namely hydrogen bond. But it is important to remember that the hydrogen bond is a physical interaction force that operates between stable molecules. Its formation or destruction leaves the molecule intact. The hydrogen bond is thereby distinct from a covalent, i. e. chemical, bond. In a hydrogen bond, a hydrogen atom acts as a bridge, the hydrogen bridge (proton bridge), between two small strongly electronegative atoms with lone pairs of electrons, such as oxygen. It is here represented by a blue dotted line (Fig. 1.11). Hydrogen bonding occurs when the element bound to the hydrogen by covalent bonding is strongly electronegative. This causes hydrogen to bear a partial positive charge. The electronegative element linked to the hydrogen must be capable of donating a lone electron pair in order to be attracted to the partially positively polarised hydrogen. With regard to paper, the most important elements that involve hydrogen bonding are oxygen and nitrogen. The strength of a hydrogen bond ranges between 5 and 50 kJ/mol and it is strongest when it is linear (Ojala 1999). As a rule of thumb, hydrogen bonds have one-tenth of the strength of chemical bonds.

As water is a polar substance, strong intermolecular interaction forces between its molecules determine its physical properties. Considerable energy is needed to overcome the strength of hydrogen bonding for the evaporation of liquid water. That is the reason why the boiling point of water, 100°C at 101.325 kPa, is relatively high in comparison to other molecules with similar or higher molecular mass. Because water molecules are able to establish hydrogen bonds to other polar molecules, they are able to swell or dissolve organic substances. Only molecules that carry a sufficient number of polar functional groups such as hydroxyl (-OH) or amino (-NH₂) groups can interact with water, i.e. make hydrogen bonding possible (Table 1.5). Among the substances that can be swollen or dissolved by water are:

• carbohydrates (via their hydroxyl groups): sugar, dextrin, starch, cellulose, cellulose ethers;
• proteins (via their amino groups): gelatine, casein, enzymes;
• synthetic products (via their hydroxyl groups): polyvinyl alcohol (PVA), polyethylene glycol (PEG);
• organic acids via their carboxyl groups -COOH: acetic acid, citric acid, lactic acid, ascorbic acid.

The role of hydrogen bonding in the chemistry and technology of cellulose processing cannot be overestimated. It is perhaps the most important molecular interaction force within paper and therefore important to the papermaking and paper conservation processes.