The nature of the anion played a major role in the dissolution of biomass. For example, [EMIM][OAc] was more effective than [EMIM] [Cl] in the dissolution of southern yellow pine [36]. The chloride anion combined with the [BMIM] cation was effective in the dissolution of maple wood flour. Substitution of the chloride anion with the tetrafluoroborate or hexafluorophosphate anions made the maple wood flour insoluble [25]. Maple wood flour pretreated in [EMIM][OAc] and [BMIM][OAc] at 90°C for 6 h resulted in a decrease in cellulose crystallinity, higher glucose, and xylose yields. In contrast, pretreatment with [BMIM][MeSO4] had little effect on the biomass cell structure, sugar yields, and cellulose crystal­linity, compared to untreated wood flour [38].

The ability to dissolve biomass was related to the anion basicity. [EMIM] [OAc] was a better solvent than [BMIM][Cl] for southern yellow pine and red oak (particle size 0.125-0.250 mm), due to the increased basicity of the acetate anion and also its lower viscosity and melting point [36]. Cork powder remained insoluble in [EMIM] [Cl] and [BMIM][Cl] after 4 h at 100°C. Replacing the chloride anion with a lactate or ethanoate anion improved the cork dissolution significantly. ILs based on the cholinium cation and alkanoate anions were more effective in the cork dissolution. Among the alkanoate anions included in the study, the increasing alkyl chain length (ethanoate, butanoate, hexanoate) led to an increase in biomass dis­solution efficiency, attributed to an increase in the basicity of the anion [39].

The structure of the cation plays a role in the melting point of the IL. Alkyl groups on the imidazole tend to lower the melting temperature, and enhance wood liquefaction and processability of the wood solution. For example, wood disso­lution was more effective in 1-butyl-3-allylimidazolium chloride ([BAIM][Cl]), then in 1-methyl-3-allylimidazolium chloride ([MAIM][Cl]) [16].