Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 110

Solid γ γ Sd γ SP

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Most of the notions and physical laws of surfaces have been obtained by the studies
of liquid-gas or liquid 1 -liquid 2 interfaces. The solid surfaces have been studied in
more detail, but this has taken place more recently. The asymmetrical forces acting
at the surfaces of liquids are much shorter than those expected on solid surfaces.
This is due to the high energies that stabilize the solid structures. Therefore, when
one examines a solid surface, the surface roughness will need to be considered.

5.2.1 S of l I d S u r f a c e T e n S I o n ( γ S o l I d )
It was described exhaustively before that the molecules at the surface of a liquid
are under tension due to asymmetrical forces, which gives rise to surface tension.
However, in the case of solid surfaces, one may not envision this kind of asymmetry
as clearly, although a simple observation might help one to realize that such surface
tension analogy exists. For instance, let us analyze the state of a drop of water (10 μL)
as placed on two different smooth solid surfaces (e.g., Teflon and glass). One finds
that the contact angles are different (Figure 5.4).
Since the surface tension of water is the same in the two systems, the difference
in contact angle can only arise due to the surface tension of solids being different.
The surface tension of liquids can be measured directly (as described in Chapter 2).
However, this is not possible in the case of solid surfaces. Experiments show that,
when a liquid drop is placed on a solid surface, the contact angle, θ, indicates that
the molecules interact across the interface. This shows that these data can be used to
estimate the surface tension of solids.

5.3 contact anGle ( θ ) oF lIquIdS on SolId SurFaceS
As already mentioned, a solid in contact with a liquid leads to interactions related
to the surfaces involved. The solid surface is being brought in contact with surface

Liquid

Solid

FIGure 5.4 Apparent or true contact angle of a liquid drop on a rough solid surface.