Thermodynamic characterization of temperature-induced micellization and demicellization of detergents studied by differential scanning calorimetry

The micellization and demicellization of surfactants can be induced not only by changes in concentration but also by changes in temperature, because the critical micellar concentration (cmc) exhibits a characteristic temperature dependence with a minimum at temperatures between 20 and 50 degreesC, depending on the nature of the surfactant. We obtained differential scanning calorimetry (DSC) curves of the three nonionic surfactants octylglucoside, nonylglucoside (NG), and decylmaltoside and the anionic surfactant sodium dodecyl sulfate at surfactant concentrations just below and above the minimal cmc. The DSC curves exhibit characteristic maxima at temperatures (critical micellization temperatures (cmt)) where the micellization or demicellization process occurs and a minimum at the same temperature where the cmc minimum is observed by isothermal titration calorimetry (ITC) measurements. The DSC curves were calculated using a mass-action model with fixed aggregation number and thermodynamic parameters for the demicellization process as obtained by simulations of the titration curves obtained from ITC measurements of the same surfactants. The results show that it is possible to obtain cmt or cmc values of surfactants directly from DSC experiments provided that the minimal cmc of the surfactant is not below 2 mM. The DSC curves of NG show a more complicated temperature-induced aggregation behavior than expected front the ITC measurements.