Cage Occupancies in Nitrogen Clathrate Hydrates from Monte Carlo Simulations

Abstract

Comparisons of Gibbs ensemble Monte Carlo simulations with experimental data for the cage occupancies in N 2 clathrate hydrates are performed to assess the accuracy of such simulations, to refine the effective potentials employed, and to help interpret recently measured large cage over small cage occupancy ratios [Petuya et al., J. Phys. Chem. C 122, 566 (2018)]. Different sets of interaction potentials for N 2 N 2 , N 2 H 2 O and H 2 O H 2 O interactions are considered. Some of them fail to reproduce the known experimental fact that some large cages are doubly occupied at 273 K and high pressures. The best agreement between simulations and experiments is obtained when using a new N O interaction potential derived in this work by averaging an ab-initio potential energy surface for the N 2 H 2 O dimer.

Adsorption of CO and N2 molecules at the surface of solid water. A grand canonical Monte Carlo study

“…The present GCMC calculations evidence the influence of the CO-water interaction model on the results, as already previously shown for N 2 . 45 Indeed, whereas the model of Piper et al (model A) 42 appears well-suited for characterizing incorporation of CO molecules into clathrates, the one proposed by Straub and Karplus 43 (model B) seems more accurate for characterizing adsorption at the surface of solid water. This enlightens the need for an interaction potential model between carbon monoxide and water molecules that could be transferable to different water environments.…”

“…On the basis of previous works devoted to trapping of CO and N 2 in clathrates, 44,45 , the water molecules have been modeled by the TIP4P-Ew model, 40 whereas the model proposed by Potoff and Siepmann on the basis of the TraPPE force field, 41 has been used for N 2 . When considering the CO molecules, two different force fields have been considered, on the basis of their ability for describing CO-water interactions in various contexts.…”

Nitrogen Hydrate Cage Occupancy and Bulk Modulus Inferred from Density Functional Theory-Derived Cell Parameters

“…The volume and energies were found to be linearly dependent on the cage occupancy, and there was no marked difference in the structure between singly and doubly occupied systems. Grand-canonical Monte-Carlo studies were performed on sI and sII nitrogen clathrate hydrates. , These studies showed that at low temperature (at 50 K), the SCs are filled before the LCs, but at a higher temperature (at 150 K) the opposite happens, i.e., the LCs are filled before the SCs. Carbon monoxide hydrate has also been simulated and, in the latter case, whatever the structure formed or the temperature, the SCs were always occupied before the LCs, although at 150 K the occupancy of both types of cages becomes simultaneous.…”

“…Grand-canonical Monte-Carlo studies were performed on sI and sII nitrogen clathrate hydrates. 25,26 These studies showed that at low temperature (at 50K), the SCs are filled before the LCs, but at higher temperature (at 150K) the opposite happens, i.e. the LCs are filled before the SCs.…”

Molecular Selectivity of CH4–C2H6 Mixed Hydrates: A GCMC Study

“…The internal degrees of freedom have not been considered and, thus, all of the molecules have been treated as rigid bodies. The water molecules have been represented by the TIP4P-Ew model, 31 already used in several hydrate studies, 32,33 in which it has been shown to give satisfactory results compared to the experimental data. In this The Lennard−Jones parameters σ (size parameter) is given in Å, like all distances, whereas ε/k B (energy parameter) is given in K, with k B being the Boltzmann constant.…”