Concentration, salt and temperature dependence of strain hardening of step shear in CTAB/NaSal surfactant solutions

Abstract

By measuring stress relaxation following a step strain, we find that strain hardening is prevalent over a temperature range of 15–25 °C for a solution of cetyl trimethylammonium bromide (CTAB) with the added hydrotrope sodium salicylate (NaSal) at hydrotrope-to-surfactant concentration ratios (CS/CD) between 0.5 and 3.0. The extent of strain hardening upon nonlinear deformation varies nonmonotonically as a function of salt-to-surfactant ratio for different temperatures. As the strain amplitude is progressively increased, a transition from strain hardening to softening or linear response is observed at strains that are dependent on temperature and concentration. Strain hardening was also found in CTAB when using sodium 3-Hydroxy-2-naphthoate as a hydrotrope with CTAB, but solutions of anionic sodium lauryl sulfate surfactants in salt with no hydrotrope showed no strain hardening, indicating that the hydrotrope is critical to obtaining strain hardening in step strains. The results indicate a stress relaxation mechanism that is more complex than that of simple disentanglement and micelle breakage and rejoining, possibly involving strain-induced associations between micelles facilitated by hydrotropes that may act as linkers.