Effect of counterions on the micellization of alkali alkyl sulfates

COLL 329

Gunther Czichocki, Marie Helene Ropers, and Gerald Brezesinski. Max-Planck-Institut for Colloids and Interfaces, Am Muehlenberg 1, Potsdam/Golm, 14424, Germany
Thermodynamics of micelle formation of alkali decyl and dodecyl sulfates was investigated by using Isothermal Titration Calorimetry (ITC). The influence of counterions (Li+, Na+, K+, Cs+) on critical micelle concentration (cmc) and enthalpy of micellization (ÄHmic) was studied between 10oC and 60oC. The enthalpy of micellization decreases strongly with increasing temperature and passes through zero (endothermic to exothermic processes) while the cmc versus temperature exhibits a minimum. Critical micelle concentration and ÄHmic decrease in the order Li+ > Na+ > K+ >Cs+ at a given temperature and for a fixed chain length. It is related to the increase of the binding of counterions to micelles. The electrostatic repulsion between ionic head groups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The cmc versus temperature reaches a minimum while the enthalpy of micellization passes through zero. The heat of dilution of micelles is markedly dependent on temperature and is correlated with the temperature dependent shape of micelles. The use of the pseudo-phase separation model including the counterion binding shows that the variations of enthalpy and entropy of micellization compensate each other and the free energy ÄGmic is only slightly dependent on counterions and temperature.