Wetting of a selective solid surface by an asymmetric binary mixture.

We consider a lattice-gas model of an asymmetric binary mixture in which the attraction between a pair of molecules of species A exceeds that between a pair of molecules of species B. The interaction between two molecules of species A and B is chosen to promote the formation of demixed A-rich liquid bulk phases. Molecules interact with a selective solid wall, preferentially adsorbing molecules of species B. Positions of molecules are restricted to sites on a simple-cubic lattice. We invoke a mean-field representation of the Hamiltonian governing all intermolecular interactions and assume only nearest-neighbor attractions. Minimizing the grand-potential functional of the lattice gas numerically, phase diagrams for films wetting the solid substrate are obtained. One of our key findings concerns B-rich mixed or demixed films forming in the vicinity of the solid surface and coexisting with demixed A-rich films. The formation of B-rich films can be understood as a result of the competition between the asymmetry of the (bulk) mixture and the selectivity of the solid surface. The concentration of component B in B-rich mixed films shows a peculiar temperature dependence. It first increases with temperature T until an "inversion" temperature T(inv) is reached, and then declines for T>or=T(inv) until the critical point between (demixed) A- and B-rich films is reached.