5 ns 9-12 ns

1.2

2-3 ns 5-6 ns 12-15 ns

1.6

r (nm)

r (nm)

Fig. 12 Radial distribution functions of the oxygen in water (OW) around the carbon chains of (a) 0.6% butyric acid, (b) 1.5% butyric acid, (c) 0.6% isovaleric acid, and (d) 1.5% isovaleric acid at different times during methane hydrate dissociation (303 K, 50 bar).


2.0

Page 33 of 33

TOC Graphic

0.4

293K, 50bar

0.3 0.2

F4ϕ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


methane

0.1

fatty acid hydrate cage

0.0 0

10

20

30

Time (ns)


40

50

Molecular Dynamics Simulation of Methane Hydrate Formation and

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