Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates

In this work, the effect of the range of the dispersive interactions in the determination of the three-phase coexistence line of the CO₂ and CH₄ hydrates has been studied. In particular, the temperature (T₃) at which solid hydrate, water, and liquid CO₂/gas CH₄ coexist has been determined through molecular dynamics simulations using different cut-off values (from 0.9 to 1.6 nm) for the dispersive interactions. The T₃ of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T>T₃. Contrary, if the hydrate grows, then T<T₃. The effect of the cut-off distance on the dissociation temperature has been analyzed at three different pressures for CO₂ hydrate, namely, $100$, $400$, and 1000\,bar. Then, we have changed the guest and studied the effect of the cut-off distance on the dissociation temperature of the CH₄ hydrate at 400\,bar. Also, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cut-off value of 0.9 nm. The results obtained in this work highlight that the cut-off distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T₃ about 2-4\,K depending on the system and the pressure.