Molecular Dynamics Insights and Water Stability of Hydrophobic Deep Eutectic Solvents Aided Extraction of Nitenpyram...

In recent times, deep eutectic solvents (DES) have received attention as an extractive media for separations. In this work, the water stability of eight menthol-based DESs and two tetrabutylammonium chloride (N₄₄₄₄Cl) based DESs with organic acid-based hydrogen bond donors (HBD) at a temperature of 298.15 K and atmospheric pressure were studied. dl-Menthol and N₄₄₄₄Cl were considered as the hydrogen bond acceptors (HBA). Molecular dynamics simulation (MD) was used as a tool to examine the distribution of molecules of DES and water in either phase. The intermolecular nonbonded interaction among the species of the systems was analyzed with radial distribution function, interaction energy, and hydrogen-bonding analysis to understand the stability of DESs in an aqueous medium. The results showed that the strong hydrogen bond plays a crucial role in the water stability of the DES. The degree of hydrogen bonding in HBD-water in terms of HBDs obtained by MD simulation can be presented in the order of acetic acid > levulinic acid > butanoic acid > pyruvic acid > hexanoic acid > octanoic acid > decanoic acid > dodecanoic acid. The strength of the hydrogen bond was attributed to the structure of solvents and the alkyl chain length of the HBD group. Overall, the order of stability of DES in water based on a "relative stability factor" was found as dl-menthol:acetic acid (1:1) < dl-menthol: levulinic acid (1:1) < dl-menthol:butanoic acid (1:1) < dl-menthol:pyruvic acid (1:2) < dl-menthol:hexanoic acid (1:1) < dl-menthol:octanoic acid (1:1) < dl-menthol: decanoic acid (1:1) < dl-menthol:dodecanoic acid (2:1). The transfer of molecules in the system from the aqueous phase to the DES rich phase was analyzed with the help of mean-square displacement and diffusion-coefficients. dl-Menthol and organic acids starting from octanoic acid and higher ones can be used in aqueous systems as solvents. Finally, dl-menthol:octanoic acid (1:1) -based DES was used to benchmark and predict the extraction efficiency of a pesticide (nitenpyram) from an aqueous feed. Hydrogen bond analysis demonstrated higher interactions of nitenpyram with dl-menthol and octanoic acid as compared to water. The MD simulation of the ternary system consisting of DES, water, and nitenpyram showed encouraging results, and gave an excellent agreement with experimental literature data in terms of extraction efficiency (∼42 to 46.7%) and distribution ratio (0.72).