V.N.3 Gas Transport Across Hyperthin Membranes

Single Langmuir-Blodgett (LB) bilayers derived from a calix[6]arene-based surfactant and poly(acrylic acid) exhibit exceptional H2/CO2 permeation selectivities (ca. 75) and significant CO2/N2 selectivities (ca. 30). In sharp contrast, hyperthin polyelectrolyte multilayers (PEMs), formed from poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrene sulfonate) (PSS) show very low H2/CO2 permeation selectivity (ca. 2) but exceptionally high CO2/N2 permeation selectivities (ca. 100). The H2/CO2 and CO2/N2 selectivity found with these LB bilayers is mainly due to differences in diffusivity, which reflect their kinetic diameters (H2, 0.29 nm; CO2, 0.33 nm; N2, 0.36 nm), and also the tightness of the membrane. The very low H2/CO2 selectivity and high CO2/N2 selectivity associated with these PEMs is mainly the result of a high solubility of CO2 relative to H2 and N2.