Spectroscopic Studies of Zwitterionic DDAPS at Planar and Droplet Oil/Water Interfaces.

Developing the knowledge on surfactant interfacial phenomena is highly valuable for the advancement of technological, commercial, and industrial products, as these applications often rely on interfacial and colloidal chemistry. Zwitterionic surfactants are a less toxic alternative to standard charged surfactants. With both positively charged quaternary ammonium and negatively charged sulfonate constituents, zwitterionic DDAPS can have diverse interfacial interactions with various coadditives. In this work, we investigate DDAPS adsorption to a planar oil/water interface and its stabilization of oil-in-water nanoemulsions. By studying both interfacial geometries with surface-specific, nonlinear spectroscopy, we gain deeper insights and a molecular perspective into DDAPS's behavior in the presence of various salts and cosurfactants. From an application standpoint, zwitterionic surfactants are often mixed with other chemicals or used in an environment with pre-existing chemicals (e.g., ocean water during oil remediation). Thus, it is important to understand how such coadditives alter DDAPS's behavior and its performance as an emulsifier. Our results show that DDAPS is nearly uninfluenced by coadditives at a planar oil/water interface, but the identical coadditives are crucial for DDAPS to form and stabilize nanoemulsions. Additionally, the surfactant packing properties vary between interfaces as well as coadditives, indicating that certain interactions with the DDAPS headgroup are stronger and play a greater role in tuning DDAPS's interfacial behavior.