Rheological behavior of wormlike micelles (WLMs) in alcohol/water mixed solvent: influence of alcohol chain length

Abstract

The influence of replacing water with ethanol, propanol, and butanol in alcohol–water mixtures on the rheological properties of wormlike micelles (WLMs) consisting of sodium salicylate (NaSal) and cetyltrimethylammonium bromide (CTAB) is investigated at 25 °C. At maximum viscosity of 100 mM CTAB and 60 mM NaSal, the alcohols at low concentrations of up to 8% ethanol, 3% propanol, and 2% butanol do not affect the WLM structure as indicated by the normal modulus values of the solutions, but they decrease the structural relaxation times and their viscosities. This behavior is explained by the fact that as alcohol chain length increases, it has lower miscibility in water, and hence, it can interact to a higher extent with WLMs even at lower contents. We hypothesize that alcohols adsorbed at the WLM surface make it more hydrophilic. The contacts between WLMs are therefore lost, the viscosity decreases, and the structural relaxation times become faster. At suitable contents, alcohol molecules can penetrate WLMs and damage their structure. For solutions with minimum viscosity, higher alcohol contents were required to destroy WLMs, and replacing water with ethanol leads to viscosity increase, followed by a gradual decrease at higher ethanol contents. Propanol and butanol drastically changed the WLM viscoelastic properties. Generally, as the alcohol chain length increases, WLMs become more pronounced at lower alcohol contents. Alcohols, at relatively low amounts, do not affect the WLM structure but lead to vanishing of the entanglement between WLMs. Butanol, due to its longer chain compared to ethanol and propanol, destroys WLMs at lower contents. Generally, as alcohol chain length increases, the damage of entanglement between WLMs becomes more pronounced at lower alcohol contents.