Unveiling interfacial molarities: In situ probing of nanoemulsion interfaces stabilized by cationic and anionic surfactants via chemical trapping

Abstract

The properties and applications of nanoemulsions are critically governed by the interfacial composition at the oil–water interface. Conventional techniques, which primarily simulate interfacial behavior, exhibit notable limitations in capturing authentic molecular arrangements. In contrast, the chemical trapping (CT) method enables in situ probing of interfacial composition, offering molecular-level insights into nanoemulsion behavior. Here, sodium lauryl methyl taurine (SLMT) and cetyltrimethylammonium bromide (CTAB) nanoemulsions were prepared under different surfactant concentrations, oil fractions, homogenization methods (high-pressure homogenization vs. ultrasonication), and salt addition conditions. The emulsions were characterized by dynamic light scattering (DLS) and stability analysis, while their interfacial compositions were investigated using the CT method, providing molecular-level insights into droplet size and stability changes. The results revealed that changes in surfactant concentration and oil fraction directly impacted droplet size and emulsion stability, corresponding to shifts in interfacial molarities. Additionally, nanoemulsions produced via ultrasonication displayed interfacial properties comparable to those produced by high-pressure homogenization, validating ultrasonication as a scalable alternative. Notably, SLMT emulsions displayed ion-specific effects: tetrapropylammonium (TPA⁺) and tetraethylammonium (TEA⁺) ions formed ion pairs with sulfonate headgroups, displacing interfacial water, enhancing molecular packing, and reducing curvature, thereby increasing droplet size. These results establish a direct structure–property relationship between interfacial composition and nanoemulsion performance, providing a rational framework for designing functional nanoemulsions in food, cosmetics, and pharmaceutical applications.