Ra. Sachleben et al., EXTRACTION OF ALKALI-METAL CATIONS BY LIPOPHILIC DIBENZO-14-CROWN-CARBOXYLIC ACIDS, Separation science and technology, 30(7-9), 1995, pp. 1157-1168
The extraction of alkali metal cations by the lipophilic crown ether,
bis-t-octylbenzo-14-crown-4 (BOB14C4), three derivatives of BOB14C4 ha
ving pendant carboxylic acid sidearms, and a lipophilic carboxylic aci
d, 2-methyl-2-heptylnonanoic acid (HMHN) was studied by two-phase pote
ntiometric titration and ion-chromatography. The lipophilic, ionizable
crown ethers, BOB14C4-acetic acid (BOB14C4AA), BOB14C4-propanoic acid
(BOB14C4PA), and BOB14C4-oxyacetic acid (BOB14C4OAA) 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 HM
HN. The mode of attachment of the ionizable sidearm, via an ether link
age (BOB14C4OAA) versus a carbon linkage (BOB14C4OAA and BOB14C4PA), h
as a significant effect on the cation selectivity and extraction effic
iency of these extractants. BOB14C4 exhibits no p[H] dependent extract
ion behavior and has no significant effect on the extraction of alkali
metal cations by HMHN in a mixture of these two compounds. Although B
OB14C4AA and BOB14C4PA extract cations at lower p[H] than HMHN, all th
ree compounds exhibit similar selectivity for Li+ over Na+, K+, Rb+, a
nd Cs+. A significant reversal in selectivity is observed with BOB14C4
OAA, which extracts Na+ and K+ selectivity over Li+, Rb+, and Cs+ and
at significanty lower p[H] than BOB14C4AA, BOB14C4PA, or HMHN. The uni
que behavior of BOB14C4OAA may be attributed to the presence of the et
her linkage between the crown ether and the pendant carboxylic acid.