Cellulose intermolecular hydrogen bonds

FIGURE 5.9 Vegetation, even huge trees such as these, is held upright by the powerful intermolecular hydrogen bonds that exist between the ribbonlike cellulose molecules that form much of its bulk. Without hydrogen bonding, these trees would collapse. 

Cellulose is a linear polymer. Despite this, it is not thermoplastic, essentially because of its extensive intermolecular hydrogen bonding which never allows the molecules to move sufficiently for the polymer to melt. 

The structure of cellulose reveals the two key components for hydrogen bonding OH functional groups (the X-H unit) and additional oxygen atoms (the Y atom). Hence, water molecules may form intermolecular hydrogen bonds with cellulose in one of two ways ... 

Thus, extensive intermolecular hydrogen bonding is possible between cellulose and water, enhancing the water capacity of a cotton towel. 

The solubilization of polysaccharides such as chitin and cellulose apparently results from the disruption of strong intermolecular hydrogen bonding by the lithium ions in the N,N-dimethylacetamide. Interestingly under identical conditions, cations such as Na+, K" ", Cs+, Ca+, Ba" "" " showed no tendency to solvate the above polymers. Additionally, some specificity was shown for the anion type, i.e., Br-, Cl-, and NO3-. These trends are under further investigation. 

By forming intramolecular and intermolecular hydrogen bonds between OH groups within the same cellulose chain and the surrounding cellulose chains, the chains tend to be arranged in parallel and form a crystalline supermolecular stracture. Then, bundles of linear cellulose chains (in the longitudinal direction) form a microfibril which is oriented in the cell wall structure. Cellulose is insoluble in most solvents and has a low accessibility to acid and enzymatic hydrolysis (Demirbas, 2008b). 

The major structural characteristics of cellulose I, determined in this and other studies, include extended chains stabilized alorig their lengths W two intramolecular hydrogen bonds per glucose residue (0(3)- 0(5) and 0(6)—0(2) ), the arrangement of the chains into sheets stabilized by one intermolecular hydrogen bond per residue (0(3)—0(6)), and the packing of the sheets into a three-dimensional structure marked by a parallel-chain arrai ement. These structural characteristics are illustrated in Fig. 2. 

To account for the phenomenon of chelation by intermolecular hydrogen bonds, Pierce [28] has suggested an alternative constitution for cellulose, as shown in Fig. 76. 

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