Determination of the ion exchange constants of four aromatic organic anions competing for a cationic micellar interface

In order to evaluate factors that affect the extent of organic anion binding to charged micellar interfaces, the ion exchange constants of four substituted aromatic anions, o-, m-, and p-nitrobenzoate and salicylate, competing for the tetradecyltrimethylammonium bromide (TTAB) micellar interface have been determined at room temperature employing a standard two-site model. The ion exchange constants were calculated using the chemical shifts of the protons of the organic anions, the fractional ionization constants, alpha, and the critical micellization concentrations (cmc). The cmc values were determined by surface tension and conductivity methods, whereas the fractional ionization constants of the micellar aggregates were calculated from conductivity data. For all systems, the H-1 NMR spectra revealed characteristic changes in chemical 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 fractional 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 replace inorganic bromide at the micellar interface than the para- and ortho-substituted nitrobenzoates and may provide information for deducing important mechanisms controlling the binding of organic ions at charged interfaces. Furthermore, the data reveal that inorganic bromide competes more strongly with a hydrophobic organic anion for binding to the charged micellar interface than is generally assumed.