FORMATION AND STABILITY OF LANTHANIDE COMPLEXES AND THEIR ENCAPSULATION INTO POLYMERIC MICROSPHERES

The complexation of lanthanides (Ln) with dicarbonyl compounds (acetylacetone, acac; ethyl acetoacetate; 3-ethyl-2,4-pentanedione; 2,4-hexanedione; 3-methyl-2,4-pentanedione; and diethyl malonate) was investigated using a potentiometric titration technique. The ability of a dicarbonyl compound to complex with the lanthanide elements was greatly dependent on its pK(a) and on the pH of the titrated solution. Selected lanthanide complexes (Ln complexes) were incorporated into spherical poly(L-lactic acid) (PLA) matrices and irradiated in a nuclear reactor with neutrons to produce short-lived high-energy beta-particle-emitting radioisotopes. The lanthanides investigated (Ho, Dy, Sm, and La) were chosen on the basis of their physical and nuclear properties. A transition element (Re) was also studied. The small decrease in the ionic radii of the lanthanides with increasing atomic number led to (a) greater ability to extract and complex from an aqueous solution with complexing agents, (b) larger formation and stability constants for the Ln complexes, (c) increased solubility of the Ln complexes in chloroform, and (d) increase in the maximum percent incorporation of the stable lanthanides in PLA spheres. Ho(acaC)3 was found to be the most promising candidate of the complexes studied on the basis of the above observations and due to the favorable physical properties of Ho-165 and nuclear properties of Ho-166.