Discrimination between Butylammonium Isomers by Calix[5]arene-Based ISEs

Abstract

Penta-O-alkylated p-tert-butylcalix[5]arenes 1−5 (R = benzyl, isohexyl, isopropoxyethyl, isopropoxycarbonylmethyl, and tert-butoxycarbonylmethyl, respectively) in a fixed C_5vcone conformation have been studied as ionophores in liquid membrane ion-selective electrodes (ISEs) for n-butylammonium against the other branched butylammonium isomers, alkali metals, and ammonium ions, in terms of detection limits, sensitivity, and selectivity. The highest levels of potentiometric selectivity and detection limits up to 3 × 10^-6 M are observed with ISEs based on ionophore 2, where selectivity follows the order n-BuNH₃⁺ ≫ i-BuNH₃⁺ > s-BuNH₃⁺ > t-BuNH₃⁺. The lower potentiometric selectivity displayed by ISEs based on ionophores 3−5 is ascribed to their affinity for the Na⁺ ion of the lipophilic salt present in the membrane, as evidenced by appropriate ¹H NMR competition experiments with Na⁺ and n-BuNH₃⁺ ions. Further investigation on the selectivity mechanism of ionophore 2 by means of frequency response analysis shows that the interaction of the linear butylammonium ion with membranes containing 2 involves a lower resistance process than that occurring with the other branched isomers, thus suggesting the presence of a favorable kinetic-controlled mechanism.