Solvation of carbohydrates in five choline chloride-based deep eutectic solvents and the implication for cellulose solubility

Abstract

Most organic solvents cannot dissolve carbohydrates due to the lack of hydrogen bonding ability of the solvent. The components of deep eutectic solvents (DESs) are held together with strong hydrogen bonds, which are capable of forming favourable interactions with carbohydrates. In this paper, we study the solute–solvent interactions of glucose, sucrose, erythritol, cellobiose, starch and cellulose in five different choline chloride-based DESs. The hydrogen bond donors used were urea, oxalic acid, ethylene glycol, glycerol, and 1,5-pentanediol. Molecular weight of starch and cellulose was determined by size exclusion chromatography, and the degree of polymerisation was noticed to influence the solubility. The enthalpy of hydrogen bond formation of the DESs was quantified by differential scanning calorimetry, and found to correlate well with the solubilities of the mono and di-saccharides. No correlation was found between void size of DESs and carbohydrate solubility, whereas high viscosity of the DESs restricted the solubility. In general, carbohydrates showed good solubilities in the studied DESs, and some similarities were observed between aqueous solutions and ionic liquids. Ethaline showed the best performance as it had lowest viscosity, and no degradation of the carbohydrate occurred. The intermolecular interactions between glucose and Ethaline molecules were investigated by spectroscopic techniques, including ¹H NMR and 2D {¹H–¹H}-NOESY and {¹H–¹⁹F}-HOESY. Results showed that the hydrogen bond acceptor ability of the choline anion was mainly responsible for the dissolution of the carbohydrate. The solubility of cellulose in ionic liquids and DESs is discussed in terms of the thermodynamics of solvation, being concluded that the entropy change of solvation was the driving force for cellulose solubility in highly ordered ionic liquids. The lower order of DESs could not enable sufficient entropy gain of the solvent resulting lower solubilisation.