Polymer−Surfactant Interactions:  An Ultrasonic Relaxation Study

Polymer−surfactant interactions in water−polyvinylpyrrolidone−sodium dodecylsulfate systems have been investigated, at 25 °C, by ultrasonic relaxation methods as a function of polymer and surfactant concentration. At fixed polymer content, three regions, characterized by significant differences in their kinetic behavior, have been observed. In the first region the relaxation time increases with the surfactant content, in the second it is nearly constant, and in the third region it decreases with increasing surfactant concentration. Such behavior has been ascribed to the binding of the surfactant aggregates on the polymer backbone and to micelle formation above the saturation threshold. To account for the reported behavior, a kinetic model has been developed from the theory of ultrasonic relaxation in micellar solutions. Some thermodynamic quantities related to polymer−surfactant interactions, i.e. the Gibbs energy of binding per monomer, ΔG bind/〈m〉, where 〈m〉 is the number of monomers in polymer-bound micelles, and the volume change of binding of the surfactant to the polymer, ΔV bind, have been obtained from the data analysis.