Critical aggregation concentration in the PAMPS (10%)/DTAB system

Abstract

Polyelectrolyte/surfactant systems of opposed charge generally show a plateau in surface tension isotherms as a function of surfactant concentration. The initial plateau is generally associated with the CAC (critical aggregation concentration). In the case of the PAMPS (10%)/DTAB system the plateau is not clearly defined. In this work dynamic surface tension (DST), and static and dynamic Kerr effect measurements are presented. These results suggest a polymer/surfactant non-cooperative association at surfactant concentrations below the CAC. At a surfactant concentration not long before that 1 mM a co-operative aggregation between neighborhood surfactant seems to take place. We associate this concentration with the CAC. Evidence in the relationship between dynamic surface adsorption and the equivalent point of charge (EP) is presented. At the EP concentration a plateau in the DST curves appears. For adsorption times above this plateau it seems that the adsorption mechanism is controlled by polymer diffusion and for times below the plateau by a mixed activation–diffusion mechanism with an energy barrier between 2 and 11 RT. Sensitivity and reliability of electro-optic techniques for this kind of study are emphasized.