Chemistry Reference
In-Depth Information
ionic forces. Because the interactions in molecular lattices are weaker, the
distance of closest approach of adjacent molecules will be considerably
greater than the bond distances between atoms within the molecule. Mole-
cular solids may be either crystalline, amorphous, or a mixture of the two.
The distinctions among ionic, homopolar, and metallic structures are not, of course,
absolute. A perfect ionic bond, for example, would require complete rigidity of the
ionic species. In fact, the ions in such crystals behave more like deformed spheres,
with the degree of deformation, or polarization, following definite patterns. Anions,
for instance, are generally more highly polarized than are cations, and polarization
susceptibility increases with the ionic radius. Because of the wide variability in the
strength of ionic interactions in such materials, ionic solids exhibit a wide range of
intensive and extensive properties, including surface energies.
Stearic acid crystals are a typical molecular solid within the current definition.
In those materials, the shortest distance between carbon atoms in neighboring
molecules is about 0.35 nm, compared to a covalent bond length of 0.154 nm.
The packing density of molecules in such a lattice, then, would be expected to
be lower than that found in the ionic, homopolar, or metallic cases. As was pointed
out in Chapter 3, the surface energy of a material is directly related to the difference
in the magnitude of the forces acting on an atom or molecule at the surface and that
in the bulk. It is not surprising to find that ionic and metallic crystals, materials in
which the magnitude of the lattice energies is high, possess high surface energies,
ranging from several hundred to several thousand millijoules per square meter
(mJ/m
2
). Table 10.1 lists the surface energies of several types of solid. Because
the lattice forces in most molecular solids are much lower than those in the other
systems, such materials exhibit much lower surface energies.
The study of the surface chemistry of solids is concerned with specific properties
of the atomic or molecular layers of material within a few molecular diameters of
the interface with a vapor (or vacuum), a liquid, or another solid. To visualize the
unique character of solid surfaces, it is helpful to compare the similarities and dif-
ferences between solid and liquid surfaces.
TABLE 10.1. Experimentally Determined Surface Energies
of Representative Solids
Surface Energy (mJ/m
2
)
Surface
Polyhexafluoropropylene
18
Polytetrafluoroethylene
19.5
Paraffin wax
25.5
Polyethylene
35.5
Polyethylene terephthalate
43
Quartz (SiO
2
)
325
Tin oxide (SnO
2
)
440
Platinum
1840
