Preferential Behaviors of Fluorinated Surface-Active Molecules at Liquid-Liquid Interfaces

Abstract

Contamination of per- and polyfluoroalkyl substances (PFAS) in water supplies will continue to have serious health and environmental consequences. Despite the importance of detecting the presence of PFAS at potential sites of contamination and at treatment plants, there are few scalable techniques that provide the necessary selectivity to distinguish fluorinated surfactants (such as PFAS) from other surface-active components and the sensitivity to quantify the low concentration often present in real-world conditions. Existing gold standards—namely liquid chromatography electrospray tandem mass spectrometry (LC-MS/MS)—can provide ultra- low limits of detection and superb selectivity. However, such methods require well-equipped laboratory with well-trained specialists. Thus, these methods are cost-prohibitive for large scale efforts to fully map the presence of PFAS in water production wells or for regular monitoring of the effectiveness of treatment plans. This thesis explores an alternative method of detecting PFAS by evaluating their behaviors at multiple liquid-liquid interfaces. Chapter 1 provides an introduction to PFAS and their surface-active properties, as well as the current method of detection. It also introduces the techniques used to measure interfacial energy and their sensing applications. Chapter 2 characterizes the preferential interfacial behaviors of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at the hydrocarbon oil-water (H/W) and fluorocarbon oil-water (F/W) interfaces. Sodium dodecyl sulfate (SDS) and Capstone FS-30 were used as model systems for hydrocarbon and fluorocarbon surfactants, respectively. The key highlights are (1) PFAS and fluorocarbon surfactants lower the interfacial tension at the F/W interface more readily than at the H/W interface and (2) by measuring the interfacial tensions at both F/W and H/W interfaces, it is possible to distinguish between fluorocarbon and hydrocarbon surfactants. Chapter 3 summarized experimental details and supporting information.