Probing the role of hydrogen bond donors on the speciation and electrochemical behavior of Eu(III) in choline chloride-based DESs

Abstract

This study explores the electrochemical properties and speciation of Eu(III) ions in choline chloride (ChCl)-based deep eutectic solvents (DESs), focusing on how different hydrogen bond donors—urea (U), ethylene glycol (EG), and malonic acid (MA)—influence the stability of europium complexes. Employing electrochemical methods, spectroscopic analysis, and theoretical approaches, the research examines how changes in hydrogen bonding during complexation affect Eu(III) coordination. Cyclic voltammetry highlights the role of complex formation and DES viscosity in controlling mass transport and electron transfer rates. Theoretical and experimental data show that the ChCl:1MA DES forms the most stable europium complex, driven by strong hydrogen bonding and robust Eu-DES interactions, as supported by optimized geometries and favorable electrochemical stability. This stability is further confirmed by the high negative binding and formation free energies of Eu(NO3)3·5H2O in all three DES systems. Photoluminescence spectroscopy reveals fluorophore formation due to interactions between hydrogen bonds and functional groups such as –NH2, –COOH, and –OH, enhancing the understanding of Eu3+-DES complexes for potential applications in materials science. By clarifying how hydrogen bond donors affect Eu(III) speciation and reactivity, this research advances lanthanide chemistry in DESs and opens avenues for designing Eu(III)-based materials with specialized properties for various technological uses.