THERMODYNAMICS OF MICELLE FORMATION AS A FUNCTION OF TEMPERATURE - A HIGH-SENSITIVITY TITRATION CALORIMETRY STUDY

Titration calorimetry was employed to measure the critical micelle con centration (cmc) and the heat of demicellization Delta H-demic of the four surfactants octyl glucoside, sodium dodecyl sulfate (SDS), sodium cholate, and sodium deoxycholate at temperatures between 10 and 70-80 degrees C. From these data, the thermodynamic parameters Delta G(demi c), Delta S-demic, and Delta C-p,C-demic associated with the demicelli zation process were calculated. Titration calorimetry has the advantag e that the cmc and the thermodynamic parameter Delta H-demic can be di rectly measured, whereas with other methods Delta H-demic has to be ca lculated from the temperature dependence of the cmc, which requires hi gh precision for the cme data. Changes in temperature caused large var iations of Delta H-demic and Delta S-demic, whereas Delta G(demic) rem ained virtually constant. Therefore, the changes in enthalpy and entro py almost completely compensate each other. At room temperature, the e ntropy was found to be the dominant factor responsible for micellizati on, whereas at elevated temperatures contributions from enthalpy domin ate. These observations are in agreement with data of other processes where hydrophobic effects play a major role and were used to discuss t he nature of the driving forces that rule micelle formation at various temperatures. Furthermore, predictions regarding the degree of hydrat ion of the micelle interior were made. It is shown that titration calo rimetry is an easy and fast method to determine the cmc and the demice llization enthalpy from a single experiment. For surfactants with low aggregation numbers the titration curves could be simulated using a ma ss action model.