Impact of Molecular Features on the Interfacial Behaviors of Fluorine-Free Firefighting Foam Surfactants

Finding fluorine-free firefighting formulations that are comparable to their fluorinated counterparts or perform even better against fires atop diverse fuels is a crucial need. To address this need, the fundamental relationships between surfactant molecular features and the interfacial properties that drive foam characteristics must be advanced. Molecular dynamics simulations enable a mechanistic understanding of the molecular phenomena driving surfactant interfacial interactions across a diverse set of fuels. In the present work, it has been shown that there are trade-offs among interfacial activity, surfactant-surfactant interactions, aggregation, and interfacial spreadability. In terms of specifically designing surfactants to retain interfacial stability against polar solvents and alcohols, the following appear to be crucial for enhancing surfactant interfacial performance: increasing the molecular size (up to at least 1200 g/mol, especially sizable hydrophobic groups), maximizing the number of hydrogen bond accepting sites in a small volume, and designing a hydrophilic tail to interact strongly with water up to the point of excluding polar fuel from hydrogen bonding at the interface. These mechanistic insights from simulations, the design of which was informed by previous experimental works, enable the research field to move forward with surfactant screening against a wide range of possible structures, mixtures, and fuels in future efforts to develop fluorine-free foams with increased performance and efficiency.