Colloidal stability of cellulose nanocrystals in aqueous solutions containing monovalent, divalent, and trivalent...

Aggregation kinetics and surface charging properties of rod-like sulfated cellulose nanocrystals (CNCs) have been investigated in aqueous suspensions containing monovalent, divalent, or trivalent inorganic salts. Electrophoresis and time-resolved dynamic light scattering (DLS) were used to characterize the surface charge and colloidal stability of the CNCs, respectively. The surface charge and aggregation kinetics of the sulfated CNCs were found to be independent of solution pH (pH range 2-10). For the monovalent salts (CsCl, KCl, NaCl, and LiCl), the critical coagulation concentration (CCC) followed the order of Cs⁺ < K⁺ < Na⁺ < Li⁺, which follows the direct Hofmeister series, indicating specific interaction of the cations with the CNCs surface. The experimental aggregation kinetics of CNCs were in very good agreement with predictions based on the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. A Hamaker constant of 3.6 × 10^-20 J for the CNCs in aqueous medium was derived, for the first time, from the colloidal stability curves with monovalent salts. This value is consistent with a previous value determined by direct force measurements for cellulose surfaces in aqueous solutions. For the divalent salts (MgCl₂, CaCl₂, and BaCl₂), the CCC values followed the order Mg²⁺ > Ca²⁺ > Ba²⁺, which is in the reverse order of the counterion ionic size. For the trivalent salts (LaCl₃, AlCl₃, and FeCl₃), the CNCs suspension was destabilized much more effectively. The observed complex stability curves with AlCl₃ and FeCl₃ are attributed to charge neutralization and charge reversal imparted by the adsorption of aluminum and ferric hydrolysis species on the CNC surface. The significant charge reversal induced by the ferric hydrolysis species led to the restabilization of suspensions. Our results on the colloidal stability of CNCs are of central importance to the nanotechnology and materials science communities working on various applications of CNCs.