Micellar Counterion Binding for Alkylpyridinium Iodides Probed by N-Sulfopropylacridinium Fluorescence Quenching

Abstract

Fluorescence quenching using N-sulfopropylacridinium (SPA) makes it possible to determine the degree of micellar counterion binding (β) of alkylpyridinium iodide in aqueous solution. But with quinoline derivatives as a halide-sensitive fluorescence probe, this was not possible. Fluorescence was quenched by iodide ions as indicated by linear Stern-Volmer plots, the slope of which was noted to decrease beyond cmc owing to micellar counterion binding with SPA excitation at 416 nm. The slope was found to significantly increase beyond cmc with SPA excitation at 357 nm. The abrupt quenching was due to absorption bands resulting from charge-transfer interactions of pyridinium and iodide ions. SPA fluorescence was quenched by iodide and bromide ions but virtually unaffected by nitrate or sulfate ions. Selective counterion binding of iodide ions is thus shown to be detectable.