The objective of this project was to develop siloxane-betaine zwitterionic additives to enhance fire suppression and burnback resistance for gasoline, which contains aromatic and aliphatic components. Specifically, vary siloxane-betaine’s molecular structure to promote bilayer formation at an interface for increased surfactant packing density, and to suppress extraction by fuel. To vary the structure, synthesize new zwitterionic silicones, and evaluate commercial zwitterionic hydrocarbons as functional additives to glycoside foams.

The project team took the following actions:

• Varied size and configuration of the surface-active molecules to optimize the structure.
• Systematically varied the siloxane-betaine additive’s chemical structure by synthesis and by using commercial zwitterionic hydrocarbon surfactants as additives to the alkylpolyglycoside foams.
• Characterized surfactant layers at air-water, air-fuel interfaces by quantifying composition and packing density.
• Quantified the effects of oleophobicity through quantification of change in contact angles due to the presence of surfactant molecules adsorbed at interfaces, solubility in fuel and water or partition coefficients, and equilibrium phase separation (or emulsification).

These are in addition to the measurements of foam dynamics and gasoline-fire suppression/burnback at bench scale. Varying molecular structure of the siloxane additive can affect the hydrolysis of the siloxane tails. The project team measured acute lethality endpoints (LC₅₀) and chronic sublethal toxicity endpoints (EC₅₀, IC₅₀).

The results presented show that surprisingly small changes to a surfactant's molecular structure can result in dramatic changes in critical micelle concentration (CMC), foam expansion ratio, foam degradation, fuel permeation through foam, and improvements in fire suppression. They appear to have only small effects on foam bubble size distributions and liquid drainage for foams generated by sparging methods at bench scale but have significant effects for foams generated by a pressurized nozzle at a large scale. Many of these changes could largely be explained by the changes to the amphiphilic balance of the surfactants involved and the project team shows that the effects of surfactant’s amphiphilicity permeates from CMC and foam stability to fire suppression. But, the degree of improvement in fire suppression is fuel specific because fuels affect the degree of synergism between the surfactants. Calculated packing parameter of 0.98 for the tetrasiloxane-sulfobetaine suggests aggregation into bilayer or vesicle shaped micelles. In the micelles, the sulfobetaine head exhibits strong electrostatic interaction with water, forming a hydration layer containing 4 to 9 units of water as determined by titration method. The hydration layer is acidic (pKa 3.3) and catalyzes hydrolysis of the siloxane tail with time. While this can help degrade the siloxane in an aqueous environment, it can limit shelf life. The project team found that a glucopon component in the formulation as well as adding a pH buffer can slow the hydrolysis.

The project team quantified acute and chronic toxicities of individual siloxane, hydrocarbon, and fluorocarbon surfactants and Reference AFFF using P. promelas and C. dubia species following standard methods (in collaboration with Dr. David Moore, U.S. Army Corps of Engineers, U.S. Army Engineer Research and Development Center, Vicksburg, MS). Based on results to date with both species, the relative order of toxicity of the tested compounds from least toxic to most toxic is as follows: DGBE<, Capstone 1157<, 502W<, Glucopon 215UP<, Glucopon 225DK<, Silwet L-77,< HMTS-PEO-TMACl, <<RefAFFF.

A fluorine-free alternative to the current aqueous film-forming foam (AFFF) will eliminate the bio-accumulative, environmentally persistent, and toxic per- and polyfluoroalkyl substances. The fluorine-free firefighting foam will lead to lower projected risks to the aquatic environment and human health. In addition, the fluorine-free formulation will meet military specification performance standards for gasoline, maintaining essential fire suppression performance of AFFF, critical to Department of Defense applications. Having low viscosity and being compatible with existing hardware, the fluorine-free formulation will be a drop-in replacement for AFFF.