Effect of pH on the aggregation behavior of cellulose nanocrystals in aqueous medium

The cellulose nanocrystals (CNCs) were obtained from microcrystalline cellulose by using sulphuric acid hydrolysis, and the effects of pH on aggregation behavior of the CNCs have been investigated by rheology-small angle light scattering (Rheo-SALS), field emission scanning electron microscopy and Zetasizer Nano ZS instrument. The results show that the CNC suspensions at different pH conditions (4.78, 7.00, and 9.00) exhibit shear-thinning behavior. The viscosity of the CNC suspensions markedly increases under the acidic or alkaline conditions, with a significant increase in the size of the CNC aggregates. Moreover, we demonstrate that different morphologies of the CNC aggregates can be obtained by adjusting the pH of the suspensions. The morphology of the CNC aggregates changes from parallelepiped platelets (about 1.5 mu m) to monodispersed spherical-like nanoparticles with increasing pH (from 0.99 to 7.00), and the morphology of the CNC aggregates becomes aciculiform flocculation (0.7 mu m) when the pH is up to 13.01. We explain it in terms of protonation or the shield of repulsion forces with the addition of H+ or Na+, and disruption of cellulose crystalline domains by high-concentration OH-.