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

TL;DR

This paper uses density functional theory to analyze how solvent-mediated interactions, especially bridging phenomena, occur near fluid-fluid phase separation in a soft core fluid, revealing long-range attractions and force discontinuities.

Contribution

It provides a microscopic treatment of bridging in a soft core fluid near phase separation, highlighting the role of the bridge function in correlations.

Findings

Bridging occurs as wetting films join, forming a fluid bridge.

The potential between big particles becomes long ranged and attractive.

Bridging causes a discontinuity in the solvent-mediated force.

Abstract

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 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 non-zero 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.