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).
The current work reports on molecular dynamics (MD) and quantum chemical (QC) calculations for two pesticide systems through extraction from an aqueous environment. Two separate deep eutectic solvents (DESs) consisting of DL-menthol as a hydrogen bond acceptor (HBA) and two carboxylic acids (octanoic acid and dodecanoic acid) as hydrogen bond donors (HBDs) at 298.15 K temperature and atmospheric pressure were adopted in our studies. The nonbonded interaction energy; radial, combined, and spatial distribution functions; and hydrogen bonding extent of various components were obtained via MD simulations that highlighted the enhanced and favorable interactions of the DES components with pesticides as compared to water. Further, transport properties, such as the mean squared displacement and diffusivity of compounds within the phases, were evaluated with the help of Einstein's diffusivity equation to obtain the affinity of the pesticides, namely, acetamiprid and imidacloprid, toward the DES-rich phase. The extractive characteristics of pesticides, such as the distribution coefficient (β), selectivity (S), and extraction efficiency (% EE), were calculated from the simulation and were fitted with the experimental data. The β values obtained by simulation were 8.67 and 6.25, respectively, and S values were 102.38 and 71.86 for acetamiprid and imidacloprid systems, respectively. Within QC, the charge-transfer (CT) process confirmed the direction of CT from DES to the pesticide, and the NBO analysis established the stable character of both the DESs. A slight increase in the O (HBA)•••H (HBD) distance confirmed the increased interaction between the DES and the pesticide. The DES−pesticide optimized clusters confirmed the interactions between the pesticide and the DES at a distance ranging from 2.896 to 3.77 Å for imidacloprid and from 1.724 to 2.03 Å for acetamiprid, which are validated by MD simulations. The highest occupied molecular orbital−lowest unoccupied molecular orbital studies displayed the active sites involved in CT and interactive operations. This was confirmed by the structural findings of the MD simulation, which were initially validated by the experimental results.
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 (C 8 to C 12 ) 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 (C 1 to C 6 ) based DESs are disrupted by water penetration to a higher extent as compared to the higher fatty acid (C 8 to C 12 ) 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.
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