Synergistic solvent extraction of alkaline earth cations by mixtures of di-n-octylphosphoric acid and stereoisomers of dicyclohexano-18-crown-6

The partitioning of Ca(TI), Sr(II), and Ba(II) in solvent extraction system s comprising di-n-octylphosphoric acid (HDOP) and individual stereoisomers of dicyclohexano-18-crown-6 (DCH18C6) in toluene has been investigated at 2 3(2) degrees C, Results of vapor pressure osmometric experiments, continuou s variation studies, single- and double-extractant dependencies, and acid d ependencies have been used to determine the stoichiometries of the extracte d complexes, Extraction of Ca(II) by HDOP alone affords Ca(H(DOP)(2))(2) HD OP as the extracted species, and no synergism is observed in the presence o f DCH18C6, presumably due to the strong complexation of Ca(II) with HDOP an d the strained complexes this cation forms with DCH18C6. In the absence of DCH18C6, Sr(II) and Ba(II) are extracted as the M(H(DOP)(2))(2) . 2HDOP (M = Sr or Ba) complexes. Upon addition of DCH18C6, the extracted complexes ar e formulated as M(DCH18C6)(H(DOP)(2))(2) (M = Sr or Ba), with Ba(II) showin g greater synergistic effects than Sr(II), The effectiveness of the DCH18C6 stereoisomers as synergists decreases in the following order: cis-syn-cis > cis-anti-cis > cis-trans > trans-syn-trans > trans-anti>trans, This seque nce has been explained by correlating the logarithm of the synergistic addu ct formation constants with the ligand strain energies of the DCH18C6 stere oisomers calculated using molecular mechanics methods. Those stereoisomers having the largest strain energies afford the lowest extraction and synergi stic adduct formation constants.