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Liquid-gas and liquid-liquid interfaces 99

Figure 4.19

Sunlight)

A surface balance (By courtesy of Unilever Research Laboratory, Port

The surface pressure of the film is determined by measuring the
force which must be applied via a torsion wire to maintain the float at
a fixed position on the surface (located optically) and dividing by the
length of the float. For precise work, the surface balance is enclosed
in an air thermostat and operated by remote control. With a good
modern instrument, surface pressures can be measured with an
accuracy of 0.01 mN m~^1.
Surface pressures can also be determined indirectly by measuring
the lowering of the surface tension of the underlying liquid (or
substrate) caused by the film. For example, the float can be replaced
by a Wilhelmy plate set-up (Figure 4.5b). This method is at least as
accurate as the Langmuir-Adarn surface balance and is particularly
useful for studying films at oil-water interfaces^156. The obvious
advantage of this method is that it is not necessary to maintain a clean
surface behind a float. The arrangement can be simplified even