Anion Effects on Interspecies Interactions in Tris-Salt/Ethylene Glycol Deep Eutectic Solvents Revealed by Dielectric Relaxation Spectroscopy

Abstract

To understand the interspecies interactions in ethylene glycol (EG)-based eutectic systems, we studied solutions of Tris [tris(hydroxymethyl)aminomethane] chloride, nitrate, and perchlorate in EG using dielectric relaxation (DR) spectroscopy over the frequency range 0.05 ≤ ν/GHz ≤ 89 at concentrations from 0.1 M to the solubility limits at 298.15 K. Among the studied systems, {Tris·HNO₃+EG} forms a deep eutectic solvent at salt mole fractions between 0.08 and 0.13. The DR spectra are described by three relaxation modes assigned to reorientation of Tris-containing species, including free Tris·H⁺ cations and contact ion pairs (slow Debye process), relatively unperturbed EG molecules (intermediate Cole–Davidson mode), and fast librations. We show that the studied salts are only weakly solvated in EG, with solvation numbers at infinite dilution decreasing in the sequence Tris·HCl (2.30) > Tris·HNO₃ (1.76) > Tris·HClO₄ (1.04). This trend is consistent with decreasing anion–EG interaction strength and with the Gibbs formation energies of the corresponding anion–EG complexes. In contrast, ion association is governed by anion charge density, yielding apparent association constants of approximately 120, 20, and 5 M^–1 for Tris·HCl, Tris·HNO₃, and Tris·HClO₄, respectively. Although the fraction of free Tris·H⁺ cations increases with concentration, all systems exhibit subideal Walden behavior within the studied composition range, suggesting that cation–cation interactions significantly reduce ionic mobility. These findings demonstrate that ion solvation, ion association, and ionic mobility in Tris-based eutectic systems are governed by different aspects of anion identity, highlighting their potential for the rational design and further application of eutectic solvents with tunable physicochemical and transport properties.