Aggregation in polystyrene-sphere suspensions in near-critical binary liquid mixtures

We study the reversible aggregation of polystyrene latex spheres (PLS) in one phase mixtures of 2,6 lutidine plus water at temperatures and solvent compositions near the solvent mixture's critical point. The aggregation occurs only on the side of the critical composition rich in the liquid component which is nonpreferred when the system is in the solvent's two-phase region. The liquid that is preferred can be changed by changing the surface charge density of the PLS. The aggregation region extends to temperatures well beyond the wetting temperature, and dynamic light scattering shows no evidence of a thick layer building up on individual particles before they join aggregates.