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The management of hydrocarbon fuel fires is relevant to military applications. Current firefighting formulations depend on perfluorinated surfactants. These toxic agents feature exceptionally poor biodegradability, propagate through the food chain, and affect multiple species over an extended period. Thus, search for alternatives to perfluorinated surfactants has been assigned high priority by the U.S. Department of Defense (DoD).
The objective in this proof of concept phase was to find readily biodegradable and non-toxic firefighting formulations, which included composites of metal carbonate nanoparticles and ionic liquid surfactants. These composite formulations would form stable foams and also release carbon dioxide that starves a fire of oxygen as a bonus. The ability to form stable foams that spread over hydrocarbon surface, the ability to form metal carbonate composites as well as biodegradability are key factors that we have considered in determining successful surfactant mixtures for the formulations.
The project team combined experiments and computational chemistry techniques to identify those surfactants that feature high spreading coefficients over cyclohexane and bind to metal carbonate surfaces. The project team used molecular dynamics simulations to predict spreading coefficients of surfactants, density functional theory to quantify the binding of surfactants to metal carbonate nanoparticle. Based on the in-silico modeling and chemical intuition, they identified, synthesized, and characterized most promising surfactant-metal carbonate composites. The project team then performed foam stability and small-scale fire tests.
The experiments indicated that aqueous solutions of mixtures of alkyl imidazolium or alkyl pyridinium ionic liquids as well as alkyl sulfates have spreading coefficients near or higher than the DoD minimum of 3 mN/m. The project team also found that these surfactants formed stable foams whose drainage time fell within the target range. They further established that metal carbonate nanoparticles, when added to the surfactant formulation, enhance the stability of foams. Finally, the project team has demonstrated that the formulations can extinguish small scale hydrocarbon fires. The surfactants are fluorine-free and immeasurably more biodegradable than perfluorinated surfactants that persist in the environment for decades.
The proof of concept research has provided strong evidence that ionic liquid-based formulations fully compliant with the DoD standards and regulations for firefighting aqueous film-forming foams are likely achievable. Upon completion of extra work to optimize their performance, ionic liquid based formulations would mitigate risks associated with exceptionally poor biodegradability of perfluorinated surfactants currently used to extinguish jet fuel fires.