DISCRIMINATION BETWEEN BUTYLAMMONIUM ISOMERS BY CALIX[5]ARENE-BASED ISES

Penta-O-alkylated p-tert-butylcalix[5]arenes 1-5 (R = benzyl, isohexyl , isopropoxyethyl, isopropoxycarbonylmethyl, and tert-butolxycarbonylm ethyl, respectively) in a fixed C-5 upsilon cone conformation have bee n studied as ionophores in liquid membrane ion-selective electrodes (I SEs) for n-butylammonium against the other branched butylammonium isom ers, alkali metals, and ammonium ions, in terms of detection limits, s ensitivity, and selectivity, The highest levels of potentiometric sele ctivity and detection limits up to 3 x 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 affinit y for the Na+ ion of the lipophilic salt present in the membrane, as e videnced by appropriate H-1 NMR competition experiments with Na+ and n -BuNH3+ ions. Further investigation on the selectivity mechanism of io nophore 2 by means of frequency response analysis shows that the inter action of the linear butylammonium ion with membranes containing 2 inv olves a lower resistance process than that occurring with the other br anched isomers, thus suggesting the presence of a favorable kinetic-co ntrolled mechanism.