The cellulose nanocrystals prepared by sulphuric acid hydrolysis form stable colloidal suspensions at low ionic strengths because of anionic sulphate ester groups on their surface. Could equally stable suspensions of cellulose nanocrystals be prepared stabilized by cationic or non-ionic substituents covalently bonded to the surface? This isn't as easy as it sounds; the nanocrystals contain a relatively small number of chains, with a significant proportion on the surface, so that attempts at surface derivation either result in no reaction, or in complete conversion of the nanocrystals to cellulose derivatives. Furthermore, it is often difficult to separate the surface-modified nanocrystals from mixtures of unmodified nanocrystals and cellulose derivatives. We use a base-catalysed ring-opening reaction to attach substituted epoxides to the surface hydroxyl groups on the cellulose nanocrystals. Cationic functionalized nanocrystals were successfully prepared and characterized by this method. [Cationic Surface Functionalization of Cellulose Nanocrystals, Merima Hasani, Emily D. Cranston, Gunnar Westman and Derek G. Gray, Soft Matter, 4, 2238-2244 (2008)]
Polarized light image of a droplet of cationically-stabilized aqueous suspension of cellulose nanocrystals with (2-hydroxypropyl)-trimethylammonium chloride groups on the surface. The droplet is drying from the top left, so that there is a concentration gradient across the image. Liquid crystalline order is appearing in the dilute region on the right.
A similar chemistry was used to attach short poly(ethylene oxide) side chains to cellulose nanocrystals, to give sterically-stabilized aqueous PEG-CNC suspensions that formed a chiral nematic ordered phase. [Surface PEG-grafting of cellulose nanocrystals in aqueous media, Elisabeth Kloser and Derek G. Gray Langmuir, 26 (16), 13450-13456 (2010)]
Aqueous suspensions of sterically stabilized cellulose nanocrystals. On left, stability of suspensions compared to unstabilized nanocrystal suspension. On right, chiral nematic fingerprint texture displayed by sterically stabilized nanocrystal suspension.