Curvature- and temperature-dependent transport of soluble surfactant mixtures to the air-aqueous surface with applications in fluorine-free firefighting foams

Understanding surfactant adsorption to air–water interfaces is crucial to eliminating toxic fluorocarbon-based surfactants while retaining firefighting performance. The adsorption of commercial siloxane and Glucopon surfactants is investigated by measuring dynamic surface tension at different length scales using a pendant drop tensiometer and capillary pressure microtensiometer (CPM) for millimeter and micrometer sized bubbles at 23 °C and 60 °C. Higher surfactant concentration and higher curvature favor surfactant adsorption. The effect of interfacial curvature can be rationalized by rescaling respective times scales for diffusion-limited adsorption. For constant area adsorption in the capillary pressure microtensiometer, surfactants relevant to firefighting foams show stepwise adsorption. Model mixtures of ethoxylated surfactants of different chain lengths also show this stepwise adsorption, suggesting heterogeneity in tail lengths in the commercial surfactants. Surfactant adsorption is modeled by treating the mixtures as a quasi-single component using the Ward-Tordai equation and the Langmuir adsorption isotherm to characterize the temperature-dependent surfactant properties. While temperature increases the diffusivity of both Dow 502W and Triton X100, Dow 502W demonstrates key differences in surfactant adsorption compared to Triton X100 and the Glucopon surfactants at elevated temperatures. A deeper understanding of how different head and tail group lengths and temperature affect surfactant adsorption will help optimize new surfactant replacements for enhanced firefighting performance.