Application of the polanyi adsorption potential theory to adsorption from solution on activated carbon. V. Adsorption from water of some solids and their melts, and a comparison of bulk and adsorbate melting points

Adsorption isotherms from water solution onto an activated carbon have been determined, both above and below the underwater melting points, for the following compounds: o-, m-, and p-nitrophenol; 2,5-dichlorophenol; coumarin; phthalide; 3,5-dichlorophenol; p-bromophenol; m-acetotoluidide; m-chloroacetanilide; and 2,2′-bipyridine. Of these compounds, adsorption isosteres on both sides of the underwater melting points have been determined for o- and m-nitrophenol, phthalide, and coumarin. The isotherms show distinctly higher adsorption on exceeding the melting points. The effect is ascribed to relatively inefficient packing of the solid phases into the carbon pores. The isosteres show first-order transitions at temperatures that are not detectably different from the underwater melting points and that are therefore taken as the melting points of the adsorbates. The identity of the bulk and adsorbate melting points is taken as evidence that the adsorbates are similar in their properties to the corresponding bulk phases. Except for the phenols, the adsorption isotherms of the liquids can be calculated by the previously published modified Polanyi adsorption potential theory, using gas-phase data on the same carbon, together with solute molar volumes and refractive indices. For the phenols (except o-nitrophenol), the theory underestimates the adsorption potential by about 20% and requires an empirical factor for each solute to account for the adsorption data.