(25b) Gibbs Ensemble Monte Carlo Simulations Probing the Miscibility Gap in Water/Hydrogen Mixtures at High Temperatures and Pressures

To explore fluid-fluid immiscibility in the binary system H₂O/H₂ under extreme conditions, NPT-Gibbs ensemble Monte Carlo simulations are carried out at temperatures above the critical point of neat water and pressures of 1.8 and 2.6 GPa. The simulations, using molecular mechanics force fields that treat both molecules as rigid and nonpolarizable but quantitatively reproduce the critical points of the neat compounds, support the presence of a miscibility gap under these conditions, in agreement with a recent experimental study that found two distinct immiscible phases in olivine inclusions. Analysis of the atom-atom radial distribution functions for H₂O in the H₂-rich phase highlights the microheterogeneous structure, with significant H₂O aggregation. Examination of the cluster size distribution of these H₂O aggregates shows that the miscibility gap is closed by the onset of a cluster percolating in space. The distribution of H-bond energies and geometries also reveals the weakening of the H-bond network in these mixtures under extreme conditions. Consideration of the angular distribution of water molecules shows the importance of many-body dipolar correlations in closing the miscibility gap and emphasizes the role of dipole-dipole interactions in determining the enthalpic difference between the two coexisting phases.