Solvent mediated interactions close to fluid-fluid phase separation: microscopic treatment of bridging in a soft-core fluid.
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.
Archer
A J
AJ
H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom. Andrew.Archer@bristol.ac.uk
Evans
R
R
Roth
R
R
Oettel
M
M