Nucleation of Droplets in a Binary Mixture

We revisit the theory of condensation of a droplet in a vapour with the aim of finding the effect that the surface tension has on the phase diagram of a binary mixture. For that purpose we consider condensation under volume control and rederive the Gibbs phase rule. The Gibbs phase rule is equivalent to the ‘common tangent construction’ for two free enthalpies corresponding to different pressures: in this case, the pressure in the vapour and the pressure in the droplet. Explicit results are calculated for a droplet mixed from incompressible liquids and for a vapour that is an ideal gas mixture. It turns out that in a certain range of volumes the equilibrium state of droplet and vapour has a higher free energy than the vapour alone. At the lower bound of that range of volumes a stable droplet of finite size will nucleate and consequently the vapour pressure will drop. The usual condensation line in a (p, X1)-phase diagram without surface tension is thus replaced by two lines: One for the incipient condensation and one for its completion; both lie above the condensation line without surface tension.