Aggregation, solvent extraction, metal complexation and supercritical carbon dioxide solubility properties of symmetrically-substituted alkylenediphosphonic acids

Alkyl-substituted diphosphonic acids in organic solvents such as toluene and oxylene have been shown to be effective solvent extraction reagents for separating actinides from acidic aqueous solution. In an effort to design and characterize diphosphonic acid ligands which are soluble in an environmentally benign solvent, supercritical carbon dioxide, the aggregation, solvent extraction, metal ion complexation and supercritical carbon dioxide solubility properties of selected of silyl- and alkyl-substituted diphosphonic acids have been determined. The aggregation of these ligands has been studied by vapor pressure osmometry in toluene at 25°C. The solvent extraction chemistry of the silyl-substituted diphosphonic acids has been investigated employing radiotracers to determine the distribution ratios for selected metal ions between an acidic aqueous phase and an organic phase containing the diphosphonic acid extractant. In order to determine the effect of incorporating a silicon functionality into diphosphonic acids, the aggregation and solvent extraction properties of the silyl-substituted diphosphonic acids has been compared with those of the conventional alkyl-substituted di-(2-ethylhexyl) alkylenediphosphonic acids. The aggregation, solvent extraction and supercritical carbon dioxide solubility properties of the symmetrically disubstituted diphosphonic acids has been explored as a function of the number of methylene groups bridging the phosphorus atoms of the extractant and the nature of the esterifying group. Factors, including aggregation, P=O basicity, POH acidity and chelate ring size formed upon metal ion complexation have been explored in order to explain features of the solvent extraction data. The supercritical carbon dioxide solubility data has been shown to be dependent on a number of factors including the number of methylene groups separating the phosphorus atoms and the degree of branching of the esterifying group.