A thermometric titration study on the micelle formation of sodium decyl sulfate in water

The critical micelle concentration (cmc) and the enthalpy of micelle format ion (Delta H-mic) of sodium n-decyl sulfate (SDeS) in aqueous solutions wer e determined in the temperature range 15-45 degreesC by isothermal titratio n microcalorimetry. An empirical sigmoidal expression providing high accura cy and internal consistency was used for the evaluation of the titration cu rves. As a function of increasing temperature, the cmc passes through a min imum at 29.5 degreesC, where Delta H-mic changes sign from positive to nega tive. The isosteric enthalpies calculated by means of the van't Hoff relati on agree very well with the directly measured calorimetric enthalpies. The emc, enthalpy, Gibbs energy, entropy, and heat capacity data of SDeS micell ization are reported and compared with the results of previous studies on t he same system. It was found that the presence of n-decanol, which appears to be a product of the natural hydrolysis of SDeS, significantly lowers the cmc. Delta H-mic is less appreciably influenced by this impurity. (C) 2001 Academic Press.