Cationic polymer/anionic surfactant interactions☆

The interaction of a water-soluble cationic substituted cellulose ether with sodium dodecyl sulfate (SDS) has been examined through a study of precipitation patterns and measurements of surface tension, foaming, and electrophoretic mobility. In addition, the effect of this polycation in the subsolution underlying a spread monolayer of sodium docosyl sulfate (SDCS) has been investigated using force/area measurements. Addition of SDS to the polymer in solution results in progressive reduction of both its solubility and electrophoretic mobility, with maximum precipitation being close to the point of zero mobility. Increasing the surfactant concentration beyond this point results in resolubilization of the precipitate and increasing electrophoretic mobility in the opposite sense. Surface tension studies show that addition of the polymer, itself weakly surface active at the air/water interface, can result in large reductions in the surface tension of an SDS solution. Correspondingly, substantial increases in foam stability can result. The presence of the polymer in the subsolution results in penetration/adsorption into a monolayer of SDCS and a pronounced change in its force/area characteristics. The results are interpreted in terms of a highly surface-active polymer/surfactant complex resulting from “head to head” adsorption of the surfactant onto the polymer. Resolubilization occurs by “tail to tail” adsorption of a second layer of surfactant ions to form a polyanion.