Fluorescence Quenching Equilibria Studies in Ionic Micelles in Aqueous Media

Abstract

The fluorescence quenching equilibria of different molecules in anionic and cationic micelles in aqueous media have been reported in the current literature. In one particular study involving SDS (Sodium Dodecyl Sulfate) - Ru(bipy)$_3^{ + 2}$ (tris(2,2’-bipyridyl)-ruthenium (II))-(9-methylanthracene), it has been asserted that the increase in aggregation number of SDS micelles on addition of NaCl could be estimated from fluorescence quenching studies. A reinvestiagtion was therefore carried out under varying experimental conditions. This study reportes that aggregation numbers cannot be estimated from spectral measurements (based on our experimental and thermodynamic analyses). Another fluorescence quenching system: SDS-Acridine orange-Methylene blue was also studied. This system also clearly showed that the fluorescence data are not dependent on aggregation number of micelles. The fluorescence quenching of anthracene and perylene in SDS was studied in micellar solutions on addition of Cu²⁺ and Ni²⁺. The binding constants of Cu²⁺ and Ni²⁺ were calculated which agreed with the literature data. In cetyl trimethyl ammonium bromide (CTAB) micelles, the fluorescence of anthracene was enhanced on the addition of KCl, while no effect was observed if perylene was used. In cetyltrimethyl ammonium chloride (CTAC) the fluorescence of anthracene was quenched on addition of KBr, while no effect was observed in the case of perylene. The latter data clearly show that the probe anthracene is able to interact with the counter-ions in both SDS or CTAB or CTAC systems. On the other hand, the probe perylene behaves differently in CTAB and CTAC systems, as compared to SDS-perylene system. This observation leads us to conclude that fluorescence probes are not present in micelles but as distinct (detergent-probe) complexes, or as ‘mixed micelles’. The structure of these mixed micelles is found to be different from that of the pure detergent micelles, as one would also expect from thermodynamic considerations.