Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 66

Although much weaker than the primary bonds, hydrogen bonds are a strong type of intermolecular bonding (Table 2.2). Hydrogen bonds result from Keesom forces, the bonding between molecules that have a permanent dipole, but in addition they have the following characteristics:

 A hydrogen atom is covalently bonded to a highly electronegative atom, thus the hydrogen atom has a slight positive character while the highly electronegative atom has a slight negative character.

 The highly electronegative atom to which the hydrogen atom is bonded has at least one active set of lone pair electrons.

A well‐known example of hydrogen bonding is between H₂O molecules in water (Figure 2.14). By sharing one electron in its outer shell with each hydrogen atom, the oxygen atom achieves an outer shell containing eight electrons, but the four unshared electrons make up two orbitals, each containing two electrons. The two unshared electrons in each orbital are referred to as lone pair electrons. The hydrogen bond involves intermolecular bonding between a hydrogen atom of one molecule and a lone pair electron in the oxygen atom of a neighboring molecule. When water freezes, the tetrahedral arrangement of sp³ orbitals of the oxygen atom coupled with hydrogen bonding between the water molecules results in an ordered three‐dimensional pattern of the molecules in ice (Figure 2.15). The hydrogen bonds keep the molecules well apart, which is why ice has a lower density than water.

Figure 2.14 Schematic illustration of hydrogen bonds between water molecules.

Figure 2.15 Illustration of the arrangement of water (H₂O) molecules in ice showing the hydrogen bonds.

Secondary bonds also play an important role in polymers, providing links between their constituent macromolecules that make them solids at ordinary temperatures. In polyethylene composed of nonpolar macromolecules, secondary bonding consists of weak van der Waals bonds (Figure 2.16a). On the other hand, polymers composed of macromolecules that have polar groups, such as OH and NH groups, form stronger hydrogen bonds. When the macromolecules of the synthetic polymer nylon 6.6 are aligned in the formation of fibers, hydrogen bonding between carbonyl oxygen and amide hydrogen atoms of neighboring molecules serve to stabilize the structure, thus contributing to the high mechanical properties of the fibers (Figure 2.16b).

Figure 2.16 Schematic comparison of (a) van der Waals bonding between nonpolar polyethylene macromolecules and (b) hydrogen bonding between polar nylon 6.6 macromolecules.