Extraction of alkali metal cations by lipophilic dibenzo-14-crown-4-carboxylic acids

The extraction of alkali metal cations by the lipophilic crown ether, bis-t-octylbenzo-14-crown-4 (BOB14C4), three derivatives of BOB14C4 having pendant carboxylic acid sidearms, and a lipophilic carboxylic acid, 2-methyl-2-heptylnonanoic acid (HMHN) was studied by two-phase potentiometric titration and ion-chromatography. The lipophilic, ionizable crown ethers, BOB14C4-acetic acid (BOB14C4AA), BOB14C4-propanoic acid (BOB14C4PA), and BOB14C4-oxyacetic acid (B0B14C40AA) extract cations efficiently from aqueous mixed alkali metal chloride solutions into 1-octanol by an ion-exchange mechanism in the range p[H] > 7, as does HMHN. The mode of attachment of the ionizable sidearm, via an ether linkage (B0B14C40AA) versus a carbon linkage (BOB14C4AA and BOB14C4PA), has a significant effect on the cation selectivity and extraction efficiency of these extractants. BOB14C4 exhibits no p[H] dependent extraction behavior and has no significant effect on the extraction of alkali metal cations by HMHN in a mixture of these two compounds. Although BOB14C4AA and BOB14C4PA extract cations at lower p[H] than HMHN, all three compounds exhibit similar selectivity for Li⁺over Na⁺, K⁺, Rb⁺’ and Cs⁺. A significant reversal in selectivity is observed with B0B14C40AA, which extracts Na⁺and K+ selectively over Li+, Rb⁺, and Cs⁺and at significanty lower p[H] than BOB14C4AA, BOB14C4PA, or HMHN. The unique behavior of B0B14C40AA may be attributed to the presence of the ether linkage between the crown ether and the pendant carboxylic acid.