Synthesis of novel bio-based polyamide surfactants and their emulsifying property

Amphiphilic polyamides represent a class of surfactants with vast potential for applications. In this work, fatty acid methyl esters (methyl oleate and methyl erucate) and biomass-derived 1,4-butanediamine were utilized as raw materials to synthesize two novel anionic polyamide surfactants (APSs, C18:1-PEA and C22:1-PEA). The structure of APSs was characterized via 1H nuclear magnetic resonance (1H NMR) spectra, Fourier transform infrared (FT-IR) spectra, and gel permeation chromatography (GPC). The physico-chemical properties of APSs were explored through thermal stability, surface activity, behavior of APSs molecules in aqueous solution, and the influence of pH on their emulsion stability. The results indicated that APSs could form stable oil-in-water emulsions under alkaline conditions (pH 11 and above). In addition, stable emulsions of liquid paraffin or simulated oil formed by the C18:1-PEA surfactant can be completely destabilized in the lower pH solution in 30 min. The pH-sensitive behavior of the emulsions could be attributed to the introduction of the polyether structure, which, under alkaline conditions, imparts additional electrostatic repulsion due to hydrogen bonding interactions. This study provides an alternative idea for designing novel pH-sensitive emulsifiers, with potential applications in scenarios where emulsions need to be formed under alkaline conditions and subsequently destabilized after use.