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 wettingThe three types of wetting are summarised by the following
equations:(6,1)(6-7)(6.9)A reduction of ySL 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. Wa 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./ A = S - ySG - ySL - yLO-AGadhesjon / A = Wa = -XSG - TSL + TLG/ A = ySG ~ ?SL = ?LG COS BGlass Hydrophobic solid
Figure 6.4The wetting of a hydrophobic solid surface by an aqueous medium
is considerably helped by the addition of surface-active agents. Wa is
increased and yLG 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