**2. Emulsion formation and stabilization by natural polymers and particles**

Emulsions are multiphasic systems of at least three main components, the oil phase, the water phase, and the emulsifier. One of the phases is dispersed into the other in the form of droplets that are stabilized by a key compound, an emulsifier. Depending on the dispersed phase, we can have either oil-in-water (o/w) or waterin-oil (w/o) emulsions, and the type of emulsion formed mainly depends on the solubility properties of the emulsifier. According to Bancroft's rule, o/w emulsions are formed when the emulsifier has a preference for water whereas the opposite applies for w/o emulsions [7]. When an emulsion is formed, a large interfacial area is created between the two phases, generating an increased energy in relation to the interfacial tension between oil and water. Therefore, emulsions seek to minimize the energy used to create such large interfacial area and break down over time by the combination of different instability mechanisms, such as, creaming, flocculation, coalescence and Ostwald ripening [7]. The role of the emulsifier is to reduce the interfacial tension and form a "protective layer" through its adsorption on the droplets surface, thus facilitating not only the formation of the droplets but also preventing/minimizing their re-association. Amphiphilic molecules and insoluble particles have both been employed as emulsifiers (**Figure 1**). Small surfactant molecules are usually good emulsifiers. Nevertheless, they are often not particularly well suited to provide long-term stability; this is because they are in dynamic equilibrium with the bulk medium. In this case, often, a stabilizer is required to achieve sufficient kinetic stability for the required shelf-life of a certain product. Polymers

**Figure 1.** *Emulsifiers: Surfactants, polymers and particles. Differences in scaling are not considered.*

#### *Cellulose as a Natural Emulsifier: From Nanocelluloses to Macromolecules DOI: http://dx.doi.org/10.5772/intechopen.99139*

are often applied as stabilizers in oil-in-water (o/w) emulsions, and they can act either via the reinforcement of the stabilizing layer, co-acting with the emulsifier at the interface, or via the viscosity enhancement of the continuous phase, thus reducing droplet mobility [8]. Certain amphiphilic polymers and particles may act as both emulsifiers and stabilizers. Good examples are surface-active polysaccharides, such as gum Arabic, pectin, galactomannans and modified starches and celluloses [8–11]. These polymers provide strong steric repulsions driven by the entropic penalty when polymer segments from two droplets start to entangle, since conformational rearrangements are hindered due to their high molecular weight [12].

Another type of stabilization is provided by insoluble particles, often called Pickering stabilization; rigid particles, Janus particles and microgels, have been described as Pickering emulsifiers [13–15]. The amphiphilicity of a typical Pickering emulsifier (rigid particles) is usually described in terms of surface wettability, which is measured by the three-phase contact angle of a particle adsorbed at an oil–water interface. Both o/w and w/o emulsions can be formed depending on the particle wettability and whether the particles are predominantly hydrophilic or hydrophobic [13]. In agreement with Bancroft's rule, the interface tends to bend towards the more poorly wetted liquid, and this becomes the dispersed phase. Pickering particles adsorb irreversibly at the oil–water interfaces due to the high-binding energy per particle, forming an effective mechanical barrier against coalescence; they may also inhibit lipid oxidation due to the thick interfacial layers formed [10, 11]. This is an important feature in what concerns food and pharmaceutical applications where polyunsaturated lipids are involved. Their double bonds are prone to oxidation leading to the deterioration of the products by the formation of rancid flavors and, eventually, toxic by-products [16]. The most widely used bioparticles are derived from biopolymers, such as cellulose, chitin and chitosan, starch and modified starches, lignin and proteins [17–21]. Bioparticles may vary widely in shape, size, aspect ratio and morphology, implying that their mechanistic behavior considerably deviates from that of both the solid sphere and the flexible polymer [22]. Nevertheless, particles with an irregular shape and higher aspect ratios have been found to have a greater ability in stabilizing emulsions and foams (and at lower concentrations) compared to synthetic particles of spherical shape [19].

A special type of particles that display some similarities to surfactants and polymers are known as Janus particles. These are amphiphilic particles, composed of two or more regions with distinct physicochemical properties, that can selfassemble in bulk media and readily adsorb to fluid interfaces, remarkably lowering the interfacial tension; for this reason, they are also called "colloidal surfactants" [14, 23, 24]. They can be synthesized in geometrically different shapes and chemical compositions with high uniformity and precision [14]. Polysaccharides, such as, alginates, chitosan, pectin, cellulose and heparin, have been used to produce biobased Janus particles [25, 26].

Another interesting type of emulsifying particles are microgels, which are soft deformable gel-like particles made up of aggregated or cross-linked polymer networks [22]. These microgels can swell in aqueous solvents and rearrange at the oil–water interface, resulting in thick and mechanically resilient layers. Owing to the amphiphilic character of their polymeric constituents, most microgels are inherently surface active at oil–water and air-water interfaces and, as rigid particles, they also irreversibly adsorb at the interfaces [22]. Synthetic microgels offer an additional feature that arises as a direct consequence of their combined polymeric and particulate character. They have the potential to effectively stabilize water-inwater emulsions, which are mixed solutions of thermodynamically incompatible polymers, producing two immiscible aqueous phases, and where the effective thickness of the interface is defined on a length scale considerably greater than the

molecular dimensions of a conventional emulsifier [22, 27]. However, microgels based on physical cross-linking of biopolymers are rather novel and much of their behavior at interfaces remains unclear [15]. Two examples of natural ingredients that exhibit microgel-like characteristics are casein micelles (in their native form) and whey proteins and gelatinized starch granules (upon heat treatment). However, in order to mimic the special features of the synthetic microgels, these traditional food-grade microgels need more pronounced long-term structural stability under conventional processing and storage conditions, which typically requires the introduction of additional covalent cross-links within the aggregated biopolymer-based entity [22].
