On the selection of the optimal plasticizer for calix[n]arene-based ion-selective electrodes: Possible correlation between the ion selectivity and the 'softness' of the plasticizer

A criterion for the selection of a suitable plasticizer for calix[n]arene-based ion-selective electrodes is discussed. The cation selectivity of plasticized membranes without the ligand was first measured as a reference. The membranes can be roughly classified into two groups. The first group shows cation selectivity in the order Cs⁺<Rb⁺>K⁺>Na⁺>Li⁺. The membranes in the second group are made of phosphorus plasticizers, which show a selectivity in the reverse order. The plasticizers in the first group featured a linear relationship between the dipole moment of the plasticizer (calculated by a PM3 method) and the ratio of cesium selectivity to lithium selectivity. The linear relationship supports the view that the polar membrane which includes a 'soft' plasticizer with a large dipole moment shows selectivity for Cs⁺, whereas the nonpolar membrane including the 'soft' plasticizer with the small dipole moment shows much lower selectivity for Cs⁺. Next, 2-fluorphenyl-2′-nitrophenyl ether (FPNPE) which showed the highest Cs⁺ selectivity and tris(2-ethylhexyl)phosphate (TEHP) which showed the highest Li⁺ selectivity were mixed in an appropriate ratio to make membranes with a different affinity for 'hard' ions. The metal selectivities of several crown-based and calixarene-based ionophores were examined in these membranes. Although a few exceptions exist, the polar 'soft' membrane is favorable when the interfering metal ion is 'hard", whereas the 'hard' membrane is favorable when the interfering metal ion is 'soft'.