Pyrene absorption can be a convenient method for probing critical micellar concentration (cmc) and indexing micellar polarity

The critical micellar concentration (cmc) of both ionic and non-ionic surfactants can be conveniently determined from the measurements of UV absorption of pyrene in surfactant solution. The results on a number of surfactants have agreed with that realized from pyrene fluorescence measurements as well as that obtained following conductometric, tensiometric and calorimetric methods. The absorbance vs [surfactant] profiles for all the major UV spectral peaks of pyrene have been found to be sigmoidal in nature which were analyzed according to Sigmoidal–Boltzmann equation (SBE) to evaluate the cmcs of the studied surfactants. The difference between the initial and the final asymptotes (aiai and afaf, respectively) of the sigmoidal profile, Δa=(af−ai)Δa=(af−ai) and the slope of the sigmoid, SsigSsig have been observed to depend on the type of the surfactant. The Δa   has shown a linear correlation with the ratio of the fluorescence intensities of the first and the third vibronic peaks, I1/I3I1/I3 of pyrene which is considered as a measure of the environmental polarity (herein micellar interior) of the probe (pyrene). Thus, Δa values have the prospect for use as another index for the estimation of polarity of micellar interior.