Thermodynamic Characterization of Temperature- and Composition-Dependent Mixed Micelle Formation in Aqueous Medium

This article provides a detailed description of micellization of a binary nonionic surfactant system (Lutensol AP9 and AP20) at different compositions and temperatures in an aqueous medium. Critical micelle concentrations of the individual components (CMCi) and the mixed surfactants (CMCm) were determined via surface tension measurements in the temperature range of 288.15-318.15 K. On the basis of the magnitude and temperature dependence of CMC values, changes in the Gibbs energy (a dagger(mic) G), enthalpy (a dagger(mic) H), and entropy (a dagger(mic) S) of micellization were evaluated. At 288.15 K the thermodynamic parameters were confirmed by isotherm titration calorimetry (ITC). Regular solution theory (RST) was used to estimate the compositions of the mixed micelles (X (M)) and molecular interaction parameters (beta (M)) for all molar ratios and temperatures. The results of evaluations based on the closed association model and RST were compared.