Counterion effect on the thermodynamics of micellization of alkyl sulfates

Thermodynamics of micelle formation of anionic surfactants was investigated by using isothermal titration calorimetry (ITC). Highly purified decyl and dodecyl sulfates have been used to analyze the effect of counterions (Li+, Na+, K+, and Cs+) on critical micelle concentration (cmc) and enthalpy of micellization (DeltaH(mic) determined between 10 and 60 degreesC. The enthalpy of micellization decreases strongly with increasing temperature and passes trough zero (endothermic to exothermic processes), while the cmc versus temperature exhibits a minimum. At a given temperature and for a fixed chain length, the decrease of cmc and DeltaH(mic) in the order Li+ > Na+ > K+ > Cs+ is related to the increase of the binding of counterions to micelles. The electrostatic repulsions between ionic headgroups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The heat of dilution of micelles is markedly dependent on temperature and is correlated with the temperature-dependent shape of micelles. The cmc concept has an exact meaning within the so-called phase separation model of micelle formation. Therefore, free energy and entropy were deduced from the cmc and enthalpy of micellation using this model by taking into account the counterion binding. The temperature changes of DeltaS(mic) indicate that the process of micellization is entropically driven. DeltaG(mic) is always negative (thermodynamically favored process) and slightly temperature and counterion dependent.