Understanding Nitric Acid-Induced Changes in the Arrangement of Monomeric and Polymeric Methacryloyl Diglycolamides on Their Affinity toward f-Element Ions

Assembled diglycolamides (DGAs) have a strong affinity toward f-element ions at high nitric acid concentrations. Small angle X-ray scattering studies revealed that nitric acid concentration dependent changes occur in the geometrical arrangement of the DGA units of monomeric methacryloyl-DGA and the corresponding polymeric DGA. Cylindrical aggregates of methacryloyl-DGA were formed in 10:1 n-dodecane:1-decanol (added for solubility reasons) upon equilibration with nitric acid. The lengths and diameters of the cylindrical methacryloyl-DGA aggregates increased on varying the nitric acid concentration from 3 to 4 mol L-1. This resulted in an increase of the distribution coefficient (D) of Eu3+ ions from 72 to 197. The physical structure of cross-linked (10 mol %) poly(methacryloyl-DGA) reorganized distinctly upon equilibration with nitric acid. In this case, also the D-Eu(3+) values increased significantly from 147 mL g(-1) at (-1) mol L-1 HNO3 to similar to 4000 mL g(-1) at 4 mol L-1 HNO3. Hydrogen bonds between the outer sphere of Eu3+/Am3+/Pu4+ nitrate and DGA units provide stabilization in the hydrophobic environment. This results in enhancement of their extraction upon increasing nitric acid concentration both in the organic phase as well as in the polymer matrix. Though monomeric and polymeric methacryloyl-DGA are different in their physical assembling, the normalized D-I values for a same f-element ion upon varying HNO3 concentrations show remarkably similar patterns in both forms. In addition, the unusual stoichiometry deduced from the slopes of the log D vs log[HNO3] curves at fixed nitrate concentration seems to suggest that the normal extraction mechanism may not be operating in the hydrogen bonded DGA assemblies.