Multiscale Investigation of Surfactant-Driven Fuel Resistance

Abstract

Aqueous film-forming foams (AFFF) are widely recognized for their exceptional performance in suppressing flammable liquid fires. Their effectiveness stems from the unique ability of fluorinated surfactants, including per- and polyfluoroalkyl substances (PFAS), to rapidly spread across fuel surfaces and form stable barriers that prevent vapor release. However, increasing evidence of PFAS toxicity, persistence, and environmental accumulation has led to growing regulatory and societal pressure to eliminate these compounds from firefighting formulations. As the fire protection community transitions toward fluorine-free alternatives, a central challenge emerges: how to develop new surfactant systems that retain AFFF's performance without compromising environmental and human health. Fuel transport at the surfactant solution-fuel interface is a key factor in foam performance, yet the underlying physicochemical mechanisms that govern fuel transport and foam resistance remain poorly understood, particularly in the context of fluorine-free formulations. In particular, limited insight exists into how surfactant molecular architecture, including headgroup type, tail length, and mixture composition, influences micelle behavior, interfacial dynamics, and ultimately, foam performance. This knowledge gap hinders rational formulation and slows the development of next-generation firefighting foams. This dissertation aims to fill this gap through a comprehensive, multiscale investigation of surfactant-fuel interactions. This research is organized into four integrated objectives: first examining how foam structure and composition affect fuel transport and foam stability; second understanding the role of additives and surfactant mixtures in modifying nanostructure and fuel transport; third isolating the influence of headgroup chemistry (anionic, cationic, non-ionic, and zwitterionic) on fuel transport in hydrocarbon surfactants; and fourth evaluating the effects of surfactant chemical diversity, including tail types such as hydrocarbon, fluorocarbon, and siloxane, tail lengths, and head types on micelle organization and fuel resistance. Together, this study provides critical insight into how surfactant formulations operate across molecular to macroscopic scales. By linking foam performance to underlying chemical structure properties, this study highlights key molecular and physicochemical features that influence fuel-blocking behavior, offering a foundation for the future development of environmentally sustainable surfactant systems.