NEW METHOD FOR ESTIMATING THE DEGREE OF IONIZATION AND COUNTERION SELECTIVITY OF CETYLTRIMETHYLAMMONIUM HALIDE MICELLES - CHEMICAL TRAPPINGOF FREE COUNTERIONS BY A WATER-SOLUBLE ARENEDIAZONIUM ION

Products from spontaneous reaction of a short-chain, water soluble are nediazonium salt, 2,4,6-trimethylbenzenediazonium tetrafluoroborate (1 -ArN2BF4), in aqueous micellar solutions of cetyltrimethylammonium hal ides ((CTA)X (X = Cl, Pr)) are used to estimate the degree of ionizati on, alpha, and the ion exchange constant, K-Br/Cl The arenediazonium i on (1-ArN2+) reacts by rate-determining loss of N-2 to give an aryl ca tion that traps available nucleophiles, i.e. H2O, Cl-, and Br-, to giv e stable phenol (1-ArOH) and halobenzene products (1-ArCl and 1-ArBr), respectively. Product yields are determined by HPLC from calibration curves obtained from independently prepared standards. Reproducible yi elds are obtained at very low halide ion concentrations, on the order of millimolar, well within the range needed to detect the ''free count erions'' in the aqueous intermicellar pseudophase. The basic assumptio n of the method is that 1-ArN2+ remains in the aqueous pseudophase at all (CTA)X and NaX concentrations. Trends in the Stern-Volmer constant for fluorescence quenching of Ru(bpy)(3)(2+) by 1-ArN2+ in (CTA)Cl/Na Cl solutions strongly support this assumption. The results obtained by this method are in good agreement with literature values: alpha = 0.2 5 and 0.29 for (CTA)Br and (CTA)Cl, respectively, and K-Br/Cl = 2.65 /- 0.4. Potential applications of the method are briefly discussed.