Relaxation processes at liquid-gas interfaces in one- and two-component Lennard-Jones systems: Molecular dynamics simulation

Abstract

The process of formation of an equilibrium liquid-gas interface, beginning with a sharp stepwise interfacial surface between phases with equal pressures, temperatures and chemical potentials, has been reproduced in molecular dynamics (MD) simulation of one- and two-component Lennard-Jones systems. In binary mixtures, the second component was a volatile impurity that is adsorbed in the interfacial layer. The tasks of this study are quantitative estimations of the establishment time of mechanical equilibrium between phases, the magnitude of the non-equilibrium surface tension and the time of its relaxation, the characteristic times of achieving equilibrium values of the composition, the shape and the thickness of the interface, as well as the effect of the volatile component on the relaxation processes in liquid-gas interfaces and nucleation in a superheated solution. Time dependences of the surface tension, the thickness of the interfacial layer and the relative adsorption of the volatile component have been obtained, and the times of their relaxation have been determined for a temperature close to the temperature of the solvent triple point, at the concentration range of the volatile component in the liquid phase cl from 0 to 0.25 mol fraction.It is shown that the maximum value of the non-equilibrium surface tension exceeds the equilibrium value 1.2 to 1.6 times. The relaxation time of the surface tension to equilibrium increases from 10 to 100 ps with an increase in cl. In a two-component system with a limited volume of the gas phase an equilibrium interfacial layer forms in two stages. At the first stage the volatile component is transferred into the interfacial layer from the near-surface regions of the gas phase and liquid phase. While achieving the equilibrium partial density of the volatile component in the gas phase, a transition to the second stage takes place, when particles are mainly transferred from the liquid phase, which leads to a considerable increase in the relaxation time of the relative adsorption and surface tension.Comparison of the data obtained with the results of an MD study of nucleation in solution shows that the time for establishment of adsorption equilibrium in a liquid-gas interfacial layer exceeds by an order of magnitude the time in which the equilibrium composition and the size of a vapor-gas bubble are established. This can lead to the fact that when the top of the activation barrier of nucleation is passed, the surface tension at the bubble-solution interface can exceed its equilibrium value.