Effect of Salt on the Dynamics of Aqueous Solution of Hydrophobic Solutes: A Molecular Dynamics Simulation Study

The effect of salt concentration on the dynamics of the aqueous solution of small hydrophobic solutes is presented. We have performed molecular dynamics simulation in an isothermal-isobaric ensemble of methane in water without and with sodium chloride of different concentrations at standard temperature and pressure. Mean square displacements of methane, water, and ions have been calculated from the analysis of simulation trajectories, and diffusion constants have been obtained from slope of the respective mean square displacement (MSD) plot. Translational mobilities of these molecules are shown to decrease with the increase of salt concentration. Whether any correlation exists between the translational dynamics and the structure of constituent particles in the solution has been investigated. Analysis of the methane-methane radial distribution function reveals that the decreasing mobility of methane molecules is a consequence of the increasing hydrophobic interaction between methane molecules as the concentration of the salt increases. For ions also, the same correlation is observed, whereas in the case of water decreasing mobility is not directly related with its structure.