Micelle formation by sodium dodecyl sulfate in water-additive systems

The conductances of sodium dodecyl sulfate (SDS) have been determined in water in the presence of various additives, such as strong nitrogen organic bases (pyridine, 3- and 4-methylpyridines, and 2,6-dimethylpyridine), carboxylic acids (acetic acid, propionic acid, and butyric), and alcohols (methanol (MeOH), l-butanol (BuOH), 2-methyl-1-propanol, 2-methyl-2-propanol, and 2-methyl-2-butanol) at 25 degrees C. The conductances of SDS in water in the presence of pyridine and acetic acid at temperatures between 15 and 45 degrees C have also been measured. From the conductivity data, the critical micellar concentration (cmc), degree of counter-ion association (alpha), molar conductivities at infinite dilution (Lambda(0)), limiting ionic molar conductivities (lambda(+/-)degrees), free energy of transfer of the surfactant hydrocarbon chain from medium to the micelle (Delta G(HP)degrees), free energy of transfer of surface contributions (Delta G(s) degrees), and standard enthalpies of micellisation (Delta H(HP)degrees) of SDS have been computed. It has been found that the presence df strong nitrogen organic bases causes strong micelle denaturation, whereas the presence of carboxylic acids and alcohols stabilizes the micelles, with the exception of MeOH and 2-methyl-2-butanol, The temperature effect on the micellisation of SDS in the presence of pyridine shows that the enthalpy of micellisation becomes less negative along with an increase in temperature. However, the enthalpy of transfer of a hydrocarbon group from an aqueous to a nonpolar environment in the presence of acetic acid is endothermic.