Noncovalently Assembled Biobased Surfactants from Fatty Acids as Green Foaming Agents: Insights into Foam Performance and Interfacial/Bulk Properties

Conventional foam extinguishing agents face increasing restrictions due to the environmental concerns associated with fluorinated foaming agents, prompting a global search for greener alternatives. Biobased pseudo-Gemini surfactants, assembled via noncovalent interactions of fatty acids, have emerged as promising candidates for sustainable foam generation. In this study, a series of eco-friendly pseudo-Gemini surfactants were constructed using polyetheramine and fatty acids of varying chain lengths. Structural validation via 1H NMR and FTIR confirmed the formation of amphiphilic architectures. A systematic study was conducted on the foaming ability and stability of these surfactants, with analysis of the differences in foam performance insights from bulk and interfacial properties. Results indicate that both excessively short and excessively long hydrophobic tails hinder foam formation, while foam stability positively correlates with tail length, attributed to the formation of large aggregates that block Plateau borders and suppress drainage. Additionally, pseudo-Gemini surfactants with extended liquid film lifetimes exhibited an enhanced foam stability. Among them, pseudo-Gemini D230-C12C14 demonstrated superior adsorption efficiency (pC20 = 1.95) and the highest interfacial thermodynamic stability (Gmin = 19.52 kJ/mol). Shear-thinning behavior, DLS data, and cryo-TEM revealed distinct micellar structures among different pseudo-Gemini. By integration of DST data, the foaming mechanisms can be elucidated: Spherical micelles facilitate rapid unimer exchange and diffusion-controlled adsorption, enhancing the foam formation. In contrast, wormlike micelles and vesicles exhibit denser molecular packing, significantly slowing unimer desorption, leading to higher t* values and reduced foaming efficiency. Additionally, fire-extinguishing tests demonstrated that foams based on pseudo-Gemini surfactants exhibit an extinguishing performance comparable to that of fluorinated ones.