**3.2 Nanocelluloses at interfaces**

Several researchers have demonstrated the ability of cellulose particles to self-assemble at oil–water interfaces and to stabilize o/w emulsions without the aid of classical surfactants [47–49]. It is believed that the amphiphilic character of nanocellulose resides in its crystalline organization at the elementary "brick" level, and thus, cellulose nanocrystals have both hydrophilic and hydrophobic edges that are preferentially wetted by water and oil phases, respectively [49]. The wettability properties of cellulose particles may be tuned by surface hydrophobization, due to the presence of many reactive hydroxyl groups, and w/o emulsions can be formed [50–52]. Most of the particles from biological origins, such as cellulose, chitosan, or starch, show an irregular shape and are polydisperse in size and morphology. However, this structural anisotropy may be very beneficial for emulsion formation and stability. Particles with high aspect ratios are capable of stabilizing biphasic systems at lower concentrations compared to systems containing spherical particles [53]. Particles with such a well-defined shape are usually derived from inorganic materials, like silica and these have been extensively studied because of their availability in different sizes with narrow size distributions and chemical surface tunability. However, their lack of biocompatibility and biodegradability restricts

their use in food and pharmaceutical applications [54]. For this reason, the study and characterization of materials from biological origins have gained increasing attention, and many efforts have been made in the food and pharmaceutical industries in order to develop new food-grade particles [19, 55]. It was early noticed that MCC particles have the ability to stabilize conventional o/w emulsions, and multiple emulsions systems of w/o/w type, with the aid of a hydrophobic surfactant for the stabilization of the internal w/o interface [56, 57]. These MCC particles form a network around the emulsified oil droplets that provides a mechanical barrier against coalescence, and, beyond that, the non-adsorbed particles may act as thickener agents in the continuous aqueous phase. MCC particles have also the ability to reduce lipid oxidation, one of the major concerns among food manufacturers due to its negative effects on food quality [55]. More recently, nanocelluloses, such as MFC/NFC and NCC, have been increasingly in focus for having a better performance than MCC, owing their smaller sizes and more regular shapes [58]. NCCs with low aspect ratios (shorter) have a dense organization at the interface and cover better the oil surface, while NCCs with high aspect ratios (longer) typically form a network around the droplet with relatively low coverage. Therefore, shorter NCCs have better emulsification efficiency and long-term stability, since higher droplet coverage usually means smaller droplet size [59, 60]. On the other hand, long nanofibrils (NFC) with a high aspect ratio also tend to form bigger droplets resultant from a lower surface coverage, but the fibers protrude in the continuous phase forming a strong network that is able to physically hinder droplet coalescence [59]. As mentioned, the colloidal stability of NCC is controlled by their surface charge resulting from the acid hydrolysis with various acids (e.g., H2SO4 or HCl). The higher the charge density the better their colloidal stability, but their ability to efficiently stabilize emulsions is reduced. Thus, the anionic charges on the surface of the nanocrystals control their tendency to be dispersed in water in relation to being adsorbed at the oil–water interface and, therefore, the particle polarity must be confined to a limited range. A surface charge density lower than ca. 0.03 e/nm2 is ideal for the effectiveness of NCC as an emulsifier and stabilizer, usually achieved by HCl hydrolysis. NCC with sulfate groups, resultant from the hydrolysis with H2SO4, possess a high surface charge density (e.g., 0.123 e/nm2 ), and the charges may undergo desulfation or may be screened by salt addition, to tune their amphiphilicity [49, 61]. Nanocellulose-stabilized emulsions are generally thermally stable, but in the presence of charges their stability against pH and ionic strength may decrease [58, 62]. NCC are able to form stable o/w high internal phase emulsions (HIPEs) containing volume fractions of oil as high as 0.9, at very low NCC concentrations (< 0.1 wt.%) [61]. Hydrophobized nanocellulose has been also explored to form w/o HIPEs [51]. Double emulsions of both o/w/o and w/o/w have been prepared by using a combination of native and hydrophobized NCC and NFC [63, 64]. Apart from the outstanding physical stability against coalescence, nanocelluloses also afford oxidative stability and lipid digestion control due to the dense interfacial layer formed [60].

### **3.3 Cellulose derivatives at interfaces**

Cellulose derivatives produced by etherification reactions are generally water-soluble and surface-active. Therefore, cellulose ethers are a major class of commercially important water-soluble polymers, from construction products, ceramics, and paints to foods, cosmetics, and pharmaceuticals [32, 33, 65, 66]. Cellulose ethers are commonly made by reacting alkali cellulose with the appropriate reagents to substitute the hydroxyl groups of the AGU monomers by either alkyl, hydroxyalkyl or carboxyalkyl groups [66]. Methyl cellulose (MC), ethyl

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

cellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), and their derivatives, are common products of those reactions. Their functionality and solubility in water depend on the type of substituent, degree and pattern of substitution, and molecular weight [33, 67]. The non-ionic cellulose ethers, such as, MC, HPMC and EHEC and their hydrophobically modified versions, have been mostly used to produce o/w emulsions due to their water-solubility [68–71], but EC can be used to stabilize w/o emulsions [72]; this change-over from MC to EC illustrates the subtle role of polarity and illustrates the applicability of Bancroft's rule. The emulsion stabilization due to cellulose ethers is the result of the combined effects of: a) reduction of the interfacial tension, arising from the balance between polar and non-polar groups; b) adsorption of thick layers, forming a physical barrier with strong steric repulsion; and c) the viscosity increase of the continuous phase, constraining droplet dynamics [7, 71, 73]. Cellulose derivatives often show a dual effect, as stabilizing and emulsifying agents [73]. However, carboxymethyl cellulose (CMC) which is an anionic polymer, cannot efficiently stabilize emulsions by itself due to its highly polar character. However, it can assist emulsion stabilization by controlling the viscosity of the continuous medium [5, 16]. In general, cellulose ethers provide good stability against droplets aggregation due to the strong steric repulsions between the adsorbed polymer layers of two approaching droplets, and due to the increased viscosity of the systems. One of the main advantages of using non-ionic polysaccharides, such as the cellulose ethers in this specific case, is their high stability against environmental stresses, such as, pH, ionic strength, and temperature. This is particularly important in food and pharmaceutical applications, where complex environments are encountered. Cellulose ethers also provide good oxidative stability to the core materials and delay lipid digestion of o/w emulsions, provided that the physical barrier and thickened aqueous phase slows down the diffusion of pro-oxidants and lipases [71, 74]. Lipid digestion is even further reduced using cellulose ethers when compared to calcium-caseinate, a common food emulsifier. Additionally, the thermo-gelling ability of cellulose ethers, in particular HMC, makes it possible to obtain emulsions with high consistency during gastric digestion, contributing to slow down the gastric digestion and increase fullness and satiety perceptions [71].

## **3.4 Molecular and regenerated cellulose at interfaces**

The behavior of molecularly dissolved cellulose at interfaces is expected to resemble that of typical cellulose derivatives or any semiflexible amphiphilic polymer that shows interfacial activity, i.e., the tendency to adsorb at oil–water interfaces and reduce the free energy between the two phases. However, due to its dissolution limitations, the properties of molecularly dissolved cellulose and its potential in emulsions formation and stability are clearly much less explored. Nevertheless, recent studies have confirmed the stated hypothesis. Molecular dynamics simulations indicate that molecularly dispersed cellulose gradually assembles at the oil–water interface eventually surrounding the oil droplet [75]. Experimentally, molecular cellulose dissolved in H3PO4 (aq.) was found to adsorb at the oil–water interface and decrease the interfacial tension (IFT) between the two phases (**Figure 2**).

The decrease in IFT is similar in magnitude to that of the non-ionic cellulose derivatives MC and HPMC, for the same polymer concentration (0.1 wt.%) and same type of oil (liquid paraffin) [34]. Yet, cellulose-stabilized emulsions formed in H3PO4 (aq.) were found to be short-lived, as oil was separating from the emulsions and floating to the top within 24 h. However, by subsequently adding water to the dispersions during emulsification, the properties of the emulsions changed

**Figure 2.** *Effect of dissolved cellulose on the interfacial tension oil-aqueous medium.*

dramatically, and there was no evidence of oil separation over one year of storage (**Figure 3**). This effect was attributed to a decrease in cellulose solvency in H3PO4 (aq.) by the addition of an anti-solvent (water), which promoted a greater affinity for the oil–water interface, leading to the outstanding stability against macroscopic phase separation of the oil.

There are two ways of using native cellulose to stabilize o/w emulsions without the need of further modifications. One, is by following the dissolution-regenerationemulsification approach, resulting in Pickering emulsions of solid or soft cellulose particles (microgels), since the oil is either dispersed in a water suspension of cellulose particles or in a water suspension of cellulose microgels, respectively [76–83].

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

Another way, is to follow the dissolution-emulsification-("in situ")regeneration approach, where the oil is directly dispersed in the cellulose solution, and regeneration takes place at the oil–water interface ("in situ") [34, 84]. This way, the oil droplets seem to be stabilized by a "cellulose film" with a smooth appearance, contrasting with the rough networks and particulate appearances of Pickering emulsions (**Figure 4**) [7, 84]. Another fundamental difference between the two described approaches is related to the existence of dissolved cellulose during the oil emulsification, which acts as a polymeric surfactant by decreasing the IFT, and possibly contributing for a reduction in droplet size. Overall, the emulsions produced by both methods display very good stability against droplet coalescence which can be referred to the irreversible adsorption of cellulose onto the droplet surfaces [34, 78, 79, 85]. Soft cellulose microgels also impart an outstanding stability against flocculation because of the thick viscoelastic layers formed at the interface [15]. The mechanism behind droplet stabilization in emulsions prepared with cellulose particles is similar to that operating for nanocelluloses as described above, i.e., a combination of Pickering adsorption and network stabilization, often showing gel-like characteristics upon a concentration increase of cellulose [79, 82–84].

Moreover, the resulting emulsions are remarkably stable against environmental changes, such as, pH, ionic strength, and temperature, which makes them good candidates for target delivering [80, 82, 86]. Cellulose regenerated particles have also been shown to improve the physical stability of emulsions stabilized by sodium-caseinate, a milk-protein commonly used as a food emulsifier, promoting adsorption of the protein and thickening the continuous phase [87]. Very little has been done regarding the stabilization of w/o emulsions since cellulose particles are better wetted by water than oil. However, it has been suggested that the presence of a water–oil interface when regenerating cellulose affects the conformation of the cellulose molecules and so the way they reassemble. Therefore w/o emulsions are possible to form, but they have poorer stability compared to o/w emulsions [78]. More recently, a "hydrophobic" cellulose microgel was developed to stabilize w/o emulsions, by coagulating cellulose in the presence of a coagulant and sunflower

#### **Figure 4.**

*Different morphologies of cellulose-stabilized emulsions. Particle-stabilized emulsions by longer NCC (bacterial cellulose) (a), shorter NCC (b), and regenerated cellulose (dissolution-regeneration-emulsification approach) (d). Emulsions prepared from molecular solutions of cellulose (dissolution-emulsification-"in-situ" regeneration approach) (c and e). Reprinted (adapted) with permission from Ref. [49, 77, 80, 84].*

oil [88]. The resultant microgels were more easily dispersed in oil than water, and stable emulsions w/o emulsions were formed. The simplicity and versatility of the dissolution-regeneration approaches open many new possibilities for the functionalization of cellulose and its applicability in both o/w and w/o emulsions.
