Abstract

Purpose:This study was carried out to examine the effect of different types of micelles on the UV absorption and acid-base dissociation of indicators and to use that information for the CMC determination of surfactants at various pH values. Methods: UV spectra of neutral red (NR) and bromothymol blue (BTB) were recorded and the absorbance at the maximum absorption wavelength (λ_max) was determined in surfactant solutions, Tween 80, cetyltrimethylammonium bromide (CTAB), and sodium lauryl sulfate (SLS) at pH values of 4, 7, and 10. The absorbance ratio in the presence of surfactants to that in the absence of surfactants was calculated. The ratio was plotted against the surfactant concentration and the break point in the plot represented the critical micelle concentration (CMC). The acid-base dissociation of the dyes was determined in the absence and presence of the micelles. Results: A shift in the UV spectra of the indicators was observed in the presence of micelles. The CMC values determined using the break point in the plot between the absorbance ratio and the surfactant concentration (e.g. 1.54±0.16 mM for SLS) were consistent with those determined using a tensiometer with an attached duNouy ring (e.g. 1.28±0.06 mM for SLS). A shift of the apparent pKa was subsequently observed owing to the shift in acid-base equilibrium which was used as evidence of the interaction between micelles and dye indicators. Conclusions: Micelles interact with indicators resulting in a shift in the acid-base equilibria and UV spectra of the indicators due to redistribution of acid and base species into the micellar pseudo-phase, especially charged micelles. These interactions can be used to determine the critical micelle concentrations of surfactants. However, the types of indicators and the pH of the systems used should be carefully selected.


Keywords:  Surfactants   Micelles   Critical micelle concentration   Dye indicators