Thickness and temperature dependence of molecular structure in stearic acid LB films studied by FT-IR reflection—absorption spectroscopy

Abstract

Abstract 1-,3-,9- and 21-monolayer Langmuir—Blodgett (LB) films of stearic acid were fabricated on silver-coated glass slides from the monolayer on water at the surface pressure of 30 mN m −1 . Fourier tranform-infrared (FT-IR) reflection—absorption spectra (RAS) of these LB films were recorded at various temperatures from 31 to 72°C. The spectra at 31°C exhibited characteristic features of high perpendicular orientation of the hydrocarbon chain. In the 1-monolayer LB film, The CO stretching band was not observed, possibly due to the image dipole effect on the silver surface. In the 1- and 3-monolayer LB films, the trans isomer of stearic acid was prominent, but the cis isomer took its place in the 21-monolayer LB film. FT-IR-RAS measurements at elevated temperatures indicated that the chain melting temperature increases and approaches the bulk melting point with an increase in the number of monolayers, except for the 1-monolayer LB film which had a higher melting temperature than the 3-monolayer film owing to the strong interaction with the silver substrate.