In this paper, a new deep eutectic solvent (DES) has been successfully synthesized that is based on benzyltriethylammonium bromide as a hydrogen bond acceptor (HBA) and urea as a hydrogen bond donor (HBD). However, its usability in modifying cellulose derivatives, especially acylating hydroxyethylcellulose (HEC) was investigated. The chemical modification (acetylation) of HEC was carried out in BTEAB/urea DES system without any additional conventional solvent or catalyst. However, the proposed structure of acetylated HEC (HECA) was confirmed according to the structural spectra analyses FTIR-ATR, 1H, 13C, and APT-NMR. The crystalline behavior of acetylated and unmodified HEC in the DES system has been evaluated using XRD patterns, where the thermal stability was evaluated basing on the TD-TGA thermograms. Hence, SEM images and EDX spectra were recorded to prove the changes that are expected at the morphological level and elemental profile. Yet, the nanometric sheets aspect was observed. The Functional Density Theory (DFT) was investigated as a useful computational tool to understand mechanism and donor-acceptor interactions. The topological parameters (electron density Laplacian, kinetic energy density, potential energy density, and energy density) at the bond critical points (BCP), between TBEAB and urea, are deducted according to Quantum Bader's theory, and Atoms-in-molecules (AIM). The non-covalent interactions and steric effect in the DES system were studied using the reduced density gradient isosurface (RDG). Theoretical and computational calculations revealed that the H-bonds and the electrostatic coexist, as predominant interactions in the BTEAB-based DES resulting chemical structure, and mechanism formation. The physical interactions between the component entities of DES lead to a new equilibrium that is more stable than that of HBA and HBD in their separate states.
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