Halide ion induced quenching and enhancement of the fluorescence of fluoranthene solubilized in cetyltrimethylammonium bromide (CTAB) micelles

Abstract

A study is made of the quenching by halide ions of the fluorescence of fluoranthene solubilized in CI'AB micelles, using both steady-state and lifetime measurements. Stern-Volmer plots or plots of fluorescence decay rate against quencher concentration are non-linear, even under conditions of constant ionic strength. Only halide ions present in the Stern layer are effective in quenching and a relationship is derived between the concentration of ions present in the Stern layer and the total ion concentration using a multiple equilibrium model. Good agreement between theory and experiment is observed for the fluorescence decay rate and intensity as a function of total quencher ion concentration. The model shows that the quenching process is controlled by the quenching rate in the micelles and the association constant of the ions to the micelles. An increase in ionic strength leads to a decrease in the association constant and, owing to changes in micelle structure, it leads to an increase in the quenching rate in the micelle. The overall quenching effect is qualitatively dominated by the variations of the association equilibrium. The product of the micellar quenching rate constant and the number of active sites per micelle at an ionic strength of 0.3 mol dm-3 is numerically identical to the quenching rate constant in water. This may indicate there being only one active site for quenching per micelle. The fluoranthene fluorescence lifetime is found to increase upon addition of chloride or fluoride ions, indicating that these ions are replacing the more strongly quenching bromide ion in the Stern layer. Iodide quenches fluoranthene fluorescence more strongly than bromide ion. Association equilibrium constants for halide ions with CTAB micelles are estimated. * A value of 8.23 x mol dnr3 is given for the c.m.c. of CTAB in water at 30°C in ref. (17). t The system is as described in ref. (19), except that we looked at transient emission rather than absorption.* In calculating the ionic strength we have ignored any contribution from the CTAB to the ionic strength. However, assuming an aggregation number of 60, and a charge of + 12 for the micelle, CTAB wouId only contribute 0.01 4 mol drn-3 to the ionic strength at the concentrations employed here.

Effect of nanosize micelles of ionic and neutral surfactants on the photophysics of protonated 6-methoxyquinoline: Time-resolved fluorescence study

“…This has been H N H 3 CO attributed to Br À ion induced fluorescence quenching [26] and this observation is in agreement with earlier reports [32][33][34][35][36]. The fluorescence of 6MQ + is quenched by Br À ions to a greater extent because 6MQ + species tend to remain outside the micellar sphere due to the electrostatic repulsion from positively charged head groups of CTAB micelle.…”

Unfolding of rabbit serum albumin by cationic surfactants: Surface tensiometry, small-angle neutron scattering, intrinsic fluorescence, resonance Rayleigh scattering and circular dichroism studies

“…The fractal dimension of the protein-surfactant complex is the measure of the compactness of such a complex, which decreases with surfactant concentration as the amount of unfolding of protein increases. It is interesting to note that the aggregation number of CTAB on the complex is fairly low in our case as observed for the typical CTAB micelle ($60) [38]. This is the indication of participation of the hydrophobic portions of the unfolded protein chain in micelle formation.…”

Selective recognition of biologically important anions using a diblock polyfluorene–polythiophene conjugated polyelectrolyte

“…16 This model takes into account diffusion of a quencher held in the Stern layer into the core of an ionic micelle. 35 This is in contrast to the buried fluorophore model, in which the buried population is completely inaccessible to the quencher. This ionic micelle model was derived using a multi-equilibrium model 35 which leads to the modified Stern-Volmer relationship between the fluorescence intensity ratio and the quencher concentration:…”