Partitioning of polystyrene latex spheres in immiscible critical liquid mixtures

We have made a systematic survey of the behavior of colloidal polystyrene latex spheres (PLS’s) in critical mixtures of 2,6-lutidine plus water at temperatures where the liquid mixture separates into two immiscible phases. By varying the particle size, particle-surface charge density, and global PLS’s concentration, we observe a robust and reversible set of phenomena that include the partitioning of the particles into a preferred phase at temperatures close to ${\mathit{T}}_{\mathit{c}}$ with a subsequent population of the interface between the liquid phases as the temperature is quenched deeper into the two-phase region. These results are shown to be in good qualitative agreement with thermodynamic models if the models are extended to include the singular temperature dependence of the particle-liquid and liquid-liquid interactions near the critical point. We also observe crystalline ordering of particles on the interface when the population of PLS’s on the liquid-liquid interface is high.