Anomalous Restabilization of Colloid-Polymer Mixture with Increasing Colloid Concentration

Polymers are often used to modulate interactions and, consequently, the stability of colloids, which depends on a range of parameters such as concentration, the nature of the polymer (adsorbing/nonadsorbing, molecular weight, charged/neutral, etc.), temperature, and so on, as has been well studied before. However, the effect of colloid concentration on the stability of colloid-polymer mixtures at high polymer concentrations has been little addressed. In this work, we studied the stability of colloidal silica (CS)-poly(vinyl alcohol) (PVA) mixtures as a function of CS concentration. It was found that in PVA dilute solutions, CS was well dispersed, and the system remained stable. However, above the PVA critical concentration (c > c*), flocculation or precipitation took place at lower CS concentrations. Counterintuitively, restabilization was achieved upon increasing CS concentration. The balance between steric repulsion from adsorbed PVA and depletion attraction from free PVA was identified as the key factor, which is determined by the PVA-CS interactions. This anomalous restabilization at high colloid concentrations may facilitate the preparation of high-filling polymer composites.