Aqueous solution degradation pathways of trimethylsiloxane surfactants†

Trimethylsiloxane (TriSil) surfactants are promising alternatives to per- and polyfluoroalkyl substances (PFAS), which are global recalcitrant and persistent environmental contaminants, in aqueous film-forming fire-fighting foams (AFFF). However, much less information is available on the environmental fate and degradation of TriSil surfactants. Thus, it is important to study the degradation chemistry of fluorine-free TriSil surfactants in the solution phase under various conditions to further assess their environmental impact. This computational study reports the prominent hydrolysis, reduction, and oxidation pathways of a truncated TriSil and proposes the major degradation products using density functional theory (DFT) calculations. We have identified the polydimethylsiloxane unit of TriSil to play a prominent role in aqueous solution reactivity initiated via hydrolysis and reduction, while oxidation mainly proceeds through H-atom abstraction along the polyethylene glycol unit. The results of this study aid in establishing the use of the alternative fluorine-free surfactant, TriSil, for fire-fighting foams from an environmental perspective.