Phase Separation and Gel-like Structures in Mixtures of Colloids and Surfactant

A colloidal suspension becomes unstable when a sufficient amount of non-ionic surfactant is added, undergoing a segregation process ascribable to depletion forces induced by the surfactant micelles. The concentrated phase which separates out is, however, less compact than theoretically expected and shows stable gel-like features. This anomaly can be imputed to a dynamic freezing of the segregation process due to the interconnection of transient particle clusters. The results are compared with the predictions for the percolation threshold of Baxter adhesive hard-sphere model. Small-angle light scattering measurements show that the "gel" phase is fractally organized over large spatial scales.