Physical Chemistry , 1st ed.

terminations of solid crystals, an idea that was mentioned briefly in the previ-

ous chapter.

Surfaces have properties that are different from bulk materials. Why? At

any boundary between two materials, there is an imbalance of interactions that

ultimately affects the properties of the boundary. This is what we will find with

surfaces. There is a property called surface tension that has no bulk equivalent,

yet it can have a major influence on the behavior of liquids. We experience

those influences every day with water. Interfaces, which are boundaries be-

tween phases, have properties that are dictated by surface effects. Curved sur-

faces, like those of liquid droplets, also have unique properties. These will be

explored briefly.

Surfaces of crystalline solids can be specifically defined, thanks to the fact

that planes of atoms in crystals can be specifically defined. Some of the ideas

from the previous chapter on crystals will be applied here. Finally, we recog-

nize the fact that the presence of certain surfaces speeds up, or catalyzes, some

chemical reactions. Again, why? It turns out that there can be an interaction

between the reactants and the surface itself that decreases the activation energy

of the reaction, and therefore speeds up the rate. Catalysis of chemical reac-

tions is an important industrial concern because in industry time is money.

The physical chemistry of surfaces provides the basis for understanding why

catalysis by surfaces occurs.

22.2 Liquids: Surface Tension

Under some circumstances, a material can spread out into a monatomic or

monomolecular layer or film.For example, solutions of stearic or oleic acid

(both long-chain fatty acids) in a hydrocarbon solvent can be carefully dripped

onto water; when the solvent evaporates, the remaining fatty acids can arrange

themselves into a monomolecular film on the water’s surface. Such films have

a definite surface coverage (that is, a definite area) depending on the number

of fatty acid molecules present.

However, the surface layer of atoms/molecules of a liquid can also be

considered such a film, shown diagramatically in Figure 22.1. Further, we

might suggest that this surface layer would have different properties than

the bulk material. This is because the surface layer isn’t really “bulk.” Bulk

atoms or molecules are surrounded on all sides by other molecules of the

same material. At the surface, atoms or molecules are surrounded by the

same molecules on one side but differentmolecules (or nothing) on the other.

Forces between different materials (or between one material and nothing)

are different, implying that the forces on the single surface layer of mole-

cules are different from those in the bulk. Therefore, surface molecules

aren’t really bulk species and their behavior might not be the same as the

bulk material.

Suppose we want to increase or decrease the amount of surface available,

perhaps by changing the shape of the liquid so that more surface area is ex-

posed. Because of the differing forces acting at the surface, it will require work

to change the surface area. Figure 22.2 shows a diagram of what we are trying

to accomplish for an idealized system. If we want to increase the size of the rec-

tangular surface area, then we have to do work onthe liquid and against the

unbalanced forces that exist at the surface. (Again, increasing surface area of a

liquid requires that work be done onthe liquid. Conversely, if the surface area

is decreased, work is done bythe liquid on the surroundings.) If the magni-

766 CHAPTER 22 Surfaces

dx

Surface

area

Liquid

F

Figure 22.1 The atoms or molecules at the
surface of a liquid can be considered a film. First
consider the liquid particle in color at the lower
left, below the surface. It interacts with other liq-
uid particles all around it, with an overall balance
of forces. However, a similar particle at the sur-
face has interactions only down and to the sides.
With no liquid particles above the surface layer,
an imbalance of forces occurs that is the ultimate
cause of surface effects.

Figure 22.2 The experimental setup to define
the surface tension . See text for discussion.