A Semi-Empirical Spectrophotometric (SESp) Method for the Indirect Determination of the Ratio of Cationic Micellar Binding Constants of Counterions X^– and Br^– (K_X/K_Br)

Abstract

The semi-empirical spectrophotometric (SESp) method, for the indirect determination of ion exchange constants (K_X^Br) of ion exchange processes occurring between counterions (X^– and Br^–) at the cationic micellar surface, is described in this article. The method uses an anionic spectrophotometric probe molecule, N-(2-methoxyphenyl)phthalamate ion (1^–), which measures the effects of varying concentrations of inert inorganic or organic salt (Na_vX, v = 1, 2) on absorbance, (A_ob) at 310 nm, of samples containing constant concentrations of 1^–, NaOH and cationic micelles. The observed data fit satisfactorily to an empirical equation which gives the values of two empirical constants. These empirical constants lead to the determination of K_X^Br (= K_X/K_Br with K_X and K_Br representing cationic micellar binding constants of counterions X and Br^–). This method gives values of K_X^Br for both moderately hydrophobic and hydrophilic X^–. The values of K_X^Br, obtained by using this method, are comparable with the corresponding values of K_X^Br, obtained by the use of semi-empirical kinetic (SEK) method, for different moderately hydrophobic X. The values of K_X^Br for X = Cl^– and 2,6-Cl₂C₆H₃CO₂^–, obtained by the use of SESp and SEK methods, are similar to those obtained by the use of other different conventional methods.