Exploring deep eutectic solvents of methane sulfonic acid and choline chloride: Formation, properties, and metal solvent effectiveness

Abstract

Methane sulfonic acid (MSA), a hydrogen bond donor (HBD), was combined with choline chloride (HBA) at varying molar ratios (1:1, 1:2 and 2:1) to explore the formation of deep eutectic solvents (DES). The resulting blends, at 1:1 and 2:1 ratio, remained liquid at ambient temperature. Among these combinations, the DES formed with a 1:1 M ratio (MSA-ChCl) exhibited the highest decomposition temperature (166 °C) followed by MSA-ChCl (1:2) and MSA-ChCl (2:1). Alongside, the glass transition temperature of the DES also remained relatively unaffected by the molar ratio of its components. Notably, both density and viscosity were observed to be temperature dependent. To elucidate hydrogen bonding interactions between choline chloride and MSA, Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy were employed. The broadening and shifting of IR peaks illustrate the formation of intermolecular bonding between MSA and ChCl. Density Functional Theory (DFT) calculations were conducted to optimise molecular structures, analyse HOMO-LUMO levels, estimate electronegativity and ionization potential, and generate electrostatic potential surfaces of the DES. As per calculated Molecular Electrostatic Potential (MEP) by DFT of pure MSA and ChCl, SO of MSA has most electron deficient region and it has potential to interact with electron positive part of OH part of ChCl, which is also demonstrated by IR spectra. MSA-ChCl (2:1) displayed the most negative electron density and proved to be the most effective metal solvent among the DES formulations tested. Exceptional solubility of lithium in MSA-ChCl (2:1) was observed, exceeding 105 mg/L. Moreover, zinc, copper, cobalt, and iron also demonstrated good solubility in MSA-ChCl (2:1), ranging from 105 to 104 mg/L.