The dissolution of Norway spruce in [BMIM][Cl] or [AMIM][Cl] depended on the size of the biomass. Whereas ball-milled powder and spruce sawdust (size 0.1-2 mm) were completely dissolved at 80°C in several hours, it took several weeks to dissolve wood chips (5 x 5 x 1 mm3) at 130°C in the same ILs. In general, dissolution was fastest for ball-milled wood, followed by sawdust (particle size 0.1-2 mm), thermomechanical pulp fibers, and wood chips [7]. A similar size effect was observed for southern pine and red oak wood chips [36], and rice straw [46]. Ball-milling of Norway spruce TMP and southern pine increased the glucose yield after IL pretreatment and enzymatic hydrolysis, by opening access to the wood structure for enzymes. The same effect, which became more significant with milling time, was also observed for corn stovers. The molecular weight of ball — milled corn stovers decreased with increasing milling time [33].

The size reduction effect could be explained by the increase of effective surface area and the improved access of enzymes to the biomass cellulose. However, feedstock size reduction through mechanical grinding is energy-intensive [36]. Also, ball-milling for several days can lead to significant degradation and chemical modification of cellulose and lignin, as well as the generation of soluble species that reduces the recyclability of the IL [31, 33, 57]. It was reported that extensive ball-milling causes cleavage of aryl-ether linkages in lignin and the generation of phenolic hydroxyl groups [57].