Determination of the Ion Exchange Constants of Four Aromatic Organic Anions Competing for a Cationic Micellar Interface

Abstract

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, α, 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 1H 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 1H 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.

Effects of inert organic and inorganic anions on the rates of intramolecular general base-catalyzed reactions of ionized phenyl salicylate (PS?) with piperidine in the presence of cationic micelles

“…The values of ion-exchange constants, obtained directly by physical techniques, for m-nitrobenzoate ion 23 and m-phthalate ( isophthalate) dianion 24 are significantly larger than those for corresponding ortho-isomers. Although the value of É for isophthalate dianion is slightly more than four times smaller than that for benzoate ion, the water solubility of benzoic acid is 34 times larger than that of isophthalic acid.…”

Kinetics and mechanism of alkaline hydrolysis of 4‐nitrophthalimide in the absence and presence of cationic micelles

“…The structural location of the nitro group in NPTH roughly corresponds to in between the p-and m-nitrobenzoate ions. Kinetic data revealed a nearly 1.4-fold larger affinity of the benzoate ion [32] compared to the o-and p-nitrobenzoate ion [28] toward cationic micelles. Thus, a ϳ 2.5-fold larger value of K S for NPTH compared to K S for phthalimide is conceivable in view of the effect of the nitro group on K S .…”

“…(5): The value of K S (ϭ 6600 M Ϫ1 ) is nearly 2.5-fold larger than K S (ϭ 2600 M Ϫ1 ) for phthalimide [7]. The CTABr micellar surface affinity of the m-nitrobenzoate ion is nearly 3-fold larger than that of the o-or p-nitrobenzoate ion [28]. The reason for the low cationic micellar surface affinity of the p-nitrobenzoate ion is attributed to the significant polarity of the nitro group.…”

Kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous micellar media

“…The addition of these salts means that we are adding ionic species having hydrophobic character. Such anions will be solubilized in the micelle palisade layer with the acidic group exposed close to the headgroup region [27,28]. Therefore, in addition to neutralization of micellar positive charge, they will restrict solubilization sites to hydrophobic substrates.…”