Separation of alkali metal cations and alkaline earth cations using ionizable crown ethers
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Proton-ionizable crown ethers are novel agents for the separation of metal ions by extraction and membrane transport processes. A series of novel crown ether carboxylic acids , crown ether phosphonic acid monoesters, and crown ether phosphonic acids which possess lipophilic pendant groups were employed for the study of extraction and membrane transport. In addition to mono- and di-ionizable macrocyclic complexing agents, monoand di-ionizable acyclic complexing agents were utilized to probe the effects of structural variation within the ionizable polyether and environmental variation upon the selectivity and effÃciency in metal ion complexation and transport. Experimental techniques of pK^ determination in dioxane-water systems, solvent extraction, and proton-coupled transport across bulk liquid membranes were utilized to assess the metal ion complexing properties of these ligands. To provide greater insight into the factors which control metal ion complexation by ionizable crown ethers in solvent extraction and membrane transport processes, thermodynamic ionization constants, ^pKa, for twenty ionizable complexing agents were determined. Thirty functionalized polyether ligands were employed in the solvent extraction of group lA and 2A metal cations. For these highly lipophilic ionizable macrocyclic multidentate ligands, the influence of structural variation upon the selectivity and efficiency of cation complexation was probed. Such structural variations included: the polyether cavity size; the lipophilic group size; the side arm length; the lipophilic group attachment site; the polarity of the lipophilic group; the ring oxygen basicity; the ring heteroatom identity; and the ionizable group identity. In addition to these parameters, an environmental variation of the organic solvent was also investigated. Competitive alkali metal transport from an alkaline aqueous source phase through a bulk liquid membrane phase into an acidic aqueous receiving phase facilitated by nineteen mono-ionizable crown ethers was investigated in three kinds of cells, i.e., Cell A (U-Tube cell), Cell B (tube-witiiin-a-beaker type cell), and Cell C (beaker-within-a-beaker type cell). In order to probe the influence of structural variations upon membrane transport, experiments were conducted for five structural parameters: variation of the polyether cavity size, variation of the lipophilic group attachment site, variation of the lipophilic group size, variation of the ring hetero atom identity, and variation of the ionizable group identity. In addition to these structural parameters, environmental variations of the organic solvent, the pH gradient between aqueous source phase pH and aqueous receiving phase pH, and the carrier concentration were also investigated, Through this complexation and transport study using forty-three functionalized organic multidentate ligands, it has been shown that ionizable crown ethers are effective agents for the solvent extraction of alkali metal and alkaline earth cations and for the proton-coupled transport of alkai metal cations across bulk liquid membranes. The selectivity and effÃ®ciency of competitive solvent extraction and membrane transport can be controlled by structural variation within the ionizable crown ethers and acyclic polyethers or by environmental variation. Such systems possess considerable potental for practical separations of specifÃ®ed alkali metal or alkaline earth cations from aqueous solutions.