Liquid-gas and liquid-liquid interfaces 107
affinity for the aqueous surface. Compression of the oleic acid film
forces the double bonds above the surface and eventually orientates
the hydrocarbon chains in a vertical position. This process is
somewhat gradual, as indicated by the form of the IT-A curve. In
conformity with this, the rate of oxidation of an oleic acid monolayer
by a dilute acid permanganate substrate is found to be greater at high
areas.
There are a number of instances in which (with the aid of sensitive
measurements) well-defined transitions between gaseous and coherent
states are observed as the film is compressed. The ir~A curves show a
marked resemblance to Andrews' p-V curves for the three-dimensional
condensation of vapours to liquids. The ir-A curve for myristic acid,
given as an example, has been drawn schematically to accentuate its
main features (Figure 4.26). Above 8 nm^2 molecule"^1 the film is
gaseous and a liquid-expanded film is obtained on compression to
0.5 nm^2 molecule"!. Fluctuating surface film potentials verify the
heterogeneous, transitional nature of the surface between 0.5 nm^2
molecule"^1 and 8 nm^2 molecule"^1.

qn

(^7) e 20
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0.2
•^

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s

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v

Condensed

Transition

Liquid expanded

Transition

Gaseous

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0.2 -0.35 0.5 8

/A/nm^2 molecule"^1

Figure 4.26 Schematic representation of the ir-A curve for myristic acid spread on
0.1 mol dm'^3 HC1 at 14°C