Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 105

5 Solid Surfaces—

Adsorption

5.1 IntroductIon
In a large variety of applications, the surface of a solid plays an important role
(e.g., active charcoal, talc, cement, sand, catalysis). Solids are rigid structures and
resist any stress effects. Many such considerations in the case of solid surfaces will
be somewhat different for liquids. The surface chemistry of solids is extensively
described in the literature (Adamson and Gast, 1997; Birdi, 2002). Mirror-polished
surfaces are widely applied with metals, where the adsorption at the surface is
much more important. Further, the corrosion of metals initiates at the surfaces,
thus requiring treatments based on surface properties. As described in the case of
liquid surfaces, analogous analyses of solid surfaces can be carried out. The mol-
ecules at the solid surfaces are not under the same force field as in the bulk phase
(Figure 5.1).
The differences between perfect cr yst a l su r faces a nd su r faces wit h defects are very
obvious in many everyday observations. The solids were the first material that was
analyzed at the molecular scale, leading to an earlier understanding of the structures
of solid substances and the crystal atomic structure. This is because while molecular
structures of solids can be investigated by such methods as x-ray diffraction, the same
analyses for liquids are not as straightforward. These analyses have also shown that
there exists surface defects at the molecular level. As pointed out for liquids, when
the surface area of a solid powder is increased by grinding, then surface energy is
needed. Of course, due to the energy differences between the solid and liquid phases,
these processes will be many orders of magnitude different from each other. The liq-
uid state, of course, retains some structure that is similar to its solid state, but in the
liquid state, molecules exchange places. The average distance between molecules in
the liquid state is roughly 10% larger than in its solid state. It is thus desirable at this
stage to consider some of the basic properties of liquid-solid interfaces.
The surface tension of a liquid becomes important when it comes into contact
with a solid surface. The interfacial forces are responsible for self-assembly forma-
tion and stability on solid surfaces. The interfacial forces present between a liquid
and solid can be estimated by studying the shape of a drop of liquid placed on any
smooth solid surface (Figure 5.2).
The balance of forces as indicated, again, were analyzed very extensively in the
last century by Young (1805), who related the different forces at the solid-liquid
boundary and the contact angle, θ, as follows (Adamson and Gast, 1997; Chattoraj
and Birdi, 1984: Birdi, 1997, 2002):

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