Investigations of ion-Size-Selective Synergism in Solvent Extraction

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Abstract
Organophilic crown ethers and other similar macrocyclic compounds produce a synergistic enhancement of the extraction of several metal ions by organic-phase—soluble carboxylic, phosphoric, and sulfonic acids. Ions examined include the alkali metals, the alkaline earths, and some first-row transition elements. In many cases, the synergistic effects are clearly related to the correspondence of the ion size to the macrocycle cavity diameter. In other cases this is less true, and occasionally, complete lack of syner-gism or even antagonism is observed. These phenomena have been investigated by systematic studies of the systems involved, by equilibrium studies of selected extraction systems, and by vapor-pressure osmometric studies of the iritermolecular bonding between the organic-phase macrocycles and acids. Lack of synergism (when it is expected from size correspondence) does not appear to be due to bonding interactions between the macrocycle and the extractant acid, Other considerations suggest that synergism may depend on the degree to which the macrocycle can surround the ion. Possibilities for metal ion separations are noted, and the directions for further, needed investigations are indicated.