... it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules ... through which small molecules, such as nitrogen and water can diffuse. 4A (pore size 4 Å): Adsorbs H2O, CO2, SO2, H2S, C2H4, ... hydrocarbons. Branched chain hydrocarbons and aromatics are excluded. 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to nC4H10, alcohols to ...