This project addresses the following two scenarios of interest: 1) Firefighting operations at extreme temperatures up to 125oF; 2) Firefighting operations involving various fuels to include polar solvents, gasoline, or gasoline-alcohol blends. Ionic liquid- and siloxane-based fluorine-free firefighting foams (FFF) have shown encouraging firefighting performance on their own. However, foam stability in the presence of fuel vapors is one aspect that diminishes fire suppression performance. The project team will investigate the role of transition metal complexation into siloxane-, ionic liquid- and chelating ligand-based surfactants in improving fuel vapor-foam compatibility for several classes of fuels: Heptane, JetA fuel, gasoline, and gasoline-alcohol blends. The preliminary investigation has shown that transition metal coordinated long-chain surfactants are better surface-active agents than the corresponding free ligands. Compared to the free ligands, metal coordinated surfactants have significantly lower surface tension values at much lower critical micellar concentration values. In addition, transition metal coordinated surfactant foams have shown better fuel-vapor compatibility. The present endeavor is a limited scope effort to develop and test metallosurfactants and transition metal coordinated surfactants as foam stabilizers to enhance foam compatibility with fuel vapors. The impact of metalation of surface-active molecules on fire suppression performance will be shared with other research groups to achieve the SERDP's goal of replacing per- and polyfluoroalkyl substances containing aqueous film forming foams with FFF.
The project team will investigate the role of transition metal complexation into siloxane-, ionic liquid- and chelating ligand-based surfactants in improving fuel vapor-foam compatibility for several classes of fuels: Heptane, JetA fuel, gasoline, and gasoline-alcohol blends. The higher foam stabilization will be achieved through the intermolecular interactions of the hydrophilic surfactant heads in the aqueous phase with a network of surfactant molecules or ions crosslinked by coordinating metal cations. Consequently, the affinity of surfactants to the aqueous phase may increase, and the transfer of surfactants into the fuel phase may be suppressed, leading to more stable foam bubbles in the presence of fuel vapors. These stabilization effects may allow 1) operations at foam formulation solution at elevated temperature before foaming, such as those requested in the statement of need, and 2) enhancement of foam stability in firefighting operations after the foam is applied to a fire. The project team will investigate fuel vapor-surfactant molecule interaction at elevated temperatures by utilizing Ross-Miller foam analysis and closed chamber test method involving a volatile organic compound sensor.
The use of metallosurfactants and transition metal coordinated surfactants in FFF formulation opens a new pathway to address the longstanding issue of fuel-vapor compatibility of FFF formulations, especially at elevated temperatures. The approach is ubiquitous and can be applied to the most promising FFF molecules that have shown acceptable fire extinguishing performance but are incompatible with fuel-vapors and therefore, fail to meet burnback resistance requirements. The use of metallosurfactants in FFF will diminish fuel vapor interactions and, thus enhance burnback resistance. Also, improvement in surface-active properties will improve fire extinguishing performance on several classes of fuels: Heptane, JetA fuel, gasoline, and gasoline-alcohol blends. The project team will develop a comprehensive metalation strategy that can be applied to the promising FFF surfactants with promising fire extinguishing performance.