The impacts of anion and cation modification on the physicochemical properties of tris-(2-hydroxyethyl) ammonium-based protic and aprotic ionic liquids

Abstract

The main distinguishing feature of protic ionic liquids (PILs) is that their cations have at least one accessible proton, which allows them to form hydrogen bonds with anions. Furthermore, their cost-effectiveness and straightforward synthesis technique make them suitable for large-scale applications. In the present work, a series of ammonium based protic ionic liquids (PILs) and aprotic ionic liquids (AILs), which are tris(2-hydroxyethyl)ammonium acetate ([HEA][Ac]), tris(2-hydroxyethyl)ammonium lactate ([HEA][La]), tris(2-hydroxyethyl)ammonium butyrate ([HEA][BU]), tris(2-hydroxyethyl)ammonium ascorbate ([HEA][AS]), allyl tris(2-hydroxyethyl)ammonium chloride ([AyHEA][Cl]), and benzyl tris(2-hydroxyethyl)ammonium chloride ([BzHEA][Cl]), were synthesized and characterized using nuclear magnetic resonance (NMR) spectroscopy, infrared spectroscopy (IR), and elemental analysis. The densities, viscosities, and refractive indices of these ILs were determined at temperatures ranging from 273.15 to 353.15 K. In addition, various thermodynamic properties, including the thermal expansion coefficient, molar refraction, standard molar entropy, and lattice energy, were estimated for these ILs. The results demonstrate that these ionic liquids (ILs) exhibit lower densities, comparable refractive indices and viscosities, and a lower decomposition temperature than their analogs based on tri-(2-hyroxyethyl) ammonium. In addition, the ILs exhibited a slight dependence on temperature for the thermal expansion coefficients, αp, which ranged from 6.27 × 10⁻⁴ to 7.07 × 10⁻⁴ K⁻¹. These findings offer valuable information on the properties and potential uses of tris(2-hydroxyethyl)ammonium-based protic and aprotic ionic liquids containing carboxylate and chloride anions.