  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 $d T_c/dX$ and $d
p_c/dX$ of the critical temperature and pressure with respect to the solute
molar fraction $X$ and the temperature-derivative $(\p \gamma/\p T)_{{\rm
cx},p}$ of the surface tension at fixed pressure $p$ on the coexistence
surface. Here $(\p \gamma/\p T)_{{\rm cx},p}$ 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.
