Equilibrium molecular dynamics of methane hydrate systems at pre-nucleation conditions to predict system transport properties*

This work intended to extend the current understanding of water and gas hydrate systems by developing improved molecular simulations to estimate the transport properties of pure water and methane hydrate systems. LAMMPS molecular dynamics package was used to implement TIP4P/2005 and TIP4P-ice water force fields for subcooled water systems. Various equilibrium formulations for transport properties were tested, including the Green-Kubo and Einstein formulations. A new Stokes-Einstein formulation for viscosity was also tested in this work. This work implemented systems consisting of approximately 7,600 atoms that were methodically equilibrated with the primary intent to develop stable and physically representative systems adequate for the estimation of bulk phase transport properties via the methods above. Finally, the data generated in this study were compared to experimental data to validate their accuracy. This includes experimental viscosity measurements of methane hydrates systems collected by our research group in previous experimental work. The present study showed that the TIP4P-ice force field for water resulted in an improved estimation of viscosity for pure water systems when compared to the TIP4P/2005 force field results. However, discrepancies in the estimations for the viscosity of methane hydrate systems indicate the necessity for re-parametrization of the TIP4P-ice force field taking new viscosity experimental data into consideration for gas hydrate applications.