Features of the Thermodynamics of Two-Phase Distribution Reactions of Americium(III) and Europium(III) Nitrates into Solutions of 2,6-Bis[(bis(2-ethylhexyl)phosphino)methyl]pyridine N,P,P ‘-Trioxide

New bifunctional and trifunctional organophosphorus ligands, 2-[(bis(2-ethylhexyl)phosphino)methyl]pyridine N,P-dioxide, DEH(MNOPO), and 2,6-bis[(bis(2-ethylhexyl)phosphino)methyl]pyridine N,P,P‘-trioxide, TEH(NOPOPO), have been synthesized. In contrast with previously reported phenyl derivatives, the increased solubility of these ligands in normal paraffinic hydrocarbon solvents make them attractive reagents for actinide partitioning. While the bifunctional reagent DEH(MNOPO) interacts with Eu3+ and Am3+ comparatively weakly, the trifunctional TEH(NOPOPO) exhibits moderate to high ability to transfer the trisnitrato complexes of these ions into n-dodecane from acidic aqueous solutions. We report here the details of TEH(NOPOPO) and DEH(MNOPO) preparation and of their ability to extract HNO3, Am(NO3)3, and Eu(NO3)3 into paraffinic hydrocarbons. The trifunctional TEH(NOPOPO) can extract up to two molecules of HNO3. The dominant extracted species for both Am(NO3)3 and Eu(NO3)3 has two TEH(NOPOPO) ligands associated over the range of temperatures 10−40 °C. From the variation in the equilibrium coefficients for the phase transfer reactions as a function of temperature, we have calculated the enthalpies and entropies for extraction of HNO3, Am(NO3)3, and Eu(NO3)3 into n-dodecane. Each metal nitrate is transferred into the organic phase in an exothermic process but opposed by an unfavorable (negative) entropy. The thermodynamic data are interpreted to indicate that the pyridine N-oxide is apparently a significantly weaker donor group for these metal ions than the phosphine oxides.