Characterization of the Structural Transitions in CTAB Micelles Using Fluorescein Isothiocyanate

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 microstructural 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, at different concentrations of FITC using a nonlinear least-squares method. The average KM for [FITC]T ≤ 5 mM 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) of fluorescence 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. ■ INTRODUCTION Micellization is a reversible self-assembly process of surfaceactive materials above a well-defined threshold concentration, known as the critical micelle concentration (cmc). The morphology of micelles relies heavily on the electrostatic/steric repulsion between the hydrophilic heads and the attractive interactions of the hydrophobic tails. In general, the tail of a surfactant may consist of 8 to 18 carbons. The average size of spherical micelles is ∼50 Å with around 40−100 surfactant molecules per micelle unit. However, manipulation of the electrostatic/steric and the hydrophobic interactions by any means may result in the formation of different structures, such as cylindrical, wormlike, and vesicles. Known as the “smart nano-materials” and “living polymers”, the flexibility of micelles has attracted people from industries as well as the researchers to explore the possibilities and advantages they might offer in various applications including drug delivery. The microstructures of micelles and their changes have been determined by several analytical techniques, such as NMR, neutron or X-ray scattering, chemical probes, electrochemistry, and electrophoresis. Micelle structures may be significantly altered by the slightest changes in experimental conditions, such as the addition of hydrophobic counterions. The growth of micelles may be induced by a decrease in the electrostatic repulsion between the micelle heads and an increase in the hydrophobic interaction between the tails. Such growth may also occur simply due to an increase in the concentration of the surfactants. Because the changes to microstructures during the concentration-dependent growth of micelles may be very subtle, the use of a very sensitive probe is recommended. It is also important to make sure that the probe itself does not significantly affect the process of either micelle aggregation or deformation. This equilibrium has been described in review articles and textbooks. Any interaction of the probe with surfactant molecules may result in probe-micelle aggregation, leading to the formation of different type of “impure” micelles, as reported in a system involving cyclodextrin and CTAB. Fluorescence probing has been a useful method for the characterization of many systems including micelles. A search in the literature results in a variety of fluorophore compounds sensitive in detecting the cmc of surfactants. This is mainly associated with the significant changes in the optical properties of the fluorophore due to the presence of surface-active molecules in monomer and micelle forms. However, a fundamental understanding of the suitability of fluorophores for Received: May 18, 2012 Revised: June 19, 2012 Published: June 22, 2012 Article