Discussion of the Physicochemical Effects of Modifiers on the Extraction Properties of Hydroxyoximes. A Review

Abstract

The physicochemical aspects of hydroxyoxime modification with various modifiers containing oxygen atoms are discussed. This kind of modification is unique to hydroxyoxime reagents, which form weak intramolecular and intermolecular hydrogen bonds. These bonds can easily be destroyed not only by modifier molecules (alcohols, alkylphenols, and esters) but also by solvating diluents. Alcohols are stronger modifiers than alkylphenols. The drawback of these modifiers is their strong self‐association, which is eliminated in esters and sterically hindered alcohols. The extraction properties of strong hydroxyoxime extractants can also be modified by adding weak reagents (hydroxyoximes or β‐diketones), which form co‐associates with strong extractants. The use of sterically hindered esters seem rational for two reasons: they inhibit hydroxyoxime adsorption at the hydrocarbon/water interface with oximino group penetrating the aqueous layers; and they show negligible adsorption. Due to these phenomena, hydroxyoxime extractants are more stable, and a retardation of the extraction is not observed in the presence of hindered esters. There is currently no general explanation of the chemistry of modification. Many published works focus only on the interactions of modifiers with extractants, neglecting the interactions with metal complexes. In order to achieve a substantial progress in our understanding of the phenomenon of modification, interactions between complexes and modifiers must be studied and explained.