oa Molecular level simulations of clathrate hydrates: Recent progress with Monte Carlo studies

Hydrates are non-stoichiometric inclusion compounds that mainly consist of water molecules that form three dimensional cavities that are stabilized by entrapped guest molecules. Hydrates have been under investigation for many years due to their wide variety of applications in engineering and scientific problems. In particular, their characteristic to encage selectively large amounts of gas into the crystalline structure has attracted significant attention for possible practical applications including the separation of gas mixtures, desalination, storage and transportation of gases (e.g., methane, hydrogen). Vast amounts of natural gas hydrates that contain significant amount of methane could possibly be used as a future energy source. Assessing thermodynamic properties of hydrate-containing systems, such as the hydrate equilibrium pressure and temperature conditions or the exact amount of gas stored in the hydrate structure has been addressed by either molecular level simulations (e.g., Monte Carlo) or continuum level modeling. In either case the Lorentz-Berthelot (LB) combining rules are by far the most common used for the parameters between different types of atoms. This is a result of their success in describing fluid mixtures of nonpolar molecules. However, the LB combining rules perform inadequately when describing the gas - H2O interactions without using a correction factor. An extensive discussion on the use of combining rules is provided in the studies of Delhommelle and Millie (2001), and Haslam et al. (2008). The effect of combining rules has not been addressed adequately in the hydrate-related literature. The vast majority of studies have considered the LB combining rules. In the current study we report two series of Grand Canonical Monte Carlo (GCMC) simulations: (i) simulations along the three phase (H-Lw-V) equilibrium curve, and (ii) simulations at pressure and temperature conditions that are off-hydrate-equilibrium. For both cases the exact geometry of hydrate crystals is known from diffraction experiments and therefore, the formation of hydrates can be simulated as a process of gas adsorption in a solid porous material. In the first case, we examine the effect of deviations from the LB combining rules on the cavity occupancy of Argon hydrates. The specific system is selected as a result of the characteristic behavior of Argon to form hydrates of different structures depending on the prevailing pressure. In particular, sII hydrate is formed at lower pressures, while sI hydrate is formed at intermediate pressures, and finally sH hydrate is formed at higher pressures. In the second case, an extensive series of GCMC simulations for the case of all the known hydrate structures, sI, sII, and sH that hydrogen is known to form, is performed. During the simulations a number of water force-fields are examined regarding their effect on the storage capacity of hydrates. In particular, the following popular water force-fields are considered: SPC/E, TIP4P, TIP4P/Ice, and TIP5P. The Langmuir constants for each type of cavity and hydrate structure are reported as a function of temperature and pressure. Therefore, the storage-capacity of the different hydrate structures can be calculated. Acknowledgment Financial support by Qatar National Research Fund (project NPRP 6-1547-2-632) is gratefully acknowledged.