Unusual Maximum in the Adsorption of Aqueous Surfactant Mixtures: Neutron Reflectometry of Mixtures of Zwitterionic and Ionic Surfactants at the Silica-Aqueous Interface.

The adsorption of two zwitterionic surfactants, dodecyldimethylammonium propanesulfonate (CPS) and dodecyldimethylammonium carboxybetaine (CCB), and of their mixtures with the cationic dodecyltrimethylammonium bromide (CTAB) and the anionic sodium dodecylsulfate (SDS) at the silica-water interface has been studied by neutron reflection (NR). The total adsorption, the composition of the adsorbed layer, and some structural information have been obtained over a range of concentrations from below the critical micelle concentration (CMC) to about 30× the mixed CMC. The adsorption behavior has been considered in relation to the previously measured micellar equilibrium of these mixtures in their bulk solutions and their adsorption at the air-water interface. CCB adsorbs cooperatively close to its CMC to form an almost complete bilayer on its own, whereas CPS adsorbs more weakly in a fragmented bilayer structure. Although SDS does not normally adsorb at the silica-water interface, SDS adsorbs strongly and cooperatively with CPS at fractional SDS compositions up to about 0.5. This cooperativity is lost when the adsorbed fraction of SDS rises above about 0.5. At this point, adsorption drops sharply, creating an unusual maximum in the variation of adsorption with a total concentration above the mixed CMC. Neither the increase in cooperativity nor the subsequent decline in adsorption results directly from variations of the independently determined monomer concentrations in the bulk solution. The adsorption maximum is predominantly the effect of strong cooperative interaction, possibly accompanied by partial segregation of SDS within the layer, followed by charge repulsion from the surface. Although the solution aggregation and adsorption at the A-W interface are similar for SDS with CCB, the addition of SDS to CCB at the silica-water interface promotes the opposite behavior to that of SDS with CPS, and SDS simply disrupts the cooperative binding of CCB. Unlike SDS, the cationic surfactant CTAB adsorbs on silica. It therefore coadsorbs at the SiO-W interface with either CCB or CPS. However, in neither case is there any pronounced cooperativity and, even though the presence of CTAB might be expected to favor adsorption, the adsorption is generally unexpectedly low.