Adsorption isotherms for dilute solutions via the mean force method

Abstract

The adsorption of solutes from a dilute liquid solution is of great technical importance but calculations of the local density of the solute and of the adsorption isotherm by standard molecular simulation yield large scattering with increasing dilution. As alternative the mean force (MF) method was suggested where the MF on a constrained solute molecule is integrated over a path from the bulk fluid to the wall. It has already been shown that the MF method gives reliable results for the relative local density, even at high dilution. Here, an extension of this method is introduced, where the absolute value of the bulk density is determined by particle balance. Thus, it is possible to calculate adsorption isotherms from the Henry regime to any finite concentration. Molecular dynamics simulations for the local density and the adsorption isotherm were performed for a model solution consisting of tetrahedral Lennard–Jones (LJ) solvent and linear LJ solute molecules in contact with a plane wall. It is found that the MF-results show less scattering than the results from standard simulations. Moreover, results for the orientation and the selectivity are given.