vapors. (Water, ethanol, and diethyl ether are all liquids at room tempera-
ture.) Ignoring the possibility of minimum-energy interactions between the
three vapors (that is, azeotropes and the like), what would you predict to be
the true surface structure of NaCl? (Ignore interactions between the NaCl and
the vapors.)Solution
If we apply the ideas immediately above, the preferred surface structure
would have the minimum surface energy. Comparing the surface tensions of
the liquid phases associated with the vapors, we find that diethyl ether has the
lowest surface tension. Thus, we suggest that the surface of NaCl might actu-
ally be covered with diethyl ether molecules. (Apart from its instructiveness,
this example is extremely naive. In reality, ion-dipole interactions would con-
tribute considerably to the formation of a molecular film on the surface of
the solid. In extreme cases—like NaOH—energies of solvation are so nega-
tive that enough water is adsorbed to form a solution; such compounds are
described as deliquescent.)The above example illustrates a point: real surfaces, exposed to real envi-
ronments, are very messy at the molecular level. Just about any surface you can
see is covered with molecules that are not of the material itself.
It actually takes special effort to get surfaces that are clean; that is, free from
adsorbed contaminants. Among other things, it is necessary to expose a surface
to a very, very high vacuum.Ultrahigh vacuums,which are usually considered
to involve pressures less than 1 10 ^8 torr, are necessary to minimize the num-
ber of gas molecules that come in contact with a surface and form a low-surface-
energy layer. This might not sound like an “ultrahigh” vacuum, even though it
represents about one hundred billionth of an atmosphere. From considerations
related to the kinetic theory of gas, we can show that even at about one mil-
lionth of a torr, in about 1 second enough gas molecules strike a surface to form
a single molecular monolayer. A pressure of 1 10 ^8 torr suggests that the
monolayer formation takes about 100 seconds, or less than 2 minutes: not a
clean surface for very long! Pressures of 1 10 ^11 to 1 10 ^12 torr are some-
times necessary, and they require special vacuum technology to achieve.
The term exposureis defined as the product of the pressure of gas in con-
tact with a surface and the time the surface is exposed to that pressure:
exposure (pressure) (time) (22.22)The unit of exposure is the langmuir.A pressure of 1 10 ^6 torr applied for
1 second gives a surface an exposure to molecules that is defined as 1 langmuir
(1 L, not to be confused with 1 liter):
1 langmuir 1 10 ^6 torr s (22.23)For scientists trying to study clean surfaces, this is a useful unit for communi-
cating exposure pressure and time, both of which relate to how long a surface
can really be considered “clean.” Roughly speaking, a monolayer of adsorbed
atoms or molecules forms when a surface experiences an exposure of 1 langmuir.
Example 22.7
A clean surface is exposed to a pressure of 2.0 10 ^11 torr. How long will it
take for about one-half of a monolayer to form on the surface?22.5 Solid Surfaces 781