Molecular-Dynamics Simulation of Relaxation Processes at Liquid–Gas Interfaces in Single- and Two-Component Lennard-Jones Systems

The formation of equilibrium liquid–gas interfaces in single- and two-component Lennard-Jones systems has been reproduced by molecular-dynamics simulation. The second component in the two-component system is a volatile impurity. The initial state is created by bringing in contact homogeneous liquid and gas phases having equal temperatures, pressures, and chemical potentials. The times required to establish equilibrium values of pressure, composition, shape and thickness of an interfacial layer, relative adsorption, and surface tension have been evaluated by the simulation. The calculations have been carried out at a temperature close to the triple point temperature of a solvent. It has been found that, in the course of relaxation, the maximum dynamic surface tension exceeds the equilibrium value by a factor of 1.2–1.6, while the relaxation time increases from 10 to 100 ns as the concentration of the volatile component in the solution grows to 0.25. In the two-component system with a limited volume of the gas phase, an equilibrium interfacial layer is formed in two stages. At the first stage, the volatile component is transferred into the interfacial layer from the near-surface regions of the liquid and gas phases. When an equilibrium partial density of the volatile component in the gas phase is achieved, the second stage begins, at which the surface layer is mainly supplied with liquid-phase particles. As a result, the relaxation times of relative adsorption and surface tension substantially increase. The role of the dynamic surface tension in the process of nucleation has been discussed.