Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations

Abstract

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2 guest molecules in order to investigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O–N2 radial distribution function, and therefore the structure in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs of molecules, the stresses on the host lattice are comparable to those in single occupation. Small changes in the intermolecular interactions do not affect the results. The clathrate hydrate destabilizes upon increasing the size of the guest molecules by more than about 10%. We also find that the clathrate hydrate remains stable when experimental conditions are closely followed, i.e., for partially doubly occupied clathrates with a pressure dependent filling fraction. The calculated compressibility is the same as in experiments.

Thermodynamic stability of hydrogen clathrates

“…The structure and stability of several multiple-occupancy ice clathrates have been examined with molecular dynamics methods (15,16). Here, we adopt a statistical thermodynamics approach to investigate the thermodynamics of H 2 ͞H 2 O clathrate formation.…”

Monte Carlo study of sII and sH argon hydrates with multiple occupancy of cages

“…Additional studies that consider the case of multiple occupancy include, among others, the experimental [24][25][26] and the theoretical [27][28][29] works for nitrogen, the experimental [30,31] and the theoretical [32][33][34][35][36][37] works for hydrogen, and the theoretical work for carbon dioxide [38].…”

Ab initio studies of (H2O)28 hexakaidecahedral cluster with Ne, N2, CH4, and C2H6 guest molecules in the cavity

“…Because of a significantly large cage size, the single and double occupancy of the cage by relatively smaller N 2 and CH 4 molecules are also examined. Such a study, especially with the N 2 guest molecules, allows one to compare results with that of the molecular dynamics by van Klaveren et al 13 which suggests significant stability of the hexakaidecahedral cluster with double occupancy of N 2 molecules. In addition, the guest molecules of different sizes 1 are considered so as to learn how well different guest molecules stabilize a large cage structure.…”