DETERMINATION OF THE ION-EXCHANGE CONSTANTS OF 4 AROMATIC ORGANIC-ANIONS COMPETING FOR A CATIONIC MICELLAR INTERFACE

In order to evaluate factors that affect the extent of organic anion b inding to charged micellar interfaces, the ion exchange constants of f our substituted aromatic anions, o-, m-, and p-nitrobenzoate and salic ylate, competing for the tetradecyltrimethylammonium bromide (TTAB) mi cellar interface have been determined at room temperature employing a standard two-site model. The ion exchange constants were calculated us ing the chemical shifts of the protons of the organic anions, the frac tional ionization constants, alpha, and the critical micellization con centrations (cmc). The cmc values were determined by surface tension a nd conductivity methods, whereas the fractional ionization constants o f the micellar aggregates were calculated from conductivity data. For all systems, the H-1 NMR spectra revealed characteristic changes in ch emical shifts of the aromatic protons at concentrations above the cmc allowing the fractions of micellar bound organic anion to be measured. Analyzing H-1 NMR spectroscopic data together with the cmc and the fr actional ionization constants, the ion-exchange constants of 3.8 for o -nitrobenzoate, of 11 for m-nitrobenzoate, or 3.3 for p-nitrobenzoate, and of 20 for salicylate were calculated. The ion exchange constants clearly reveal that salicylate and m-nitrobenzoate more readily replac e inorganic bromide at the micellar interface than the para- and ortho -substituted nitrobenzoates and may provide information for deducing i mportant mechanisms controlling the binding of organic ions at charged interfaces. Furthermore, the data reveal that inorganic bromide compe tes more strongly with a hydrophobic organic anion for binding to the charged micellar interface than is generally assumed.