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Some properties of mobile and immobile adsorbed films

Published online by Cambridge University Press:  24 October 2008

J. K. Roberts
J. K. Roberts
Affiliation:
Trinity College
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Extract

The essential difference between mobile and immobile adsorbed films of gases on solids when there is repulsive interaction between adsorbed particles is discussed. Ordinary statistical methods can be applied to the equilibrium configurations of the particles in a mobile film but not to immobile films. Films formed by the adsorption of diatomic molecules with dissociation are then considered. It is pointed out that for a mobile (equilibrium) film there are differences between the true heat of adsorption and the apparent heat obtained from the temperature variation of the evaporation rate. A detailed comparison is made by obtaining a formula for the apparent heat of evaporation and comparing it with that for the true heat obtained earlier by Wang, and the physical reasons for the differences are discussed. A model suitable for studying the pseudo-equilibrium states in immobile films is described and formulae and diagrams in connexion with the kinetics of the formation of such films and the variation of heat of adsorption with fraction of surface covered are given.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 34 , Issue 3 , July 1938 , pp. 399 - 411
Copyright
Copyright © Cambridge Philosophical Society 1938

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References

* Roberts, , Proc. Roy. Soc. A, 161 (1937), 141.CrossRefGoogle Scholar

† Fowler, , Proc. Cambridge Phil. Soc. 31 (1935), 260CrossRefGoogle Scholar and 32 (1936), 144.

‡ Peierls, , Proc. Cambridge Phil. Soc. 32 (1936), 471.CrossRefGoogle Scholar

§ Wang, , Proc. Roy. Soc. A, 161 (1937), 127.CrossRefGoogle Scholar

* Roberts, loc. cit. equation (25).

* Wang, , Proc. Roy. Soc. A, 161 (1937), 133,CrossRefGoogle Scholar equation 15.

† Langmuir, and Villars, , Journ. Amer. Chem. Soc. 53 (1931), 486;CrossRefGoogle ScholarJohnson, and Vick, , Proc. Roy. Soc. A, 151 (1935), 308;CrossRefGoogle ScholarBosworth, and Rideal, , Physica (2), 4 (1937), 915CrossRefGoogle Scholar and 5 (1938), 170.

‡ Roberts, loc. cit. equation (9).

* Apart from any heat of activation in the adsorption process, see Roberts, loc. cit. p. 144.

* Roberts, , Proc. Roy. Soc. A, 152 (1935), 473.Google Scholar

† Roberts, , Proc. Roy. Soc. A, 161 (1937), 148.CrossRefGoogle Scholar

‡ Roberts, , Proc. Roy. Soc. A, 152 (1935), 455Google Scholar and 161 (1937), 148.

§ It may be noted that, for a random distribution of single occupied sites, γ(θ) = 1 − θ. This is not true for a random distribution of pairs of occupied sites, since γ(θ) refers to the distribution of particles round a pair of vacant sites, and the fact that a given site is vacant affects the probability of occupation of neighbouring sites. When θ is small, γ(θ) = 1 − 0.75θ see equation (18).