Measurement of excess functions of binary gas mixtures adsorbed in zeolites by adsorption calorimetry

Adsorption equilibria and heats of adsorption were measured for mixtures of ethylene and ethane on NaX at 298 K. The pure-component isosteric heat of adsorption of ethane increases with loading due to gas-gas interactions; the heat of adsorption of ethylene is approximately constant with loading because of a balance between cooperative interactions and gas-solid energetic heterogeneity. This mixture, which is nearly ideal on carbon, exhibits moderate negative deviations from ideality on NaX. The nonideality is explained by a difference in the polarities of the molecules: ethylene has a quadrupole moment but ethane is nonpolar. The infinite-dilution activity coefficients are unity in the Henry's law region and decrease exponentially to a value of 0.56 at high loading. Regular-solution theory fails to agree with experiment. All three excess functions (free energy, enthalpy, and entropy) are negative; thus, activity coefficients are less than unity and the enthalpy of mixing in the adsorbed phase is exothermic. These results are consistent with an adsorbed solution in which the molecules are segregated into regions of different composition.