Auto-catalyzed acidic desulfation of cellulose nanocrystals

The surface charge density of cellulose nanocrystals (CNCs) extracted by sulfuric acid hydrolysis is determined by its anionic sulfate half-ester content. Sulfate half-ester moieties are relatively labile, and can be removed by acid- or alkaline-catalyzed de-esterification at high catalyst concentration and temperatures above 50 degrees C. Auto-catalyzed desulfation in acidic (H-CNC) suspensions also occurs slowly at ambient conditions. Desulfation poses a problem because loss of surface charge alters CNC suspension stability, liquid crystal properties and rheological behaviour. The rate of acid-catalyzed desulfation depends on the local concentration of sulfate half-ester groups with proton counterions at the surface of the cellulose nanocrystal. High temperatures and low moisture content significantly increase the reaction rate constant. Temperatures just above 0 degrees C substantially slow desulfation in H-CNC suspensions during long-term storage. Neutral salt forms of CNCs do not undergo significant in situ desulfation even under very harsh conditions.