Page 115 - Introduction to Colloid and Surface Chemistry

Liquid-gas and liquid-liquid interfaces 105

Condensed films

Palmitic, stearic and higher straight-chain fatty acids are examples of
materials which give condensed films at room temperature. At high
film areas the molecules of fatty acid do not separate completely from
one another, as the cohesion between the hydrocarbon chains is
strong enough to maintain the film molecules in small clusters or
islands on the surface (Figure 4.23). Because of this strong cohering
tendency the surface pressure remains very low as the film is
compressed and then rises rapidly when the molecules become tightly
packed together.

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Figure 4.23

For stearic acid spread on dilute HC1 an initial pressure rise is
2 1
observed at about 0.25 nm molecule" , corresponding to the initial
packing of the end-groups (Figure 4.24). The ir-A curve becomes
1
2
very steep at about 0.205 nm molecule" , when it is supposed that
more efficient packing has been achieved by staggering of the end-
groups and interlocking of the hydrocarbon chains. A limiting area of
0.20-0.22 nm 2 molecule" 1 is observed for straight-chain fatty acids
irrespective of the chain length. The packing of the molecules in the
film at this point is not far short of that in the crystalline state. The
cross-sectional area of stearic acid molecules from X-ray diffraction
measurements is about 0.185 nm 2 at normal temperatures. Any
attempt to compress a condensed film beyond its limiting area will
eventually lead to a collapse or buckling of the film.

Expanded films
Oleic acid (Figure 4.25) gives a much more expanded film than the
corresponding saturated acid, stearic acid - i.e. TT is greater for any
value of A. Because of the double bond there is less cohesion
between the hydrocarbon chains than for stearic acid and a greater