Cationization of Cellulose Nanocrystals with Ionic Liquids and Polymers: From Electrostatic Screening to Adsorption-Controlled Aggregation.

Noncovalent surface modification of sulfated cellulose nanocrystals (sCNCs) and its effects on colloidal stability were investigated in the presence of alkylimidazolium ionic liquids (ILs) and cationic polymers. The addition of ILs resulted in the aggregation of sCNCs, and the exact mechanism of dispersion destabilization depended on the hydrophobicity of the ILs. Moderately hydrophobic ILs of short alkyl chains induced sCNC aggregation like inorganic salts. ILs bearing octyl or decyl groups adsorbed onto the sCNCs, leading to charge reversal of the particles, which was not sufficient to prevent aggregation. Adding water-soluble polycations in appropriate doses also reversed the charge of sCNCs upon surface adsorption without changing the morphology. The surface-coated particles maintained positive charge in the presence of salts and were stable at low ionic strengths, while ion-specific aggregation at higher salinity followed the trends predicted by the Hofmeister series of ions and Schulze-Hardy rule in mono- and multivalent salt solutions, respectively. The major interparticle forces and aggregation kinetics were rationalized within the DLVO model.