Page 169 - Introduction to Colloid and Surface Chemistry

158 The solid-liquid interface
In a related method 176 the contact angle is determined by measuring
the rate of penetration of liquid through a powder packed into a glass
tube.
Factors affecting contact angles and wetting

The three types of wetting are summarised by the following
equations:

/ A = S - y SG - y SL - y LO (6,1)

-AG adhesjon / A = W a = -XSG - TSL + TLG (6-7)

/ A = y SG ~ ?SL = ?LG COS B (6.9)

A reduction of y SL facilitates all of these wetting processes, but a
reduction of y\_ G is not always helpful.
Hie contact angle between water and glass is increased considerably
by even less than an adsorbed monolayer of greasy material such as
fatty acid. W a is decreased, since some of the glass-water interface is
replaced by hydrocarbon-water interface (Figure 6.4a); hence, from
the Young-Dupre" equation, 0 increases.

Glass Hydrophobic solid
Figure 6.4

The wetting of a hydrophobic solid surface by an aqueous medium
is considerably helped by the addition of surface-active agents. W a is
increased and y LG is decreased (Figure 6.4b), so that, from the
Young-Dupre equation, 0 is reduced on two counts.
Surface roughness has the effect of making the contact angle
further removed from 90°. If 0 is less than 90°, the liquid will