108 Liquid-gas and liquid-liquid interfaces
Liquid-expanded films obey the equation of state,
A 0 ) = kT (4.36)
which resembles the van der Waals equation. The accepted theory of
the liquid-expanded state, suggested by Langmuir, is that the
monolayer behaves as a duplex film in which the head groups are in a
state of two-dimensional kinetic agitation while the attractive forces
between the hydrocarbon chains keep the film coherent.
Factors influencing the physical state of monomolecular films
As remarked previously, the physical state of a monolayer depends
on the lateral cohesive forces between the constituent molecules. By
a suitable choice of chain length and temperature, straight-chain fatty
acids and alcohols, etc., can be made to exhibit the various
monolayer states, one CH 2 group being equivalent to a temperature
change of c. 5-8 K.
Lateral cohesion also depends on the geometry and orientation of
the film molecules, so that the following factors will favour the
formation of an expanded film:
- Bulky head groups, which prevent efficient packing and, hence,
maximum cohesion between the hydrocarbon chains.
- More than one polar group - e.g. unsaturated fatty acids, hydroxy
acids. A film pressure is required to overcome the attraction
between the second polar group and the aqueous substrate before
the molecules can be orientated vertically.
- More than one hydrocarbon chain orientated in different directions
from the polar part of the molecule - e.g. esters, glycerides.
- Bent hydrocarbon chains - e.g. brassidic acid (trans-
CH 3 (CH 2 ) 7 CH=CH(CH 2 )iiCOOH), which has a straight hydro-
carbon chain, gives a condensed film, whereas erucic acid (cis-
CH 3 (CH2)7CH=CH(CH 2 )iiCOOH), which has a bent hydrocarbon
chain, gives a very expanded flim.
- Branched hydrocarbon chains.
The nature of the substrate, particularly pH, is important when the
monolayer is ionisable. When spread on alkaline substrates, because
of the ionisation and consequent repulsion between the carboxyl
