Stability Mechanism of Menthol and Fatty Acid Based Hydrophobic Eutectic Solvents: Insights from Nonbonded Interactions

Abstract

Deep eutectic solvents (DESs), being ionic liquid (IL) analogues, are easy to prepare, cost-effective, recyclable, and biocompatible and have low toxicity. Hydrophobic deep eutectic solvents (HDESs) composed of natural compounds are highly effective in the extraction of micropollutants (metal ions, pharmaceuticals, and so on) from an aqueous stream. In this work, the formation mechanism of dl-menthol and carboxylic acid (acetic, butanoic, hexanoic, octanoic, nonanoic, decanoic, and dodecanoic acid) based DESs in a molar ratio of 1:1 has been presented along with their structural composition, charge transfer analysis, chemical stability, and aqueous phase solvation analysis using density functional theory (DFT) calculations. The intermolecular hydrogen bonding and dispersion interactions between menthol (−OH group) and carboxylic acids (−COOH) at a short to medium range are responsible for the stable DES formation. CHELPG and natural bonding orbital (NBO) analyses suggest that the acetic acid based DES is more closely structured and has lesser resistance to charge transfer as compared to higher chain length acid (C8 to C12) based DESs. Frontier molecular orbital (FMO) analysis confirms higher chemical stability and lesser reactivity of long-chain fatty acid based DESs as compared to their short-chain counterparts. The atom-in-molecules (AIM) and noncovalent interaction (NCI) analyses suggest that a strong network of hydrogen bonding and dispersion interactions are solely responsible for the formation and strength of the DESs. The DFT-based solvation study supported by spatial distribution function (SDF) from molecular dynamics (MD) indicates that the lower fatty acid (C1 to C6) based DESs are disrupted by water penetration to a higher extent as compared to the higher fatty acid (C8 to C12) based DESs. According to the solvation-based NCI analysis, van der Waals and dispersion connections are the interactions that are largely responsible for the chemical and physical stability of the DESs in the aqueous phase. The overall findings from the computational study have a remarkable degree of coherence with the findings of the experiments that were previously reported.