Perfluoroalkyl surfactants are the key ingredients in aqueous film forming foams (AFFF) which are used by the Department of Defense (DoD) and others to fight hydrocarbon (Class B) pool fires. Perfluoroalkyl surfactants work extremely well for this application, however there are growing concerns about these materials because they are highly persistent in the environment and may be toxic to plants and animals or increase their risk to certain diseases. The objective of this project was to explore hydrolysis-resistant siloxane surfactants as replacements of perfluoroalkyl surfactants found in current AFFF concentrates used in fire-fighting by the DoD. The new, stable siloxane surfactants produced in this research will contain only the elements carbon, silicon, hydrogen and oxygen. Foams containing the new surfactants will extinguish small-scale, unleaded gasoline pool fire in 45 seconds or less as dictated by MIL-F-24385F.

Technical Approach

In previous work, it was discovered that the inexpensive 3- aminopropylmethylbis(trimethylsiloxy)silane can be converted into surfactants with a positive spreading coefficient on cyclohexane. However, these surfactants could not extinguish a heptane pool fire themselves but could extinguish such fires in synergistic mixtures with other hydrocarbon surfactants such as alkyl polyglucosides. There were obvious stability issues with these simple siloxane surfactants, with apparent hydrolysis or other chemical changes occurring at the siloxane head group. This chemical reactivity was most likely the result of the water-labile trimethylsiloxy groups of the siloxane head. The hydrolysis of the siloxane would probably be enhanced when applied on pool fires and the thermal stress they cause. Therefore, the project team will synthesize new siloxane surfactants with hydrolysis-resistance by adding methyl groups to the terminal silicon atoms. Thus, surfactants with trisiloxane head groups terminating in bis(ethyldimethylsiloxy), bis(isopropyldimethylsiloxy) and bis(tert-butyldimethylsiloxy) will be synthesized. The project team will quantify the effects of changes to the surfactant chemical structures on surfactant/fuel diffusion at fuel-aqueous interface, micelle diffusion, and bubble coalescence rate at foam-fuel interface, in addition to the foam dynamics and fire extinction for heptane and gasoline fuels at bench-scale. For leading surfactant candidates, the project team will scale-up synthesis to generate 200 mL of surfactant and conduct 28 ft2 MilSpec gasoline pool fire testing.


New siloxane surfactants were synthesized by a four-step route starting from an inexpensive, commercially available silane and silanol reagents. The new siloxanes had tert-butyldimethylsilyl rather than trimethylsilyl groups as the terminus. For water solubility of the new siloxanes, it was critical to incorporate the proper number of aldonic acid substituents. The fire extinguishment results showed that the experimental mixture containing the hydrolysis-resistant surfactant required 61 seconds, or four times longer application time to reach 100% extinction versus military specification AFFF solution which required only 11 seconds. It was also shown that the hydrolysis-resistant surfactant was a vital component in the experimental mixture because in its absence the mixture of glucopon 225DK and diethylene glycol monobutyl ether was unable to extinguish the fire. Remarkably, the siloxane surfactant mixture composed of 2062–139 was able to extinguish the fire in 11 seconds just like the AFFF solution, albeit at a slightly higher application rate.


This new technology could help the DoD eliminate perfluoroalkyl surfactants from AFFF concentrates since these compounds appear to be persistent in the environment. Eliminating perfluoroalkyl surfactant use could also mitigate future legal issues since it is not clear what their biological effects really are. Using hydrolysis-resistant siloxane surfactants, it may be possible to extinguish fires with smaller quantities of chemicals than previous AFFF formulations. (Project Completion - 2022)