Equilibria and effect of diluent in the solvent extraction of lithium salts by highly alkylated 14-crown-4-ethers

As shown by survey experiments, 14-crown-4 ethers bearing certain aliphatic substituents exhibit strong selectivity for lithium. Both selectivity and overall extraction efficiency depend markedly on type of ring substituents and on diluent properties. To understand such effects in greater detail, extraction of LiCl by the crown ether 2,2,3,3,6,9,9,10,10-nonamethyl-14-crown-4 (NM14C4) was subjected to equilibrium analysis. By use of the program SXLSQI (a solvent-extraction modeling program), the extraction behavior as determined by ion chromatography has been modeled quantitatively in terms of four equilibrium in 1-octanol. The following neutral and ionic organic-phase species have been considered: LiCl, Li+, Cl{sup {minus}}, LiCECl, and LiCE+ (CE = crown ether). Parallel measurements of the same system by {sup 7}Li NMR techniques agree with the ion-chromatography results. The NMR experiment affords the advantage of distinguishing between free and bound lithium and thus provides a check on the species indicated by the modeling. Extraction of LiCl by NM 14C4 correlates with diluent properties, including the Shmidt-Marcus diluent parameter and Reichardt`s E{sub T} parameter; as diluent polarity increases, LiCl extraction increases steeply