Local solubility of nonpolar molecules in the liquid-vapor interfaces of water and simple liquids

The local solubility Σ of nonpolar molecules at the liquid-vapor interface is examined for model systems of water and simple liquids. For Lennard-Jones (LJ) solutes in model water and those in the LJ solvent, Σ is obtained by the Widom test particle method as a function of the coordinate normal to the interface. In order to examine the effect of the solvent structure on Σ, we compare the dependence of Σ on a reduced coarse-grained density of the solvent ρcg⁎ for water with that for the LJ solvent under the condition that ε/T is common for the two systems, where ε is the LJ energy parameter between solute and solvent molecules and T is the temperature. We found that the solubility Σ of the LJ solute is higher in water than in the LJ solvent at any given ρcg⁎, i.e., structure of water has higher ability to dissolve the nonpolar molecule than that of simple liquids, not only in their liquid states but also in their interfacial regions. This is partly because the probability of forming a sufficiently large cavity is higher in water than in the LJ fluid, as confirmed by the calculation of the local solubility of the hard-sphere solute. In addition, it is found that variation of the probability of cavity formation with ρcg in water is very similar to that in the LJ solvent if ρcg is scaled with the bulk liquid density.