Pickering oil-in-water emulsions stabilized by carboxylated cellulose nanocrystals – Effect of the pH

Abstract

Carboxylated cellulose nanocrystals (cCNC) were prepared by oxidation of microcrystalline cellulose with ammonium persulfate and characterized by AFM. Zeta potential was measured at different pH and ionic strength, in presence of mono- and divalent cations. With a length ranging from 50 to 450 nm and a thickness varying between 20 and 60 nm, the cCNC had a surface charge that appeared to be more sensitive to the presence of divalent cations and exhibited a strong pH dependence. The nanocrystals were capable of forming stable oil-in-water emulsions at three different pH of 2, 4 and 7 with a triglyceride oil. The size of emulsion droplets was dependent on oil and cCNC contents. Emulsification was, however, mainly influenced by the pH of the continuous phase, which can be related to reduction of charge on the cCNC surface with decreasing pH. Responsiveness of emulsions towards pH changes was not as dominant as expected, and lowering of pH did not trigger the release of oil from droplets. This can be explained by the strong adsorption of the cCNC, relatively polar triglyceride oil and the limited possibility to induce desorption of nanocrystals from oil surface.