Solvation shell dynamics of supercritical water–cyclohexane mixtures in relation to the translational and rotational dynamics as studied by molecular dynamics simulation

Abstract

The translational dynamics of water and cyclohexane in supercritical binary mixtures were investigated using molecular dynamics simulations. The effects of the local composition were examined through a decomposition scheme of the conditional time-correlation functions based on the solvation numbers for water and cyclohexane. The self-diffusion of water was found to be largely controlled by the continuous and collective attractive interactions with surrounding water molecules, while interactions with cyclohexane have minimal impact on water diffusion. On the other hand, the self-diffusion of cyclohexane is dominantly determined by uncorrelated collisional interactions with neighboring cyclohexane molecules. The results demonstrate the dynamic aspect of microscopic inhomogeneity and highlight the significance of interactions between molecules of the same species. An examination of the dependence of self-diffusion on the lifetime of the solvation shell indicated that the self-diffusion of water is confined within the solvation shell. This is attributed to the hydrogen bond interactions with neighboring water molecules, which create an energy barrier to the water molecules diffusing out of the hydration cage. In contrast, diffusing cyclohexane molecules migrate beyond the solvation shell, particularly at large water contents.