Thermodynamics of Micelle Formation as a Function of Temperature: A High Sensitivity Titration Calorimetry Study

Abstract

Summary and Conclusions Titration calorimetry can be used as a routine method for the determination of all thermodynamic quantities for the micellization of surfactant^.'.^^^^-'^ Particularly, the availability of power compensated microtitration calorimeters with high sensitivity has improved the speed and precision of the method.I6 The cmc and the micellization enthalpy can be determined from a single experiment. The temperature dependence of these quantities is easily accessible in the temperature range between 0 and 80 “C. From the temperature dependence of the demicellization enthalpy, udemic, the heat capacity change, ACp,demic, can be directly determined and thus information on the change in hydrophobic contacts upon demicellization can be estimated. The temperature at which the cmc minimum occurs can be determined with great precision from the temperature where AHdemic is zero. The shape of the titration curves also contains information on the aggregation number. A simulation of the titration curves for deoxycholate was attempted using a simple mass action model with a low aggregation number and the association constant and three enthalpic terms as adjustable parameters. The calculations support previous findings that cholates and deoxycholates form micelles with low aggregation numbers.