Phase behaviour and droplet size of oil-in-water Pickering emulsions stabilised with plant-derived nanocellulosic materials

Abstract

The preparation, stability and phase behaviour of oil-in-water emulsions formed by dodecane and water and stabilised by naturally amphiphilic, chemically unmodified cellulose nanofibrils (CNF), TEMPO-oxidized cellulose nanofibrils (T-CNF) and cellulose nanocrystals (CNC) were investigated. The stability towards prolonged storage, high temperature (maximum 85°C), shear and dilution with water was evaluated. Droplet size distributions were determined from micrographs and by NMR diffusion measurements. Oil-in-water Pickering emulsions were formed at all stabiliser and O/W ratios (20–35% oil, 0.1–1.5% nanocellulose) for all three nanocellulosic materials, without the use of any additives. All emulsions creamed rapidly; the creaming layers remained stable for at least one month. The volume of creaming layers formed by CNF and T-CNF are larger and the stability towards coalescence, low shear and high temperature of CNF and T-CNF are higher than the corresponding properties of emulsions stabilised with CNC. This is probably due to the networks formed by the longer fibrils. T-CNF forms a dilute stable emulsion in equilibrium with the creaming layer. No emulsion droplets were visible in the water phases in equilibrium with the creaming layers formed by emulsions stabilised by CNC or CNF. The stability of the dilute T-CNF emulsions is probably due to the ionic charge of the nanofibrils, which is higher than that of the CNF or CNC. It seems that nanocellulosic materials primarily act as stabilisers against coalescence rather than flocculation.