Adsorbtion of binary vapor mixtures into solids

The adsorption isotherms for ethanol-cyclohexane, ethanol-benzene, and benzene-cyclohexane vapor mixtures on Cab-O-Sil and on Graphon at 20, 30, and 40°C were measured at constant total pressure. The adsorption isotherms for the pure components were also obtained. The ethanol/Graphon isotherms were intermediate between Types II and III; for the other systems, Type II isotherms were obtained. The amounts of the pure vapors adsorbed on Cab-O-Sil followed the order ethanol > benzene > cyclohexane. The adsorption isotherms for benzene and cyclohexane on Graphon were almost identical. Except at low relative pressures, the amount of ethanol adsorbed on Graphon was greater than the amounts of benzene or cyclohexane adsorbed.

Isosteric heats of adsorption and BET cross-sectional areas were calculated for the pure adsorbates on Cab-O-Sil and on Graphon. The data indicated that the three adsorbates do not form close-packed monolayers on the Cab-O-Sil surface. The isosteric heat and the integral entropy of adsorption for ethanol on Graphon suggested that ethanol forms a hydrogen-bonded structure on the Graphon surface.

The binary vapor adsorption isotherms were compared to the pure component isotherms. In several instances, the amounts of the components adsorbed from the mixtures were greater than from the pure states. For the Cab-O-Sil systems, selective adsorption of ethanol occurred from ethanol-cyclohexane and ethanol-benzene mixtures; benzene was selectively adsorbed from benzene-cyclohexane mixtures. The adsorbate-vapor composition diagrams for benzene-cyclohexane mixtures on Graphon all contained adsorption azeotropes. Very little selective adsorption of either component occurred. In general, benzene and cyclohexane were selectively adsorbed from benzene-ethanol and cyclohexane-ethanol mixtures on Graphon. Adsorption azeotropes occurred in the 20°C isotherms for these two systems at high ethanol mole fractions. The temperature dependence of the selectivity for the systems studied followed no consistent trend. Comparison of the binary vapor adsorption isotherms with the analogous solution adsorption isotherms indicated that selectivity is generally higher in adsorption from solution.

The experimental binary vapor adsorption isotherms were compared to those calculated from the pure vapor adsorption isotherms using the ideal adsorbed solution model. It was found that the adsorbed solutions were ideal or slightly nonideal for all three mixtures on Cab-0-Sil and for benzene-cyclohexane mixtures on Graphon. The nonideal behavior observed for benzene-ethanol and cyclohexane-ethanol mixtures on Graphon was attributed to the presence of benzene or cyclohexane disrupting the hydrogen-bonded structure of ethanol on the Graphon surface. It was concluded that the ideal adsorbed solution model is a useful one for predicting binary vapor adsorption equilibria.