Mixtures of methane and water under extreme conditions

Abstract

The hydrophobic effect has been a topic of research for decades, not only due to its importance as the primary building block of much of chemistry (it dictates which solvent can dissolve which solutes) and biology (guiding protein binding and gene expression) but also due to it being a fundamental physical process.

The commonly held opinion is that 'like dissolve like', implying polar substances can readily mix with other polar substances, and similarly for apolar ones, but polar and apolar would separate and tend to stay isolated from one another (like oil in water). We have developed a quantitative imaging method that can be used in tandem with Raman spectroscopy in order to investigate the effect of high pressure on a model hydrophobic system - water and methane. Our study revealed an unexpectedly large increase in the amount of methane that can readily mix with water once a rather modest pressure has been applied to the system. Thus, the solubility of CH4 in H2O starts abruptly increasing at 1.3 GPa and reaches a maximum of 44(3) mole % at 2.1 GPa, showing no pressure dependence upon further compression. We have tried to reproduce the observed experimental behaviour using classical molecular dynamics simulations deploying a range of widely used water potentials (SPC/E, TIP4P, TIP3P), but unfortunately no quantitative or even qualitative agreement was reached with experiments. Finally, in order to understand the atomic level changes that enable this increased amount of methane to dissolve in water, we have performed neutron scattering measurements along with EPSR (empirical potential structure refinement) fits to the data in order to solve the structure of the fluid mixture.

These revealed a tendency towards maintaining the H-bond network present in water and homogeneous mixing. Despite the network staying similar to the one found in pure fluid water at milder pressures and temperatures (close to ambient conditions), the H-bonds seem more disordered and show a greater variability in their lengths.