Diffusion at the liquid-vapor interface

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 latter 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.

Intrinsic profiles and the structure of liquid surfaces

“…The physical relevance of the outmost liquid layer may be understood using molecular dynamics simulations [30], namely, through the analysis of the surface kinetics [31] and hydrodynamics [32]. It is found that the rate at which the list of molecules identified as surface pivots is renewed shows a minimum at the optimal value of n s .…”

“…The advantages and drawbacks of these techniques have been discussed in a recent review [28]. The ISM, including its extension to complex liquids [29], surface kinetics [31] and hydrodynamics [32], is computationally more demanding than others, but gives the most comprehensive view of the liquid surfaces.…”

Reactivity and Dynamics at Liquid Interfaces

“…For example, the typical time required for a Lennard-Jones atom to traverse a molecular diameter is similar to its surface residence time. 213 Thus, the validity of using the asymptotic behavior of the parallel and perpendicular MSDs breaks down and, in particular,…”

“…[214][215][216] Berne and coworkers proposed a method to overcome this problem. 217 The diffusion coefficient parallel to the interface is determined from the MSD and from the An approach was reported by Duque et al 213 for calculating the diffusion coefficient perpendicular to the interface. It takes into account the finite residence time (by employing a solution to a stochastic differential equation), but avoids using slabs and considers the molecule positions relative to the intrinsic surface.…”

Nonequilibrium molecular dynamics simulation of diffusion at the liquid-liquid interface

“…15 Another method proposed by Liu et al, 16 also applied to a liquid-vapour system, creates a set of virtual boundary conditions and computes the survival probability of a molecule in a region of interest. An alternative approach was proposed by Duque et al, 17 in which the intrinsic sampling method is applied to analyse the dynamical process at the liquid-vapour interface by tracking the displacements of the surface molecules. All these approaches are based on an equilibrium description of the system.…”