Aggregation, Metal Ion Extraction, and Solubility Properties of Silyl-Substituted Alkylenediphosphonic Acids

Abstract

Partially esterified alkylenediphosphonic acids were shown to be effective solvent extraction reagents for the removal of actinide cations from acidic aqueous solutions into conventional organic solvents. As a first step in an effort to design diphosphonic acid extractants that are soluble in supercritical carbon dioxide, we prepared and characterized a novel series of silyl-substituted alkylenediphosphonic acids. The aggregation of these extractants in toluene at 25°C was studied by vapor pressure osmometry. Their solvent extraction chemistry was investigated by 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. To determine the effect of incorporating a silicon functionality into diphosphonic acids, the aggregation and solvent extraction properties of the silyl-substituted alkylenediphosphonic acids were compared with those of the conventional alkyl-substituted di(2-ethylhexyl) alkylenediphosphonic acids. The aggregation and solvent extraction properties of these ligands were shown to vary dramatically as a function of the number of methylene groups bridging the phosphorus atoms of the extractant. Factors including aggregation, P=O basicity, POH acidity, and chelate-ring size were explored to explain features of the solvent extraction data. Additionally, the solubility of the silyl- and alkyl-substituted ligands in supercritical carbon dioxide was studied.