Highly Viscoelastic Wormlike Micellar Solutions Formed by Cationic Surfactants with Long Unsaturated Tails

TLDR

The study examined cationic surfactants with long C22 mono‑unsaturated tails in salt solutions via steady and dynamic rheology. The erucyl bis(hydroxyethyl)methylammonium chloride surfactant self‑assembles into giant wormlike micelles, producing a factor‑of‑10⁷ viscosity increase, gel‑like solids at low temperatures that become Maxwellian viscoelastic at higher temperatures, viscosities exceeding 10 Pa·s up to 90 °C, and its self‑assembly is strongly enhanced by penetrating counterions such as sodium salicylate, while a trimethylammonium headgroup reduces high‑temperature viscosity.

Abstract

Cationic surfactants having long (C22) mono-unsaturated tails were studied in aqueous solutions containing salt using steady and dynamic rheology. The surfactant erucyl bis(hydroxyethyl)methylammonium chloride self-assembles into giant wormlike micelles, giving rise to unusually strong viscoelasticity. Under ambient conditions, the viscosity enhancement due to surfactant exceeds a factor of 107. Some samples behave as gel-like solids at low temperatures and revert to the viscoelastic (Maxwellian) response only at higher temperatures. These samples display appreciable viscosities (>10 Pa·s) up to very high temperatures (ca. 90 °C). Salts with counterions that penetrate into the hydrophobic interior of the micelles, such as sodium salicylate, are much more efficient at promoting self-assembly than salts with nonbinding counterions, such as sodium chloride. Changing the surfactant headgroup to the more conventional trimethylammonium group reduces the viscosity at high temperatures.