Pore Size, Shape and Composition

To understand gas transport phenomenon it is critical to consider the interactions between the gas molecules and the pore wall. The van der Waals interactions between particles are explained well by the Lennard-Jones function containing two parameters, the kinetic diameter g (the distance where the potential energy between the particles is zero) and the well depth e (the deepest potential minimum between the particles). These parameters were used, e.g. by Freeman [52], to establish a theoretical basis for the relationship between selectivity and permeability for a range of polymers and gases (so-called first upper bonnd empirically determined by Robeson in 1991 [53] see also more recent paper by Robeson [54]). The rate of diffnsion of a gas is dependent on its kinetic diameter while its solnbility mainly depends on the condensability of the gas and consequently on the well depth for gas-gas interactions. 

1 Adsorption onto surface and flux from position 0o,surf to 9q at the pore entrance via surface diffusion (f2.J). 

3 Flux directly to pore entrance with no adsorption taking place (F1.J). 

4 Entrance of adsorbed molecules at position Qq to position 9i within the pore (F2.J). 

6 Desorption of the molecules from within the pore to the external surface or directly to the gas phase. 

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