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 been prepared from a neutral anion binding receptor, meso-octamethylcalix[4]pyrrole (1). Analogous systems were also prepared from the novel pyridine-containing analogues of 1, namely dichlorocalix[2]pyrrole[2]pyridine (2) and tetrachlorocalix[4]pyridine (3). At lower pH values (i.e., 3.5 and 5.5), ISEs derived from 1 display strong anionic (negative slope) responses toward Br-, 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 slope) responses toward chloride and bromide anions but also selectivities (i.e., Br- < Cl- < OH- ≈ F- < HPO4 2-) that are non-Hofmeister in nature. This is considered consistent with the PVC-supported receptor 1 behaving as a direct anion binding agent at low pH but acting, at least in part, as an hydroxide-complexing receptor at higher pH. For the ISEs based on 2 and 3, no special non-Hofmeister selectivity is observed at pH 9.0. However, at lower pH values both increased anionic responses and improved selectivities for hydrophilic anions (e.g., F- and H2PO4 -) are observed. These observations are rationalized in terms of protonation effects involving the pyridine-containing receptors from which these ISEs are derived. For all the receptors discussed in this paper, the addition of tridodecylmethylammonium chloride (TDDMA), a known lipophilic additive, serves to increase the magnitude of the ISE response but only at the price of greatly reduced anion selectivity.