Immersion heat relationships for homogeneous surfaces

Abstract

The thermodynamic relations between quantities obtained from an immersional heat curve and the heats derived from adsorption isotherms are examined from the point of view of the Gibbsian formalism. The usual interpretation of the immersional heat decrement as an integrated isosteric heat (taking the liquid phase as a reference state) is verified. It is found that, contrary to some treatments in the literature, the immersional heat decrement is not related to the film pressure and its temperature derivative by means of an equation of the Gibbs-Helmholtz type. With the use of the two-dimensional van der Waals equation as a model for monolayer adsorption on homogeneous surfaces, the contributions of lateral interactions to the integral energy and equilibrium heat curves are discussed. The model predicts that the equilibrium heat versus coverage curve will exhibit a maximum prior to the completion of the first layer. It is also pointed out that in the critical region pronounced deviations from the behavior predicted by the model can be expected to occur. In order to assess the role of lateral interactions in determining the character of immersional heat curves, a relationship between the gas-phase adsorption energy of a single molecule and the immersion heat at zero coverage is developed. This relationship is based on a quasi-continuum model, which is restricted to adsorbate-adsorbent systems for which the interaction forces are nonpolar. It is found that when the value of the zero coverage immersion heat is sufficiently low, the effect of increasing coverage is first to increase the immersion heat. This effect, coupled with that of lateral interactions, may produce a maximum in the immersion heat curve.