Solvent quality dependent continuum van der Waals attraction and phase behavior for colloids bearing nonuniform adsorbed polymer layers

A liquid-gel transition measured for a polymerically stabilized dispersion as a function of solvent quality and particle volume fraction is compared with theoretical predictions of phase behavior. The experimental liquid-gel transition is interpreted from rheological measurements of 360 nm polystyrene (PS) particles with adsorbed F 108 Pluronic (PEO-PPO-PEO) layers in aqueous 0.5 M NaCl as a function of temperature, which controls solvent quality for the adsorbed Pluronic. The measured liquid-gel transition occurs at temperatures when attractive interactions are not expected to occur from either core PS particle van der Waals forces or Pluronic mixing interactions. To consider an alternative temperature dependent attraction, an adhesive sphere (AS) phase diagram is constructed using a theoretical potential that includes the solvent quality dependent continuum van der Waals attraction due to the nonuniform dielectric properties of the adsorbed Pluronic. An impressive correspondence is found between the experimental liquid-gel transition and the theoretical AS percolation threshold with no adjustable parameters using the nonuniform film model. This work indicates conditions when adsorbed polymeric van der Waals interactions are critical for interpreting and predicting solvent quality dependent phase behavior in polymerically stabilized systems.