Polyelectrolyte−Micelle Coacervation:  Effects of Micelle Surface Charge Density, Polymer Molecular Weight, and Polymer/Surfactant Ratio

Abstract

The effects of micelle charge density, polymer molecular weight, and polymer-to-surfactant ratio on coacervation were studied by turbidity, dynamic light scattering, and electrophoretic mobility in the system composed of the strong cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and oppositely charged mixed micelles of Triton X-100 (TX100) and sodium dodecyl sulfate (SDS). Phase boundaries in the range of SDS mole fraction from 0.30 to 0.50 and in the range of polymer molecular weight from 8.2 × 10 3 to 4.28 × 10 5 were obtained, and coacervate volume fraction as a function of polymer molecular weight was subsequently determined. Three-dimensional phase boundaries were used to represent the effects on coacervation of micelle surface charge density, polymer molecular weight, and PDADMAC-to-SDS ratio. The coacervation region is seen to increase with micelle surface charge density and polymer molecular weight (MW). Both higher and lower polyelectrolyte-to-surfactant ratio can suppress coacervation. An increase in MW reduces the micelle charge required for coacervation and also increases coacervate volume fraction. Coacervation is found to occur when the following conditions are satisfied: the electrophoretic mobility is close to zero, and the size of polyelectrolyte−micelle complex is at least about 45 nm.