Insight into structural properties and molecular interactions of maline (choline chloride + malonic acid) and 1, 4- butanediol based pseudo-binary mixture: A thermophysical, spectral, and simulation portrayal

Abstract

The current study elucidate the thermophysical, spectral, and simulation depiction of the prepared deep eutectic solvent (DES) named as maline comprising of an equimolar mixture of choline chloride (ChCl) as hydrogen bond acceptor (HBA) and malonic acid (MA) as hydrogen bond donor (HBD) in the presence of 1, 4-butanediol (BTD) as other HBD (cosolvent). Various thermophysical properties viz., (densities, ρ, speeds of sound, u, viscosity, η and refractive index, nD) were comprehended for the examined pseudo-binary (maline and BTD) mixtures as a function of composition and temperature. The derived thermophysical properties viz., excess molar volume, VmE, excess isentropic compressibility, κsE, excess intermolecular free length, LfE, deviation in viscosity, Δη, and deviation in molar refraction, ΔRM were found to be negative while the excess speed of sound, uE, excess acoustic impedance, ZE, and deviation in refractive index, ΔnDwere found to be positive which signified the favourable hydrogen bonding interactions between maline and BTD. The partial molar volumes, V¯m,1, V¯m,2 and excess partial molar volumes, V¯m,1o and V¯m,2o of the components at infinite dilution exhibited the negative value due to the contraction in volume over the entire concentration thereby strengthening the presence of specific interaction. The molecular interaction in the examined pseudo-binary system conferred in terms of excess parameters with composition and temperature have also been correlated using the Redlich-Kister (R-K) equation. The significant shifting in the spectral profile as observed from FT-IR (Fourier-transform infrared) spectroscopy evidently inferred the favorable inter-molecular interaction within the maline and BTD based pseudo-binary mixture. A noteworthy change in the chemical shifts as deduced from1H- and 13C- NMR (Nuclear Magnetic Resonance) spectral peaks confirmed the hydrogen bonding networks between maline and BTD. Additionally, the 2D 1H–1H COSY (correlation spectroscopy) outcomes exposed the degree of variation in the inter-molecular interactions and structural arrangement between maline and BTD system. The spectral results were further validated employing theoretical (computational simulation) analysis which accounted the extensive hydrogen bonding from the evaluated computed descriptors Also, the 3D-molecular electrostatic potential (3D-MEP) explicated the effect of charge delocalization in the proximity of hydrogen bonding networks.