Insight into different roles of chitin nanocrystals and cellulose nanocrystals towards stabilizing Pickering emulsions

Rod-like polysaccharide particles such as chitin nanocrystals (ChNCs) and cellulose nanocrystals (CNCs) have been widely used for the preparation of Pickering emulsion. Both are believed playing good role of particle emulsifiers. However, these two kinds of nanocrystals have completely different surface properties: electronegative CNC is more hydrophilic and has higher surface charge level as compared to electropositive ChNC. To reveal their difference as emulsifiers in Pickering emulsions is therefore of interest. In this work, ChNCs and CNCs with the same structural parameters (aspect ratio = 28) were used to prepare olive oil-in-water emulsions for a comparative study. The two kinds of nanocrystals show the same mechanical state in emulsions, behaving like rigid rods, instead flexible fibers because they have 104 level of effective stiffness. Therefore, their roles of emulsifying oil/water system are dominated by their surface properties: ChNCs reveal higher emulsifying capacity relative to CNCs due to better ChNC-oil affinity, and can even emulsify the oil/water (7/3 w/w) system with high oil fractions. However, ChNC-stabilized emulsions are not that stable to centrifugation as compared to CNC-stabilized emulsions at low oil fractions due to the formation of the cluster structure resulted from less coating of droplets. Thus, the former reveals higher strain sensitivity and stronger thixotropy than the latter in shear flow. The correlation between viscoelasticity and morphology/phase behavior of the two kinds of Pickering systems was assessed, then. This work provides valuable information on formulating preparation of Pickering emulsions by using different types of rod-like polysaccharide nanocrystals.