Membrane Composites Derived from Porous versus Nonporous Surfactants: Evidence for Uniqueness of Calix[6]arene-Based Surfactants

摘要

We have previously outlined a strategy for the synthesis of ultrathin membranes that possess size selectivity toward per-meants. In essence, it involves the deposition of monolayers or multilayers of "porous" surfactants onto highly permeable supports. When defects within the surfactant assembly are negligible, diffusion across the composite is expected to be governed by pores that exist within and between the surfactants. Recently, we have demonstrated the feasibility of this approach by showing that composites made from Langmuir—Blodgett (LB) multilayers of calix[6]arene-based surfactants plus poly-[1-(trimethylsilyl)-1-propyne] (PTMSP) supports discriminate between He and N_2 on the basis of their size. While this work showed the uniqueness of PTMSP as support material, as compared with commonly-used polymers such as Celgard (stretched forms of polypropylene) and Nuclepore membranes (track-etched polycarbonate film), the uniqueness of the calix-[6]arene surfactants, themselves, was not firmly established. In this paper we provide evidence for uniqueness of calix-[6]arene surfactants as building blocks for the construction of permeation-selective composites. Specifically, we show that membranes made from PTMSP plus multilayers of calix[6]arene I exhibit significant permeation selectivity toward He and N_2, but that those based on arachidic acid (AA), cadmium arachidate (AA/Cd~(2+)), a polyion complex of AA plus poly(ethyleneimine) (AA-PEI), and stearamide oxime (SA) show no permeation selectivity and no alteration in the barrier properties of the support. We also report results of surface analyses of PTMSP/I and PTMSP/SA by X-ray photoelectron spectroscopy (XPS) that offer insight into the origin of these striking differences.