The wide range of biomass solubility in ILs reported in the literature could be partially explained by the contamination with water, which can significantly affect their physicochemical properties [58]. Even hydrophobic ILs, such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]), are hygroscopic and can contain up to 25% of water (molar ratio) when exposed to an environment with a relative humidity of 81% [59]. Water can also be produced during the reaction of biomass with IL by the hydrolysis of acetate groups [47]. Traces of water can be detected by :H NMR or IR spectroscopy [60]. They can be quantified by Karl-Fischer titration or gravimetrically [59]. Water can be removed in a vacuum oven or in a freeze dryer [7, 61]. Its presence complicates the IL recycling and removal is energy-intensive.

Addition of water above 4 wt% to loblolly pine wood before its pretreatment in [BMIM][Cl] led to a notable decrease in soluble products, monosaccharides, and

5- hydroxymethylfurfural. This was attributed to the competition with the cellulose hydroxyl groups to form hydrogen bonds with Cl — ions [53]. The presence of water reduced the solubility of wood in ILs and the yield of sugars released in the dissolution of maple wood flour in [BMIM][OAc] and [EMIM][OAc] [7]. Water also prevented the IL from effectively reducing the cellulose crystallinity [38]. Water can also prevent the formation of by-products such as 5-hydroxymethyl­furfural during the dissolution of cellulose in [EMIM][Cl] catalyzed by HCl or H2SO4 [62].

The IL hygroscopicity is the result of the adsorption of water on the IL surface, diffusion from the surface and/or the formation of complexes through hydrogen bonding [59, 63, 64]. The hygroscopicity depends on the IL composition and structure [65]. Adsorption would depend on the charge distribution and structure of the cation and anion, while diffusion would be affected by the IL viscosity [59]. The length of alkyl chains and substitution on the cation ring (e. g., pyridinium, imidazolium) affected the mutual solubility of the IL with water [65, 66]. For ILs with the [EMIM] cation, water uptake increased with different anions in the fol­lowing order: dicyanamide < diethyl phosphate < chloride < acetate [61].