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 onl y by changes in concentration but also by changes in temperature, because t he critical micellar concentration (cmc) exhibits a characteristic temperat ure dependence with a minimum at temperatures between 20 and 50 degreesC, d epending 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 minim al cmc. The DSC curves exhibit characteristic maxima at temperatures (criti cal micellization temperatures (cmt)) where the micellization or demicelliz ation process occurs and a minimum at the same temperature where the cmc mi nimum is observed by isothermal titration calorimetry (ITC) measurements. T he DSC curves were calculated using a mass-action model with fixed aggregat ion number and thermodynamic parameters for the demicellization process as obtained by simulations of the titration curves obtained from ITC measureme nts of the same surfactants. The results show that it is possible to obtain cmt or cmc values of surfactants directly from DSC experiments provided th at the minimal cmc of the surfactant is not below 2 mM. The DSC curves of N G show a more complicated temperature-induced aggregation behavior than exp ected front the ITC measurements.