The Perturbation of Solids by Adsorbed Molecules

As in so many other fundamental aspects of the thermodynamics of surfaces, we are indebted to Gibbs 6) for pointing out that for solids the surface tension (7) and surface free energy (F ) are not equivalent quantities. Nevertheless, as Shuttleworth (7) mentions in an excellent review, the two terms have been (8) and still are (9) confused. The relationship between the quantities is 

Theoretical calculations of the surface free energy of solids date back to 1928 with the work of Lennard-Jones and Dent 10). Displacements of the positive and negative ions when a given interior layer becomes a surface layer were allowed for by Verwey 11). Calculations by Shuttleworth 12) showed that van der Waals terms make a significant contribution to surface energy. Benson and his co-workers have made an extensive study of alkali-halides 13-16) and of magnesium oxide 17). 

Some of the difficulties involved may be gathered from the fact that the values calculated for LiF vary from -f550 ergs/cm. IS) to a negative value 16). Similar difficulties occur with MgO 17), variations in the van der Waals and repulsive terms produced values for F ranging from —151 to -1-885 ergs/cm.2 according to the model used. 

Following the original measurements of the surface energy of sodium chloride by heats of solution by Lipsett et al. 18), much more accurate determinations have been carried out recently by Benson and his co-workers 19-22). Other substances which have been investigated include magnesium oxide 23), calcium oxide and hydroxide 24), and silica, unhydrated and hydrated 25). 

When precautions are taken with the purity of the solids 20) and care 

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