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 (C12PS) and carboxybetaine (C12CB), their mixtures with the cationic dodecyltrimethylammonium bromide (C12TAB) anionic sodium dodecylsulfate (SDS) at silica–water interface has been studied by neutron reflection (NR). total adsorption, composition adsorbed layer, some structural information have obtained over a range concentrations from below critical micelle concentration (CMC) to about 30× mixed CMC. behavior considered in relation previously measured micellar equilibrium these bulk solutions air–water interface. C12CB adsorbs cooperatively close its CMC form an almost complete bilayer on own, whereas C12PS more weakly fragmented structure. Although SDS does not normally adsorb interface, strongly fractional compositions up 0.5. This cooperativity is lost when fraction rises above At this point, drops sharply, creating unusual maximum variation Neither increase nor subsequent decline results directly variations independently determined monomer solution. predominantly effect strong cooperative interaction, possibly accompanied partial segregation within followed charge repulsion surface. solution aggregation A–W are similar for C12CB, addition promotes opposite that C12PS, simply disrupts binding C12CB. Unlike SDS, surfactant C12TAB silica. It therefore coadsorbs SiO2–W either or C12PS. However, neither case there any pronounced and, even though presence might be expected favor generally unexpectedly low.