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

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.