Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 7

bonds, covalent bonds, ionic bonds, and weak bonds. Among the weak bonds, there
is a distinction between polar bonds or hydrophilic bonds (hydrogen bonds and van
der Waals bonds) and nonpolar or hydrophobic bonds. From these properties will
come the spatial form of the associated atoms and the molecules and then, at a larger
scale, of the crystal, and finally of the organism as a whole.
Metallic bonds are formed by the sharing of electrons in the outer layer of the
atom in an electron cloud, where they are free and delocalized. This free-electron
gas ensures the cohesion of the remaining cations and enables electrical conduction
in metals and alloys.
Covalent bonds are formed by the sharing of pairs of valence electrons in order to
fill the outer electron shells of each atom. They are very strong bonds that are found
in non-metals such as semiconductors, certain ceramics, polymers, and biological
materials.
Ionic bonds are formed by the transfer of an electron from one atom to the other.
They are strong bonds that appear, for example, between a metal atom that has
released an electron and a non-metal atom that has captured the free electron. After
bonding, both atoms become charged. These bonds are found in minerals, ceramics,
biological materials, and certain polymers (ionomers).
Weak polar bonds are electrostatic and correspond to simple attractions between
dipoles in compounds or molecules with inhomogeneous or polarizable charges.
They act over long distances but with less intensity than strong bonds. Among
them, for example, are van der Waals bonds between molecules and hydrogen bonds
between water molecules in liquid water and ice. These bonds are found in all bio-
logical materials, certain hydrated minerals, polymers, and some mixed-composite
materials.
Weak nonpolar bonds or hydrophobic bonds are formed by repulsion. In a polar
liquid, the molecules try to establish a maximum number of bonds between each
other. If nonpolar molecules are added to the solution, their presence disrupts the
formation of this network of bonds, and they will be rejected. Uniquely nonpolar
molecules are rare in nature and for the most part are found in hydrocarbons. Fatty
acids are amphiphilic molecules, containing a polar end and a nonpolar end. These
molecules will then form complex structures, with the polar end on the outside in
contact with the water and the nonpolar end on the inside, completely isolated from
the water. Depending on the nature of the molecule, these structures will either
be small globules called micelles or be membranes. These bonds are found in all
biological materials.
Among all materials, only biological materials, certain synthetic polymers, and
certain mixed-composite materials or biomaterials have three types of strong, ionic,
and weak chemical bonds coexisting together. The simultaneous presence of these
three types of bonds gives the material a particular sensitivity to the effects of radi-
ation and to thermal, mechanical, or chemical treatments. The damage induced can
bring about their partial or total destruction.
Furthermore, biological materials have specific bonds combining both chemical
bonds and spatial conformations. Proteins are complex constructions with specific
spatial conformations.