Convenient estimation for counterion dissociation of cationic micelles using chloride-sensitive fluorescence probe

Chloride-sensitive fluorescence probe provides a new approach to studying the degree of micellar counterion dissociation (alpha). The fluorescence of N-ethoxycarbonylmethyl-6-methoxyquinolinium bromide (MQAE) is quenched by chloride ion with linear Stern-Volmer plots. Thus the fluorescence intensity can be used to monitor the concentration of free chloride ion in micellar solutions. The Stern-Volmer plot gave a distinct break at critical micelle concentration (CMC) due to the counterion binding to micelles. The estimated alpha and CMCs of cationic surfactants including fluorocarbon ones were in fair agreement with the reported experimental values. The MQAE has greater sensitivity to bromide ion of CTAB than chloride ion of CTAC. The alpha of 0.16 for CTAB micelles was almost constant up to 0.2 M CTA-B at 35 degreesC. The alpha values of CTAB micelles decreased with increasing the concentrations of CTAB and NaBr along with micellar growth. (C) 2001 Academic Press.