Specific ion-dependent attraction and phase behavior of polymer-coated colloids

This paper presents results demonstrating the role of temperature and specific ions in mediating attraction between polymer-coated colloids and determining their equilibrium phase behavior. In particular, theoretical predictions of continuum van der Waals attraction between poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)-coated polystyrene colloids are used to explain measured temperature and specific ion-dependent fluid-gel transitions in dispersions of these particles. Building on previous studies of PEO-PPO-PEO-coated polystyrene colloids dispersed in aqueous NaCl media, this work reports rheologically measured fluid-gel transitions as a function of temperature and NaCl/MgSO4 composition. Adhesive-sphere predictions of percolation thresholds are fit to measured fluid-gel data by allowing the adsorbed copolymer layer thickness as a single adjustable parameter. This allows the attraction between the PEO-PPO-PEO layers to be interpreted as a function of temperature and NaCl/MgSO4 composition. Quantitative predictions of a polymeric van der Waals attraction associated with the layer collapse in diminishing solvent conditions provides a simple mechanism for explaining the measured fluid-gel data as a dynamic percolation transition. Ultimately, this work identifies the importance of continuum polymeric van der Waals attraction for explaining specific ion-dependent phenomena.