Insight into the permeation selectivity of calix[n]arene-based Langmuir-Blodgett films: Importance of headgroup association and the solid phase

This paper reports the design and synthesis of 5,11,17,23,29,35-hexaacetyl-37,38,39,40,41,42-hexakis(1-hexadecyloxy)calix[6]arene (2) and 5,11,17,23-tetraamideoxime-25,26,27,28-tetrakis(1-hexadecyloxy)calix[4]arene (3), plus a detailed comparison of the membrane properties of corresponding Langmuir-Blodgett films with those assembled from 5,11,17,23,29,35-hexaamideoxime-37,38,39,40,41,42-hexakis(1-hexadecyloxy)calix[6]arene (1a). Surface pressure-area isotherms that were recorded for 1a and 2 over a pure water subphase at 25 C showed significant hysteresis that was eliminated after two compression-expansion cycles; the limiting area in both cases was ca. 162 {angstrom}{sup 2}/molecule. In sharp contrast, 3 showed no hysteresis at 25 C and a limiting area of 107 {angstrom}{sup 2}/molecule. Careful examination of the dependence of surface viscosity on temperature has provided compelling evidence for a solid-analogous to liquid-analogous phase transition occurring at 31 and 28 C for monolayers of 1a and 2, respectively. Permeation measurements that were made across Langmuir-Blodgett (LB) films [four monolayers supported on poly-[1-(trimethylsilyl)-1-propyne] with respect to helium and nitrogen] revealed high selectivity in the case of 1a and much lower selectivity with 2 and 3 at ambient (21--23 C) temperatures. The implications of these findings, in terms of the further development of LB films as membranes for gas separations, are briefly discussed.