Convective instability mechanism for a binary mixture at a vertical surface

It is shown that the mechanical equilibrium state of a two-component medium (e.g., salt sea water) in a gravity field can be unstable even in the case of indefinitely strong hydrostatic stability of this medium due to different boundary conditions for two substances (temperature and admixture concentration) at vertical boundaries. Instability criteria for long-wave perturbations at a vertical surface and in a vertical layer are obtained in analytic form.