International Conference on Gas Hydrates (ICGH) (6th : 2008)


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  A MICROSCOPIC VIEW OF THE CRYSTAL GROWTH OF GAS HYDRATES   Peter G. Kusalik! Department of Chemistry University of Calgary 2500 University Dr. NW Calgary, Alberta  T2N 1N4 CANADA  Jenel Vatamanu Department of Chemistry University of Calgary 2500 University Dr. NW Calgary, Alberta  T2N 1N4 CANADA   ABSTRACT In this paper we will discuss the first successful molecular simulation studies exploring the state- steady crystal growth of sI and sII methane hydrates. Since the molecular modeling of the crystal growth of gas hydrates has proven in the past to be very challenging, we will provide a brief overview of the simulation framework we have utilized to achieve heterogeneous growth within timescales accessible to simulation. We will probe key issues concerning the nature of the solid/liquid interface for a variety of methane hydrate systems and will make important comparisons between various properties. For example, the interface demonstrates a strong affinity for methane molecules and we find a strong tendency for water molecules to organize into cages around methane at the growing interface. The dynamical nature of the interface and its microfaceted features will be shown to be crucial in the characterization of the interface. In addition to the small and large cages characteristic of sI and sII hydrates, water cages with a 51263 arrangement were identified during the heterogeneous growth of both sI and sII methane hydrate and their potential role in cross-nucleation of methane hydrate structures will be discussed. We will describe a previously unidentified structure of methane hydrates, designate structure sK, consisting of only 51263 and 512 cages, and will also show that a polycrystalline hydrate structure consisting of sequences of sI, sII and sK elements can be obtained. In this paper we will also detail a variety of host defects observed within the grown crystals. These defects include vacant cages, multiple methane molecules trapped in large cages, as well as one or more water molecules trapped in small and large cages. Finally, preliminary results obtains for THF and CO2 hydrates will be presented and their behaviour contrasted to that of methane hydrate.  Keywords: gas hydrates, crystal growth, molecular simulation   ! Corresponding author: Phone: +1 403 220 6244 Fax +1 403 289 6003 E-mail: NOMENCLATURE  MH – methane hydrate z – direction of heterogeneity in systems Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. INTRODUCTION Methane hydrates (MH) are crystalline inclusion compounds of methane and water, in which water molecules form cages around methane [1]. Great interest in methane hydrates is being stimulated by several important considerations, including the fact that they contain the most abundant source of carbon-based energy on Earth [2,3]. Additionally, these compounds are of relevance in global warming [4], geology [5], astronomy [6] and marine systems [7].      The most common gas hydrate structures are the so called sI, formed by dodecahedra (512) and tetradecahedra (51262) cages, and sII, containing 512 and larger hexadecahedra (51264) water cages [1]. Methane hydrates are stable at relatively high pressure [8]. The most stable form of methane hydrate at low to moderate pressures is sI, where the relatively small sized methane molecules stabilize the 512 and 51262 water cages. The larger 51264 cages of sII gas hydrates are normally stabilized by larger inclusion molecules (e.g. tetrahydrofurane) [9]. However, it has been experimentally shown that at relatively high pressure structure sII can form for smaller guest molecules like methane [10]. Also, in situ transformations between sI and sII have been reported for methane hydrates [11]. In addition, one of our recent studies [12] presented findings of an interconversion from sI to sII during the heterogeneous crystal growth of the [001] face of MH-sI.      Previous simulation studies exploring molecular level properties of methane hydrates have addressed issues such as the interfacial properties or interfacial relaxation of methane hydrate crystals [13], melting [14], properties of the crystalline hydrate phase [15], hydrate stability and their unusual self-preservation [16], the influence of inhibitors on the methane hydrates [17], and the nucleation of methane hydrate cages [18]. As well, larger scale and more phenomenological models based on phase field theory [19] have been used to predict the (macroscopic) kinetics of CO2 and CH4 gas- hydrate crystal growth [20], interconversion of CH4 to CO2 hydrate  [21] and the morphological structure of CO2 hydrate  [22].  METHODOLOGY The molecular simulations performed in this work utilize two methodologies recently developed [23,24] and successfully employed in several other studies [12,25,26,27] to investigate steady-state heterogeneous crystal growth. The main difference between the two methods can be viewed as that one approach uses Newtonian dynamics to evolve the growing interface, while in the second method a canonical sampling of the fluctuations is achieved. The observed kinetics of growth and interfacial properties are essentially identical from the two methodologies confirming that heterogeneous crystal growth is a stochastic process that is independent of the details of the underlining dynamics. For further details of the methodologies, potential models and system setup employed see Refs. 12, 23, 24, and 26.  RESULTS AND DISCUSSION Initial sI crystal growth studies In our initial MD simulation work explored the heterogeneous crystal growth of type sI methane hydrate crystal along the [001] face. The difficulties inherent to the simulation of such systems, specifically the low solubility of methane and its low diffusion coefficient in aqueous solutions, proved particularly challenging [12].   Figure 1 Cropped images of averaged configurations from the MD trajectory of a sI system growth at 4 Å/ns.       We have observed [12] methane hydrate crystal growth at rates as high as 4 Å/ns (see Figure 1), significantly faster than that obtained for ice Ih. This is a somewhat surprising result since in the case of the crystallization of a mixture one might expect smaller growth rates as the system must sample a more complex configurational space. This result underlines the importance of the behavior of methane at the interface and the strong tendency of water to order around methane as cages. Indeed, we have observed that once a methane molecule gets trapped in a partially formed cage at the solid/liquid interface it will usually stay in this cage, suggesting a strong affinity of partially formed cages for methane.   Figure 2 Two observed guest defects during growth of sI methane hydrate.      We found that, at the growth rates employed, the methane occupancy in the crystal grown was about 90%, this is roughly 10% of the water cages were not filled with methane. We have also found an interesting defect consisting of two water molecules trapped within one cage (see Figure 2). In this set of runs, this appeared as a unique event, although it was not clear whether it represents a possible defect to be found in real crystals.      Within the present approach it was possible to measure various physical properties of the interface within the moving frame of the steady state crystal growth of the system. The profile functions thus obtained provide a clear picture of the interface. The position and width of the interface can be determined and were found to depend on the choice of measured property, consistent with previous simulation and experimental data. From an analysis of the fluctuations within the interface and the timescales over which they occur, we conclude that multi- nanosecond simulations are required to provide meaningful results regarding the kinetics of hydrate crystal growth.  Polycrystalline structures and sK [25] One of the most interesting observations in our initial crystal growth studies of sI methane hydrate was the formation of a [001] MH-sII structure on the template provided by [001] face of MH-sI (see Figure 3). However, the rather striking question of how a [001] MH-sI face could actually accommodate the [001] face of MH-sII (since these two faces are sterically incompatible) was raised by this result. A subsequent careful inspection of the layer connecting the MH-sI and MH-sII structures revealed an interesting fact: a new kind of cage made from 3 hexagonal faces and 12 pentagonal faces provided the required steric template to connect the two structures. Although a 51263 cage has been reported in several tertiary-amine inclusion compounds [27], to our knowledge it has not been considered as possible structural component of methane hydrates (or gas hydrates in general). In MH-sI, the larger 51262 cages have a cubic arrangement in space and are connected via their hexagonal faces. In MH-sII, its 51264 cages are connected into a tetrahedral arrangement via their hexagonal faces. These two different kinds of crystal can be linked structurally through an intermediate layer of 51263 cages, where one hexagonal face is shared with a hexagon of a 51262 cage of MH-sI and the two other hexagonal faces are shared with two hexagonal faces of the larger cages of MH-sII.   Figure 3 Formation of sII hydrate on the grow face of sI methane hydrate   Figure 4 The growing [001] surface of the MH-sII showing formation of both intermediate 51263 and 51264 cages during growth.       We have shown a molecular configuration of the interface during the crystal growth of the [001] face of MH-sII in Figure 4. We can see that at this point in its growth, the interface contains both kinds of cages, the larger 51264 cage of MH-sII and the ‘intermediate’ 51263 cage. Depending on which cage will survive the ordering-disordering fluctuations taking place at the interface, the next layer of crystal to form will be either sII or sI; if the 51263 survives, the next layer will be MH-sI, otherwise the MH-sII will continue to growth. We suggest that through this mechanism it may be possible to obtain (under specific conditions of temperature and pressure) a polycrystalline methane hydrate solid formed by sequences of hydrate MH-sI and MH-sII connected via layers with the ‘intermediate’ 51263 cages. Additionally, we postulate that on the [001] MH-sI surface, the [001] MH-sII can be heterogeneously nucleated via intermediate layers of 51263, thereby allowing a system to transform between the two structures, as in the in-situ transformations observed experimentally between MH-sI and MH-sII.   Figure 5 The appearance of MH-sK during growth of methane hydrate.   Figure 6 View of the [001] face of MH-sK.       In some of our simulations of the MH-sI growth, we have obtained a sequence of two intermediate layers containing 51263 cages with subsequent growth of MH-sI crystal, but now misaligned with respect to the initial crystal (see Figure 5). A closer inspection of the hydrate structure composing these two layers reveals it is a new kind of methane hydrate structure (which we label sK). MH-sK consists of six 512 cages, four 51263 cages and four 51262 cages per repeating unit, where 80 molecules of water and 14 molecules of methane are contained in a 12.118Åx12.118Å x20.824Å orthorhombic box (see Figure 6).      This hydrate structure, with its 80H2O14CH4  stoichiometry and apparent hexagonal symmetry, appears consistent with one of the three structures under hydrate structure IV from the Jeffrey’s Table (see pages 148 and 150 of [28]), and we are aware of no report of its experimental existence. We suggest that it may be possible for methane hydrates (or gas hydrates in general) to adopt this sK structure under appropriate conditions. To provide further insight when it might be possible to observe sK hydrates, we have compared the energies of sI, sII and sK hydrate crystals as a function of the size (spherical) of the guest molecule, at a pressure of 100 atm and a temperature of 245 K (see supporting information). Although sI has the lowest energy (as expected) above a certain molecular diameter, the sK does become more stable than the sII, suggesting that it might be possible to identify sK for certain guest compounds (gases) at certain conditions. Additionally, a polycrystal of MH-sI and MH-sK, as well as an in-situ transformation of MH-sI to MH-sK could be possible at appropriate conditions. It is important to note that, even if the percentage of 51263 cages (as defects) in a MH-sI structure is small (and therefore difficult to detect experimentally) it could nonetheless have significant consequences in the stacking structure of the crystal as well as in its morphology.  Growth on a sII [001] face Very recently we have reported [26] the results of a set of molecular dynamic simulations of the [001] crystallographic face of MH-sII where steady-state crystal growth from supersaturated solutions of methane and water is achieved. Both constant pressure and constant volume conditions are examined, and we are successful in observing steady-state growth (at a rate of 1Å/ns) at two different temperatures and various pressures. For the methane supersaturation levels employed, the observed kinetics of growth are essentially independent of the applied pressure over the range 0 and 2000 atm. The MH-sII structure was found to be stable at these pressures over timescales of several tens of nanoseconds. Several interesting structural features, including [110] microfacets, were found.   Figure 7 Configurations from simulations of sII crystal growth.       Results of several molecular simulations of the heterogeneous crystal growth of methane hydrate sII were examined in detail (see Figure 7). From detailed examination of profile functions across the interface, the MH-sII/aqueous methane solution interface was found to be between 11-12 Å wide from the point of view of particle density, and 12-17 Å wide from the potential energy viewpoint. The measured profile functions for potential energies and densities across the interface were not symmetric, and they did not exhibit a temperature or pressure dependence.   Figure 8 Molecular structure at the solid/liquid interface of a sII hydrate crystal.      Consistent with previous work [29], the topology of the sII hydrate interface was found to be complex with the [001] face exhibiting [110] microfacets (see Figure 8). Not surprisingly, intermediate 51263 cages and a polycrystalline transition from MH-sII to MH-sI via such cages were identified.  (a) (b) Figure 9 Examples of observed crystal defects: (a) two methane molecules and (b) three molecules of water trapped in large cages.       Crystals grown were found to contain a number of defects. Typically about 20% of the hydrate cages were unoccupied, apparently independent of the applied temperature and pressure, while defects consisting of cages containing two methane molecules trapped in a large 51264 cage were also seen, primarily in simulations at higher pressures. Additionally, cages containing one or three water molecules (instead of methane) were observed. Perhaps future careful experimental studies, perhaps utilizing x-ray or neutron scattering, may be able to verify the presence of such defects in methane hydrate crystals. Although the populations of these defects can be expected to be low, they may have reasonable impacts upon properties such as thermal conductivities.   CONCLUSIONS This work emphasizes further that the crystal growth of systems such as methane hydrates is rich with molecular level detail and demonstrates that molecular simulation is an excellent means by which to probe this behavior. Certainly, an understanding of observed macroscopic properties will be built on such molecular level insights.       REFERENCES [1] Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Structural Aspects of Inclusion Compounds Formed by Inorganic and Organic Host Lattices; Inclusion Compounds, Vol. 1; Oxford University Press: Oxford, UK, 1991. [2] Sloan, E. D. Jr., Nature, 2003, 426, 353. [3] (a) Claypool, G. E., Kaplan, I. R., Natural Gases in Marine Sediments; Plenum: New York, 1974; Vol. 3, pp 99-139. (b) Barnes, R. O., Goldberg, E. D., Geology, 1976, 4, 297. (c) Boetius, A., Suess, E., Chem. Geol., 2004, 205, 291. (d) Kevnvolden, K. A., Chem. Geol., 1988, 71, 41.  (e) Chapman, P. K., Haynes, W., Acta Astron., 2005, 57, 372. (f) Chatti, I., Delahaye, A., Fournaison, L., Petitet, J. P., Energy Convers. Manage., 2005, 46, 1333. (g) Pooladi-Darvish, M., J. Pet. Technol., 2004, 56, 65. (h) Laherrere, J., Energy Exploration & Exploitation, 2000, 18, 349. [4] (a) Wignall, P. B., Newton, R.J., Little, C. T. S., Am. J. of Science, 2005, 305,  1014. (b) Chazelas, B., Leger, A., Ollivier, M., Science of the Total Environment, 2006, 354, 292. (c) McElwain, J. C., Wade-Murphy, J., Hesselbo, S.P., Nature, 2005, 435, 479. (d) Buffett, B., Archer, D., Earth and Planetary Science Letters, 2004, 227, 185. (e) Beauchamp, B., Comptes Rendus Geoscience, 2004, 336, 751. (f) Svensen, H., Planke, S., Malthe-Sorenssen, A., et al., Nature, 2004, 429, 542. (g) Renssen, H., Beets, C. J., Fichefet, T., et al., Paleoceanography, 2004, 19, Art. No. PA2010. (h) Dickens, G. R., Earth and Planetary Science Letters, 2003, 21, 169. (i) Benton, M. J., Twitchett, R. J., Trends in Ecology & Evolution, 2003, 18, 358. (j) Judd, A. G., Hovland, M., Dimitrov, L. I., et al., Geofluids, 2002, 2, 109. (k) Wignall, P. B., Earth-Science Reviews, 2001, 53, 1. (l) Brewer, P. G., Annals of the New York Academy of Science, 2000, 912, 195. (m) Paull, C. K., Ussler, W., Dillon, W. P., Geophysical research Letters, 1991, 18, 432. [5] (a) Kleinberg, R. L., Flaum, C., Griffin, D. D., et al., Journal of Geophysical Research- Solid Earth, 2003, 108 (B10),  Art. No. 2508. (b) Fluteau, F., Comptes Rendus Geoscience, 2003, 335, 157. (c) Rohl, U., Bralower, T. J., Norris, R. D., et al., Geology, 2000, 28, 927. (d) Gornitz, V., Fung, I., Global Biogeochemical Cycles, 1994, 8, 335. (e) Kvenvolden, K. A., Reviews of Geophysics, 1993, 31, 173. [6] (a) Lunine, J. I., Stevenson, D. J., Icarus, 1987, 70, 61. (b) Lunine, J. I., Stevenson, D. J., Astrophys. J. Suppl. Ser. 1985, 58, 493.  (c) Grasset, O., Sotin, C., Deschamps, F., Planet. Space Sci. 2000, 48, 617. [7] (a) Kelleher, B. P., Simpson, A. J., Rogers, R. E., et al., Marine Chemistry, 2007, 103, 237. (b) Farabegoli, E., Perri, M. C., Posenato, R., Global and Planetary Change, 2007, 55, 109. (c) Chuang, P. C., Yang, T. F., Lin, S., et al., Terrestrial Atmospheric and Oceanic Sciences, 2006, 17, 903. (d) Matsushima, J., Journal of Geophysical Research – Solid Earth, 2006, 111(B10),  Art. No. B10101. (e) Hill, T. M., Kennett, J. P., Valentine, D. L., et al., PNAS, 2006, 103, 13570. (f) Leifer, I., Luyendyk, B. P., Boles, J., et al., Global Biochemical Cycles, 2006, 20, Art. No. GB3008. (g) Inagaki, F., Nunoura, T., Nakagawa, S., et al., PNAS, 2006, 103, 2815. (h) Priest, J. A., Best, A. I., Clayton, C. R. I., Geophysical Journal International, 2006, 164, 149. [8] Sloan, E. D. Jr., Clarthrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. [9] Mak, T. C. W., McMullan, R., J. Chem. Phys. 1964, 42, 2732. [10] Loveday, J. S., Nelmes, R. J., Guthrie, M., Belmonte, S. A., Allan, D. R., Klug, D. D., Tse, J. S., Handa, Y. P., Nature, 2001, 410, 661. [11] (a) Shimizu, H., Kumazaki, T., Kume, T., Sasaki, S., J. Phys. Chem. B, 2002, 106, 30. (b) Chou, I. M., Anurag Sharma, A., Burruss, R. C., Shu, J., Mao, H. K., Russell, J., Hemley, R. J., Goncharov, A. F., Stern L. A., Kirby S. H., PNAS., 2000, 97, 13484. [12] Vatamanu, J., Kusalik, P. G., J. Phys. Chem. B, 2006, 110, 15896. [13] (a) Rodger, P. M., Ann. N.Y. Acad. Sci. 2000, 912, 474. (b) Nada H., J. Phys. Chem. B., 2006, 110, 16526. [14] (a) Ripmeester, J. A., Ratcliffe, C. I., Klug, D. D., Tse, J. S. Ann. N.Y. Acad. Sci. 1994, 715, 161. (b) Forrisdahl, O. K., Kvamme, B., Haymet, A. D. J., Mol. Phys. 1996, 89, 819. (c) Rodger, P. M., Forester, T. R., Smith, W., Fluid Phase Equilib. 1996, 116, 326.  (d) English, N. J., Johnson, J. K., Tylor, C. E., J. Chem. Phys. 2005, 123, 244503. [15] (a) Alavi, S,. Ripmeester, J. A., Klug, D. D., J. Chem. Phys. 2006, 124, 014704. (b) Tse, J. S., Klein, M., McDonald, I. R., J. Chem. Phys. 1984, 81, 6146. (c) Sizov V.V., Piotroskaya E. M., J. Phys. Chem. B., 2007, 111, 2886. (d) Miyoshi T., Ohmura, R., Yasuoka, K., J. Phys. Chem.. C., 2007, 111, 3799. (e) English, N. J., MacElroy, J. M. D., J. Comput. Chem., 2003, 24, 1569. (f) Jiang, H., Jordan, K. D., Taylor, C. E., J. Phys. Chem. B., 2007, 111, 6486. [16] (a) Tse, J. S., Klug, D. D., J. Supramol. Chem. 2002, 2, 467. (b) Alavi S., Ripmeester J. A., Klug D. D., J. Chem. Phys., 2007, 126, 124708. [17] (a) Carver, T. J., Drew, M. G. B., Rodger, P. M., J. Chem. Soc., Faraday Trans., 1995, 91, 3449. (b) Kvamme, B., Huseby, G., Forrisdahl, O. K., Mol. Phys., 1997, 90, 979. (c) Wallqvist, A., J. Chem. Phys. 1992, 96, 5377. (d) Storr, M. T., Rodger, P. M., Ann. N.Y. Acad. Sci., 2000, 912, 669. (e) Anderson, B. J., Tester, J. W., Borghi, G. P., Trout, B. L., J. Am. Chem. Soc., 2005, 127, 17852. [18] (a) Moon, C., Taylor, P. C., Rodger, P. M., J. Am. Chem. Soc. 2003, 125, 4706. (b) Guo, G. J., Zhang, Y. G., Zhao, Y. J., J. Chem. Phys. 2004, 121, 1542. (c) Guo G. J., Zhang Y. G., Liu H., J. Phys. Chem. C., 2007, 111, 2595. [19] Granasy L., Pusztai T., Warren J. A., Journal of Physics – Condensed Matter, 2004, 16, R1205, and references therein. [20] Svandal, A., Kvamme, B., Granasy, L., Pusztai, T., Buanes, T., Hove, J., J. Cryst. Growth., 2006, 287, 486. [21] (a) Moon, C., Hawtin, R. W., Rodger, P. M., Faraday Discussions, 2007, 136, 367. (b) Lee, H., Seo, Y., Seo Y.-TMoudrakovski., I. L., Ripmeester, J. A., Ang. Chem. Int. Ed., 2003, 42, 5048. (c) Park, Y., Kim, D.-Y., Lee J. W., Huh D.- G., Park, K.-P., Lee, J., Lee, H., Proc. Natl. Acad. Sci. U.S.A., 2006, 103, 12690. (d) Tegze, G., Granasy, L., Kvamme, B., Phys. Chem. Chem. Phys., 2007, 9, 3104. [22] Tegze, G., Pusztai, T., Toth, G., Granasy, L., Svendal, A., Buanes, T., Kuznetsova, T., Kvamme, B., J. Chem., Phys., 2006, 124, 234710. [23] Gulam Razul, M. S., Tam, E. V., Lam, M. E., Linden, P., Kusalik, P. G., Mol. Phys. 2005, 103, 1929. [24] Vatamanu, J., Kusalik, P. G., J. Chem. Phys., 2007, 126, 124703. [25] Vatamanu, J., Kusalik, P. G., J. Am. Chem. Soc., 2006, 128, 15588. [26] Vatamanu, J., Kusalik, P. G., J. Phys. Chem. B, 2008, 112, 2399. [27] Brownstein, S., Davidson, D. W., and Fiat, D., J. Chem. Phys., 1967, 46, 1454; Calvert, L. D., and Srivastava, P., Acta Crystallogr. A, 1969, 25, 131. [28] Atwood, J.L., Davies, J.E.D., MacNicol, D.D. Inclusion Compounds, Volume 1, “Structural Aspects of Inclusion Compounds Formed by Inorganic and Organic Host Lattices”, (Oxford University Press, Oxford, 1991. [29] Vatamanu, J., Kusalik, P. G., Phys. Rev. B., 2007, 76, 035431.   


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