Influence of combining rules on the cavity occupancy of clathrate hydrates by Monte Carlo simulations

Assessing the exact amount of gas stored in clathrate-hydrate structures can be addressed by either molecular-level simulations (e. g. Monte Carlo) or continuum-level modelling (e. g. van der Waals-Platteeuw-theory-based models). In either case, the Lorentz-Berthelot (LB) combining rules are by far the most common approach for the evaluation of the parameters between the different types of atoms that form the hydrate structure. The effect of combining rules on the calculations has not been addressed adequately in the hydrate-related literature. Only recently the use of the LB combining rules in hydrate studies has been questioned. In the current study, we report an extensive series of Grand Canonical Monte Carlo simulations along the three-phase (H-L-w-V) equilibrium curve. 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. We examine the effect of deviations from the LB combining rules on the cavity occupancy of argon hydrates and work towards quantifying it. The specific system is selected as a result of the characteristic behaviour of argon to form hydrates of different structures depending on the prevailing pressure. In particular, an sII hydrate is formed at lower pressures, while an sI hydrate is formed at intermediate pressures, and finally an sH hydrate is formed at higher pressures.