Ideal Gas Selectivity

The ideal gas selectivity of a membrane, ? ij , is defined as the ratio of the permeability of two pure gases, measured separately under the same conditions: ?ij = Pi/Pj (1) where P i and P j are the permeability (or the permeance) of the two pure gases, respectively, with i being the most permeable gas. Rarely the real selectivity is equal to the ideal gas selectivity. Most commonly the ideal selectivity of a membrane is lower than the real selectivity, especially when the more permeable gas species plasticizes the polymer matrix, making it relatively more permeable for the slower species. In some cases, in particular in high free-volume polymers, strong sorption of the more permeable species may obstruct the transport of the less permeable species, making the mixed gas selectivity higher than the ideal selectivity.