SURFACE VISCOELASTICITY IN MIXED POLYCATION ANIONIC SURFACTANT SYSTEMS STUDIED BY A SIMPLE TEST

The talc particle test, performed on mixed solutions of polyelectrolyt e and oppositely charged surfactant, was used to examine the changes i n rheology of the air/water interface of these solutions over a large range of compositions. The systems scrutinized consisted of sodium dod ecyl sulfate (SDS, concentration ranging from 0.005 to 0.5 g L(-1)) an d either the cationic cellulosic polymer, Polymer JR400, or the corres ponding hydrophobically modified polymer, Quatrisoft(TM) LM200, of con centration ranging from 0.001 to 1 g L(-1). Clear indications of visco elasticity at the air/water interface were obtained over a wide compos ition domain in both instances. Viscoelasticity provides evidence of a synergistic adsorption of the two components at the air/water interfa ce. When the SDS was increased sufficiently with respect to the polyel ectrolyte concentration, the viscoelasticity vanished, and the interfa ce became fluid; the fluidity recovery is an indication that the inter face is denuded of polyelectrolytes by growing competition from micell ar interfaces in solution.