Charge Control on the Surface of Ionic Cellulose Nanocrystal for the Stable Pickering Encapsulation

The charge of surface-active particles is one of the critical factors in Pickering emulsion. It is difficult for charged surfactant particles to form emulsion droplets due to the repulsive force of Coulomb's law against oils with high polarity. In order to form stable emulsion droplets and introduce thermodynamically stable colloidal dispersion between droplets, it is necessary to control the charge of the surface-active materials. Here, we present the CNC surface modification method for controlling charge density from high to low and its effect on the stabilization of Pickering emulsion. The sulfated cellulose nanocrystal (S-CNC) was hydrolyzed with sulfuric acid, and the S-CNC was desulfated with hydrochloric acid at different reaction times to observe the surface charge effect of CNCs on the Pickering encapsulation. In desulfation, we used a low concentration of hydrochloric acid, which does not affect the morphologies of CNCs. Then, the morphologies and surface properties of CNCs were analyzed by TEM, AFM, conductometric assay, zeta potential, FT-IR, and XRD. As a result, the ionic groups on the S-CNCs were successfully substituted for the non-ionic groups without changing CNCs' length and crystal size. After preparing CNCs, to understand the surface effect of CNCs on Pickering encapsulation, we chose n-decane, n-dodecane, n-tetradecane, and n-hexadecane as non-polar oils and chloroform and 1-octanol as polar oils. The stability of Pickering emulsion was evaluated by emulsion phase stability, surface tension, emulsion droplet size, and zeta potential. The dS-CNCs desulfated for 30 h formed small emulsion droplets with high stability for long-term storage. CNCs are potential emulsifiers based on particles. It is critical to control the surface chemistry of CNCs to form a stable Pickering emulsion of oils. These results will find potential utility in pharmaceutical, cosmetic, agriculture, food chemistry, and other related fields.