Characterization of the Structural Transitions in CTAB Micelles Using Fluorescein Isothiocyanate

Abstract

A fluorescence-based method has been developed to detect the structural changes that occur in micelle systems. The sensitivity of fluorescein isothiocyanate (FITC) has been evaluated for (i) detecting the micellization of cetyltrimethyl ammonium bromide (CTAB) and (ii) probing the concentration dependent aggregation, leading to micro- structural changes that occur within CTAB micelles. The critical micelle concentration (cmc) of CTAB has been determined to be 1.35 ± 0.35 mM using the fluorescence spectral characteristics of FITC. Because the experimental conditions have been altered to optimize FITC probing, the cmc is also validated by surface tension and conductivity measurements. To make sure FITC does not affect the properties of micelles, we calculated the micelle binding constant, KM ,a t different concentrations of FITC using a nonlinear least-squares method. The average KM for (FITC)T ≤ 5m M is found to be 6575 ± 233. The optical properties of FITC have also been found to be sensitive in response to the changes in the polarity of the microenvironment, caused by the structural changes in CTAB/water system. Two significant observations are noticed from the fluorescence spectra of FITC in CTAB solutions: (i) a decrease followed by an increase in the maximum intensity (Imax )o ffluorescence and (ii) a red shift of maximum wavelength (λmax) with increasing concentrations of CTAB. These observations could be correlated with the concentration-dependent microstructural changes in CTAB micelles. On the basis of the experimental observations, FITC is found to be a suitable fluorescent probe for monitoring the changes in CTAB micelle structures.