The polymer polarity and density of sulfonated poly (phenylene oxide) (SPPO) membranes were manipulated by exchanging the proton of the sulfonic groups with mono-, di-, and tri-valent metal cations. The performance of the TFC membranes coated with ion-exchanged SPPO, toward the separation of CO2/CH4 and O-2/N-2 gas systems, showed to correlate with the location of the metal in the universal periodic table of the chemical elements. Altering the metal cation that replaces the proton of the sulfonic groups has altered: the.,cross-linking forces, the hindrance effects due to the voluminous metal size, and the SO3-Metal groups polarity due to the change in the metal electronegativity.