Identifying non- per-and polyfluoroalkyl substances (PFAS) surfactant or surfactant mixtures that combines the necessary low surface tensions against fuel and air to make the spreading coefficient positive is a significant hurdle; hence new paradigms and formulations to suppress fuel fires are urgently needed.

This work will aid in the identification and development of PFAS-free firefighting formulations from mixtures of soluble anionic siloxanes and either cationic siloxanes or cationic quaternary ammonium surfactants capable of synergistically lowering the surface tension. Previous work has shown that many non-equimolar mixtures of soluble anionic and cationic surfactants ion pair in solution to form pseudo double-tailed catanionic surfactants, resulting in 2-3 order of magnitude decrease in the critical micelle concentration (CMC) and a 5 - 10 mN/m decrease in surface tension relative to the individual surfactants. Recently developed anionic siloxanes and commercial cationic siloxanes have surface tensions in the 20 – 25 mN/m range which may decrease sufficiently on mixing to reach those of perfluorinated surfactants leading to a positive spreading coefficient to aid in sealing of the foam blanket and prevention of burn back (Task 1). In parallel, the project team will use fundamental rheology-based design principles to enhance foam stability against coarsening by meeting the Gibbs criteria for bubble stability by reducing surface tension and increasing the dilatational modulus, particularly at low frequencies (long time scales) using the catanionic surfactant mixtures (Task 2). Coalescence, or rupture of the aqueous surfactant stabilized films between bubbles can be inhibited due to the increase in the film disjoining pressure through nanoparticle-like bilayer vesicles formed at concentrations above the catanionic mixture CMC. The combination of lower surface tension, higher dilatational moduli and spontaneous vesicle formation will result in rapidly formed yet stable foam blankets to suppress fires, even if the criterion of spontaneous spreading is not achieved. The project team will make use of capillary micro tensiometers and microfluidic devices unique to the labs to measure the equilibrium surface and interfacial tensions, rates of surfactant adsorption, and the frequency dependent dilatational moduli of these catanionic mixtures and correlate these parameters with foam generation, stability and lifetimes measured with microfluidic devices from WP19-1407 and a conventional foam tester. Finally, the project team will incorporate ecotoxicity and life cycle analysis studies, to ensure long term sustainability of the candidate surfactant mixtures (Task 3).

Successful completion of this project will result in new paradigms for creating PFAS-free firefighting foams with enhanced stability against coarsening and coalescence from synergistic mixtures of anionic and cationic surfactants. The project will also provide fundamental understanding of the rheological properties of catanionic surfactant mixtures and how these properties are related to molecular parameters. The work will focus on scenarios described in the Statement of Need to determine optimized surfactant formulations at temperatures from 0°C to 60°C, in saltwater and freshwater environments, and in the presence of varied fuel types and the role of transport of fuels and vapors through the catanionic surfactant foam layers.