Enhancing emulsion stability and adsorption of Pickering emulsions using alkylated cellulose nanofibers

Cellulose nanofibers (CNFs) extracted from plant cell walls form a three-dimensional network in water and stabilize the dispersion of droplets, which are used as emulsion stabilizers. In this study, four types of alkyl-grafted CNFs (ACNFs) with different lengths of dialkyl chains (diC4, diC6, diC8, and diC10) were synthesized by the surface modification of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized CNF (TOCNF) to improve the stability of oil-in-water Pickering emulsions (PEs). The ACNF-stabilized emulsions with a 20/80 ratio of oil/water were prepared using two different oils (olive oil and eucalyptus oil) and the ACNF 1.0 % dispersion, and the changes over time were investigated at approximately 25 °C. Compared to TOCNF, the ACNFs provided more stable olive oil-PE (o-PE) and eucalyptus oil-PE (e-PE) although the stabilization behaviors of o-PE and e-PE differed. Olive oil, which contained rich long-chain fatty acids, was more likely to interact with the ACNFs, thereby promoting the interfacial adsorption of oil droplets. On the other hand, eucalyptus oil is mainly composed of 1,8-cineole and does not have alkyl chains, so the interaction with the ACNFs was weak and therefore, sufficient interfacial adsorption could not be obtained. The results of the particle size distribution and viscosity measurements also showed that the ACNFs contributed significantly to the stabilization of o-PE. Based on the overall experimental results, it was determined that the diC6-ACNF gave the most significant stability improvement. Therefore, we designed an alkylated CNF that mimics diC6-ACNF for MD simulations. MD simulations showed that hydrophobic modification created a hydrophobic interaction between the alkyl chain and dodecane molecules as the oil, leading to the enhancing adsorption of ACNF at the oil droplet interfaces.