Adsorbate-adsoibate interaction

The Type II and Type III isotherms shown in Figure 5.7 can both be described by the so-called infinity-form of the BET expression, which allows for an infinite number of adsoibate layers. The difference in the shsqpes of these two isotherms arises from differences in the strength of the adsorbate-adsorbent interaction. 

J. M. Cases, Normal Interaction Between Adsoibed Species and Adsorbing Surface, Trans. 

Microcalorimetry measurements are combined with Grand Canonical Monte Carlo simulations in order to understand more deeply the interactions between methane and two types of faujasite systems. The modelling study, based on newly derived force fields for describing the adsorbate/adsorbate and adsoibate/adsorbent interactions, provide isotherms and evolutions of the differential enthalpy of adsorption as a fimction of coverage for DAY and NaX which are in very good accordance with those obtained experimentally. The influence of the location of the extra-framework cations within the supercages on these thermodynamics properties is also pointed out. Furthermore, the microscopic mechanisms of CH4 adsorption is then carefully analysed in each faujasite system which are consistent with the trend observed for the differential enthalpies of adsorption. 

The adsorptives were modelled using the 12-6 potential model with the parameters given in Tabie 1, where e , and e, o are the potential well depths and hard sphere diameters for the adsoibate and adsorbate adsorbent potentiais respectiveiy. Ciearly this is less satisfactory for than for Ar since, in spite of the fact that free rotation is to be expected to reduce the quadrupole effects in the bulk phase above 40K, it is not necessarily the case that this also applies to the adsorbate. This reservation seems to be borne out by the results reported below. Nevertheless it has proved valuable here to be able to compare two similar models which differ only in their molecular parameters. Because of its particular combination of molecular size and interactions the Ar-graphite system is difficult to model with precision at the temperature of interest. A useful touchstone for evaluating model potentials is provided by the liquid to incommensurate solid transition exhibited by this system at 77.5K. It is also of interest in the present context to know how porosity affects this transition. 

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