Surface viscoelasticity in mixed polycation anionic surfactant systems studied by a simple test

The talc particle test, performed on mixed solutions of polyelectrolyte and oppositely charged surfactant, was used to examine the changes in rheology of the air/water interface of these solutions over a large range of compositions. The systems scrutinized consisted of sodium dodecyl sulfate (SDS, concentration ranging from 0.005 to 0.5 g L(-1)) and either the cationic cellulosic polymer, Polymer JR400, or the corresponding hydrophobically modified polymer, Quatrisoft(TM) LM200, of concentration ranging from 0.001 to 1 g L(-1). Clear indications of viscoelasticity at the air/water interface were obtained over a wide composition domain in both instances. Viscoelasticity provides evidence of a synergistic adsorption of the two components at the air/water interface. When the SDS was increased sufficiently with respect to the polyelectrolyte concentration, the viscoelasticity vanished, and the interface became fluid; the fluidity recovery is an indication that the interface is denuded of polyelectrolytes by growing competition from micellar interfaces in solution.