Cryogenic Viscoelasticity of a C22-Tailed Amine Oxide across an Extremely Broad pH Range

Sustaining viscoelasticity of surfactant solutions at subzero temperatures and across extreme pH regimes remains challenging, as freezing and harsh acidic or alkaline environments inhibit aggregate growth and retard structural relaxation. This study examines microstructural evolution and rheological response of C22-tailed amine oxide (C22AO) in a 50/50 (v/v) ethylene glycol-water mixture spanning pH 2.0-13.0 and 20 °C to -20 °C. Potentiometric titration identifies an apparent pKa at 6.8, with ζ-potential declining from +18.53 mV (pH 2.0) to +2.35 mV (pH 6.8) and -1.66 mV (pH 13.0). All formulations retain thermal stability, freezing near -42.0 °C. Microscopy and scattering confirm that pH dominates aggregate morphology in 0.5 wt % C22AO solutions. Acidic conditions (pH 2.0) yield heterogeneous lamellar domains with a 57.0 nm repeat distance at 20 °C, evolving into coexisting 62.8 and 48.3 nm spacings at -20 °C. Near neutrality (pH 6.8), compact lamellar bilayers shrink from 37.0 to 31.4 nm upon cooling, whereas alkaline conditions (pH 13.0) promote disordered, entangled wormlike micelles. All samples exhibit pronounced shear-thinning and elasticity-dominated rheology. At -20 °C across these three pH values, zero-shear viscosities reach 2.45 × 106, 1.96 × 106, and 1.10 × 106 mPa·s, with corresponding elastic moduli of 65, 10, and 28 Pa at 1 rad·s-1 and loss tangent values of 0.24, 0.40, and 0.75. These findings demonstrate that pH governs aggregate morphology, while subzero cooling strengthens molecular packing and decelerates structural relaxation, enabling robust thickening and marked viscoelasticity in cryogenic extreme-pH environments.