Estimate of the ionization degree of ionic micelles based on Krafft temperature measurements

The Krafft temperature, T-K, of two classical ionic surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), were measured by conductivity, in the absence and in the presence of added salt, to test a recent suggestion (J. Phys. Chem. B 2002, 106, 9033) that T-K measurements can be used to estimate the degree of counterion dissociation of ionic micelles, alpha. The method is based on the fact that T-K of SDS and CTAB are functions of the concentration of counterions in the aqueous phase, C-aq, whether the counterions are provided by the surfactant or both the surfactant and added salt. The values of alpha are estimated by requiring that the values of T-K fall on a common curve when plotted against any function of C-aq and are found to be in good agreement with the literature. The present results confirm that the Krafft temperature of ionic surfactants is one of the properties from which alpha can be derived for both anionic and cationic micelles. In the case of SDS, because the aggregation number, N, is known as a function of C-aq, T-K may be plotted versus N. The plot showed excellent linear correlation; however, it is not yet known if the linear behavior is theoretically significant.