More than 1.3 million tons of chemical waste is produced globally per year and its amount continues to increase. Hazardous waste is mainly generated in the chemical industry, the production of petroleum products and coal, in the processing and disposal of waste, the production of agricultural chemicals such as pesticides, fertilizers, etc. The main source of waste is liquid waste from academic laboratories and industrial chemical processes; it's mostly from extraction, separation, chemical synthesis and pretreatment processes. These chemical processes consume a lot of solvents. Solvents include acids, bases, organic solvents of petroleum origin and other inorganic solvents . But hazardous waste also comes from the products we use every day, such as batteries, cosmetics, cleaning products, paints, pharmaceuticals, and electronics.
Consequently, there is a need to develop more environmentally friendly, cheaper, non-toxic solvents that are harmless to humans and the environment. In this regard, deep eutectic solvents (DES) and their derivatives so called natural deep eutectic solvents (NADES) are a new field in the search for green alternative solvents. Ease of manufacture, low cost of components, environmental friendliness are the advantages of DES . Some DES are completely non-toxic or less toxic than conventional organic solvents and ionic liquids (IL).
Since the introduction of the first solvent of choline chloride and urea in 2003 by Abbott at all  many other solvents, including binary, ternary and quaternary components of the solvent system have developed. And interest in these solvents continues to grow, especially in the past decade. This is due to the prospects for the use of these materials in electrochemistry [4, 5], catalysis and gas capture [6–8], polymer synthesis [9, 10], purifications, and extraction [11–14] processes applications. For example, DES using polyols such as glycerin or ethylene glycol typically has lower freezing points and even exists as a liquid below room temperature, and these polyols are also widely used in many industrial applications. The high activity and stability of enzymes in DES can be used to dissolve metal oxides, which is necessary for their extraction, processing, and catalyst preparation .
However, insufficient knowledge of the structure and thermodynamic properties of DES and the effect of solvents such as water, alcohols, and alkanes on these properties hinder the possibility of large-scale application of these materials in industry .
Garcia et al. , in study using Density Function Theory and AIM and with ChCl: urea (1: 2), ChCl: glycerin (1: 2), ChCl: glycerin (1: 3) and ChCl: malonic acid (1: 1) proposed that charge delocalization caused by hydrogen bonding is thought to cause low electron density, which leads to the low melting points seen in DES. However, Zan et al.  question this assumption. In a recent study by Ashworth et al.  choline Chloride and urea form several complexes with the participation of choline and urea, as well as urea and chloride. This casts doubt on the popular assertion that chloride anion and urea form hydrogen bonds, which are known to be the main force the interaction underlying the formation of DES. Therefore, there is a need for further studies of the molecular basis of DES formation.
The structural properties of a 1:2 mixture of choline chloride and urea and a 1:2 mixture of butyltrimethylammonium chloride and urea, have been investigated by means of Molecular Dynamics simulations in . It was found that the presence or absence of a hydroxyl group on the organic cation strongly affects the DES hydrogen bond network, causing a different three-dimensional arrangement of all particles present in the mixtures. These results may be important for the future development of DES for specific applications. The combination of neutron reflectometry and molecular dynamics (MD) simulations has been shown to provide fresh insights into the structure of solid/DES interfaces . The electrosorption of water limited by graphene in the 1:2 choline chloride - urea (Reline) system in a wide range of surface polarizations has been investigated using atomistic molecular dynamics . It was found that the interfacial structure and water distribution are sensitive to the polarization of the electrode surface. Local intermolecular interaction with reline particles and electrostatic interaction with a graphene electrode strongly affect the electrosorption of water. A theoretical study using both ab initio molecular dynamics and quantum chemistry calculations to clarify the role of water in the nanostructure of a mixture of urea and betaine was carried out in Ref. 23. Facilitating the water association between urea and betaine, increasing the network of hydrogen bonds and inhibiting the aggregation of urea molecules, was established from preliminary modeling results. In situ studies of the formation of iron oxide nanoparticles in DES and observation of the effect of water on the reaction were carried out in  using SANS, SAXS, EXAFS, neutron and X-ray diffraction, and atomistic modeling. Solute-solvent interactions of glucose, sucrose, erythritol, cellobiose, starch and cellulose in five different choline chloride-based DES are studied in  using size exclusion chromatography, NMR and differential scanning calorimetry. In general, carbohydrates showed good solubility and some similarities were observed between aqueous solutions and ionic liquids. Ethylene showed the best performance because it had the lowest viscosity and did not degrade carbohydrates. The effect of ethylene glycol, malic acid, tartaric acid, glycerol and oxalic acid with choline chloride acceptor in the formation of supramolecular structures have been studied in  using the molecular dynamics simulations approach. In compared to other solvents, choline chloride with tartaric acid at a 2: 1 ratio has a better polarizability, entropy, thermal stability and heat capacity. At the same time, choline chloride with Ethylene Glycol at a ratio of 1: 2 has the highest conductivity, dipole moment, electron mobility and hole mobility.
In this regard, we have done DFT calculations, and all-atom classical Molecular Dynamics (MD) simulations to study the intermolecular interactions within the NADESs at the molecular level. In particular, we have selected NADESs, that is, ChCl/Glu (1:1) as theoretical model for ab initio calculation and all-atom classical MD simulations to understand its formation mechanism at the molecular level. The choice of the NADESs is based on the available experiments and simulation parameters, as we explain below.