Effect of steric, double-layer, and depletion interactions on the stability of colloids in systems containing a...

Experiments carried out by Stenkamp et al. [Stenkamp, V. S.; McGuiggan, P.; Berg, J. C. Langmuir 2001, 17, 637.] have shown that polystyrene latexes can be restabilized at sufficiently high electrolyte concentrations in the presence of an amphiphilic block copolymer [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)] At even higher electrolyte concentrations, the systems can again be destabilized. The present paper attempts to explain the restabilization through the dominance of steric interactions and the destabilization through the dominance of depletion interactions. Because of salting out, as the concentration of electrolyte increases, the polymer molecules are increasingly precipitated onto the surface of the latex particles and, at sufficiently high electrolyte concentrations, form, in addition, aggregates. The precipitation onto the latex particles generates steric repulsion, which is responsible for the restabilization, whereas the formation of aggregates generates depletion interactions, which are responsible for destabilization.