Effect of surfactant-polymer association on the stabilities of foams and thin films: Sodium dodecyl sulfate and poly(vinyl pyrrolidone)

The foaming behavior of the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in the presence and in the absence of the nonionic polymer poly(vinylpyrrolidone) (PVP). A current model of surfactant-polymer aggregations in bulk solution and at the air/water interface is related to the foam and thin-film stabilities. Tensiometry, foaming tests, and a thin-film balance are used to obtain this relationship. It is found that, at very low surfactant concentrations, where the surfactants are present as unimers in the bulk solution, there is association between surfactants and polymer at the liquid/air surface, giving increased foam and thin-film stabilities as compared to cases for the same surfactant concentrations but without polymer. As the surfactants and polymers associate in the bulk solution, there is desorption of surfactants and polymers from the surface, rendering decreases in foam and thin-film stabilities. At higher surfactant concentrations, the bulk viscosity is significantly increased owing to the presence of both micelles and saturated micelle-polymer complexes. Also, the surfactant surface coverage at the liquid/air surface has reached its maximum value and is similar to that of SDS solution above the cmc when no polymer is present. Both the increased surface viscosity and the increased bulk viscosity contribute to the observed foam and film stabilities. In the thin-film studies, several stratification steps are observed, probably owing to micelles that are being pushed out of the film.