Study On Pickering Emulsions Stabilized By Bacterial Cellulose Nanofibrils

With the development of nanotechnology, more attentions have been focused on the Pickering emulsions stabilized by solid particle emusifiers in place of traditional surfactants. We choose environmentally benign and biomass-based bacterial cellulose (BC) and TEMPO-oxidized BC (TOBC) as the particle emulsifier. Characterizations of TOBC were made in terms of oxidation degree, morphology, degree of crystallinity and lattice spacing et al. The influences of fibril content, oxidation degree, and storage time et al. on emulsion stability and particle size distribution for Pickering emulsions stabilized by BC or TOBC were explored. Macro rheological measurements of fibril dispersions and emulsions stabilized by TOBC with different oxidation degrees were made. Microrheological measurements of above-mentioned emulsions help us to develop a better understanding of the stabilization mechanism.The oxidation degrees of TOBC were determined using an electric conductivity titration method, which increased with the dosage of NaClO. It is proved that carboxylate group linked to the cellulose molecular chain via FT-IR spectrum. It is shown in SEM images that the width of TOBC fibrils is 5 to 6 nm, distinctly thinner than BC, resulting from the cleavage of fibers during oxidation. TEMPO-oxidation reaction is mainly happened on the surface of cellulose crystalline region and there is no obvious difference of degree of crystallinity and lattice spacing between BC and TOBC. The absolute value of Zeta potential and charge density increase with the carboxylate content. Adding BC or TOBC can reduce water surfacial tension and water-liquid paraffin interfacial tension, but BC is more effective.Pickering emulsions prepared by sonication are all oil-in-water type. In terms of emulsion stability, it increases with the higher fibril content while decreasa with the increasing carboxylate content. Increasing the fibril dosage, larger mean droplet diameter would produce. SEM images show that fibrils distributed evenly at the surface and bent to follow the curvation of each droplet. Zeta potential and charge density of emulsions increase with the carboxylate content.Macro rheological measurements show that 2-TOBC and 6-TOBC dispersions have the viscoelastic character. All of the tested dispersions present the shear-thinning behavior. BC dispersion has the largest shear viscosity while 2-TOBC,6-TOBC and 10-TOBC dispersions show closed shear viscosity. The yield stress values of fibril dispersions have a similar trend.Pickering emulsions stabilized by 2-TOBC and 6-TOBC show obvious viscoelastic property by macro rheological measurements, and 2-TOBC has a stronger viscoelasticity. With the storage time prolonging, G’ of emulsions firstly decrease and then will recover, but not exceeding G’ fresh prepared.Microrheological parameters, MSD, SLB, El, and MVI, show that 2-TOBC emulsions have the strongest stability, structure and viscoelasticity; and 10-TOBC emulsions are the most unstable. The conclusion corresponds well with that from the macro rheology.In summary, outstandingly stable Pickering emulsions have been prepared; changes can hardly be observed for 8 months’ storage at room temperature. Fibril size and wettability are two contradictory factors influencing the stability of emulsions. Besides, viscoelastic structure also contributes to the stability. Rheology and droplet size of emulsions are tunable by ajusting fibril content and wettability. Safe, nontoxic, and green TOBC is a kind of promising particle emulsifier for food and pharmaceutical emulsions.