**3. Cellulose: a versatile source of emulsifiers**

### **3.1 Physicochemical characteristics of cellulose and the various morphological forms**

Cellulose is a polysaccharide composed of glucose monomers, the anhydroglucose units (AGUs), linked by β -(1–4) glycosidic bonds. These β-linked AGUs adopt the 4C1 chain conformation, which is the conformation with the lowest free energy of the molecule. Consequently, the three polar hydroxyl groups in each AGU are located on the equatorial positions of the rings, and the hydrogen atoms of the non-polar C–H bonds are located on the axial positions [1]. This structural anisotropy is what gives cellulose its amphiphilic nature [28]. Due to the large number of hydroxyl groups within a cellulose molecule, both intra- and intermolecular hydrogen bonding occur and various types of supramolecular semi-crystalline structures can be formed. It is believed that intramolecular hydrogen bonding is responsible for the single-chain conformation and stiffness, while the intermolecular hydrogen bonding would be responsible for the sheet-like arrangement of the native polymer [1]. However, the stacking of these sheets into the three-dimensional crystalline supramolecular structures must involve hydrophobic interactions, as it was shown trough molecular dynamic simulations, and moreover was observed, many years ago, in native cellulose biosynthesis [1, 29, 30].

Hydrophobic interactions between cellulose molecules make, in combination with favorable packing conditions (and thus a low energy) in the crystalline state, cellulose insoluble in water [31]. Solubility can be achieved by ionizing cellulose, which occurs at extreme pH's. Solubility in water can also be aided by addition of co-solutes that weaken hydrophobic interactions. Derivatization of cellulose is also found to generally enhance solubility strongly, which can be referred to packing constraints in the solid state. Thus, to make cellulose soluble in aqueous solutions, the crystalline packing has to be disrupted, and this can be achieved, for example, by chemical modifications via etherification reactions in alkaline media, resulting in water-soluble cellulose ethers [32, 33]. These cellulose derivatives keep the amphiphilic properties of cellulose, as can be seen from their association with surfactants and their adsorption at the air-water and oil–water interfaces [34–39].

A fairly simple way of converting cellulose into a versatile class of new materials is through a dissolution-regeneration process. The regeneration of cellulose occurs when a coagulant ("anti-solvent") gets in contact with a cellulose solution or dope, leading to a solvent exchange and subsequent aggregation of the cellulose chains. The organization of the molecules in the regenerated materials (e.g., fibers, films, foams, particles) and their properties are strongly influenced by the dissolution state of cellulose (molecular cellulose, partially dissolved, crystalline or amorphous aggregates), as a well as the nature of the coagulant used [40, 41].

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

Cellulose can also be shaped into micro- and nanoparticles of different colloidal structure. Acid or mechanical treatments are usually applied to deconstruct the cellulose fibers into crystalline or semi-crystalline nanocelluloses [20, 42–44]. Partial decomposition of cellulose fibers, by acid treatment and cellulase-catalyzed hydrolysis, yields powdery microcrystalline cellulose (MCC), such as commercial Avicel®, with DP values between 150 and 300 [45]. Avicel® still contains both amorphous and crystalline portions. On the other hand, nanocrystalline cellulose (NCC) is obtained by strong acid hydrolysis. During the chemical process, the more readily accessible amorphous regions are completely disrupted deliberating rod-like crystal sections, whose sizes are dependent on the time and temperature of the reaction. The dimensions of the isolated NCC are also found to be strongly influenced by the degree of crystallinity of cellulose, which, in turn, is dependent on the natural source. Cotton, wood and Avicel® usually yield highly crystalline nanorods (90% crystallinity) with a narrow distribution of sizes (5–10 nm in width and 100–300 nm in length), whereas sources, such as tunicin (extracted from the tunicates), bacteria and algae, yield crystals with higher polydispersity and larger dimensions [42]. NCC forms stable suspensions in water by application of a mechanical force, typically sonication. Its surface properties are determined by the mineral acid and the reaction conditions used during its extraction. NCC prepared with hydrochloric acid (HCl) is weakly negatively charged, while it exhibits a strong repulsive character if prepared with sulfuric acid (H2SO4), since approximately one tenth of the glucose units is functionalized with the anionic sulfate ester groups. Thus, NCC prepared with H2SO4 give suspensions with higher colloidal stability than NCC prepared with HCl.

Micro- and nanofibrillated (MFC/NFC) celluloses are obtained by extruding wood pulp suspensions trough mechanical devices (high-pressure homogenizers), which results in fiber delamination and deliberation of the fibrils, usually being tens of nanometers wide and lengths ranging from several nanometers to several micrometers (i.e., 5–60 nm in width and 100 nm to several micrometers in length) [42]. This type of nanocelluloses are usually less crystalline than NCC, since they still own part of the amorphous domains, and have higher aspect ratios [5, 46]. In aqueous solutions, the fiber-like morphology and high aspect ratio, typically drive gel-like behavior due to entanglements between the microfibrils.
