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

Abstract

Aqueous solutions of five lanthanide salts: LaCl₃, La(NO₃)₃, La₂(SO₄)₃, Eu(NO₃)₃ and Eu₂(SO₄)₃ have been studied at 25 °C by dielectric relaxation spectroscopy over the frequency range 0.05 ≤ ν/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.