Dilational rheology of polymer/surfactant mixtures at water/hexane interface

The present work is devoted to the interaction in mixed solutions of the cationic polyelectrolyte PAH and the counter-charged anionic surfactant SDS at the water/oil interface. For this purpose interfacial tension and dilational rheology studies were performed to describe the formation of complexes of PAH and SDS. The dilational elasticity values depend on the concentrations of surfactant and polyelectrolyte and their mixing ratio. As expected the concentration dependence of the dilational elasticities of SDS and PAH-SDS mixtures are similar in the SDS concentration range 10−5 M till 10−3 M (at a fixed amount of PAH) and have a maximum which correlates with an observed minimum in the interfacial tension isotherm. The sharp decrease of the dilational elasticity values in a narrow range is presumably due to the destruction of two-dimensional rigid structures and the formation of microaggregates in the interfacial layer. From dilational viscosity measurements as a function of oscillation frequency one can conclude that with increasing surfactant concentration the SDS dominates in the interfacial adsorption layer whereas the polyelectrolyte–surfactant complexes remain in the bulk phase.