Abstract
A new type of isotherm equation is presented for mobile molecules adsorbed on a heterogeneous surface of random topography. The basic idea of the adsorption model is that molecules moving around on the surface spend more time on sites interacting with lower energy; the probability on each site is determined from the energy term that pulls the system toward more stable states and the entropy term that maximizes the number of combinatorial configurations on sites. The hybrid isotherm equation was applied to single- and mixed-gas adsorptions on MS-13X and on H-mordenite by assuming two sites. The site fractions were found to be important quantities for correlating the single-gas isotherms, while the assignments of sites were crucially important for the interpretation of mixed-gas adsorptions. This theory suggests that the large negative deviations from Raoult’s law observed for systems of C3H8 + CO2 and C3H8 +H2S on H-mordenite are due to the negative site correlations, which means that the lower-energy sites do not compete with each other.
