Self-Consistent Field Analysis of Segregative Aqueous Dextran─Poly(ethylene glycol) Solutions: (3) Polymer-Induced Loop-to-Bridge and Capillary Bridge Forces.

When colloidal particles are added to a two-phase system such that the minority phase forms capillary bridges between the particles, then─above the percolation threshold─a strong gel is formed. We focus on an aqueous two-phase system (ATPS) composed of dextran-water-poly(ethylene glycol) (PEG) with dispersed silica particles that are preferentially wetted by the minority PEG-rich phase. This ATPS is characterized by an ultralow interfacial tension between the PEG-rich and dextran-rich phases, especially when the water content (control parameter) in the phases is high. The Scheutjens-Fleer version of self-consistent field (SF-SCF) theory is used to investigate these systems. We focus on the (equilibrium) force-distance relations between the particles using a molecularly detailed model with parameters established by previous works and pay attention to the effects of particle size, the interfacial tension, the Laplace pressure, and the contact angle. The interparticle interactions appear to be strongly dominated by proximal polymer-induced loop-to-bridge attractive forces, and the capillary contributions that dominate at larger interparticle distances, are, as expected, comparatively weak. The three-gradient SCF results indicate that the pairwise additivity approximation, usually assumed in coarse-grained computer simulations, may be compromised because capillary bridges, especially when the contact angle vanishes, can easily merge. As an illustration we present a three-gradient SCF result for a capillary suspension.