c	Concentration (mol/m3)	P	The permeability coefficient (Barrer)
D	Diffusion coefficient (cm2/s)	p	The pressure (Pa)
Ds	Surface diffusion coefficient (cm2/s)	R	The gas constant (J/mol K)
d1/d2	The membrane thickness changes due to the surface adsorption	rp	The membrane pore radius (m)
E int	The interfacial energies of the composite membrane (kcal/mol)	S	The solution coefficient (cm3 (STP)/(cm3 kPa))
E layer,1-2	The total energies of the composite membrane (kcal/mol)	T	The temperature (K)
E layer,1	The energies of separation layer (kcal/mol)	V s	The reciprocal density
E layer,2	The energies of support layer (kcal/mol)	V VdW	The van der Waals volume
FFV	Fractional free volume (%)	Greek
fk	The gas flux due to Knudsen diffusion (kg/(m2 s))	ρ	The density (kg/m3)
fs	The gas flux due to surface diffusion (kg/(m2 s))	ε	The porosity of porous media
M	The gas molecular weight (kg/mol)	μs	The shape factor
N α	The atoms number	α	The selectivity