Rigid-rod Langmuir-Blodgett films from [n]staffane-3-carboxylates

Abstract

The rigid-rod surfactants [n] staffane-3-carboxylates (1[n]) readily form Langmuir-Blodgett films when n = 4 and, with some effort, also when n = 3. The area per molecule, extrapolated to zero pressure, is nearly independent of the counterion (26.0 Å2/molecule). Bilayer thickness in a multilayer assembly is 25.8 ± 0.1 Å for Ca2+ 1[3], 32.2 ± 0.1 Å for Ca2+ 1[4], and 31.4 ± 0.1 Å for Cd2+ as determined by low-angle X-ray reflection. Ellipsometry yields the values 27.2 ± 1.0, 33.2 ± 1.0, and 32.6 ± 1.0 Å, respectively. The average tilt angle deduced from the difference between 1[3] and 1[4] is 17 ± 5°. Independent IR measurements at normal incidence on Ge and grazing angle incidence on Au substrate yield the values 28 ± 2° and 19 ± 2°, respectively. Only IR measurements were possible for a monolayer. They yielded the value 27 ± 2° at normal incidence on Ge and a distinctly different result at grazing incidence on Au, 0-10°. The larger average tilt values on the Ge surface are attributed to surface roughness. © 1992 American Chemical Society.