Molecular dynamics study of methane in water: diffusion and structure

Abstract

We present molecular dynamics simulation results for the diffusion coefficients and structure of water–methane mixtures in constant NPT ensembles, at T = 270, 300 K and P = 8.104 × 107 Pa. The systems we have studied consist of one, four and eight CH4 molecules and varying H2O molecules per unit cell, which correspond to methane concentration of 0.081, 0.324 and 0.643 mol/l, respectively. The intermolecular potentials used in all the simulations were the four-site TIP4P model of water [1] and the fitted Lennard-Jones 12-6 potential for CH4–H2O [2]. Our results show that the methane concentration has little impact on the structure of water and the formation of hydrogen bonds (H-bonds) between water molecules. The H-bond numbers, H-bond length and the H-bond angle are independent of the methane concentration at the temperatures and densities examined in this study. We also find that the number of H-bonds and angles are sensitive to the temperature. The rise of temperature produces a decrease in the number and an increase in the angle of the H-bonds. Enhanced structuring of the hydration-shell water molecules is indicated by an increase of the first and second peak in the water oxygen–oxygen radial distribution function as temperature is decreased. The self-diffusion coefficient of water is sensitive to the methane concentration and temperature.