Molecular recognition at an organic-aqueous interface: Heterocalixarenes as anion binding agents in liquid polymeric membrane ion-selective electrodes

Poly(vinyl chloride) (PVC)-derived ion-selective electrodes (ISEs) have bee n prepared from a neutral anion binding receptor, meso-octamethylcalix[4]py rrole (1). Analogous systems were also prepared from the novel pyridine-con taining analogues of 1, namely dichlorocalix[2]pyrrole[2]pyridine (2) and t etrachlorocalix[4]pyridine (3). At lower pH values (i.e., 3.5 and 5.5), ISE s derived from 1 display strong anionic (negative slope) responses toward B r-, Cl-, and H2PO4- and, to a much lesser extent, F-. By contrast, at high pH (i.e., pH 9.0) ISEs derived from 1 not only display cationic (positive s lope) responses toward chloride and bromide anions but also selectivities ( i.e., Br- < Cl- < OH- approximate to F- < HPO42-) that are non-Hofmeister i n nature. This is considered consistent with the PVC-supported receptor 1 b ehaving as a direct anion binding agent at low pH but acting, at least in p art. as an hydroxide-complexing receptor at higher pH Fnr thp ISEs based nn 2 and no special non-Hofmeister selectivity is observed at pH 9.0. However , at lower pH values both increased anionic responses and improved selectiv ities for hydrophilic anions (e.g., F- and H2PO4-) are observed. These obse rvations are rationalized in terms of protonation effects involving the pyr idine-containing receptors from which these ISEs are derived. For all the r eceptors discussed in this paper, the addition of tridodecylmethyl-ammonium chloride (TDDMA), a known lipophilic additive, serves to increase the magn itude of the LSE response but only at the price of greatly reduced anion se lectivity.