THE FREE-ENERGY OF A BINARY MIXTURE NEAR A SURFACE WITH APPLICATIONS TO SURFACE ENRICHMENT

An expression for the free energy of a binary mixture near a surface has been derived by a method analogous to Debye's molecular-based model for unbounded mixtures (J. Chem. Phys. 31, 680 (1959)). This new expression is valid when the wall interaction is relatively long-ranged. The expression has terms containing first and second derivatives of the concentration at the surface. Our form is different from the expression for the case of short-range wall interactions which was derived by Cohen and Muthukumar (J. Chem. Phys. 90, 5749 (1989)). We report an explicit expression for the wall interaction term. In addition, a more general equilibrium boundary condition for the surface enrichment problem has been derived. © 1992.