Exceptional Gas Permeation Selectivity of a Glued Langmuir−Blodgett Bilayer by pH Control

The monolayer properties of 5,11,17,23,29,35-hexakis[(N,N,N-trimethylamonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexadecyloxy-calix[6]arene hexachloride (1) have been characterized over aqueous solutions of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) as a function of pH. At high pH values (e.g., pH 10), such monolayers show relatively low surface viscosities. At low pH (e.g., pH 4.4), these monolayers exhibit relatively high surface viscosities. The barrier properties of single Langmuir−Blodgett bilayers of 1, which have been ionically cross-linked (i.e., “glued together”) with PAA were found to correlate with changes in surface viscosity. Thus, bilayers that were fabricated under low pH conditions exhibited high permeation barriers and high permeation selectivity with respect to He and N2. Given the extreme thinness of these glued bilayers (ca. 6 nm), the optimized He/N2 selectivity of ca. 1000 is extraordinary. These results, taken together, demonstrate the feasibility of fine tuning the surface viscosity of monolayers of 1, and also the barrier properties of corresponding glued bilayers, by adjusting the pH of an aqueous subphase that contains a weak polyacid.