Revealing foam stability for cationic and zwitterionic triethylsilyl-containing surfactants

Abstract

A fundamental understanding of surfactant structure–property–performance relationships will inform the design of next-generation alternatives to perfluoroalkyl substances (PFAS) in aqueous film-forming foams. This manuscript describes the synthesis, solution properties, and foam stability of novel triethylsilyl-containing surfactants, which elucidated the influence of the hydrophilic head group on critical micelle concentration (CMC), surface tension, and foam stability. Photocatalyzed hydrosilylation of triethylsilane and N,N-dimethyl allylamine yielded N,N-dimethyl-3-(triethylsilyl)propane-1-amine. Subsequent functionalization with either propane sultone or bromoethane afforded zwitterionic sulfobetaine surfactant, 3-(dimethyl(3-(triethylsilyl)propyl)ammonio)propane-1-sulfonate (TESDMAPS) and cationic quaternary ammonium surfactant, and N-ethyl-N,N-dimethyl-3-(triethylsilyl)propane-1-ammonium bromide (TESDMABr), respectively. Dynamic light scattering and cryo-transmission electron microscopy (TEM) characterized micelle size and shape in solutions above the CMC. Surface tensiometer analysis determined minimum TESDMAPS and TESDMABr solution surface tensions of 37.7 and 35.9 mN/m, respectively. Molecular dynamics simulations related this decrease in surface tension to a larger average interfacial area of 88 Å2 per TESDMABr molecule compared to 66 Å2 per TESDMAPS molecule. Steady-shear rheological measurements showed consistent exponential viscosity-scaling relationships between TESDMAPS and TESDMABr solutions ≤ 30 wt. %. Above this concentration, TESDMAPS displayed solution viscosities greater than TESDMABr, and a mixture of surfactants provided an intermediate concentration dependent viscosity scaling. Dynamic foam analysis revealed TESDMABr foams displayed longer 25% foam drainage times than TESDMAPS. The oscillatory rheology of TESDMABr solutions demonstrated solid-like solution behavior at low shear rates. Finally, polarized light-imaging rheology highlighted the formation of birefringent structures in TESDMABr solutions under shear. For the first time, this work relates solution viscoelasticity from shear-induced surfactant assembly to foam stability with implications on fluorine-free, next-generation, fire-fighting foams.