Page 110 - Introduction to Colloid and Surface Chemistry

100 Liquid-gas and liquid-liquid interfaces

further by doing away with the movable barrier as well and regulating
the area of the film by the amount of material spread.

Surface film potential

In heterogeneous systems, potential differences exist across the
various phase boundaries. The surface film potential, A V, due to a
monolayer is the change in the potential difference between the bulk
substrate liquid and a probe placed above the liquid which results
from the presence of the monolayer. Surface film potentials can be
measured by air-electrode and vibrating-plate methods.
The air electrode consists of an insulated metal wire with its tip
1-2 mm above the liquid surface and with polonium deposited on the
tip to make the air gap conducting.
In the vibrating-plate method, a small gold or gold-plated disc is
mounted about 0.5 mm above the surface. The vibration of the disc
(~ 200 Hz) produces a corresponding variation in the capacity across
the air gap, an alternating current being thus set up, the magnitude of
which depends on the potential difference across the gap. This
method is more accurate than the air-electrode method, being
capable of measuring AV to 0.1 mV, but is subject to malfunctioning.
It can also be used at oil- water interfaces.
Surface film potential measurements can yield useful, if not
absolute, information about the orientation of the film molecules,
Treating the film as a parallel plate condenser leads to the
approximate expression

where n is the number of film molecules per unit area, /u. is the dipole
moment of film molecules, 0 is the angle of inclination of dipoles to
the normal and e is the permittivity of the film (see page 179).
Surface film potential measurements are also used to investigate
the homogeneity or otherwise of the surface. If there are two surface
phases present, the surface film potential will fluctuate wildly as the
probe is moved across the surface or as one blows gently over the
surface,