Phase Diagram of Methane and Carbon Dioxide Hydrates Computed by Monte Carlo Simulations

Molecular Monte Carlo simulations are used to compute the three-phase (hydrate-liquid water-gas) equilibrium lines of methane and carbon dioxide hydrates, using the Transferable Potentials for Phase Equilibria model for carbon dioxide, the united atom optimized potential for liquid simulations model for methane, and the TIP4P/Ice and TIP4P/2005 models for water. The three-phase equilibrium temperatures have been computed for pressures between 50 and 4000 bar via free-energy calculations. The computed results are as expected for methane hydrates but deviate from the direct-coexistence molecular dynamics (MD) studies for carbon dioxide hydrates. At pressures higher than 1000 bar, both the methane and carbon dioxide hydrates dissociate at lower temperatures than expected from experiments and MD studies. The dissociation enthalpy is found to be largely independent on water models, and its values are measured to be 7.6 and 6.0 kJ/mol of water for methane hydrates and carbon dioxide hydrates, respectively. We evaluate the effect of systematic errors on the determination of chemical potentials and show that systematic errors of 0.1 kJ/mol in the chemical potential of water correspond to deviations of 5 K in the three-phase equilibrium temperatures.