Page 162 - Introduction to Colloid and Surface Chemistry
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6 The solid-liquid
interface
Contact angles and wetting 75 77
Wetting is the displacement from a surface of one fluid by another. It
involves, therefore, three phases, at least two of which must be fluids.
The following account will be restricted to wetting in which a gas
(usually air) is displaced by a liquid at the surface of a solid. A
wetting agent is a (surface-active) substance which promotes this
effect.
Three types of wetting can be distinguished:
1. Spreading wetting.
2. Adhesional wetting.
3. Immersional wetting.
Spreading wetting
In spreading wetting, a liquid already in contact with the solid spreads
so as to increase the solid-liquid and liquid-gas interfacial areas and
decrease the solid-gas interfacial area. The spreading coefficient, S
(cf. equation 4.31), is defined by the expression
6 1
(7sL + TLG) < ' )
where AG S is the free energy increase due to spreading. The liquid
spreads spontaneously over the solid surface when S is positive or
zero.
