Effects of Salt-Induced Charge Screening on AOT Adsorption to the Planar and Nanoemulsion Oil-Water Interfaces

Nanoemulsions, nanosized droplets of oil, are easily stabilized by interfacial electric fields from the adsorption of ionic surfactants. While mean-field theories can be used to describe the impact of these interfacial fields on droplet stability, the influence of these fields on the adsorption properties of ionic surfactants is not well-understood. In this work, we study the adsorption of the surfactant sodium dioctyl sulfosuccinate (AOT) at the nanoemulsion and planar oil-water interfaces and investigate how salt-induced charge-screening affects AOT adsorption. In the absence of salt, vibrational sum-frequency scattering spectroscopy measurements reveal the ΔG_ads and the maximum surface density is the same for AOT at the hexadecane nanoemulsion surface as at the planar hexadecane-H₂O interface. Upon the addition of NaCl, an increase in AOT surface density is detected at both interfaces, indicating that repulsive electrostatic interactions between AOT monomers are the dominant force limiting surfactant adsorption at both interfaces. The bulky alkyl chains of AOT molecules cause our observations in this study to differ from those found in previous studies investigating the adsorption of linear-chain ionic surfactants to the nanoemulsion surface. These results provide necessary information for understanding factors limiting the adsorption of ionic surfactants to nanodroplet surfaces and highlight the need for further studies into the adsorption properties of more complex macromolecules at nanoemulsion surfaces.