Generally, the high viscosity of the IL affects negatively the overall efficiency of the pretreatment [7]. The IL viscosity depends on the IL chemical composition and temperature. For example, [AMIM][Cl] has a lower viscosity than [BMIM][Cl], which enabled wood dissolution at a lower temperature (80°C instead of 110°C). Dissolution in ILs with aromatic substituents, such as [BzMIM][Cl] and 1-methyl — 3-m-methoxylbenzylimidazolium chloride, required higher temperatures (130°C) to achieve the same wood solubility, which was attributed to their higher melting temperatures and viscosities [7]. However, the viscosity of the wood/IL mixture also increases with the wood dissolution over time with the accumulation of extracted products and generation of by-products [4]. The viscosity of cellulose solutions in [EMIM][OAc] or [BMIM][Cl] was found to increase with the cellu­lose concentration [51, 52].

One way to decrease the viscosity of the wood/IL mixture is to increase the temperature, but this solution is energy-intensive and can accelerate the degra­dation of the IL [22]. Another method consists in the addition of a co-solvent with lower viscosity. The viscosity of a wood/[BMIM][Cl] mixture was reduced by the addition of deuterated dimethyl sulfoxide, which had no noticeable effect on the wood dissolution efficiency [4]. In another study, [BzMIM][Cl] was blended with [AMIM][Cl] to reduce its viscosity without significant efficiency loss. Biomass dissolution could occur at a lower temperature and even at room temperature in the less viscous [AMIM][Cl] [7].

During wood dissolution in [BAIM] [Cl] and [MAIM][Cl] with AlCl3 as a catalyst, increased acidity led to a decrease in viscosity, which was attributed to the formation of AlCl — and Al2Cl- that weakens the hydrogen bonds in ionic liquids [16].