Effect of low pressure on the properties of short-chain fluorocarbon surfactants and their application in aqueous film-forming foam extinguishing agent

Surface properties of three fluorocarbon surfactants with different carbon chain lengths were studied at low pressures of 94.6, 74.1 and 61.0 kPa, respectively. The results showed that the saturation adsorption capacity and the critical micelle concentration (CMC) decreased, while the average molecular area (A_S) and the surface tension at CMC (γ_CMC) of fluorocarbon surfactants increased with decreasing pressure. The surface tension (γ) decreased with the decrease of pressure when the concentration of fluorocarbon surfactants was low. However, γ tended to be stable and increased with the drop of pressure when the concentration of surfactants was close or reach to CMC. In addition, the foaming and spreading performance of fluorocarbon surfactants decrease with the decrease of pressure, and the combination of fluorocarbon and hydrocarbon surfactants is beneficial to the improvement of performance (mixed systems: foam expansion E>6, 25% drainage time t_25%>241 s, 1/2 defoaming time t_1/2>851 s, spreading amount V_s>223 μL, spreading time t_s<2 s). Further fire extinguishing experiment found that the influence of low pressure environment on fuel combustion characteristics is the key factor to determine fire extinguishing performance, while the influence of aqueous film-forming foam (AFFF) performance is small. And AFFF based on the environmental-friendly surfactant C₄F₉SO₂NH(CH₂)₂N⁺(CH₃)₂CH₂BrCH₂OH (FC-4) is an effective fire extinguishing agent under low pressure.