Hydration and ion association of La3+ and Eu3+ salts in aqueous solution

Aqueous solutions of five lanthanide salts: LaCl3, La(NO3)(3), La-2(SO4)(3), Eu(NO3)(3) and Eu-2(SO4)(3) have been studied at 25 degrees C by dielectric relaxation spectroscopy over the frequency range 0.05 /GHz 89. Detailed analysis of the solvent-related modes located at higher frequencies showed that both La3+ and Eu3+ are strongly hydrated, even including partial formation of a third hydration shell similar to that of Al3+(aq). Up to two solute-related modes could be detected at lower frequencies, due to the formation of various types of 1:1 ion pairs (IPs). All five salts showed modest levels of association in the order Cl- < NO3- << SO42-, mostly in the form of double-solvent-separated IPs with small amounts of solvent-shared IPs. Overall association constants, calculated from the stepwise IP formation constants were consistent with literature values