Light scattering method for the study of close range structure in coagulating dispersions of equal sized spherical particles

Abstract

A light scattering method is proposed for studies of close range structure in flocs. This involves an analysis of the changes in the light scattered from coagulating dilute dispersions of equal sized spherical particles. From studies of initial coagulation behaviour, the size of particles, strictly their hard core interaction diameter, and the radial distribution function relating to particle contact can be determined. An approximate insight can be gained into average cumulative co-ordination numbers of particles in growing flocs.Studies of the rapid flocculation of polystyrene particles, in the joint presence of nonionic dextran flocculant and simple electrolyte coagulant, indicate close packed structure at low ionic strength and “liquid” floc arrangements at higher electrolyte concentrations. This suggests a weakening of the hard core interaction on addition of electrolyte which is likely to be a consequence of shielding of the electrostatic repulsion between the particles. Prolonged storage of the latex particles at 60°C or adsorption of anionic surfactant can lead to a strengthening of hard core interactions.The method can be used to size spherical particles of non-uniform refractive index and has been exploited in measurements of the thickness of adsorbed polymer and surfactant layers. It has indicated that storage at 60°C can increase the size of polystyrene particles (of unswollen diameter 207 nm) by 4 ± 2 nm.