Tuning the stability and microstructure of fine Pickering emulsions stabilized by cellulose nanocrystals

Cellulose nanocrystals (CNCs) as Pickering emulsion stabilizers have attracted fast increasing interests in the food, pharmaceutical and cosmetic fields, due to their sustainability and biocompatibility. However, it still remains a debate for unmodified CNCs to act as a kind of effective Pickering stabilizers, due to their high hydrophilic nature. This work for the first time reported that CNCs were compatible with using microfluidization as the emulsification process to produce a kind of fine Pickering emulsions, with surface-average droplet sizes as low as 0.22 μm. As compared with other emulsification processes, the Pickering emulsions by the microfluidization were more stable against coalescence but readily prone to droplet flocculation and creaming. The droplet flocculation in these emulsions was mainly associated with the formation of bridging droplets with two different droplets sharing a same monolayer. The droplet size, microstructure and stability (against creaming and coalescence) of these emulsions by the microfluidization could be well tuned by varying the particle concentration (c; 0.3–1.2 wt%) and/or oil fraction (ø; 0.1-0.4). In general, the finer Pickering emulsions with more serious bridging flocculation were formed at lower ø and/or higher c values, while the emulsions at lower ø and c values were more prone to creaming. The Pickering emulsions with smaller droplet sizes exhibited a much better coalescence stability. Interestingly, the amount of adsorbed CNCs at the interface was relatively high, and the percentage of surface coverage at interface was low (ranging from 11.3 to 50.0%). The results confirmed that using the microfluidization as the emulsification technique, CNCs exhibit a good ability to stabilize the Pickering emulsions, with the microstructure and stability well-modulated by varying the c and/or ø. The findings would be of great relevance for the development of nanoscale CNC Pickering emulsions suitable for food and pharmaceutical formulations.