*2.2.1 Isolation of cellulose nanocrystals*

Cellulose nanocrystals are obtained by applying a chemical treatment to cellulose fibers: mild acid hydrolysis. Typically employing strong sulfuric acid H2SO4 is going to penetrate into accessible amorphous domains of cellulosic chains and dissolve them to release crystalline parts. Amorphous domains are randomly oriented and arranged inducing a lower density of these domains which are thus more vulnerable to acid hydrolysis [14] and especially to the infiltration of hydronium ions H3O<sup>+</sup> leading to hydrolytic cleavage of glycosidic bonds. In this sense, Ranby et al. were the first to prove the preparation and the presence of CNCs, the smallest cellulosic building blocks. **Figure 5** synthesizes the different steps of CNC isolation using sulfuric acid hydrolysis.

As previously mentioned, cellulose fibrils are exposed to a sulfuric acid hydrolysis, with defined concentration, temperature, and reaction time. Once amorphous domains are dissolved, a sonication step allows the separation between intact crystalline domains, leading to isolated CNC bearing half-sulfate ester groups on their surface. These charges come from the reaction between sulfuric acid and surface hydroxyl groups of cellulose and induce repulsive forces between negatively charged CNC leading to colloidal stability and dispersion in water [15]. While sulfuric acid is the most common acid used for cellulose fibers hydrolysis, other researches have focused on the use of other organic or mineral acids, like phosphoric acid, hydrobromic acid, or hydrochloric acid [16–19], generally leading to less stable suspension due to the lack of charges at the surface of the CNC. Moreover, the use of deep eutectic solvents is the subject of the next part of this chapter. When

#### **Figure 5.**

*Schematic representation of sulfuric hydrolysis of cellulose fibers (This scheme was extracted from an unpublished work (PhD manuscript of E. Gicquel, 2017)).*
