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 Θ. At a CTAB concentration φ_c = 0.75 mM, comparable to the critical micelle concentration, a dilute NP monolayer is found with Θ ∼ 128°. At lower surfactant concentration (φ_c = 0.05 mM) we find a dense ML of close-packed NPs with an unexpectedly high Θ ∼ 146°. 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.