Physical Chemistry , 1st ed.

fabrics are made so by a coating or by using fabric fibers that do not show cap-
illary action. All of these examples are ultimately based on surface effects.

22.4 Surface Films

Some systems can be defined as having a very thin film on the surface of a bulk
material. For example, a tiny drop of oil will spread over a larger amount of
water and produce some wonderful optical effects (caused by interference of
light reflecting off the top and bottom interfaces of the thin oil film). A film
that is one molecule thick is called a Langmuir-Blodgett film.(Irving Langmuir,
an American physical chemist who worked for General Electric, pioneered the
study of monomolecular films in 1918; his work was improved upon by an-
other GE scientist, Katherine Blodgett, in 1934.) If suspended over water, these
films are usually composed of materials that are water-insoluble and have a
negligible vapor pressure.
In the case of a surface film composed of a specific amount of material, a
sort of surface tension can be measured, using an experimental setup similar
to that in Figure 22.2. In this case, however, the “surface tension” measures the
ability of the molecules to compress or spread apart as the area of the film is
changed. For surface films, a surface pressure is defined as the difference be-
tween the surface tension ° of the pure solvent (usually water) and the sur-
face tension of the solvent with the surface film on it:

° (22.20)
The surface pressure varies tremendously with the area that a film is con-
strained to; see Figure 22.11 for a plot of for various materials versus A, the
area of coverage per molecule. In such a plot, a film of a certain amount of ma-
terial is compressed or expanded, and the surface pressure is measured. In re-
gions where each molecule is calculated to cover an area larger than itself, the
surface pressure can be approximated by the equation

ART (22.21)

22.4 Surface Films 777

0.8

0

0

A ( 10 ^20 m^2 )

A  kT

1

2

3

4

4500

^

(10

^3

J•

m

2 )

500 1500 2500 3500

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Figure 22.11 Surface pressure versus area. For each material, the x-axis corresponds to the
average area per molecule in the film.Source:Ya. Gerasimov,Physical Chemistry,Mir Publishers,
Moscow, 1974.