Dynamic and thermodynamic aspects of polymer-surfactant interactions: The SDS-PVP-water system

The water-PVP (polyvinylpyrrolidone)-SDS (sodium dodecylsulfate) system has been investigated by ultrasonic relaxation measurements. In water-PVP mixtures the relaxation time is independent on the polymer content, whereas in water-SDS solutions it decreases on increasing the amount of surfactant in micellar form. In the ternary water-PVP-SDS systems, at fixed PVP content, three different regions have been observed on increasing the amount of SDS: namely, at very low surfactant content the relaxation time, tau(1), increases then remains nearly constant and finally decreases. The above behaviour has been ascribed to surfactant binding onto the polymer and to micelle formation, respectively. By analysing the dependence of relaxation time(s) on composition, it was possible to divide the investigated region in three domains (i.e. non interaction region, polymer-surfactant interaction region and micellar region). A phase diagram for the above system was drawn according to the experimental kinetic results. From the data obtained by ultrasonic relaxation experiments the number of SDS molecules bound to PVP, [m], was evaluated and the Gibbs energy of binding, [Delta G(bind)/[m]], calculated.