Morphology of Surfactant Mixtures at Solid/Liquid Interfaces: High-Speed AFM Observation

Abstract We characterized time-dependent morphological changes in cationic monomeric/gemini surfactant mixtures adsorbed on silica using high-speed atomic force microscopy (HS-AFM). The gemini surfactant employed in this study was 1,2-bis(dodecyldimethylammonio)ethane dibromide (12-2-12). This surfactant was mixed with a monomeric surfactant, hexadecyltrimethylammonium bromide (HTAB), in the presence of a background electrolyte. The HS-AFM measurements revealed that wormlike surface aggregates that formed in an HTAB solution transformed into a bilayer within a few hundred seconds after the HTAB solution was replaced by an HTAB + 12-2-12 mixed solution. This morphological change was reversed within a few hundred seconds after this solution was replaced by the initial HTAB solution. These time-dependent morphological changes were supported by simultaneous quartz crystal microbalance with dissipation monitoring (QCM-D) and ellipsometry measurements. The energy dissipation, monitored by QCM-D, changed reversibly in response to the solution exchanges.