A radioactive tracer study of the behaviour of long chain organic molecules on solid surfaces

Abstract

The adsorption and possible mechanisms for the surface transport of stearic acid and some of its derivatives on solids have been investigated using radioactive tracers.  mono- and multimolecular layers of these compounds were deposited on quartz glass, mica, and metal specimens, which were then held at various temperatures for a short period. The resulting changes in amount and distribution of activity were determined by autoradiography or an appropriately designed Geiger tube. On unreactive surfaces such as platinum or quartz glass the pure acid is only weakly adsorbed and thermal desorption, in general, readily occurs, especially of layers deposited on an initial monolayer. Layers of copper or calcium soaps on all surfaces, either deposited as such or formed in situ, are very much more stable. There is also some evidence that desorption is favoured by the presence of water vapour. Both pure acid and soap layers have some resistance to mechanical abrasion, but none to dissolution by hot benzene unless the adsorption involves a chemical bond. Part B of the present paper describes an experimental search for a long-range surface diffusion, using the same surfaces and adsorbates. It is shown that, under favourable conditions, material desorbed from one part of a surface will readsorb elsewhere after movement over considerable distances in the vapour phase. This mechanism may account for some examples of apparent surface diffusion reported by earlier workers. If, however, this vapour phase transport is suppressed, then no evidence can be detected of a true surface diffusion over macroscopic distances.