Annular and threadlike wormlike micelles formed by a bio-based surfactant containing an extremely large hydrophobic group

Abstract

A novel bio-based anionic surfactant containing a large rigid group and a flexible alkyl chain, namely, sodium N-dodecyl-maleimidepimaric carboxylate (C₁₂-MPA-Na), was synthesized from rosin. The molecular structure of C₁₂-MPA-Na was identified using ¹H NMR, FT-IR spectroscopy and MS. Despite containing 36 carbon atoms, C₁₂-MPA-Na showed good water solubility at room temperature. Large spherical aggregates with diameters of 100–200 nm were formed by C₁₂-MPA-Na when its concentration was above 0.1 mM, which was slightly higher than the critical micelle concentration (0.078 mM). Annular wormlike micelles were discovered with increasing C₁₂-MPA-Na concentration, and began to change into extremely long threadlike wormlike micelles when the C₁₂-MPA-Na concentration reached approximately 58 mM. The viscoelastic properties of the wormlike micelle solutions were investigated using steady state and oscillatory shear sweep rheological measurements. The zero-shear viscosity (η₀) strongly depended on the concentration of C₁₂-MPA-Na, and the scaling exponent was 34.1. Cryo-TEM confirmed the formation of large spherical aggregates and wormlike micelles. ¹H–¹H 2D nuclear Overhauser effect spectroscopy (NOESY) was used to detect the molecular interactions of C₁₂-MPA-Na. The results indicated that the alkyl chain of C₁₂-MPA-Na was partially overlapped with its non-planar rigid structure in aqueous solution, and the possible aggregation process for C₁₂-MPA-Na was proposed.