Steady State and Time-resolved Fluorescence Spectroscopy of Cinchonine Dication in Sodium Dodecylsulphate Micellar System

Abstract

This paper reports the influence of surface charge of the micelles on to the photophysical properties of a cinchonine dication (C²⁺) fluorophore in anionic, sodium dodecylsulphate (SDS), surfactant at premicellar, micellar and post-micellar concentrations in aqueous phase at room temperature. In steady state there is no change in absorbance of C²⁺ in all the micelles studied. However, a continuous increase in emission intensity of C²⁺ has been observed as the concentration of SDS increases up to critical micellar concentration (cmc) of the surfactant, whereas, no change in intensity has been observed beyond cmc. Edge excitation red shift (EERS) in the fluorescence maximum of C²⁺ is observed in the surfactant. The magnitude of EERS in CTAB is lesser than that in bulk. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules the solvent relaxation rate increases which results in decrease in net magnitude of EERS. The fluorescence decay curve of C²⁺ fits with multi exponential functions in the micellar system. The increase in lifetime of C²⁺ in SDS has been attributed to the increase in radiative rate due to the incorporation of C²⁺ at the micelle –water interface. The location of the probe molecule in micellar systems is justified by a variety of spectral parameters such as dielectric constant, E_T (30), viscosity, anisotropy, EERS, average fluorescence decay time, radiative and non-radiative rate constants, and rotational relaxation time. All experimental results suggest that the C²⁺ molecule binds strongly with the SDS micelles and reside at micellar–water interface. The binding constant (K_b) calculated for C²⁺ in SDS revealed that the electrostatic forces mediate charge probe-micelle association.