Note: A simple correlation to locate the three phase coexistence line in methane-hydrate simulations

Abstract

In this Note, the three phases coexistence (hydrate-water-methane) temperature is determined by using direct coexistence simulations for a force field in which methane is described by a single Lennard-Jones site, and water is described by either SPC or SPC/E models. Results were obtained for two pressures: 100 and 400 bars. The results of this note along with those obtained in our previous work [M. M. Conde and C. Vega, J. Chem. Phys. 133, 064507 (2010)]10.1063/1.3466751 allow to present the three phases coexistence line for a number of water models: SPC, SPC/E, TIP4P, TIP4P/2005, and TIP4P/Ice. A correlation between the three phases coexistence temperature T3 of the methane hydrate and the melting point of ice at normal pressure Tm of the water model is found. Models with a good prediction of the melting temperature of ice Ih, have also good predictions of the three phases coexistence line. Models with a bad prediction of the melting temperature of ice Ih, provide poor estimates of T3. As a rule of thumb, the three phase coexistence line of the methane hydrate at 100 bars for a water-methane force field is located approximately at about 15(7) K above the melting temperature of ice Ih of the considered water model. In summary, when modeling the methane-hydrate, if you are looking for a three phases coexistence line in good agreement with experiment, you need first a water model that describes accurately the melting temperature of ice Ih.