Ion-Induced Hydrogel Formation and Nematic Ordering of Nanocrystalline Cellulose Suspensions

Nanocrystalline cellulose (NCC) is a promising material for formation of hydrogels and nematic liquid crystals. While salt addition is known to facilitate hydrogel formation, it remains unclear whether this originates from cationic bridging or charge screening effects. Herein, we demonstrate the effect of mono- and divalent salts on NCC gelation and nematic ordering. A strong correlation of NCC suspension zeta-potential and rheological behavior was found. Lower concentrations of divalent cations were needed to decrease NCC zeta-potential and form hydrogels. The same zeta-potentials and gel strengths were achieved at higher concentrations of monovalent salts. Salt-induced NCC aggregation is thus caused by intermolecular attractive forces rather than cationic bridging. Against excluded volume argumentation, salt addition was found to promote NCC nematic phase formation. Increased nematic ordering was observed in a transition regime of moderate salt addition before complete aggregation occurs. This regime is governed by an equilibrium of repulsive and attractive forces. Small angle neutron scattering suggests lateral orientation of NCC. Hence, NCC gelation and nematic ordering can be modulated via its zeta-potential by targeted salt addition.