"From Molecule to Foam: Structure-Property Relationships, and Fluorine-Free Firefighting Foam Performance of Cationic Tri- and Tetrasiloxane Surfactants" 

Ronaldo Franjul, MSE PhD Candidate 

Professors Jeffrey Youngblood & Carlos Martinez

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ABSTRACT

Organosilicone surfactants are attractive fluorine-free candidates for applications demanding ultralow surface tension and rapid spreading, yet systematic data on cationic siloxane surfactants bearing quaternary ammonium headgroups remain scarce. This work examines a matched pair of quaternized surfactants differing only in siloxane backbone length: a tetrasiloxane (QAT) and a trisiloxane (QAB), each bearing a trimethylammonium headgroup and an iodide counterion, a design that isolates the effect of a single siloxy unit on surface activity, aqueous self-assembly, and foam performance.

Physicochemical characterization established Krafft temperatures of 29.2 and 28.4 °C, critical aggregation concentrations of 2.9 and 32.4 mM, and equilibrium surface tensions near 20 mN/m. Conductimetry yielded counterion binding degrees between 0.87 and 0.94 and aggregation free energies 9.7 kJ/mol more negative for QAT, quantifying the contribution of the additional siloxy unit. Hydrolysis followed a U-shaped rate–pH profile, with QAB the more stable of the two.

Below their Krafft temperature both surfactants formed vesicular dispersions, resolved by cryogenic transmission electron microscopy and dynamic light scattering, that remained optically homogeneous for weeks despite being supersaturated. These dispersions ultimately converted into crystalline aggregates, needles for QAT and plates for QAB, with induction times ranging from 60 days without perturbation to 24 h after two gentle inversions, indicating nucleation-limited kinetics. Small-angle X-ray scattering revealed lamellar stacking combined with in-plane crystalline order, with spacings contracting from 27.65 to 20.31 Å in QAT and from 23.72 to 17.49 Å in QAB. Comparison with the solid lamellar phase of an iodide-paired trisiloxane homologue, together with a geometric argument, established that this contraction cannot arise from interlamellar dehydration and instead reflects reorganization of molecular packing within the lamellae.

Screening a panel of co-surfactants and co-solvents identified a commercial nonionic siloxane surfactant, Silsurf A008, as the optimal partner for both compounds, with fresh water consistently outperforming salt water. The bulky siloxane hydrophobe responsible for the low-curvature aggregation above also leaves interstitial gaps in the adsorbed monolayer, which a smaller co-surfactant can occupy to yield denser packing. Foam half-lives reached 8500 min for QAT at 25:75 and 5500 min for QAB at 50:50, against roughly 30 min for either surfactant alone and 425 min for the co-surfactant alone. Together these results demonstrate that a single siloxy unit governs bulk aggregation, crystallization pathway, and foam stability without altering interfacial efficacy, providing structure-property guidelines for the rational design of fluorine-free siloxane surfactant formulations.