Kinetic model for transport of liquid molecules in the solid-liquid interface region: a molecular dynamics view

Abstract

Liquid molecules form ordered structures in the vicinity of the solid-liquid interface. Mass transport in the ordered structure exhibits decidedly different characteristics from the ordinary diffusion property governed by Fick's law. In order to analyze mass transport at this nanoscopic scale, we have monitored the PMF (potential of mean force) obtained from the number density of liquid molecules, which corresponds to the free energy change with respect to the molecular migration. Using this PMF profile, the kinetic process model to describe the molecular mobility inside these ordered structures was developed in this study. In order to verify whether the suggested kinetic model works correctly, molecular dynamics (MD) simulations of the liquid-solid interface system were performed. In a simple system consisting of a LJ liquid and flat solid walls, which are modeled by the continuum media interacting with liquid molecules, molecular adsorption and desorption phenomena were observed. In each process in which molecules migrate to the neighboring adsorption layer, the height of the free energy barrier was calculated using the PMF distribution. It was found that the PMF has a dominant influence on the molecular transport perpendicular to the wall and that the developed kinetic model can accurately predict the molecular mobility over the free energy barrier due to the adsorption and desorption process.