Acidified ZnCl2 molten salt hydrate systems as hydrolytic media for cellulose I and II nanocrystal production: from rods to spheres

Exploring environmentally benign and economic methods for the preparation of nanocelluloses with controllable morphology would promote their industrial production and utilization. Herein, cellulose I and II nanocrystals (CNC I and CNC II) of rod and sphere shapes were prepared directly from microcrystalline cellulose (MCC, cellulose I) via treatment by acidified zinc chloride molten salt hydrates. Upon adding a small amount of HCl (0.50/0.75/1.0 M) to ZnCl2 (55/60/65%), the levels of cellulose swelling and hydrolysis could be tuned. Rod-like CNCs I (length: 181.3-279.3 nm) were obtained with 55% ZnCl2. Rod-like CNCs with size reduction along with spherical CNCs were produced as ZnCl2 concentration increased to 60%. Further raising the ZnCl2 concentration to 65% resulted in CNC II with exclusively spherical shape (diameter: 25.8-292.3 nm). Under the same ZnCl2 concentration, CNCs produced with high HCl concentration exhibited improved crystallinity (up to 68.4%) and thermal stability (T-max up to 343.5 degrees C). Furthermore, spherical CNC II displayed excellent suspension stability and emulsifying ability to stabilize oil/water Pickering emulsions. The best one stabilized 26% more emulsion volume than the commercially available CNCs and the emulsions showed shear and heat stability. Therefore, this study provides a sustainable method for production of CNCs with tailored characteristics and broad application potentials. [GRAPHICS] .

Automated summary

This study explored environmentally friendly and cost-effective methods for producing nanocelluloses with controllable morphology. By treating microcrystalline cellulose with acidified zinc chloride molten salt hydrates, rod and sphere-shaped cellulose I and II nanocrystals were obtained. The levels of cellulose swelling and hydrolysis could be controlled by adjusting the concentration of HCl and ZnCl2. The resulting nanocelluloses exhibited different sizes and properties, with improved crystallinity and thermal stability observed at higher HCl concentrations. The spherical nanocellulose II showed excellent suspension stability and emulsifying ability to stabilize oil/water Pickering emulsions. This study provides a sustainable method for producing nanocelluloses with tailored characteristics and broad application potentials.