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

TL;DR

This work targets the three-phase coexistence line $T_3$ for the ethane hydrate (structure I) in a water–ethane system. It employs direct coexistence molecular dynamics in GROMACS with TIP4P/Ice water and TraPPE-UA ethane, using Lorentz–Berthelot cross interactions, a 1.6 nm LJ cutoff, and PME for Coulomb, constructing a three-phase box (hydrate, water, and ethane) to locate $T_3$ across $P=1000$–$4000$ bar. The hydrate seed is fully occupied ($8$ C2H6 per $46$ H2O), and $T_3$ is inferred from the midpoint between the highest temperature causing growth and the lowest temperature causing melting. The resulting $T_3$ values agree closely with experimental data, validating the direct coexistence method for this system and revealing occupancy near $8/46$ with a consistent phase diagram that includes quadrupole points $Q_1$ and $Q_2$. This approach demonstrates the reliability of established molecular models for predicting hydrate dissociation behavior in computationally underexplored systems.

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.

Figures (5)

• Figure 1: Representation of the initial simulation box used in this work. green spheres represent the C$_2$H$_6$ molecules and red and white licorice representations correspond to water molecules. From left to right, the simulation box is conformed by a hydrate phase, a water phase, and a C$_2$H$_6$ phase.
• Figure 2: Evolution of the potential energy as a function of time for the NPT runs of the three-phase system at 1000 bar (a),1500 bar (b), and 2000 bar (c) and various temperatures (see legends).
• Figure 3: Evolution of the potential energy as a function of time as obtained from the NPT runs of the three-phase system at 2500 (a), 3000 (b), 3500 (c), and $4000\,\text{bar}$ (d) and various temperatures (see legends).
• Figure 4: Ethane and water density profiles and snapshots at $2500\operatorname{bar}$ at 308 (top), 310 (middle), and $312\operatorname{K}$ (bottom). The density profiles show the initial and final density distribution of water and C$_2$H$_6$ along the simulation box, while the snapshots show the final configuration obtained from the $NPT$ simulations.
• Figure 5: Pressure-temperature projection of the dissociation line of the C$_{2}$H$_6$ hydrate. Blue diamonds are the results obtained in this work using the direct coexistence method, the TIP4P/Ice model for water, and the TraPPE model for C$_{2}$H$_6$. Red circles correspond to experimental data taken from the literature. Roberts1940aDeaton1946aReamer1952aGalloway1970aFalabella1974aHolder1980aHolder1982aNg1985aAvlonitis1988aSong1989aNakano1998bYang2000aMorita2000a