Pressure dependence of molecular aggregation and phase behavior in methane-water mixtures

Abstract

Methane in liquid water under ambient conditions is rarely soluble but its solubility increases seriously at extremely high pressures above 2 GPa. Here, we investigate the methane aggregation behavior in a methane -water mixture at various pressures to clarify the molecular origin of a remarkable change of methane solubil-ity in liquid water caused by the pressure. The molecular dynamics simulation and relevant analyses reveal a bifurcating methane aggregation pathway with respect to the pressure, that is, methane tends to form self -associate hydrophobic aggregates separated from liquid water at lower pressure. As the pressure much further increases, spatially extended methane aggregates, which are mixed well with liquid water, are formed. The results of graph theoretical analysis and spatial inhomogeneity measurement demonstrate that the pressure -varying methane aggregation behavior influences local water structure and even the spatial distribution of component molecules, despite that the global water H-bond network structure is not greatly perturbed. This observation on the relationship between methane solubility and pressure-induced aggregation behavior in methane-water mixture is anticipated to crucially contribute to exploring the fundamental issues, such as gas solubility in liquid water and fluid-fluid phase separation in aqueous mixtures.