Study on thermally-induced aggregates transformation and its mechanism in cetyltrimethyl ammonium bromide/sodium dodecyl sulfate surfactants mixtures

Abstract

Thermally-induced formation of wormlike micelles (WLMs) in mixed surfactant solutions is investigated. The mixtures are composed of cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and anionic surfactant sodium dodecyl sulfate (SDS). The rheological characteristics, thermal curves and transmittance spectrum are obtained with Rheometer, differential scanning calorimeter (DSC) and TURBISCAN Lab stability analyzer, respectively. The Arrhenius equation is introduced to calculate the activation energy. Microscopic observation is captured with freeze-fracture transmission electron microscope (FF-TEM) at various temperatures. The results reveal that the zero-shear viscosity of the mixed solutions fluctuates at 20°C −50°C and a specific thermal-thickening phenomenon occurs in the aforementioned temperature range. When the temperature reaches 35°C, the rheological properties of the mixed solution behave distinctly compared with those beyond 35°C. There are two endothermic peaks in the DSC curve and a dramatic change in the transmittance spectrum. The FF-TEM microscopic images show that the bundled anisotropic aggregates convert into WLMs as the temperature increases from 20°C to 40°C. This study reveals that the transformation of aggregates is induced by temperature. Moreover, the WLMs form during the process of temperature elevation as is proved by the rheological behavior, FF-TEM microscopic images and the activation energy (236.5KJmol−1). The current study also suggests a simple approach to obtain the WLMs at accurate temperatures.