Dissociation Line and Driving Force for Nucleation of the Multiple Occupied Hydrogen Hydrate from Computer Simulation

In this work, we determine the dissociation temperature of hydrogen (H₂) hydrate by computer simulation using two different methods. In both cases, the molecules of water and H₂ are modeled using the TIP4P/Ice and a modified version of the Silvera and Goldman models, respectively, and the Berthelot combining rule for the cross water-H₂ interactions has been modified. The first method used in this work is the solubility method, which consists of determining the solubility of H₂ in an aqueous phase when in contact with the H₂ hydrate (H-L_w) phase and when in contact with the pure H₂ phase (L_w-L_H₂ ) at different temperatures. At a given pressure value, both solubility curves intersect at the temperature (<i>T</i> ₃) at which the three phases coexist in equilibrium. Following this approach, we determine the dissociation temperature of H₂ hydrate at 185 MPa finding a good agreement with the data previously reported in the literature. We also analyze the effect of the multiple occupancy of the D, or small, and H, or large, cages of the sII hydrate structure. We conclude that the <i>T</i> ₃ value is barely affected by the occupancy of the H₂ hydrate at 185 MPa. From the analysis of the solubility curves and performing extra bulk simulations of the three phases involved in the equilibrium, we also determine the driving force for nucleation (Δμ_N ^EC) at 185 MPa as a function of the supercooling degree and the H₂ hydrate occupancy. We determine that, thermodynamically, the most favored occupancy of the H₂ hydrate consists of 1 H₂ molecule in the D cages and 3 in the H cages (i.e., 1-3 occupancy). We also conclude that the double occupancy of the small D cages is not favored because the Δμ_N ^EC values obtained for this occupancy are the most positive ones. The second approach used in this work is the direct coexistence technique using an initial H₂ hydrate phase with 1-3 occupancy. We also propose a new modification of the Berthelot combining rule to improve the predictions of the <i>T</i> ₃ values. Following this method, we determine the <i>T</i> ₃ at 100, 185, and 300 MPa finding excellent agreement with the experimental data.