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, α. 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 α 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 α 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.