Pickering emulsions by combining cellulose nanofibrils and nanocrystals: phase behavior and depletion stabilization†
In this study, cellulose nanofibrils (CNF) were used to induce depletion stabilization of oil-in-water Pickering emulsions formed by interfacial adsorption of another type of nanocellulose, namely, cellulose nanocrystals (CNC). This depletion effect triggered solely by the non-adsorbing CNF was achieved in a novel system that contained spherical droplets and the high-aspect cellulose nanorods. By exploiting the synergies between the two nanocelluloses, green emulsions were prepared either by sequential or by one-step, simultaneous addition. A battery of techniques to unveil the properties of the emulsions (droplet size and morphology by AFM and confocal microscopy, stability and creaming by light backscattering and rheological behavior) and related surface interactions (quartz crystal microgravimetry), were applied for a comprehensive evaluation of the emulsification phenomena. Three regimes of concentration-dependent depletion stabilization were established, including (1) creaming of non-flocculated droplets at low CNF concentration; (2) flocculation at intermediate CNF content and, (3) stabilization of the emulsions with characteristic micron-sized droplets at the highest CNF concentrations used. Remarkably, the latter regime was achieved at CNF concentrations as low as 0.1%. Above a critical stabilization concentration, the emulsion droplet size became independent of CNF content, following depletion and gelation mechanisms. The universality of the findings was tested with a natural oil, sunflower oil, as well as a paraffinic alkane. Overall, we unveiled the depletion interactions in Pickering emulsions induced by the long, flexible cellulose nanofibrils, opening the way to green, edible Pickering emulsions for foodstuff, cosmetic and other related formulations. This is mainly by combining the shape-anisotropy and amphiphilicity of nanocelluloses extracted from plant cell walls.