the three crystalline axes are spatially different. Even for a cubic crystal,
the exception applies only to the planes that contain the three crystalline
axes of cubic crystals. (However, for planes that are not coincident with
the axes, this exception does not apply.)- Solids are, well, solid. Their very definition is that their atoms or mole-
cules are in fixed positions. Therefore, even if a surface is not the lowest-
energy surface, the surface usually remains in that higher-energy struc-
ture. (In some situations, surface rearrangements occur that lower the
surface energy, but we will not consider these in detail here. See Figure
22.17 below and the discussion of it in the text, however.)
In some cases, the second factor can be overcome by heating the solid, a
process called annealing.Annealing implies that a solid is heated to a temper-
ature belowits melting point so the formation of a liquid phase is avoided.
However, enough thermal energy is usually present that some of the solid
atoms or molecules can slowly move, or diffuse,a short distance and adopt a
lower-energy structure. The solid is heated and then cooled slowly, giving the
atoms or molecules time to adopt a new structure. Annealing is common in
the production of glass objects, so that the glass molecules can form a stable,
less-strained solid structure. Figure 22.17 shows an example of how annealing
leads to lower-energy solid structures; similar effects are seen in solid surfaces
as well.
So far in our discussion of solid surfaces, we have assumed that the surface
of a solid is actually formed by the solid material itself. For example, a piece of
solid iron metal has a solid surface that is composed of iron atoms, with bulk
iron on one side of the surface and air or atmosphere on the other side, right?
Unfortunately, in this instance and for almost all other surfaces, this is defi-
nitely not the case. In reality, at the atomic and molecular level, solid surfaces
are very messy.
Why is that? Well, look at Tables 22.1 and 22.2 and compare the surface
energies/tensions of solids and liquids. Note that many liquids—including
water—have much lower surface energies than solids. Applying the idea that
materials tend toward lower energies, in a system of solid and liquid the surface
will be covered with the lower-surface-energy liquid. Consider what this means
at the atomic and molecular level for a solid surface: when exposed to an envi-
ronment, the “surface” of the solid is actually covered with materials that tend
to lower the total surface energy. Consider a piece of crystalline magnesium
oxide exposed to a moist environment. MgO has a surface energy of about
1200 erg/cm^2 and water has a surface energy of about 73 erg/cm^2 , so the low-
energy scenario has a thin layer of water on the surface of the crystalline MgO.
This water can be as thin as a single monolayer (that is, a surface film) and is
considered adsorbedon the solid. (The word adsorbedshould be compared to be
word absorbed,which would mean that one material has been incorporated in-
side something else, like water in a sponge.) Consider all of the surfaces around
you: at the atomic and molecular level, they all have something adsorbed on
them. Thus, what you may be perceiving as the surface of a plastic laminate
tabletop isn’t actually plastic, but a surface that has water or other organic, sili-
cone, fluorocarbon, or other low-energy material as the true “surface” material.
Example 22.6
Consider Tables 22.1 and 22.2. A crystal of sodium chloride, NaCl, is exposed
to an air sample that has water vapors, ethanol vapors, and diethyl ether780 CHAPTER 22 SurfacesFigure 22.17 Annealing a solid allows the
atoms or molecules to adopt a different, more
stable structure. The first picture shows the solid
(glass) before annealing, and the second picture
shows the same solid after annealing.Created by Dr. George B. Hares, Research Fellow andMr. Henry E. Hagy, Senior Research Associate (ret.),Corning, Inc.