Emulsion Formation and Stabilization by Biomolecules: The Leading Role of Cellulose

Abstract

Emulsion stabilization by native cellulose has been mainly hampered because of its insolubility in water. Chemical modification is normally needed to obtain water-soluble cellulose derivatives. These modified celluloses have been widely used for a range of applications by the food, cosmetic, pharmaceutic, paint and construction industries. In most cases, the modified celluloses are used as rheology modifiers (thickeners) or as emulsifying agents. In the last decade, the structural features of cellulose have been revisited, with particular focus on its structural anisotropy (amphiphilicity) and the molecular interactions leading to its resistance to dissolution. The amphiphilic behavior of native cellulose is evidenced by its capacity to adsorb at the interface between oil and aqueous solvent solutions, thus being capable of stabilizing emulsions. In this overview, the fundamentals of emulsion formation and stabilization by biomolecules are briefly revisited before different aspects around the emerging role of cellulose as emulsion stabilizer are addressed in detail. Particular focus is given to systems stabilized by native cellulose, either molecularly-dissolved or not (Pickering-like effect).

Keywords: adsorption; amphiphilicity; cellulose; emulsion stability; oil–water interface.

Figure 1

Breakdown mechanisms of an emulsion. Note that different processes may occur simultaneously. Adapted with permission from [13]. Copyright John Wiley and Sons.

Figure 2

Interfacial stabilizing layers formed by surfactants, particles and polymers. Differences in scaling are not considered. Adapted with permission from [15]. Copyright CRC Press, Taylor and Francis Group.

Figure 3

Two established approaches of preparing emulsions stabilized by cellulose. Adapted from [91].

Figure 4

(a) Confocal laser scanning micrograph of hexadecane droplets stabilized by bacterial cellulose nanocrystals (BCN) with double staining (oil stained with BODIPY564/570 and BCN stained with calcofluor white). (b,c) Scanning electron micrographs of styrene-polymerized Pickering emulsion stabilized by BCN. (d) Confocal laser scanning micrograph of citrus oil droplets stabilized by regenerated cellulose with double straining (oil stained with Nile red and cellulose shell stained with calcofluor white). (e,f) Scanning electron micrographs of styrene-polymerized emulsion stabilized by cellulose regenerated from an 85 wt% phosphoric acid solution. (g) Fluorescence microscope image of double stained cellulose-coated emulsion droplets of n-hexadecane/toluene (1/1) (oil mixture stained with Nile red and cellulose shell stained with calcofluor white). (h) Cryo-SEM image of the fractured surface of an empty cellulose-shell of an emulsion prepared from a molecular solution of cellulose in 1-ethyl-3-methylimidazolium acetate (EMIMAc). Adapted with permission from [70] and [93]. Copyright 2011 and 2018 American Chemical Society.