Solvent mediated interactions close to fluid-fluid phase separation:Microscopic treatment of bridging in a soft-core fluid

摘要

Using density functional theory we calculate the density profiles of a binary solvent adsorbed around a pair of big solute particles.All species interact via repulsive Gaussian potentials.The solvent exhibits fluid-fluid phase separation,and for thermodynamic states near to coexistence the big particles can be surrounded by a thick adsorbed "wetting" film of the coexisting solvent phase.On reducing the separation between the two big particles we find there can be a "bridging" transition as the wetting films join to form a fluid bridge.The effective (solvent mediated) potential between the two big particles becomes long ranged and strongly attractive in the bridged configuration.Within our mean-field treatment the bridging transition results in a discontinuity in the solvent mediated force.We demonstrate that accounting for the phenomenon of bridging requires the presence of a nonzero bridge function in the correlations between the solute particles when our model fluid is described within a full mixture theory based upon the Ornstein-Zernike equations.