Acumen into the effect of alcohols on choline chloride: L-lactic acid-based natural deep eutectic solvent (NADES): A spectral investigation unified with theoretical and thermophysical characterization

Abstract

Deep eutectic solvents (DESs), an alternative to hazardous solvents is grabbing an outsized attention due to their adaptable structure and amended thermophysical properties. Recently, remarkable and new collection of nature-based solvents also termed as natural deep eutectic solvents (NADESs) have been put forth for large scale applications. Considering these facts, the current study offer an insight into the preparation of NADES from Choline Chloride (CC) as hydrogen bond acceptor (HBA) and L-lactic acid (LA) as hydrogen bond donor (HBD) in 1: 2 M ratio and abbreviated as CC: LA. In view to its broad hydrophilicity, the effect of alcohols such as ethanol (ET) and ethylene glycol (EG) as additional HBD (cosolvents) were examined in the prepared NADES (CC: LA) in different molar ratios as a function of temperature to tailor-made the thermophysical properties of pure constituents that may oblige such systems for highly advantageous industrial and commercial applications. This cosolvents-modified binary mixtures were fathomed using spectroscopic (FT-IR, NMR (1H- and 13C-) and 2D 1H1H COSY) techniques which rooted the specific nature of interactions i.e., new hydrogen bonding networks involved therein as well as the interstitial accommodation of alcohols within the cavity formed by NADES. The computational simulation approach evaluated Total Energy (T.E.) to explicate the effect of the energy of molecular orbitals which further significantly validated our experimental findings. Also, the concept of 3D-molecular electrostatic potential (3D-MEP) is presented to predict the coordination (electrophilic and nucleophilic) sites between HBA and HBD. In addition, various thermophysical properties viz., density (ρ), speed of sound (u), viscosity (η) and refractive index (nD) of the prepared NADES-based binary mixtures (CC: LA + ET/EG) were rationalized as a function of temperature. Also, excess molar volume (VmE), excess isentropic compressibility (κsE), excess speed of sound (uE), deviation in viscosity (Δη), and deviation in refractive index (ΔnD) are correlated and fitted using Redlich-Kister (R-K) equation which inferred how alcohols as cosolvents influence the plausible hydrogen bonding interactions within the examined systems.