Heterogeneity of adsorption energy of water, methanol and diethyl ether on activated carbons: effect of porosity and surface chemistry

Three samples of activated carbon of wood origin with various amounts of surface functional groups and different porous structure were used as adsorbents of water, methanol, and diethyl ether. Structural and chemical characteristics of the samples' surfaces were obtained using adsorption of nitrogen and Boehm titration, respectively. Adsorption isotherms of water and methanol were measured using a volumetric apparatus whereas adsorption of diethyl ether was measured by means of inverse gas chromatography at finite concentration. Then the isotherms at three different temperatures were used to calculate distributions of adsorption potential, free energy of adsorption and adsorption energy. The results showed that the strength of interactions depends on both the porosity of the sample and its surface chemistry. The effect of surface chemistry is the most important in the case of water and the least important in the case of diethyl ether. The high-energy centers are related to adsorption in narrow pores containing functional groups. For molecules consisting of hydrocarbon moiety and oxygen containing groups (methanol and diethyl ether) the surface chemistry plays a role at low surface coverages.