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-ethaneJiylbis(oxy)]bisben zoic 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. 1.0(2,6).0(3,10).0(5,9). 0(8,11)]dodecane (4), and 1,3-bis[2'-(2 "-carboxyphenoxy)ethyl]-adamantane (5) are evaluated as selective Pb(II) extractants. The solvent extraction o f Pb(II) and of Cu(II) from buffered aqueous solutions of varying pH into c hloroform by ligands 1-5 is examined in relation to the molecular structure of the dicarboxylic acid extractant. Ligand 1, with an ethylene spacer bet ween two lipophilic salicylic acid units, exhibits excellent extraction sel ectivity for Pb(II) over Cu(II). Lengthening the spacer in ligands 2 and 3 diminishes both the extraction efficiency and selectivity. Ligands 3 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 stoic hiometry with Pb(II) in extraction, which differs from the 1:1 stoichiometr ies for 2 and 3. The differences in the half extraction pH (Delta pH(1,2)) values for Pb(II) and Cu(II) extraction are 1.29, 0.49, and 0.48 for 1-3, r espectively. (C) 2000 Elsevier Science B.V. All rights reserved.