Diffusion at the liquid-vapor interface

TL;DR

This paper uses the intrinsic sampling method to analyze molecular diffusion and residence times at the liquid-vapor interface, revealing the importance of exchange processes alongside diffusion in surface dynamics.

Contribution

It applies the intrinsic sampling method to study dynamical processes at the liquid-vapor interface, providing detailed diffusion and residence time data for Lennard-Jones fluids and alkali metals.

Findings

Diffusion coefficients are computed for surface molecules.

Residence and permanence times are quantified.

Exchange processes are as crucial as diffusion for surface dynamics.

Abstract

Recently, the intrinsic sampling method has been developed in order to obtain, from molecular simulations, the intrinsic structure of the liquid-vapor interface that is presupposed in the classical capillary wave theory. Our purpose here is to study dynamical processes at the liquid-vapor interface, since this method allows tracking down and analyzing the movement of surface molecules, thus providing, with great accuracy, dynamical information on molecules that are "at" the interface. We present results for the coefficients for diffusion parallel and perpendicular to the liquid-vapor interface of the Lennard-Jones fluid, as well as other time and length parameters that characterize the diffusion process in this system. We also obtain statistics of permanence and residence time. The generality of our results is tested by varying the system size and the temperature; for the later case, an existing model for alkali metals is also considered. Our main conclusion is that, even if diffusion coefficients can still be computed, the turnover processes, by which molecules enter and leave the intrinsic surface, are as important as diffusion. For example, the typical time required for a molecule to traverse a molecular diameter is very similar to its residence time at the surface.