Competitive adsorption at the air-water interface from a self-assembling polymer-surfactant mixture

The relationship between polymer-surfactant aggregation in bulk fluid and adsorption at the air-water interface is investigated in aqueous solutions of sodium dodecyl sulfate (SDS) and Luvitec VA 64 (a random copolymer of vinyl pyrrolidone and vinyl acetate). The polymer exhibits strong interaction with SDS and significant surface activity. The free energy of micellization of SDS in a solution containing 0.1% w/v polymer is reduced by approximately 2.3 RT. The addition of Luvitec VA 64 to a solution of SDS above the critical micelle concentration (CMC) results in an increase in surface tension from 40 dyn/cm to 47 dyn/cm. Neutron reflectivity measurements show that the change in surface tension is accompanied by a substantial (greater than 40%) reduction in the volume fraction of SDS at the air-water interface and concomitant adsorption of polymer. The transformation is driven by the decrease in concentration of SDS from the CMC to the critical aggregation concentration (CAC), consistent with the reduction in free energy of micellization.