Amide bond-mediated rapid CO₂ responsiveness of cleavable carboxylate surfactants in alkaline media

C₁₄N-TCA, as a kind of carboxylate surfactant, could response to CO 2 below pH 8.6. • Unique bond cleavage mechanism enables faster and more efficient CO₂-induced demulsification. • Benefited from hydrogen bonds, C₁₄N-TCA possesses high solubility and surface activity Alkaline environments are widely encountered in fields such as metal cleaning, textiles and papermaking. However, it remains a challenge to develop temporarily stable emulsions that can not only maintain satisfactory stability in alkaline media but also achieve rapid demulsification under the similar alkaline conditions. In this paper, a cleavable carboxylate surfactant (sodium N-tetradecyl-1-carboxylate-1-hexylene-2-carboxamide, abbreviated as C 14 N-TCA) was synthesized. C 14 N-TCA stabilized O/W emulsions at only 0.03 mM and exhibited excellent foaming properties (initial volume 58 mL, half-life 40 h at 5 mM in 10 mL samples), owing to its intramolecular amide groups that formed intermolecular hydrogen bonds. Unlike other carboxylate surfactants that required a pH below 5 to convert from carboxylate to carboxylic acid, C 14 N-TCA underwent structural dissociation at approximately pH 8.6 upon brief CO₂ bubbling, thus enabling rapid demulsification. The CO₂ response rate of C 14 N-TCA was 5-fold and 30-fold faster than that of sodium palmitate (SP) and sodium 3,3′-(dodecylazanediyl) dipropionate (Di-NDBIa), respectively. C 14 N-TCA was applicable to the cleaning of oil-contaminated alkaline soil, as it not only exhibited superior washing efficiency to the conventional surfactant sodium dodecyl sulfate (SDS), but also allowed demulsification and surfactant recovery through CO₂ trigger post-cleaning. This study provided insights into the structural design of CO₂-responsive carboxylate surfactants and held potential applications in fields requiring the temporary stabilization of emulsions or foams under alkaline conditions. Download: Download high-res image (242KB) Download: Download full-size image