The formation of surface films by spontaneous spreading from solids

The spreading pressure Fe varies in an interesting way with temperature. Fig. 26a (redrawn from the original to show surface pressures directly) shows the behaviour of several fatty acids. 

Similar results were obtained on other long-chain compounds. Below a certain temperature there is no spreading. Above this temperature the pressure Fe increases linearly with the temperature until the solid melts. 

The figure shows that many of the surface films, spread by the aid of solvents, are not thermodynamically stable, though they are often sufficiently stable in practice to be examined without serious collapse during some hours. The establishment of equilibrium between the film and the crystal is very slow from the film side perhaps this is due to the absence of crystal nuclei of sufficiently large perimeter to allow collapse or condensation on the crystal to occur at an appreciable rate or perhaps the aggregates formed when a film collapses have a considerably higher potential energy than properly formed crystals. 

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