Page 170 - Introduction to Colloid and Surface Chemistry

The solid-liquid interface 159

penetrate and fill up most of the hollows and pores in the solid and so
form a plane surface which is effectively part solid and part liquid;
since liquid has zero 6 with liquid, 0 will, therefore, decrease. On the
other hand, if 0 is greater than 90°, the liquid will tend not to
penetrate into the hollows and pores in the solid and can, therefore,
be regarded as resting on a plane surface which is effectively part
solid and part air; since there is practically no adhesion between the
liquid and the entrapped air, 9 will increase. Surface roughness is a
possible cause of contact angle hysteresis on this basis.
The method used to prepare the solid surface may affect the
contact angle; for example, substances which have crystallised in
contact with water often have a lower contact angle than if they
crystallise in air, owing to the orientation of the water-attracting
groups outwards in the former case. Penetration and entrapment of
traces of water in the surface layers, which decreases 0, is also
probable in these circumstances.
Fluorocarbon surfaces have characteristically low critical surface
tensions (see Table 6.1) and have found well-known application in
the production of 'non-stick' surfaces. It is probable that fluorocarbon
surfaces show much more pronounced non-wetting characteristics
than the corresponding hydrocarbon surfaces, mainly on account of
the large size of the —CF 2— groups compared with that of the
—CH 2— groups. Since fewer — CF 2- groups than — CH 2— groups can
be packed into a given area of the solid surface, W a is less and 6* is
75
greater for the fluorocarbon surface .
A major difference between the wetting of 'hard' (e.g. glass and
metal) and 'soft' (e.g. textile) solid surfaces is that, in the former,
equilibrium tends to be established rapidly, whereas, in the latter,
kinetic effects may be of considerable importance.

Wetting agents

Surface-active materials, particularly anionics, are used as wetting
agents in^many practical situations. For example, in dips for sheep
and cattle and in the application of insecticide and horticultural
sprays, the surfaces in question tend to be greasy or wax-like, thus
presenting unfavourable conditions for satisfactory surface coverage
unless a wetting agent is incorporated. However, in these cases
complete wetting is not desirable either, since it causes over-
efficiency in the drainage of excess liquid from the surface. Wetting