Critical Incorporation Concentration of Surfactants Added to Micellar Solutions of Hydrophobically Modified Polyelectrolytes of the Same Charge

Abstract

Polymer−surfactant interactions in aqueous solutions of the surface-active copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(acrylic acid) and a homologous series of alkyl sulfates are studied. When surfactants of the same charge are added to copolymer solutions that already have micelles, the surfactant starts to incorporate into the existing polymer micelle at the critical incorporation concentration (CIC). Surface tension measurements determine the CIC as a local minimum in surface tension as a function of added surfactant. Fluorescence emission measurements using a selective fluorescent probe indicate that the surfactant incorporation is noncooperative. Because the initial incorporation at the CIC only involves a single surfactant entering a nonionic micelle, no counterion condensation is needed, making the tail-length dependence of the CIC similar to the tail-length dependence of the critical micelle concentration of nonionic surfactants.