Micellar Counterion Binding Probed by Fluorescence Quenching of 6-methoxy-N-(3-sulfopropyl)quinolinium

Abstract

The halide sensitive fluorescence probe, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), provides a direct approach for measurement of halide concentrations dissociated from the surfactant monomer and micelle. The gel filtration method confirms that the water-soluble SPQ probe will tend to partition in aqueous bulk phase without trapping in cationic micelles. The fluorescence of SPQ is quenched by free halide ions with a linear Stern-Volmer relation below the CMC of cationic surfactants. The Stern-Volmer plot gave a distinct break at CMC owing to the counterion binding to micelles. The CMC and degree of micellar counterion binding (β) are generally in agreement with those obtained by conductivity method. The SPQ fluorescence quenching had high sensitivities with Stern-Volmer constants of 125 M^-1 (chloride) and 223 M^-1 (bromide), while it was insensitive to nitrate ion. The selective quenching behavior shows that the nitrate ions bind to micelles displacing bromide for tetradecyltrimethylammonium micelles. The increase in counterion binding to micellar surface lower the CMC of surfactants, TTAOAc (tetradecyltrimethylammonium acetate) > TTAC (tetradecyltrimethylammonium chloride) > TTAB (tetradecyltrimethylammonium bromide) > TTAN (tetradecyltrimethylammonium nitrate).