Self-Consistent Field Analysis of Segregative Aqueous Dextran-Polyethylene Glycol Solutions: (2) Adsorption and Wetting.

Aqueous two-phase systems (ATPS), with dextran-water-poly(ethylene glycol) (PEG) as the main example, are water-continuous polymer segregated systems. How such ATPS behaves near solid interfaces is studied using the Scheutjens Fleer self-consistent field (SF-SCF) theory. We have analyzed the adsorption isotherms of the minority (wetting) component PEG at either a fixed solvent- or a fixed dextran bulk volume fraction and focused on wetting and polymer displacement transitions. When the driving force for segregative phase behavior is sufficiently strong (repulsive interactions between dextran and PEG: the major driving force), a jump-like transition displacing adsorbed dextran by PEG may take the form of a prewetting transition when the PEG-rich phase wets the surface. Interestingly, the first-order displacement transition manifests as a pure surface phase transition at supercritical conditions. This explains why the wetting transition is robust first order. Only when the segregative phase behavior is triggered by a solvent quality disparity (minor driving force), one may find isotherms with two consecutive transitions, that is, a smooth displacement transition followed by a jump-like prewetting transition. Explained by the low interfacial tension between the PEG-rich and dextran-rich phases, the parameter window for partial wetting is small. Remarkably, for an experimentally realistic ATPS and carefully tuned adsorption parameters, it is possible that upon a change of the solvent content there is a sequence of two wetting transitions (going from partial wet to wet and back to partial wet) followed by a drying transition.