92168.pdf

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,