The adsorption of alcohols and phenols from nonpolar solvents on to alumina

Abstract

1. 1. The isotherms for adsorption from benzene solution on to dry alumina were measured by means of the surface balance for a number of surface-active materials, chiefly alcohols. The isotherms were of Langmuirian form, described by θ c (1 − θ) = K , where K is the Langmuir adsorption coefficient. 2. 2. Competitive adsorption was followed between two alcohols, and between an alcohol and a phenol. The two adsorbates followed the Langmuirian equation independently, and the coefficient of adsorption of one could be deduced from the coefficient of the other. 3. 3. When one of the competing substances was water, competitive adsorption did not appear to follow the Langmuir isotherm, since the water did not desorb normally. 4. 4. The isosteric or differential heats of adsorption of several surface-active substances were measured from the temperature dependence of the Langmuir adsorption coefficient. The calorimetric or integral heats were also measured and were somewhat higher than the differential heats. 5. 5. The heats and free energies of adsorption were in all cases independent of the length of the hydrocarbon chain. The free energy and probably also the heat of adsorption in the series alcohol, phenol, fatty acid increased as the hydroxyl group became more acidic. 6. 6. The molecules are believed to be adsorbed on fixed sites of approximately uniform energy. At low concentration of adsorbate its nonpolar region remains dissolved in the solvent and is not affected by adsorption. The presence of solvent minimizes lateral forces and thus allows the Langmuirian relationship to hold good over a wide range of surface coverage. 7. 7. The Traube rule, if applied to energies of adsorption, is probably generally valid for adsorption on solid surfaces. The so-called exceptions are thought to be due to the smaller surface area often available to larger molecules rather than to any decrease in their molar energy of adsorption. 8. 8. The adsorption of polar substances from nonaqueous solutions appears to be little influenced by the hydrocarbon chain with silica as the adsorbent just as with alumina. On the other hand, the energy of adsorption of organic molecules from aqueous solution on to charcoal increases with increasing length of the hydrocarbon chain.