Page 104 - Introduction to Colloid and Surface Chemistry

94 Liquid-gas and liquid-liquid interfaces

form two separate liquid-air interfaces (Figure 4.15a), and is given
by the Dupre equation

W, = T A + y B ~ TAB (4.29)

(b)

Figure 4.15 Work of adhesion (a) and of cohesion (b)

The work of cohesion for a single liquid corresponds to the work
required to pull apart a column of liquid of unit cross-sectional area
(Figure 4.15b) - i.e.

= (4.30)
W c 2y A
Spreading of one liquid on another

When a drop of an insoluble oil is placed on a clean water surface, it
may behave in one of three ways:
1. Remain as a lens, as in Figure 4.16 (non-spreading).
2. Spread as a thin film, which may show interference colours, until it
is uniformly distributed over the surface as a 'duplex' film. (A
duplex film is a film which is thick enough for the two interfaces -
i.e. liquid-film and film-air - to be independent and possess
characteristic surface tensions.)
3. Spread as a monolayer, leaving excess oil as lenses in equilibrium,
as in Figure 4.17.