92168.pdf

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

observed at about 0.25 nm^2 molecule"^1 , corresponding to the initial

packing of the end-groups (Figure 4.24). The ir-A curve becomes

very steep at about 0.205 nm^2 molecule"^1 , 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