Interaction of dilute colloidal particles in a mixed solvent

Abstract

We have measured the second virial coefficient ${\mathit{B}}_{2}$ of very dilute colloidal dispersions of charge-stabilized polystyrene latex spheres in the one-phase region of the mixed solvent 2,6-lutidine plus water. These measurements were made as a function of temperature for two solvent compositions, both of which are richer in 2,6-lutidine than the binary liquid mixture's critical composition. The temperature ranges started deep in the one-phase region and approached the coexistence curve but did not penetrate the region of reversible aggregation near the coexistence curve. Very large, positive (repulsive-interaction) virial coefficients are observed at temperatures far from the aggregation zone and for calibration samples whose solvent is pure water. These large values of ${\mathit{B}}_{2}$ are impossible to model without invoking long-range repulsive interactions whose origin is difficult to explain. As the temperature is brought nearer to the aggregation zone, the virial coefficient plunges through zero to large negative (attractive-interaction) values. Crude modeling suggests that the observed changes in the interactions are not inconsistent with a temperature-dependent attraction arising from adsorption layer energetics operating at distances of a few solvent-fluctuation correlation lengths from the particle surfaces.