THERMODYNAMICS IN SURFACTANT SOLUTIONS - DETERMINATION OF THE MICELLIZATION ENTHALPY AND ENTROPY OF ALCOHOL/SURFACTANT MIXED MICELLES - A COMPARISON OF CALORIMETRIC METHODS WITH TEMPERATURE DIFFERENTIATION OF THE IN X(CMC) VALUES

The variation of the critical micelle concentration (cmc) has been determined for sodium dodecyl sulfate (SDS) in water and in increasing concentrations of 1-butanol (C4OH) at temperatures ranging from 25 to 55 degrees C. Estimates of the thermodynamic parameters of micellization (Delta(mic)G(o), Delta(mic)H(o) and Delta(mic)S(o)) are obtained from the variation of the In X(cmc) with the absolute temperature (method 1). In addition, the enthalpies of micellization have been measured directly for the above systems and SDS/1-pentanol (C5OH) mixed micelles, by use of isoperibol solution calorimetry (method 2) over the same temperature range. The results from both methods, as a function of increasing temperature, indicate a shift in the driving force for the formation of micelles in the direction of decreasing enthalpy, in agreement with the literature, However, as a function of increasing alcohol concentration, the enthalpies from methods 1 and 2 are not comparable. Some possible reasons for the discrepancy will be discussed. Finally, we have examined the influence of the cosurfactant (alcohol) chain length on the thermodynamics of mixed micellization.