Degree of micelle ionization and micellar growth for gemini surfactants detected by 6-methoxy-N-(3-sulfopropyl)quinolinium fluorescence quenching

The degree of micelle ionization of gemini surfactants has been investigated by using halide-sensitive fluorescence probes (e.g., 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ)). The fluorescence is quenched by the free bromide ions dissociated from surfactants. The degree of micelle ionization increased with increasing spacer chain length, but it decreased with increasing surfactant concentration. The Stern-Volmer plot gave two inflection points (i.e., not only at the cmc but also far above the cmc). The second inflection point suggested spherocylindrical micellar growth with decreases in the degree of micelle ionization. The spherocylindrical micellar growth was depressed with increasing spacer chain length, whereas it was enhanced with increasing tail chain length. The degree of micelle ionization of spherocylindrical micelles depended on the concentration and chain length of gemini surfactants. The change in SPQ fluorescence spectra upon hydrogenation was utilized to evaluate the solubilization site in micelle solutions. The dissolved SPQ in water was instantly reduced by the addition of NaBH4, resulting in abrupt changes in fluorescence intensity and spectral shift. All of the SPQ in micelle solution was also instantly reduced by NaBH4, indicating the existence of SPQ in the water bulk phase, but its fluorescence intensity increased upon the solubilization of hydrogenated SPQ into micelles.