ZnCl2:ChCl:2urea as a new ternary deep eutectic for clean production of high content zwitterionic micro- or nano-cellulose by passing to the binary DES of ZnCl2:2urea

Abstract

Deep eutectic solvents (DESs) are clean alternatives for chemical reagents and reaction media. Herein, a novel method for zwitterionzation of micro-/nano-cellulose (MC/NC) by a novel ternary DES (TDES) from ZnCl2, choline chloride (ChCl), and urea was established. In the characterized TDES of ZnCl2:ChCl:2urea, by phase diagram, viscosity, density, FT-IR, and TGA analyses, ZnCl3−Ch+ is hydrogen bond-acceptor (HBA) and two urea molecules are hydrogen bond-donors (HBDs). The theoretical investigations on donor–acceptor mechanism by Functional Density Theory, the reduced density gradient isosurface, and electrostatic potential surface (EPS demonstrate the co-existences of the H-bonding and electrostatic-interactions in formation of ZnCl2:ChCl:2urea. These interactions between ZnCl3−Ch+ complex as HBA and urea HBDs provide a higher equilibrium than the related binary DESs. By this new TDES, alkalized MC or NC was zwitterionized without any additional reagent and the products were characterized by FT-IR, FESEM, EDX, PZC, TGA, and XRD analyses. Dual positively/negatively sites on cellulose and decrease in crystallinity-index (7%) and thermal-stability detected for the given zwitterionized celluloses. In this new protocol, the simply prepared ZnCl2:ChCl:2urea served as both of reaction medium for dissolution of cellulose and cholinating agent for the zwitterionization of cellulose by nucleophilic attack of the alkalized MC/NC to activated-ChCl by TDES.