Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 112

by using photography. Recently, digital photography has also been used since these
data can be analyzed by computer programs.
It is useful to consider some general conclusions derived from these data. One
defines a solid surface as wetting if θ is less than 90. However, a solid surface is
designated as nonwetting if θ is greater than 90. This is a practical and semiquanti-
tative procedure. It is also seen that water, due to its hydrogen-bonding properties,
exhibits a large θ on nonpolar surfaces (PE, PTE, PE). On the other hand, one
finds lower θ values on polar surfaces (glass, mica). However, in some applications,
the surface properties may be changed by chemical modifications of the surface.
For instance, PS has some weak polar groups at the surface. If one treats the sur-
face with H 2 SO 4 , it forms sulfonic groups. This leads to values of θ lower than 30
(depending on the time of contact between sulfuric acid and the PS surface). This
treatment (or similar) has been used in many applications where the solid surface
is modified to achieve a specific property. Since only the surface layer (a few mole-
cules deep) is modified, the bulk of the solid properties do not change. This analysis
shows the significant role of studying the contact angle of surfaces in relation to the
application characteristics.
The magnitude of contact angle of water (for example) is found to vary depend-
ing on the nature of the solid surface. The magnitude of θ is almost 100 on a waxed
surface of car paint. The industry strives to create such surfaces to give θ > 150, the
so-called superhydrophobic surfaces. The large θ means that water drops do not wet
the car polish and are easily blown off by wind. The car polish also is designed to
leave a highly smooth surface.
In many industrial applications, the concern is for both smooth and rough sur-
faces. The analyses of θ on rough surfaces will be somewhat complicated than those
on smooth surfaces. The liquid drop on a rough surface (Figure 5.4) may show the
real θ (solid line) or some lower value ( apparent ; dotted line), dependent on the ori-
entation of the drop.
However, no matter how rough the surface, the forces will be the same as those
that exist between a solid and a liquid. The surface roughness may show contact
angle hysteresis if one makes the drop move, but this will arise from other param-
eters (e.g., wetting and dewetting). Further, in practice, the surface roughness is not
easy to define. A fractal approach has been used to achieve a better understanding
(Feder, 1988; Birdi, 1993).
In industry, many systems are found in which contact angle analyses are useful.
In Table 5.3, some unusual data for different systems are given.

table 5.3
contact angle data of different
liquid-Solid Systems

System liquid γ contact angle

Liquid Na (100°C)—glass 222 6

Hg (100°C)—glass 460 143