Understanding surfactant mixture adsorption towards the next generation fluorine-free firefighting foams.*

The primary ingredient in the current aqueous film forming foams (AFFFs) is per- and polyfluoroalkyl substances (PFAS) which poses a threat to environment and human health. A better understanding of surfactant's role in foam properties can lead to new formulations and elimination of PFAS in firefighting foams. In this work, a mixture of commercial alkyl polyglycoside and siloxane-based surfactants are studied which have shown promising fire suppression characteristics on various fuel-water interfaces. Dynamic surface tension of these systems at fluid-fluid interfaces are measured across length scales to understand the curvature dependence of surfactant transport to bubble surfaces, using pendant drop and capillary pressure micro-tensiometer (CPM) methods. Both the alkyl polyglycoside and siloxane surfactants present a characteristic two-step adsorption over CPM when the area of the interface is held constant, which we explain as due to the polydispersity in the commercial product. To test this explanation, we characterize mixtures of pure surfactants and under some conditions, observe the same two-step behavior in mixtures on the CPM. This work lays a solid foundation in providing key properties responsible in the study of surfactant transport of the next generation fluorine-free firefighting foams.