Essential Factor of Perfluoroalkyl Surfactants Contributing to Efficacy in Firefighting Foams

Two simple model aqueous foams, one made from a perfluoroalkyl surfactant and one from a silicone polyether surfactant, are compared with regard to their stability in contact with heptane as a model fuel oil. The observed foam stabilities are explained in terms of the equilibrium phase behavior of the system water-surfactant-heptane. It is demonstrated that the fundamental enabling factor that makes perfluoroalkyl surfactant perform exceedingly well in stabilizing foams on hydrocarbon fuel oil is its oleophobicity. For hydrocarbon or silicone surfactants, the propensity for the surfactant phase to solubilize hydrocarbon oil and be solubilized in the oil destabilizes the foam. This is particularly so if the surfactant's phase inversion by temperature (PIT) range falls within the application temperature range.