Abstract:

Ultra-long-chain nonionic surfactants, featuring hydrophobic tails with carbon numbers beyond C₁₈, have emerged as a significant frontier in viscoelastic surfactant research due to their capacity to form wormlike micelles at remarkably low concentrations without auxiliary hydrotropes. This review systematically delineates recent advances in the synthesis, solubility behavior, rheological properties, and application prospects of this class of amphiphiles. Two primary synthetic routes are highlighted: Esterification of fatty acids with polyoxyethylene monomethyl ether to produce polyether-type derivatives, and amidation with N-methyl-D-glucamine or N-ethyl-D-glucamine to afford glucosamide-based counterparts, both utilizing renewable plant-derived feedstocks such as erucic acid. Solubility modulation is achieved by tailoring hydrophilic headgroup size, hydrophobic tail length, and the number of unsaturated bonds, with quantitative structure-property correlations established to predict dissolution behavior across a wide temperature window. The glucosamide series further exhibits pronounced freezing-point depression, enabling subzero fluidity. Rheologically, these surfactants assemble into entangled wormlike micellar networks or hexagonal liquid crystalline phases, imparting significant shear-thinning characteristics and viscoelastic responses even at low concentrations and under freezing conditions. A representative glucosamide surfactant, UC₂₂GEt, demonstrates a zero-shear viscosity as high as 4.17 × 10⁵ mPa·s at 0 ℃ while retaining injectability at -14 ℃. Exploiting these unique features, the UC₂₂GEt system has been successfully applied as a cryoprotectant for frost-sensitive plants, forming a uniform and stable coating on hydrophobic petal surfaces and effectively mitigating freeze-induced damage through synergistic mechanisms of freezing-point suppression, enhanced wettability, and shear-responsive viscosity recovery. Future directions are envisioned toward molecular topology engineering, greener synthetic methodologies, and deeper mechanistic understanding of the thermodynamic driving forces and network dynamics governing wormlike micelle formation.

Key words: ultra-long-chain surfactant, nonionic surfactant, viscoelastic surfactant, wormlike micelles