Prediction of the three-phase coexistence line of the ethane hydrate from molecular simulation

Abstract

We investigate the three-phase coexistence line of ethane (C$_2$H$_6$) hydrate through molecular dynamics simulations using the direct coexistence approach. In this framework, C$_2$H$_6$ sI hydrate, aqueous, and pure guest phases are constructed within a single simulation box, allowing us to monitor their mutual stability. From the temporal evolution of the potential energy, we identify the equilibrium temperature (T$_3$) at which all three phases coexist, across pressures ranging from 1000 to 4000 bar, in accordance with available experimental data. Simulations are performed with the GROMACS package (version 2016, double precision) in the $NPT$ ensemble. Water and C$_2$H$_6$ molecules are represented using the TIP4P/Ice and TraPPE-UA models, respectively, while unlike non-bonded interactions are computed with the Lorentz-Berthelot combining rule. Dispersive Lennard-Jones and Coulomb interactions are truncated at 1.6 nm, with long-range Coulombic contributions treated via Particle-Mesh Ewald summation. The predicted three-phase coexistence line shows excellent agreement with experimental measurements within the investigated pressure range. These results demonstrate the suitability of the direct coexistence methodology, combined with established molecular models, for reproducing hydrate dissociation behavior in systems that have received little prior computational attention.