Understanding the structure of surfactants at the water/oil interface

APEs (alkyl phenol ethoxylates) are versatile and cost-effective surfactants widely used in a variety of consumer and industrial applications, including detergent and cleaners, agrochemical, and emulsion polymerization. The historic annual market size of NPE (nonylphenol ethoxylate) surfactants is over 1 billion pounds worldwide. NPE surfactants and their breakdown products, notably nonylphenol, however, can harm aquatic animals and plants and are resistant to natural degradation. The environmental and health risks have triggered restrictions of APE use in many applications. Great effort has been devoted to identifying and developing alternative surfactants to replace APEs.  Despite the great progress and success, it is challenging to match the high performance of NPEs with alternative surfactants in some applications. The objective of this study is to gain more fundamental understanding on the unique performance of NPE surfactants in stabilizing emulsions. We prepare stable swollen micelles by solubilizing oil into NPE micelles. Then we use dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) to study the molecular structure of nonylphenol ethoxylate surfactants at the water/oil interface. The high-brilliance X-ray beamlines enable detailed structural modeling of high-quality data from the micelle solutions. The insights on the interfacial structure from this work will help us understand how the NPE structure translates into interfacial properties such as interfacial tension, critical micelle concentration and emulsion stability.