A NEW METHOD FOR ESTIMATING COUNTERION SELECTIVITY OF A CATIONIC ASSOCIATION COLLOID - TRAPPING OF INTERFACIAL CHLORIDE AND BROMIDE COUNTERIONS BY REACTION WITH MICELLAR BOUND ARYLDIAZONIUM SALTS

Yields of aryl halides and phenolic products from dediazoniation of a hydrophobic aryldiazonium salt bound to cetyltrimethylammonium halide, CTAX, micelles in 0.1 to 0.2 M HX with added NaX(XCl- and Br-) are used to estimate, simultaneously, the quantity of halide counter-ions and water at the micelle surface. The interfacial counter-ion concentration increases, both in solutions of CTABr with added NaBr and CTACl with added NaCl with a proportionate decrease in interfacial water. The selectivity of CTAX micelles toward Br- and Cl- in solutions containing mixtures of the two ions is obtained directly, unlike all previous methods which require an estimate of the fraction of bound counter-ions. Counter-ion exchange constants, KBrCl, estimated from aryl bromide/aryl chloride yield ratios at high NaX, are consistent with published values. Our results show, for the first time, that KBrCl is insensitive to ionic strength up to 3.0 M NaX when [Br-]/[Cl-]=1.0, but has modest dependence on the mole fraction of Br- at constant total NaX. KBrCl is also independent of surfactant concentration, after correcting for the fraction of bound counter-ions. In principle, product ratios from reaction of hydrophobic aryldiazonium salts can be used to estimate the relative concentrations of all nucleophiles at the surfaces of association colloids, provided stable aryl substituted products are formed via rate determining loss of N2. © 1990.