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CH4 Hydrate Formation between Silica and Graphite Surfaces: Insights from Microsecond Molecular Dynamics Simulations Zhongjin He, Praveen Linga, and Jianwen Jiang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02711 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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CH4 Hydrate Formation between Silica and Graphite Surfaces: Insights from Microsecond Molecular Dynamics Simulations Zhongjin He, Praveen Linga and Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore

ABSTRACT: Microsecond simulations have been performed to investigate CH4 hydrate formation from gas/water two-phase systems between silica and graphite surfaces, respectively. The hydrophilic silica and hydrophobic graphite surfaces exhibit substantially different effects on CH4 hydrate formation. The graphite surface adsorbs CH4 molecules to form a nanobubble with a flat or negative curvature, resulting in a low aqueous CH4 concentration; and hydrate nucleation does not occur during 2.5 µs simulation. Moreover, ordered interfacial water bilayer forms between the nanobubble and graphite surface thus preventing their direct contact. In contrast, the hydroxylated-silica surface prefers to be hydrated by water, with a cylindrical nanobubble formed in the solution, leading to a high aqueous CH4 concentration and hydrate nucleation in the bulk region; during hydrate growth, the nanobubble is gradually covered by hydrate solid, separated from water phase, and hence slowing growth. The silanol groups on the silica surface can form strong hydrogen bonds with water, and hydrate cages need to match the arrangements of silanols to form more hydrogen bonds. At the end of simulation, the hydrate solid is separated from the silica surface by liquid water, with only several cages forming hydrogen bonds with the silica surface, mainly due to the low CH4 aqueous concentrations near the surface. To further explore hydrate formation between graphite surfaces, CH4/water homogenous solution systems are also simulated. CH4 molecules in the solution are adsorbed onto graphite and hydrate nucleation occurs in the bulk region. During hydrate growth, the adsorbed CH4 molecules are gradually converted into hydrate solid. It is found that the hydrate-like ordering of interfacial water induced by graphite promotes the contact between hydrate solid and graphite. We reveal that the ability of silanol groups on silica to form strong hydrogen bonds to stabilize incipient hydrate solid, as well as the ability of graphite to adsorb CH4 molecules and induce hydrate-like ordering of the interfacial water, are the key factors to affect CH4 hydrate formation between silica and graphite surfaces.

INTRODUCTION Gas hydrates are crystalline inclusion compounds in which gas molecules are encapsulated in hydrogen-bonded water polyhedral cages. In recent years, gas hydrates have attracted considerable interest for the scientific importance and potential industrial applications.1,2 Particularly, natural gas (mainly CH4) hydrate exists abundantly in the permafrost areas and marine sediments, and it can serve as a new energy resource.3,4 Naturally, CH4 hydrate forms in complex geological environments such as unconsolidated clays or sedimentary rocks with different hydrophilic and hydrophobic properties. How these solids surfaces affect the formation of CH4 hydrate remains poorly understood. On the other hand, gas hydrates have great potential applications in gas storage and separation.5,6 Nevertheless, the slow formation kinetics of gas hydrates is the major bottleneck in practical applications. Therefore, several approaches have been proposed to accelerate the formation of gas hydrates. For instance, the formation kinetics was reported to increase in silica sand beds or porous silica gel.7-9 Using additives of hydrophobic solid particles, including fumed silica, graphite and carbon nanotubes, is another feasible approach to promote gas hydrate formation.10-13 It was found that the surfaces of solid particles play a critical role in the promotion effect, though the underlying mechanism is elusive.14,15 Consequently, fundamental understanding in the effects of solid surfaces on gas hydrate formation is indispensable to the detection of hydrate reservoirs and natural gas extraction from hydrate-bearing sediments, as well as the development of hydrate-based technologies. 1

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Toward this end, experimental and theoretical studies have been conducted. Compared with bulk phase, the behavior of hydrate formation between solid surfaces or in porous environments is much more complex. Cha et al.16 reported that bentonite surfaces could thermodynamically and kinetically promote CH4 hydrate formation, thus inferred that solid surfaces might serve as nucleation sites. It was observed by Kim et al. that the confinement of porous clay sediments changed the phase equilibria of gas hydrates.17,18 Casco and coworkers demonstrated that CH4 hydrates could form at milder conditions with faster kinetics in the nanospace of carbon materials.19 Several groups performed molecular dynamics (MD) simulations to determine the structures of CH4 hydrate intercalated in Na-montmorillonite and found that the clay surface could stabilize CH4 hydrate by forming clathrate-like water structures near the surface to enclose CH4 molecules.20,21 MD studies of Bagherzadeh et al. showed that the presence of silica surface could affect the dissociation behavior of CH4 hydrate, the shape of gas-water interface and gas distribution.22,23 Experimental studies have revealed different effects of hydrophilic and hydrophobic surfaces on gas hydrate formation. Takeya et al. found that CH4 hydrate confined in the void space of hydrophilic beads was more stable, but destabilized in hydrophobic beads.24 Nguyen et al. compared hydrophilic and hydrophobic surfaces, and identified the latter had a much better promoting effect on the formation of CO2 bubble and hydrate.25 Several MD simulations have investigated gas hydrate formation in the presence of solid surfaces. Bai et al. observed a three-stage process in the nucleation of CO2 hydrate from CO2 solution between rigid silica surfaces.26 Based on the simulation of CH4 hydrate growth on rigid silica surfaces, Liang et al. found that hydrate growth was preferential from bulk solution and the newly-formed hydrate was not in contact with the silica surfaces.27,28 CH4 hydrate formation was also examined by Yan et al. in the nanopores of clay sediments with different pore sizes.29 It is worthwhile to mention that these simulation studies adopted highly supersaturated gas solutions, which might alter the mechanism of hydrate formation from the naturally occurring one. On the other hand, DeFever et al. recently explored the effects of –CH3 and –OH terminated self-assembled monolayers on the hydrate formation of a water-soluble guest using coarse-grained MD simulations.30 Currently, there are few simulation studies on gas hydration formation from gas/water two-phase systems in the presence of solid surfaces. Previous studies revealed that gas nanobubbles can easily form when a small gas phase is surrounded by water, and Young-Laplace pressure arising from water/nanobubble interfacial curvature also contributes to gas dissolution in water, apart from external gaseous pressure.31,32 It is unclear how the hydrophilic and hydrophobic properties of surfaces affect the shapes of gas nanobubbles and gas concentration in water, whether a solid surface triggers hydrate nucleation and alters nucleation mechanism, and what role the interfacial water plays in mediating the interactions between hydrate and surface. To elucidate these important and intriguing issues, herein, we perform microsecond MD simulations to investigate the nucleation and growth of CH4 hydrate from gas/water two-phase systems between hydrophilic silica and hydrophobic graphite surfaces. This study provides molecular insights into the effects of solid surfaces on CH4 hydrate formation, and facilitates in-depth understanding of the underlying mechanism for promoting effect.

MODELS AND METHODS Two types of initial configurations for CH4/water two-phase systems were constructed with 640 CH4 and 3680 water molecules between the nanopores of slit-like silica and graphite surfaces. The silica surfaces were hydroxylated by grafting silanol groups (-OH), with the [100] crystallographic plane facing the CH4 solution. The dimensions of the silica and graphite surfaces in the x and y directions were 7.22×4.60 nm2 and 7.26×4.91 nm2, respectively, and the initial distances between the surfaces were about 4.5 nm in the these systems. The first type of initial configurations were built up manually: a slab of CH4 phase was placed in the systems and the remaining region was filled with water molecules. In these initial configurations, the CH4 phase was in contact with two 2

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surfaces (Graphite_Contact in Figure 1A_I and Silica_Contact in Figure 3A), one surface (Graphite_Contact1 in Figure 2A_I and Silica_Contact1 in Figure S2A_I), or without any surface (Graphite_Contact0 in Figure 2B_I and Silica_Contact0 in Figure S2B_I). The second type of initial configurations (Graphite_Bubble in Figure 1B_I and Silica_Bubble in Figure 4A) were obtained by simulating a CH4/water homogenous mixture at 300 K and 10 MPa for 20 ns to spontaneously form CH4/water two phases in the nanopores. In addition, another two initial configurations (Silica_Homo and Graphite_Homo) were also considered, which contained a homogeneous mixture of 512 CH4 and 2944 water molecules between smaller graphite and silica surfaces (5.11×5.28 nm2 and 5.08×5.08 nm2, respectively). The compositions of CH4 and water in all the systems were corresponding to the stoichiometric composition of sI hydrates. All these initial configurations were used to initiate the simulations at 250 K and 50 MPa to examine hydrate formation. CH4 and water were represented by the OPLS-UA33 and TIP4P-Ice model,34 respectively. The SETTLE algorithm was used to constrain the rigid geometry of water molecules. The carbon atoms of graphite surface were modeled as sp2-like aromatic carbons using the CHARMM27 force filed,35 and the hydroxylated silica surface was described by the force filed developed by Lopes et al.36 The Lorentz-Berthelot combining rules were used to calculate the Lennard-Jones parameters of unlike atoms. The periodic boundary conditions were imposed in three directions. The electrostatic interactions were calculated with the particle mesh Ewald method.37 A cutoff of 1.0 nm was used to estimate the short-range nonbonded interactions with the long-range corrections applied to energy and pressure. All the MD simulations were performed under isothermal-isobaric (NPT) ensemble at 250 K and 50 MPa. Pressure coupling was applied semi-isotropically, thus the z-dimension (surface normal direction) and xy dimensions were allowed to fluctuate independently. Different from previous studies,22,23,26-29 where the surfaces were fixed or treated to be rigid during simulations, here the graphite and silica surfaces were mobile and flexible with the bonds, angles and dihedrals estimated by the adopted force fields. Such setup was based on two considerations: implementing the pressure coupling needs to scale the coordinates of surface atoms, and the surface flexibility is essential for the surfaces to freely interact with hydrate, especially for the silanol groups of silica surface to form hydrogen bonds with water molecules of hydrate. Repeated independent simulation runs were conducted using GROMACS v.5.0.6.38 Trajectories were integrated using the leapfrog scheme with a time step of 2 fs and the coordinates were stored every 10 ps. Similar results were obtained in these repeated runs. Thus, the representative results in 2.5-µs Run1 are presented, while those in additional runs (Run2 to Run4) with shorter durations (0.5-1.62 µs) are provided in the Supporting Information.

RESULTS AND DISCUSSION Effects of Silica and Graphite Surfaces on Nanobubble Curvature and CH4 Aqueous Concentration It is well recognized that water droplets possess different curvatures (contact angles) on hydrophobic and hydrophilic surfaces due to different wettability.39 Similar behavior also occurs for gas bubbles in solution adsorbed to surfaces by varying hydrophobicity.40 As shown in Figures 1 to 4, the hydrophobic graphite and hydrophilic hydroxylated silica differ significantly in their effects on the shape and curvature of adsorbed CH4 nanobubbles. In the Graphite_Contact system (Figure 1 A_I to A_IV), the CH4 phase quickly evolves into a concave meniscus and retains this shape, and always spans across the simulation box to keep in contact with the graphite surface. In the Silica_Contact system, however, the CH4 phase quickly develops into a convex shape (Figure 3B), opposite to the Graphite_Contact system (Figure 1 A_II), and soon separates from one silica surface and forms a partially cylindrical nanobubble on the other silica surface (Figure 3C and 3D). In the Silica_Contact1 system, the CH4 phase also forms a partially cylindrical nanobubble on the surface (Figures S2 A_II). However, in Graphite_Contact1 system, the CH4 phase quickly spreads on one graphite surface, covers the whole surface, and 3

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generates a planar water/gas interface (Figure 2 A_I to A_IV). In Graphite_Contact0 system, without contacting the surface, the CH4 phase quickly evolves into a bulk nanobubble (Figure 2 B_II); surprisingly, the nanobubble cannot adsorb onto the graphite surface during the simulation at 250 K and 50 MPa, due to the formation of very ordered and stable interfacial water bilayer between the nanobubble and graphite surface (Figure 2 B_III and 2 B_IV). More details about the structure of the interfacial water bilayer are discussed below. Nevertheless, as shown in Figure S1, at a higher temperature (300 K and 10 MPa), the nanobubble quickly adsorbs onto the graphite surface and forms a planar water/gas interface, due to the absence of the ordered interfacial water bilayer. In the Graphite_Bubble system, CH4 molecules are adsorbed onto the graphite surface and form two small nanobubbles (Figure 1 B_I). By contrast, in the Silica_Bubble system, a large cylindrical nanobubble forms in bulk solution without contacting with the silica surface (Figure 4A). In the Silica_Contact0 system, the CH4 phase also evolves into a bulk cylindrical nanobubble. The CH4 nanobubbles on the graphite surface (Figure 1 B_I to B_IV) show convex shape and are much flatter than those on the silica surface (Figure 3C). These phenomena are also observed in the repeated simulation runs (Figures S3 and S4). The above results indicate that the adsorption and curvature of CH4 nanobubble on graphite surface are affected by the presence of ordered interfacial water (formed under hydrate formation condition), the size of adsorbed nanobubble, and the number of surfaces initially contacted with CH4 phase; while for the silica system, they are only affected by contact with the silica surface. However, the determinant factors for the adsorption and curvature of CH4 nanobubble are the hydrophobicity/hydrophilicity of the surface and the matching between the surface and guest gas. Particularly, the graphite surface is hydrophobic acting as excellent sorbent for CH4, while the hydroxylated silica surface is hydrophilic and prefers to be hydrated by water. The preferential formation of gas bubbles on hydrophobic surfaces was previously found in experiments.25

Figure 1. Evolution of the shape and curvature of CH4 nanobubble in the Graphite_Contact (Upper, A) and Graphite_Bubble (Lower, B) systems. CH4, graphite and water are shown as green balls, cyan balls and light blue lines, respectively.

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Figure 2. Evolution of the shape and curvature of CH4 nanobubble in the Graphite_Contact1 (Upper, A) and Graphite_Contact0 (Lower, B) systems. CH4, graphite and water are shown as green balls, cyan balls and light blue lines, respectively. In (B_III), the interfacial water bilayer between CH4 nanobubble and graphite surface is highlighted as sticks and the side, top and rotated views of their hydrogen-bond networks are shown in (B_IV).

Figure 3. Evolution of the shape and curvature of CH4 nanobubble and CH4 hydrate formation process in the Silica_Contact system. Hydrate cages are shown as sticks with different colors (green for 512, blue for 51262, red for 51263, orange for 51264, cyan for 4151062, purple for 4151063 and pink for 4151064).

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Figure 4. Evolution of the shape and curvature of CH4 nanobubble and CH4 hydrate formation process in the Silica_Bubble system. Hydrate cages are shown as sticks with different colors (green for 512, blue for 51262, red for 51263, orange for 51264, cyan for 4151062, purple for 4151063 and pink for 4151064). Gas concentration in aqueous solution has been demonstrated to be a critical factor in governing hydrate nucleation, and the nucleation is faster at a higher gas concentration.32,41-43 Previous studies found that gas concentration is profoundly influenced by gas/water interfacial curvature due to the Young-Laplace pressure (Pinterface) at the interface, i.e., a larger curvature results in a higher gas concentration.31,32 Thus, the graphite and silica surfaces can affect the curvatures of CH4 nanobubbles, as discussed above, and then the pre-nucleation mole fraction of CH4 in water (xCH4), finally the nucleation of CH4 hydrate. As illustrated in Figure 5, the typical shapes of CH4 nanobubbles in the systems are described by the pre-nucleation 2D number density of CH4, and the corresponding number density of water is shown in Figure S5. Hydrate nucleation is tracked by the F4 order parameter, which describes the torsion angle between the oxygen atoms of two water molecules within 0.35 nm and the outermost hydrogen atoms. The average values of F4 for liquid water, ice and hydrate are −0.04, −0.4 and 0.7, respectively.44 CH4 nanobubble in the Graphite_Contact system shows a negative curvature (concave shape), while in the other systems it exhibits a positive curvature (convex shape) or a planar curvature. The direction of Pinterface for a negative curvature is toward the nanobubble, and toward water phase for a positive curvature, as shown in Figure 5A and 5B. Thus, xCH4 in the Graphite_Contact system is lower, as it is more difficult for CH4 to diffuse into water in the presence of a negative curvature. On the graphite surface, water contact angle θH2O is > 90o but < 90o on the silica surface. The curvature of the CH4 nanobubble on the silica surface is larger than on the graphite surface (Figure 5B and 5C); thus, xCH4 in the Silica_Contact system is higher than in the Graphite_Bubble system (Figure 5E). On the other hand, xCH4 in the Silica_Bubble system is similar to that in the systems without surface (about 0.03),32,42,45 as CH4 nanobubble is not in contact with surface. As expected, xCH4 in the Graphite_Contact1 system with a planar nanobubble curvature is lower than in the Graphite_Bubble system and higher than in the Graphite_Contact system. As shown in Figure 5E, xCH4 in the Graphite_Contact0 system is slightly lower than in the Graphite_Bubble system. Comparing xCH4 in the repeated runs of Silica_Contact and Silica_Bubble systems (Figure S4A and S4B), we find that the adsorption of CH4 nanobubble onto the silica surface does not ensure a higher xCH4. Nevertheless, a higher xCH4 observed in two runs of the Silica_Contact system is probably because the manual set-up of CH4/water two-phase in the Silica_Contact system is more prone to developing highly metastable state than the spontaneously formed CH4/water two-phase in the Silica_Bubble 6

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system.

Figure 5. (A-D) 2D number densities of CH4 on the xz plane characterizing the curvatures of CH4 nanobubbles in the graphite and silica systems; the white arrows show the directions of Young-Laplace pressure Pinterface at water/CH4 interface, and θH2O and θCH4 are the contact angles of water and CH4 nanobubbles, respectively. The graphite and silica surfaces are indicated by the white dash lines. The negative curvature (concave shape) and positive curvature (convex shape) of CH4 nanobubbles are observed in (A and B), respectively. Evolution of (E) CH4 mole fraction in water xCH4 and (F) F4 order parameter in the graphite and silica systems. The black and red dash lines in (E) and (F) indicate the staring of nucleation in the Silica_Contact and Silica_Bubble, repectively. Therefore, we infer that the graphite and hydroxylated silica surfaces can affect xCH4 by changing the curvatures of CH4 nanobubbles. Between the graphite surfaces, xCH4 is lowered and CH4 hydrate nucleation does not occur within 2.5 µs simulation, while a bulk-like (or even higher) xCH4 is present between silica surfaces and nucleation occurs, as shown in Figure 5F. These surface effects on xCH4 and CH4 hydrate nucleation are also supported by the repeated runs (Figure S4). Formation Mechanism of CH4 Hydrate between Silica and Graphite Surfaces The formation processes of CH4 hydrate between silica and graphite surfaces are analyzed by tracking various hydrate cages as proposed by Jacobson et al.46 Figures S6 and S7 plot the evolution of seven types of cages during simulation. In the Silica_Contact and Silica_Bubble systems, as shown in Figure 6 for Run1 and Figure S3 for the repeated runs, CH4 hydrate nucleation is always initiated from the bulk liquid phase, specifically, the local region with a high CH4 density. The nucleation processes are visualized in Videos S1 and S2. The small 512 cages are the most popular during nucleation and stabilized by the adsorption a shell of CH4 molecules to cage faces. These observations are similar to those reported for CH4 hydrate nucleation without surface,42,45,47-49 and the presence of silica surface seems not to alter the nucleation mechanism of CH4 hydrate. As displayed in Figures 3D-3H and Figure 4B-4F, after nucleation in the bulk region, CH4 hydrate rapidly grows toward CH4 nanobubble and then the nanobubble gradually shrinks and is covered by hydrate solids. Thus, mass-transfer barrier increases for CH4 to diffuse into water and significantly slows down the CH4 hydrate growth. During the growth, several cages (no more than 8, see Figures 7A and 7B) are in contact with the silica surface by forming hydrogen bonds with the silanol groups; at the end of the simulations, the hydrate solids are obviously separated from the silica surface by 7

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liquid water, only with some cages contacting with the surface (Figures 3H and 4F). The entire formation processes of CH4 hydrate in the Silica_Contact and Silica_Bubble systems are visualized in Videos S3 and S4.

Figure 6. (A and C) 2D number densities of CH4 on the xz plane and (B and D) snapshots showing the locations of hydrate cages during nucleation in the Silica_Contact and Silica_Bubble systems. The silica surfaces in (A and C) are indicated by the white dash lines.

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Figure 7. Numbers of cages contacted with the surface (i.e. forming hydrogen bonds with the silanol groups of silica surface or within 0.5 nm of graphite surface) in (A) Silica_Contact, (B) Silica_Bubble, (C) Silica_Homo and (D) Graphite_Homo systems. Due to the presence of low xCH4, CH4 hydrate nucleation is not observed for the CH4/water two phases in the graphite systems. Thus, homogeneous CH4 solutions between the graphite and silica surfaces are further considered to explore the effects of surfaces on CH4 hydrate formation. We notice that although CH4 hydrate form in very short time in these systems due to a very high xCH4, simulations on these systems have successfully captured the effects of surface on CH4 hydrate formation. Apparently, the two surfaces have different effects on CH4 hydrate formation. As shown in Figures 5E and S8, CH4 solution between the silica surfaces remains homogeneous and shows a very high xCH4 during simulation, and the cages are uniformly formed (see Figure 8 A_I to A_IV and Video 5). By contrast, some CH4 molecules are adsorbed onto the graphite surface and form surface bubbles, thus leading to a decrease in xCH4 at the early stage of the simulation, and then hydrate nucleation occurs in the bulk region (see Figures 8 B_I to B_IV, Figure S8 and Video 6); along with hydrate growth, CH4 molecules adsorbed on the graphite surface gradually convert into hydrate solids (Figure S9) and the number of cages interacting with the graphite surface increases. Much more cages interact with the graphite surface than with the silica surfaces at the end of the simulation (Figure 7C and 7D). These different surface effects on the formation 8

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process of CH4 hydrate are also observed in the repeated runs (Figures S10 and S11).

Figure 8. CH4 hydrate formation process in the Silica_Homo (upper) and Graphite_Homo (lower) systems. Hydrate cages are colored as in Figure 2. CH4 hydrate solids formed between the graphite and silica surfaces share certain similarities (e.g. cage number and hydrate structure) with those formed without surfaces reported in previous MD simulations.42,45,47-49 As shown in Figures S6 and S7, in the Silica_Contact and Silica_Bubble systems, the 512 small cages are the most abundant and followed by the 51262 large cages, and the metastable 4151062 cages are the third, which is consistent with previous MD studies on the formation of CH4 hydrate from gas/water two-phase systems;42,49,50 however, in the hydrate solids formed from the homogeneous CH4 solutions in the Silica_Homo and Graphite_Homo systems, the 4151062 cages are next in abundance and 51262 cages are the third; this is reasonable as the size (22 waters) of 4151062 cage is closer to the size (20 waters) of the hydration shell of CH4 (which is abundantly dispersed in the homogeneous CH4 solution) than that of 51262 cage (24 waters).51 Moreover, in the Silica_Homo system with a very high xCH4, as shown in Figures S6F and S12, much fewer water molecules are converted into the seven dominant cage types (512, 51262, 51263, 51264, 4151062, 4151063 and 4151064) observed in the formation of CH4 hydrate from gas/water two-phase;49,52 the formation of 512 cage is also suppressed when compared with the Graphite_Homo system with a lower xCH4, instead, distorted meta-stable cages are formed (Figure S12D). These observations hints that an extremely high CH4 concentration in solutions may affect the kinetic pathway of CH4 hydrate formation. Note that in order to observe nucleation within short simulation duration, high CH4 concentrations are often used in previous MD studies.26,28,29,53,54 In addition, CH4 hydrate solids formed between the silica and graphite surfaces are amorphous with a short-range order. Nevertheless, certain sI and sII motifs are observed in these amorphous structures (Figure S13). Interfacial Water-Mediated Interactions between Hydrate Solids and Surfaces Interfacial water near a surface has been reported to play a critical role in the surface effect.55 To understand how CH4 hydrate solids are in contact with the silica and graphite surfaces, water structuring, hydrogen bonds (HBs) and F4 order parameter are examined for the interfacial water. It is found that water molecules form a high density peak at about 0.3 nm to the hydroxylated-silica surface (Figure 9A) due to the formation of strong HBs between the silanol groups and water molecules (Figure 9B). Experimental and theoretical studies22,23,56 have demonstrated 9

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that such strong HBs cause the interfacial water near silica surface more structured and less mobile than bulk water. Silanol surface density, protonation state of silanols, surface flexibility and crystallinity have been reported as the important factors affecting the properties of interfacial water near silica surface.57-60 During the simulations of the Slica_Contact and Silica_Bubble systems, the interfacial water near the silica surfac initially maintains a liquid state with F4 = -0.04, which then increases slightly with hydrate growth (Figure 9D). The silica surface (including the silanol groups) in this study are flexible and mobile, it is observed that HBs can form among silanols (Figure 9B). In the Silica_Homo system, the F4 of the interfacial water increases at the initial stage of the simulation and then reaches a plateau (Figure 9D), as the cages are uniformly formed in the system including near the silica surface. Additionally, the F3 order parameter, which probes the deviation from the tetrahedral angle in hydrogen bonding network, was analyzed to further investigate the structure of interfacial water near the silica surface.44 The average values of F3 for liquid water, ice and hydrate are 0.1, 0.01 and 0.01, respectively. As shown in Figure S14, during the simulations of silica systems, the change of F3 supports the observation on the structural change of the interfacial water as characterized by F4.

Figure 9. Water structuring at the surfaces. (A) Density profiles of water versus the distance to the silica and graphite surfaces. The inset shows the interfacial water near the surfaces. Hydrogen-bond networks near (B) silica and (C) graphite surfaces; note that hydrogen bonds can form among the silanols of silica surface; silanols and hydrogen bonds are in blue and red, respectively; CH4 and water are shown as green balls and red-white rods, respectively. (D-G) Evolution of F4 order parameter of the interfacial water near the silica and graphite surfaces. There are two peaks of water density at about 0.35 and 0.6 nm to the graphite surface (Figure 9A); water layering near the surface can be also clearly observed in the inset of Figure 9A and in the 2D water density map in Figure S5. Different from the silica surface, the formation of the first water layer near the graphite surface is not due to the formation of HBs with surface, but to facilitate HB formation in the interfacial water. It is found that the average number of HBs per water in the first water layer (3.64) is bulk-like,61 and the contributions of HBs formed in the first water layer and those between the first and second layer water are 2.72 and 0.92, respectively. More interestingly, the ordered water structures with square, pentagon and hexagon rings are observed in the first water layer near the graphite surface (Figure 9C) to maximize HB formation. The ability of hydrophobic surfaces to induce interfacial water ordering has also been demonstrated by Raman spectroscopic studies.62 To analyze the 10

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phase state of the first water layer near the graphite surface, the bond orientational order parameter q3 was calculated, which can effectively distinguish liquid water, ice and hydrate.63,64 As shown in Figure 10, the distribution of q3 for the interfacial water near the graphite surface shifts from liquid water to sI hydrate; moreover, the F4 values for water in the first and second layers (0.06 and 0.00, respectively, see Figures 8E and 8F) change from that of liquid water (-0.04) toward hydrate (0.7). These analyses indicate that the special arrangements of interfacial water near the graphite surface tend to resemble that of hydrate. In the Graphite_Homo system, the F4 values for water near the graphite surface increase along with hydrate growth toward the surface (Figure 9G). 0.6 hexagonal ice sI hydrate liquid water interfacial water bilayer water hydrate_Silica hydrate_Graphite

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q3 Figure 10. Comparison on the distributions of the bond orientational order parameter q3 for the interfacial water near the graphite surface, the water bilayer between CH4 nanobubble and the graphite surface, the hydrates formed in the silica and graphite systems with those of hexagonal ice, sI hydrate and liquid water. The F3, F4 and q3 parameters were calculated for the ordered interfacial water bilayer formed between CH4 nanobubble and the graphite surface at hydrate forming condition (Figure 2 B_III). Shown in Figures 9G, S14 and 10, these parameters (F3 = 0.055, F4 = 0.42 and q3 close to that of sI hydrate) suggest that the water bilayer resembles hydrate structure. The water bilayer comprises two superimposed water monolayers, each of which contains pentagon, hexagon and octagon rings (Figure 2 B_IV). Several adsorbed CH4 molecules on the surface are entrapped in the bilayer octagon rings. These hydrogen-bond network of the interfacial water bilayer is more like those of bilayer ice and bilayer clathrate hydrates formed in narrow hydrophobic slit pores under high lateral pressures reported in previous MD studies.65,66 Recent experiment study indicates that few-layer ice may exist in graphene nanocapillaries.67 More interestingly, the formation of the interfacial water bilayer prevents the direct adsorption of the CH4 nanobubble onto the graphite surface, highlighting the importance of interfacial water in the effects of graphite surface on CH4 hydrate formation. Figure 11 shows the interactions of CH4 hydrate solids with silica and graphite surfaces in the Silica_Homo and Graphite_Homo systems, where more cages interact with surfaces than in other nucleated systems (Figure 7). It is found that cages contact with the silica surface by forming HBs with silanol groups, and only a few cages are observed to directly contact with the surface (Figure 11A and 11B). Figure 11C illustrates different contact modes: (I) A silanol group constitutes a cage; the number of such cage is a few and its shape is distorted. (II) One face of a cage directly forms HBs with silanol groups. (III) Water bridges form between a cage and silanol groups. (IV) Interfacial water form HBs with silanols to form a semi-cage on the surface; several isolated CH4 molecules are entrapped in the semi-cage and show as high density loci near the silica surface (see Figures 6A and 6C). Mode (II) 11

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is considered as the strongest contact with the silica surface as it forms the largest number of HBs with the surface. To form strong contacts with the silica surface, the cages need to match the arrangements of silanols to form more HBs. Moreover, previous studies have demonstrated that structural match with a surface is essential to ice nucleation at surfaces.68-70 Thus, a comprehensive investigation on the interactions between different facets of hydrate solid and silica surface warrants further simulations. The ordered interfacial water near the graphite surface is found to play a crucial role in mediating the contact of hydrate cages with the surface. As shown in Figures 11D, 11E and11F, some interfacial water molecules near the graphite surface are converted into cage faces at the surface; mor interfacial water molecules are converted into interfacial cage faces with the growth of CH4 hydrate toward the surface (Figure S15). Interestingly, such ordered interfacial water was found to promote the heterogeneous nucleation of ice near the graphite surface.71

Figure 11. Hydrate cages and hydrogen-bond networks at the (A,B) silica and (D,E) graphite surfaces. Contact modes of cages with the (C) silica and (F) graphite surfaces. Silanols and hydrogen bonds are in blue and red, respectively. The interfacial water molecules forming cage faces, which contact directly with surfaces, are highlighted as thick bonds in (A), (B), (D) and (E). Implications for Silica and Graphite Surfaces to Promote Gas Hydrate Formation Previous experiments have demonstrated that the formation kinetics of gas hydrate can be effectively promoted in the presence of silica sands or hydrophobic particles. In this context, the molecular insights revealed by our simulation study can provide useful implications for the promoting effects of solid surfaces on CH4 hydrate formation. Gas hydrate is usually formed in gas/water two-phase systems and aqueous gas concentration is extremely low due to small solubility. It is found that gas hydrate only nucleates at gas/water interface because of a high local gas concentration. However, a thin layer of hydrate solid at the interface causes an enduring issue on mass transfer, as it isolates a water phase from a gas phase. This simulation study shows that the hydrophobic graphite surface can adsorb CH4 molecules to form a gas phase. Thus, the addition of hydrophobic particles into solution can promote gas molecules into a water phase, and increase the gas/water contact area (otherwise only the gas/water interface). This promoting effect is evidenced by the enhanced CH4 dissolution rate in water upon the addition of carbon nanotubes.72 Furthermore, experiments have indicated that a high degree of water ordering can decrease the induction time for hydrate formation;14,62 thus, the ice-like water ordering near the graphite surface is able to promote gas hydrate formation.

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In contrast, the hydroxylated-silica surface does not show affinity for CH4 molecules, instead it prefers to be hydrated by water due to its hydrophilicity. As shown in Figure S16, xCH4 near the silica surface is slightly lower than average xCH4 in the system prior to nucleation and obviously lower than average xCH4 during hydrate growth; at the end of the simulation, CH4 density is almost zero in some regions near the surface. Thus, high xCH4 is not present near the silica surface and hydrate nucleation is not initiated from the silica surface (Note that cages can form at the silica surface in the Silica_Homo system due to the artificially-constructed high xCH4). Therefore, the silica surface may not promote CH4 molecules into water. However, the silanol groups of silica surface can form strong HBs with water, which may stabilize the incipient hydrate solids and promote CH4 hydrate formation. Thus, the presence of silica in the system may have an effect on hydrate nucleation pathway, though it seems not to directly affect the formation of very incipient cages (Figure 6).

CONCLUSIONS We report a microsecond MD simulation study to investigate the nucleation and growth of CH4 hydrate from the gas/water two-phase systems between silica and graphite surfaces. Different from previous studies, the surfaces here are treated as flexible and mobile. The hydrophilic silica and hydrophobic graphite surfaces are revealed to exert substantially different effects on CH4 hydrate formation. As a favorable sorbent for CH4, graphite can adsorb CH4 molecules to form a surface nanobubble, which shows a very flat or even negative curvature, causing a low aqueous CH4 concentration due to the small Young-Laplace pressure at the gas/water interface. As a consequence, no hydrate nucleation is observed in the graphite system during 2.5 µs simulation. Nevertheless, under hydrate formation conditions, direct adsorption of the nanobubble onto the graphite surface is prevented by the ordered interfacial water bilayer. In contrast, the hydrophilic hydroxylated-silica surface prefers to be hydrated by water, while a bulk cylindrical nanobubble with a large gas/water interfacial curvature is formed in solution (once located on the surface, the nanobubble is partially cylindrical), resulting in a high aqueous CH4 concentration and hydrate formation is observed. In the silica system, hydrate nucleation occurs in the bulk region followed by hydrate growth toward CH4 nanobubble, which is gradually covered by hydrate solid and separated from water phase, leading to significant slowing down of hydrate growth. At the end of 2.5 µs simulation, the hydrate solid is clearly separated from the silica surface by liquid water, only with several cages forming hydrogen bonds with the silanol groups on the surface. It is found that the silanol groups of the silica surface can form strong hydrogen bonds with water. The initial liquid-like interfacial water near the silica surface tends to resemble hydrates when hydrate cages interact with the surface. Four contact modes for the cages with the silica surface are identified, and it shows that cages need to match the arrangements of silanols to form strong interactions with the silica surface. In CH4/water homogenous solution systems, CH4 molecules can be adsorbed onto the graphite surface and hydrate nucleation occurs in the bulk region; during hydrate growth, the CH4 molecules adsorbed are gradually converted into hydrate solid. It is found that hydrate-like water ordering near graphite promotes the contact of hydrate solid with graphite. However, the silica surface can maintain homogenous CH4 solution and hydrate cages are uniformly formed in the system (including near the silica surface). This simulation study provides useful implications for graphite and silica to promote gas hydrate formation, as experimentally demonstrated. The hydrophobic graphite has strong ability to adsorb CH4 molecules and facilitate to introduce CH4 into water. Furthermore, graphite can induce hydrate-like water ordering near the surface. These two factors may promote CH4 hydrate formation. On the other hand, the silanol groups on hydroxylated-silica surface can form strong hydrogen bonds with water, which may stabilize the incipient hydrate solid and promote 13

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CH4 hydrate formation. The molecular insights revealed here into the complex effects of solid surfaces on CH4 hydrate formation would help us to understand the formation of natural gas hydrates in sediments and the underlying mechanism of promoting effect.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Evolution of the shape of CH4 nanobubble in the Graphite_Contact0, Silica_Contact0 and Silica_Contact1systems; cage locations upon hydrate nucleation; xCH4 and F4 order parameter; 2D number densities of water; cage numbers; number of CH4 molecules adsorbed on graphite surface; system configurations for CH4 solution systems; meta-stable cages in the Silica_Homo system; sI and sII motifs; F3 order parameter; percentage of interfacial water converted into interfacial cage faces; xCH4 near silica surface and CH4 distribution in silica systems; videos.

AUTHOR INFOMRAITON Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National University of Singapore for the Natural Gas Center (R261-508-001-646/733) and the National Natural Science Foundation of China (No. 21506178).

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