Effect of counterion size on wormlike micelles formed by a C22-tailed anionic surfactant

Wormlike micelles (WLMs), generally formed by a surfactant in the presence of a hydrotrope, have gained much interest over last decades; however, less emphasis was thrown on investigating the influence of hydrotrope size on the morphology of WLMs, and WLMs based long-chain anionic surfactants (> C18) have been seldom noticed. In the present work, rheology, phase observation and cryo-TEM imaging were used to get insight into the dependence of morphology and rheological properties of WLMs formed by a C22-tailed anionic surfactant, sodium erucate (NaOEr), on hydrodynamic volume of hydrotrope by comparing tetramethylammonium (TMA+) and tetrabutylammonium (TBA+). It was found on one hand, TBA+ presents superior ability over TMA+ to reduce Krafft point of NaOEr; on the other hand, the impact of hydrophobe size on rheological responses varies with NaOEr concentration. Comparatively, TMA+ is more effective in inducing micellar growth at low NaOEr concentration; while TBA+ predominates at high NaOEr concentration. The molecular interaction between NaOEr and TBA+ shows high dependency on NaOEr concentration. The hydrophobicity and the volume factor of TBA+ alternately influence the micellar arrangement. When at high NaOEr concentration, the hydrophobicity plays a critical role only and the volume factor begins to dominate at low concentration.