Ethane clathrates using different water-ethane models: Molecular dynamics

Abstract

Ethane clathrates are studied in a temperature window where the stable-unstable transition takes place by means of molecular dynamics simulations in an isothermal–isobaric ensemble. For this purpose a temperature range [200–440] K and a pressure of 2 MPa are considered. Firstly, structural analysis of the ethane clathrates is carried out at a fixed temperature of 200 K, where clathrates are stable for all considered water models. Here, it is found that structural properties of all stable clathrates do not strongly depend on the water model. As a next step, temperature is increased upon the clathrate turns unstable. This decomposition temperature is found by monitoring coordination numbers, total number of hydrogen-bonds, potential energy, potential of mean force and mean-square displacements. All properties consistently point out to the same temperature at which the stable–unstable transition takes place for each water model. As a part of our results, we notice that by using the standard Lorentz-Berthelot combining rules, the obtained temperature at which the clathrate becomes unstable is higher than the experimental reference value for all used water models. However, we have found that a reasonable way to approach the experimental-decomposition temperature is by including a re-scaling factor in the combining rules in such a way that both methyl– oxygen size and interaction energy turned out decreased. Our data indicates that the decomposition temperature is sensitive to both parameters.