The interaction of colloidal polystyrene latex spheres with a critical binary liquid mixture

A systematic study of the global phase behavior of a colloidal system consisting of monodisperse, charge stabilized, polystyrene latex spheres (PLS) in a critical binary liquid mixtures at temperatures corresponding to one and two phase coexistence of the liquid mixture, has been made as a function many control parameters including: sphere concentration, PLS surface charge, sphere radius, and solvent composition of the binary mixture. Results for a mixture phase separated close to the critical temperature, $T\sb{c}$, demonstrate a strong partitioning of a majority of the spheres into a preferred phase, with a subsequent temperature dependent migration to the liquid/liquid interface as the temperature is quenched deeper into the two phase region. The migration phenomenon is interpreted in the context of a Cahn wetting transition on the basis of simple thermodynamic arguments. Additional features observed at high PLS concentrations at the liquid interface are also reported. At temperatures and compositions where there is one liquid phase, an aggregation of the PLS particles occurs for mixtures whose composition is rich in the non-preferred component and at temperatures below the liquid coexistence temperature. Data from experimental trajectories along the critical isopleth, and measurements of the single particle size, especially at temperatures where aggregation is observed, rule out flocculation mechanisms based on critical effects (1) or prewetting transitions as had previously been suggested (2). Capillary condensation phenomena are regarded as the most likely mechanism for sphere aggregation in the one phase region. Changes in the measured critical exponents for the bulk correlation length for samples containing fixed volume fractions of PLS particles are also reported.