Interaction Of Dilute Colloidal Particles In A Mixed Solvent

Abstract

IntroductionColloidal particles in mixed solvents can show reversible aggregation in the one-phase regime of the mixture near the mixture's phase separation temperature [1–5]. This aggregation condition has been shown to be related to the affinity of the colloidal surfaces for one of the solvent components. In particular, for a 2,6 lutidine plus water (LW) mixture with colloidally dispersed polystyrene latex spheres (PLS) in a temperature range near the critical temperature, Tc, in the mixture's two-phase region, the particles will partition into one of the solvent phases, with the meniscus between the liquid phases clear to the eye and showing no sign of population by colloidal particles. Which phase of the solvent attracts the particles depends on the surface charge density of the particles, with high surface charge density particles preferring the water-rich phase and low charge density particles preferring the lutidine rich phase. As temperature is advanced deeper into the two-phase region (all effects discussed here are equilibrium effects), there is a temperature, Tw, at which particles appear on the meniscus (most particles remain in the preferred phase, whose population depletion is too small to measure). Tw changes with the surface charge density of the particles [4], but not with radius or with number density of the particles in the sample. The aggregation observed in the one-phase region [5] is then restricted to the side of the solvent's coexistence curve poor in the component which is rich in the partitioning-favored phase.