Interaction between surfaces with solvophobicity or solvophilicity immersed in solvent: Effects due to addition of solvophobic or solvophilic solute

Integral equation theories with bridge functions incorporated in the closure equations are employed to analyze how the solvent-induced interaction between surfaces is influenced by solute addition to the solvent. The solvent particles interact through a hard-core plus attractive potential. The surfaces are solvophobic or solvophilic, and the solute has rather high solvophobicity or solvophilicity: A total of four combinations of the surface and solute properties are considered. The solute addition always leads to a downward shift, a shift in a more attractive direction, of the surface interaction (except at very small surface separations). The shift becomes more pronounced as the solute solvophobicity or solvophilicity increases and the solute concentration becomes higher. Overall, the solute effects are the smallest when the solute is neither solvophobic nor solvophilic. The physical origins of the shift are discussed in detail by relating the interaction to the structure of the solvent-solute mixture confined between two surfaces. (C) 2003 American Institute of Physics.