Surface and bulk properties of surfactants used in fire-fighting

Hypothesis: Reports on the colloidal and interfacial properties of fluorocarbon (FC) surfactants used in fire-fighting foam formulations are rare. This is primarily because these formulations are complex mixtures of different hydrocarbon (HC) and fluorocarbon (FC) surfactants. By developing a greater understanding of the individual properties of these commercial FC surfactants, links can be made between structure and respective surface/ bulk behaviour. Improved understanding of structure property relationships of FC surfactants will therefore facilitate the design of more environmentally responsible surfactant replacements. Experiments: Surface properties of three partially fluorinated technical grade surfactants were determined using tensiometry and neutron reflection (NR), and compared with a research-grade reference surfactant (sodium perfluorooctanoate (NaPFO)). To investigate the bulk behaviour and self-assembly in solution, small-angle neutron (SANS) scattering was used. Findings: All FC surfactants in this study generate very low surface tensions (< 20 mN m⁻¹) which are comparable, and in some cases, lower than fully-fluorinated surfactant analogues. The complementary techniques (tensiometry and NR) allowed direct comparison to be made with NaPFO in terms of adsorption parameters such as surface excess and area per molecule. Surface tension data for these technical grade FC surfactants were not amenable to reliable interpretation using the Gibbs adsorption equation, however NR provided reliable results. SANS has highlighted how changes in surfactant head group structure can affect bulk properties. This work therefore provides fresh insight into the structure property relationships of some industrially relevant FC surfactants, highlighting properties which are essential for development of more environmentally friendly replacements.