Agglomeration of cellulose nanocrystals: the effect of secondary sulfates and their use in product separation

This study was aimed at the development of a better understanding of the agglomeration behavior of sulfated cellulose nanocrystals (CNCs) in the presence of sulfates with monovalent (NH₄⁺, K⁺, Na⁺) and divalent (Ca²⁺) cations, and to demonstrate their potential in simple and efficient product separation. Protonated CNCs were counterion-exchanged and their ionic strength was increased by adding sulfates of the respective cation to trigger agglomeration. The critical concentrations of agglomeration (CAC) and peptization (CPC) were determined. We found that the agglomeration behavior of CNCs could be attributed to matching affinities between the cations and the sulfate half-ester groups on the CNC surfaces. Based on these findings, a facile and efficient downstream process was designed to separate CNCs from neutralized reactant solutions using CAC and CPC. This method provides colloidally stable CNCs at high yield provided by centrifugation. When salt concentrations in the product are maintained below the CAC, as prepared CNCs from neutralized reactant solutions might be used in hydrogels and emulsions.