The effect of flexibility on thermodynamic and structural properties in methane hydrates

Abstract

The purpose of the present study is to compare two different schemes for treating the intramolecular degrees of freedom in molecular dynamics simulation of methane hydrates by studying their effect on the simulated properties like energy, density and radial distribution functions at different thermodynamics conditions. The first scheme supposes molecules with rigid geometry and resembles the assumption made by van der Waals and Platteeuw of a lattice which is not distorted by guest molecules. The second scheme supposes a more realistic description from the hydrate by introducing translational and rotational degrees of freedom. The NPT molecular dynamic simulations include temperature ranges from 40 to 200 K for two different pressure conditions, 0.1 and 500 MPa. The results show that increments of temperature increase the energy and decrease the density of the hydrate in both models, rigid and flexible. The effect of temperature on the lattice parameter agrees with the experimental behaviour reported in other studies. The radial distribution functions showed that the hydrate with flexible molecules presents major distortion with regard to the rigid model. However, the radial distribution functions in both rigid and flexible models reproduce the experimental peaks at the same distance. In general, the effect of flexibility concerns in major measure the thermodynamic properties in comparison with the structural properties analysed in this work.