Translational diffusion of hydrophobic solutes in supercritical water studied by molecular dynamics simulations

Abstract

The diffusion process of hydrophobic solutes (oxygen and methane) in water at various temperatures and densities has been studied by molecular dynamics simulation. We found anomalous temperature dependence of the self-diffusion constants of hydrophobic solutes in water in the medium-density region, i.e., the diffusion constants are almost independent of the temperatures. In the case of oxygen, even the inversion of the temperature dependence is observed. To investigate the reason of this anomaly, we have analyzed the velocity auto correlation function (VACF) and memory function of the friction on the diffusion based on the generalized Langevin theory. The VACFs of hydrophobic solutes decay almost exponentially, which suggests that the Enskog theory holds. According to the analysis of the memory functions, it has been revealed that the binary contribution of the friction decreases with decreasing temperature from 973 to 647 K in the density region below 663 kg m−3, which is the main reason for the anomalous temperature dependence of the diffusion constant of the hydrophobic solutes. The radial distribution function of water around the hydrophobic solutes shows the water deficient structure. This deficiency is enhanced with decreasing the temperature which causes the decrease of the binary friction.