Adsorption characteristics at a solid-solution interface

Abstract

The adsorption characteristics at an alumina-solution interface have been studied for solutions consisting of series of p-substituted phenols (methyl-, chloro-, bromo-, tertiary-butyl-, and nitro-) dissolved in cycloherane and tetrahydrofuran. Characterisation of the alumina surface has been attempted by x-ray diffraction photography, electron microscopy, low temperature nitrogen adsorption and dehydration experiments. The presence of pores in the alumina sample has been confirmed and a pore size distribution analysis carried out. Consideration of the porosity and surface area values obtained by the use of gas adsorption and solution adsorption methods, has enabled the area of the alumina surface available to the adsorptives to be assessed. The molecular area requirements of the adsorptives on the alumina surface have been determined from the adsorption isotherms and indicate that the phenols are perpendicularly orientated to the adsorbent surface. From the nature of the alumina surface and the orientation of the adsorptives, a probable mechanism of adsorption has been suggested. An assessment of the influence of the solvent on the adsorptive capacity of the alumina surface for the phenols has been made resolving the composite adsorption isotherms into the individual isotherms using experimentally determined monolayer values for the adsorptives from vapour and solution phase measurements. In order to compare the adsorptive affinity of the alumina surface for the phenols, an index of adsorption has been defined as "the number of moles of adsorptive present in the adsorbed phase at constant equilibrium mole fraction of the mobile phase". The values of K(_A) for the substituted phenols are seen to reflect the changes occurring in the electrical character of the hydroxyl group of the adsorptive due to the presence of thep-substituent, and are in accord with the relationship observed between hydrogen-bonding association in solution and p-substituent character.