pH-Responsive wormlike micelles based on microstructural transition in a C22-tailed cationic surfactant–aromatic dibasic acid system

Abstract

pH-Responsive wormlike micelles based on microstructural transition, and formed by complexation of N-erucamidopropyl-N,N-dimethylamine (UC₂₂AMPM) and potassium phthalic acid (PPA) at a molar ratio of 2 : 1, were developed and compared with CTAB/PPA at the same molar ratio. Phase behavior, viscoelasticity, and microstructural transitions of solutions were investigated by observing their appearance, rheological characteristics, dynamic light scattering, and ¹H NMR measurements. It was found that the phase behavior of UC₂₂AMPM/PPA solutions undergoes transitions from transparent viscoelastic fluid to phase separation with white floaters upon increasing pH. By increasing pH from 2.01 to 6.19, the viscosity of wormlike micelles in the transparent solutions continuously increased and reached ∼1.4 × 10⁶ mPa s at pH 6.19. As pH was adjusted to 7.32, the opalescent solution showed a water-like flowing behaviour and the η₀ rapidly declined to ∼1.7 mPa s. The viscosity of the CTAB/PPA solutions had a maximum at pH 2.98 and then decreased with increasing pH. This radical variation in rheological behavior is attributed to the pH dependent hydrophobicity of PPA and ultra-long hydrophobic chain of UC₂₂AMPM. Additionally, dramatic viscosity changes of about 6 magnitudes can be triggered by varying pH without any deterioration for the UC₂₂AMPM/PPA system.