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

Area of Science:

• Computational chemistry
• Materials science
• Physical chemistry

Background:

• Hydrogen hydrate (H₂ hydrate) plays a crucial role in various natural and industrial processes.
• Accurate determination of its dissociation temperature is essential for understanding and predicting its behavior.
• Previous studies have reported dissociation data, but further computational validation is needed.

Purpose of the Study:

• To determine the dissociation temperature of hydrogen hydrate using two distinct computational simulation methods.
• To analyze the impact of guest molecule (H₂) occupancy within hydrate cages on dissociation temperature.
• To investigate the thermodynamic favorability of different H₂ hydrate structures.

Main Methods:

• Computer simulations employing the TIP4P/Ice model for water and a modified Silvera and Goldman model for H₂.
• Utilizing the solubility method by analyzing H₂ solubility in aqueous and hydrate phases at equilibrium.
• Employing the direct coexistence technique to simulate H₂ hydrate phases in direct contact.
• Modification of the Berthelot combining rule for improved interaction predictions.

Main Results:

• The solubility method determined the H₂ hydrate dissociation temperature at 185 MPa, showing good agreement with literature data.
• Cage occupancy, specifically multiple occupancy of small (D) and large (H) cages, had a minimal effect on the dissociation temperature at 185 MPa.
• The 1-3 occupancy (1 H₂ in D cages, 3 H₂ in H cages) was identified as the most thermodynamically favored structure.
• The direct coexistence method, using a modified Berthelot rule, yielded results in excellent agreement with experimental data at 100, 185, and 300 MPa.

Conclusions:

• The study successfully determined the dissociation temperature of H₂ hydrate through two simulation approaches.
• Thermodynamically, H₂ hydrate formation is favored by a 1-3 occupancy configuration, while double occupancy of small cages is disfavored.
• The modified Berthelot combining rule and direct coexistence method provide accurate predictions for H₂ hydrate dissociation temperatures across various pressures.