Small angle neutron scattering investigation of the species formed in the extraction of Sr(II) by mixtures of di-n-octylphosphoric acid and dicyclohexano-18-crown-6

Deuterated toluene solutions of di-n-octylphosphoric acid, HDOP, containing Sr+2 extracted from 0.1 M LiNO3 aqueous solutions, were investigated by small angle neutron scattering (SANS). The extractions were also performed in the presence of either the cis-syn-cis or the cis-anti-cis stereoisomer of dicyclohexano-18-crown-6 (DCH18C6) in combination with HDOP in the toluene phase. The addition of the crown ether increased metal extraction into the organic phase over that observed with HDOP alone (synergistic extraction). The cis-syn-cis isomer was more effective than the cis-anti-cis isomer in enhancing Sr(II) extraction in the synergistic system. The SANS results confirmed that when the metal cation is extracted by HDOP present in large stoichiometric excess, the predominant species formed in the organic phase can be described as Sr(H(DOP)(2))(2) with 2 extra HDOP molecules solvating the complex. In the synergistic crown ether-HDOP system, the SANS data are consistent with solvent extraction data suggesting the formation of complexes containing one crown ether and two HDOP dimers. In the extraction of Sr(II) by HDOP alone, the SANS data reveal the presence of small amounts of large aggregates. Both the cis-syn-cis and cis-anti-cis isomers of DCH18C6 inhibit the formation of these aggregates. The cis-anti-cis isomer, however, despite its weaker complexation of strontium, is more effective in preventing aggregate formation.