The gas–solid virial expansion is developed for argon adsorbed on the (100) sodium chloride surface. Limiting parameters for the attractive components of the gas-ion interaction are suggested and the corresponding repulsive parameters for the Li
+
, Na
+
, K
+
, Rb
+
, Cs
+
, F
–
, Cl
–
, Br
–
and I
–
ions are determined from an analysis of alkali halide bulk crystal data. The second two dimensional virial coefficient for argon absorbed on (100) sodium chloride is determined over a range of temperatures by using the generalized ‘exponential-six’ equation for all interatomic interactions. The results indicate large changes from the ideal perfectly mobile two-dimensional second virial coefficient caused by the inclusion of perturbation terms that take account of the periodic structured surface. These changes are shown to be sensitive to the exact structure of the crystal at the interface as shown by adsorption of argon on the (100) sodium chloride surface which has been allowed to relax to its equilibrium conformation. The third body (surface) interactions at the interface are considered together with the applicability of the two dimensional approximation for the adsorbate self interactions. The zero coverage isosteric enthalpy and Henry’s law constants at 80 K are computed for each of the gas-ion potentials that are chosen. The results are found to agree with those obtained by using the hindered translation model and from a comparison with experimental data, suggest a larger attractive gas–crystal interaction than previously thought.
