Evaporation of Lennard-Jones Monomer-Dimer Mixtures

Evaporation and condensation are important physicochemical processes for the conversion between liquids and gases. We use Lennard-Jones (LJ) liquids as model systems to study the nonequilibrium physics of evaporation. The evaporation of pure LJ fluids has been studied extensively. However, the studies on evaporating liquid mixtures are limited and the relevant physics is poorly understood. Here we employ molecular dynamics simulations to study the evaporation behavior of LJ monomer-dimer mixtures. During fast evaporation, the concentration of LJ dimers, which are less volatile, increases significantly from the bulk of the mixture to the evaporating liquid-vapor interface. Correspondingly, the concentration of LJ monomers exhibits a negative gradient in the same region. In other words, the LJ monomers and dimers stratify in the region below the evaporating interface. Furthermore, the degree of stratification is stronger at higher evaporation rates. The effect of evaporative cooling on stratification is also clarified on the basis of thermophoresis. This stratification phenomena may be utilized to separate suspended particles interacting differently with the components of an evaporating liquid mixture.