Solute Kinetics at Liquid-Liquid Interfaces: A Molecular Dynamics Simulation

Abstract

A model of liquid-liquid interface (LLI) has been investigated by molecular dynamics (MD) technique. The model is made of two "Lennard-Jones" liquids. The interface has been characterized via thermodynamic properties: interfacial tension, density and pressure profiles.This model has then been used to study the kinetics of adsorption of a simple amphiphilic molecule at the LLI. The kinetics of the adsorption of this molecule at the interface is studied by generating trajectories starting in the bulk of one liquid. We tried to interpret the behaviour of the amphiphilic molecule as a diffusive motion in a potential well. To achieve this, we have calculated the mean force acting on the molecule fixed at a given distance from the interface. The potential of mean force deduced from this calculation has then been used in a Langevin simulation. The agreement of the MD and Langevin approaches tend to support the idea of a diffusion controlled mechanism for the adsorption.A similar study has been performed for the transfer of a solute across the interface. In this case a modified Lennard-Jones potential has been used to model a solute particle solvated in the two liquids. In order to evaluate the rate of transfer of the solute through the LLI we calculated the free energy barrier height. As in the case of the amphiphile, the mean force acting on the solute particle is plotted as a function of the distance to the average position of the interface. The potential of mean force deduced from this calculation gives the free energy variation for a solute crossing the interface. The profile of mean potential energy has also been calculated and compared to the potential of mean force. The comparison of the two profiles show that in this case the value of mean potential energy is a good approximation to evaluate the barrier height.