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J. Phys. Chem. 1996, 100, 9500-9505
Equilibria and Speciation in the Solvent Extraction of Lithium Chloride by Nonamethyl-14-Crown-4 Ether in 1-Octanol Yunfu Sun, Zhihong Chen,† Kerri L. Cavenaugh, Richard A. Sachleben, and Bruce A. Moyer* Chemical and Analytical Sciences DiVision, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6119 ReceiVed: NoVember 14, 1995; In Final Form: January 25, 1996X
The equilibria and speciation in the solvent extraction of LiCl by nonamethyl-14-crown-4 ether in 1-octanol have been determined by use of ion chromatography (IC) and inductively coupled plasma (ICP) analyses, equilibrium modeling with the program SXLSQI, and 7Li NMR measurements. SXLSQI modeling of the extraction data indicates the formation of a 1:1 metal:crown complex in the organic phase. The complex apparently exists as an ion-pair Li(crown)Cl, which partially dissociates to the complex cation Li(crown)+and free chloride anion Cl-. Extraction of LiCl by 1-octanol to give the organic-phase ion-pair LiCl and its dissociated ions Li+ and Cl- must be included in the data analysis. 7Li NMR measurements verify the existence of free and crown-complexed lithium species in 1-octanol, and the measurements quantitatively agree with results from the SXLSQI model.
Introduction A general understanding of speciation and equilibria in extractive processes involving designed ligands such as crown ethers must be obtained as a basis for thoughtful improvement of selectivity and other relevant properties. Toward this end, a wealth of information has been gathered on the extraction and transport of alkali and other metal ions by crown ethers,1,2 but only a small fraction of these data have been analyzed and reduced to equilibrium constants corresponding to definite species. Generally, such equilibrium analysis has been performed at an elementary level, and often the identity of the postulated species has not been thoroughly tested for consistency with extraction behavior. Thus, much of our understanding of the chemistry of crown ethers in solvent extraction remains in a rather qualitative state. As theoretical tools and structural methods continue to provide an increasingly accurate description of the bonding of crown ethers to metal ions, it becomes correspondingly imperative to be able to accurately describe the equilibrium state of the complexes in solution. Several research groups,3-14 including ours,15-17 have been studying highly alkylated 14-crown-4 ethers in an effort to understand aspects of the design and use of crown ethers for solvent extraction. Alkyl-substituted 14-crown-4 ethers in particular exhibit selectivity for lithium vs sodium exceeding 1000:1,3 but both selectivity and extraction efficiency depend markedly on the nature of the ring substituents and their positions on the ring. Structural studies of some of the free ligands and their complexes with LiSCN have shown18,19 that the ligands envelop the Li+ cation in basket-like complexes having a square-pyramidal coordination geometry. The free ligands, however, are not preorganized in the solid-state structure, but rather, their “cavities” are filled with inwardpointing methylene groups. It has been shown by molecular mechanics that the conformational strain (∆U) of the free ligand as it “untwists” upon complexation correlates with the extraction efficiency for the lithium chloride salt.20 This structurefunction relationship presents many exciting possibilities for † Now at Olin Research Center, 350 Knotter Drive, P.O. Box 586, Cheshire, CT 06410-0586. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03353-3 CCC: $12.00
correlation and prediction of complexation and extraction phenomena, but it also raises fundamental questions. Of primary concern here, the question of species and equilibria has not hitherto been examined, and thus, some fundamental assumptions in the theoretical analysis have not been confirmed. Indeed, our initial efforts have indicated that four organic-phase lithium species must be considered.15 In addition, the extraction depends markedly on the diluent, raising the issue of the role of solvent and solvation phenomena. To address these questions, we have examined in detail the extraction of LiCl from water by the crown ether 2,2,3,3,6,9,9,10,10-nonamethyl-14-crown-4 ether (NM14C4, 1). 1-Octanol
(dielectric constant ) 8.1 when saturated with water21) was selected as a representative water-immiscible diluent having sufficient polarity and donor/acceptor properties to effect extraction of the simple LiCl salt. As an extraction diluent, 1-octanol plays an important role in the determination of the so-called lipophilicity parameters22 and in certain process applications involving crown ethers.23 Employing the techniques of ion chromatography (IC) and inductively coupled plasma (ICP) spectrometry to quantitatively measure the concentration of extracted LiCl, we have performed and report herein an equilibrium analysis of the extraction data using the program SXLSQI. This software and its predecessor, SXLSQA,24,25 have been used to investigate the speciation in a number of solvent extraction systems,24-27 including systems involving crown ethers. SXLSQI possesses the novel capability to treat organic-phase ions, a needed feature here, since 1-octanol has a sufficiently high dielectric constant to dissociate ionpairs.21 Since the equilibrium model obtained is based only on the measured total organic-phase Li concentration, it was desirable to test it with an independent physical technique. © 1996 American Chemical Society
Solvent Extraction of LiCl Previous workers employed conductance measurements to convincingly demonstrate the dissociation of LiCl and other ionpairs in 1-octanol.21 For present purposes, 7Li NMR offered additional advantages in identifying both free and crown-etherbound lithium in the solvent phase. Fortunately, the slow exchange of free and bound lithium in the organic phase enabled us to quantify both types of species and thus to verify the postulates of the equilibrium model. The agreement between the model and the NMR results supports the continued use and development of equilibrium modeling for investigating speciation in a solvent extraction system involving crown ethers. Experimental Section Reagents and Solvents. The substituted lipophilic 14crown-4 ether (1) used in this study was synthesized in this lab.16 Lithium chloride (Aldrich, ACS reagent grade) was used as received without any treatment other than drying at 110 °C. HPLC grade 1-octanol (Sigma-Aldrich) was used as received. Aqueous solutions were prepared from deionized distilled water. Procedures. To carry out the solvent-extraction experiments, equal volumes (1 mL each) of organic and aqueous solutions of certain concentrations were equilibrated in 4 mL vials by repeated inversion on a rotating vertical wheel for at least 2 h in a thermostatic air box at 25 ( 0.5 °C. This procedure was found in kinetic tests to be sufficient for equilibrium to be reached. Centrifugation at 3000 rpm for 5 min thereafter ensured complete phase separation. For NMR measurements, samples of the organic layers (top phases) were directly transferred into NMR tubes. For IC and ICP analysis, aliquots (0.5 mL) of the organic layers were each stripped by 5 mL of deionized distilled water following the same procedure as for the extraction. (The 5 mL volume was found to be sufficient for quantitative stripping.) The stripping solution was then analyzed by IC or ICP to determine the lithium concentrations. To prevent alkali metal contamination from the container walls, especially glass, all stripping and analysis procedures were carried out in plastic vials prewashed with acid solution. At least three identical extraction experiments were run independently, and each sample was analyzed 3 times. The reproducibility of the extraction, measurement, and integration procedures was within acceptable range (overall error
