Molecular design of lipophilic disalicylic acid compounds with varying spacers for selective lead(II) extraction

Lipophilic disalicylic acids 5,5′-decyl-2,2′-[1,2-ethanediylbis(oxy)]bisbenzoic acid (1), 5,5′-decyl-2,2′-[1,3-propanediylbis(oxy)]bisbenzoic acid (2), 5,5′-decyl-2,2′-[oxybis(1,2-ethanediyl-oxy)]bisbenzoic acid (3), 3,5-bis[2′-(2″-carboxyphenoxy)ethyl]-4-oxahexacyclo-[5.4.1.02,6.03,10.05,9.08,11]dodecane (4), and 1,3-bis[2′-(2″-carboxyphenoxy)ethyl]adamantane (5) are evaluated as selective Pb(II) extractants. The solvent extraction of Pb(II) and of Cu(II) from buffered aqueous solutions of varying pH into chloroform by ligands 1–5 is examined in relation to the molecular structure of the dicarboxylic acid extractant. Ligand 1, with an ethylene spacer between two lipophilic salicylic acid units, exhibits excellent extraction selectivity for Pb(II) over Cu(II). Lengthening the spacer in ligands 2 and 3 diminishes both the extraction efficiency and selectivity. Ligands 4 and 5, with rigid spacer units, show significant reductions in both Pb(II) and Cu(II) extraction. Slope analysis reveals that ligand 1 reacts in a 2:1 stoichiometry with Pb(II) in extraction, which differs from the 1:1 stoichiometries for 2 and 3. The differences in the half extraction pH (ΔpH1/2) values for Pb(II) and Cu(II) extraction are 1.29, 0.49, and 0.48 for 1–3, respectively.