Weak aggregation of non-ionic surfactant stabilised dispersions

Abstract

An equilibrium treatment based on a form of the second Virial coefficient is used to describe the aggregation state of an unsheared dispersion. The important parameters are particle concentration, particle size and interparticle attraction. Highly (number) concentrated dispersions of large particles are found to require, relatively, the least attraction to bring about a given state of aggregation. Quantitative predictions are expressed in the form of a ‘phase diagram’ showing regions of stability, instability and weak aggregation. The approach has been used to infer effective adsorbed layer thickness from the aggregation data for a series of octaoxyethylene glycol dodecyl ether (C₁₂E₈) stabilised polystyrene latex dispersions of different sizes at different temperatures. At temperatures well below the cloud point of the surfactant the existence of repulsion forces significantly beyond the physical reach of the adsorbed molecules is indicated. Nearer to the cloud point, the range of the repulsion is more consistent with the dimensions of the surfactant molecule.