Non-Arrhenius form of the Henry adsorption on inhomogeneous substrates: the effect of frozen disorder

Usually, when considering sub-monolayer adsorption at different temperatures T , the logarithm of the Henry constant is represented as ln K H ∼ U 0 / T with some constant “heat of adsorption” U 0 > 0. However, such Arrhenius-type form has a clear base only for an ideal translational-invariant crystal substrate. In more real situation, e.g. for a structurally disordered substrate, the “heat of adsorption” will be some random function of two-dimensional coordinates characterized by some distribution function of its different values. Starting from general principles of the theory of Gaussian fluctuations, we show that the adsorption at the substrate with bulk inhomogeneous structure leads to expression ln K H ∼ U 0 / T + Δ 2 / T 2 with some fluctuation quantity Δ. A possibility to observe such “non-Arrhenius” additive seems most favorable at rather low temperatures for substrates modeled by substitutional solid solutions. The theoretical predictions are illustrated by comparison with the experimental data obtained for some disordered adsorbents.