Henry's law, surface tension, and surface adsorption in dilute binary mixtures.

Equilibrium properties of dilute binary fluid mixtures are studied in two-phase states on the basis of a Helmholtz free energy including the gradient free energy. The solute partitioning between gas and liquid (Henry's law) and the surface tension change Delta gamma are discussed. A derivation of the Gibbs law Delta gamma = -T Gamma is given with Gamma being the surface adsorption. Calculated quantities include the derivatives dT(c)/dX and dp(c)/dX of the critical temperature and pressure with respect to the solute molar fraction X and the temperature-derivative (partial derivative gamma/partial derivative T)(cx,p) of the surface tension at fixed pressure p on the coexistence surface. Here (partial derivative gamma/partial derivative T)(cx,p) can be both positive and negative, depending on the solute molecular size and the solute-solvent interaction, and diverges on the azeptropic line. Explicit expressions are given for all these quantities in the van der Waals model.