Nanoscale structure of surfactant-induced nanoparticle monolayers at the oil-water interface

Water-dispersed silica nanoparticles (NPs) do not adsorb to the interface between immiscible bulks of water and hexane. Adding, however, a surfactant (cetyltrimethylammonium bromide, CTAB) to water induces the formation of a NP monolayer (ML) at this model liquid-liquid interface. We determined the ML's structure in situ at deeply buried planar water-hexane interfaces with sub-nanometer resolution by high energy X-ray reflectivity. Detailed modeling of the data yields the NPs' interfacial concentration and water immersion depth, allowing calculation of the NP average contact angle Theta. At a CTAB concentration phi(c) = 0.75 mM, comparable to the critical micelle concentration, a dilute NP monolayer is found with Theta similar to 128 degrees. At lower surfactant concentration (phi(c) = 0.05 mM) we find a dense ML of close-packed NPs with an unexpectedly high Theta similar to 146 degrees. The structure and adsorption scenarios of the NPs at the interface are discussed, highlighting the relevance of the present method for quantitative studies of a broad range of systems and applications.