Thermoresponsive Properties of a Mixed Aqueous Solution of Cationic Surfactant and Organic Acid

Rheological properties of aqueous mixtures of the traditional cationic surfactant cetyltrimethylammonium bromide (CTAB) and organic acid 3-methylsalicylic acid (3MS) were studied as a function of concentration and temperature using steady-state and frequency sweep-rheological measurements. Upon being heated, the solutions exhibited three different types of response. Among them, the most interesting response was that light blue dilute solutions formed over the 3MS concentration range of 80 to 100 mmol·kg-1. These samples changed from dilute pale blue solutions to transparent viscoelastic ones as their aggregation state transitioned from vesicles to long worm-like micelles with increasing temperature. Moreover, the threshold temperature of the transition increased with 3MS concentration. The results of rheological temperature scanning and conductivity measurements verified this trend. A qualitative explanation for this transformation is that bound 3MS molecules dissociate from the vesicles and join the bulk aqueous phase at high temperature.