Abstract
Fourier transform infrared (FTIR) -attenuated total reflection (ATR) spectra have been studied of Langmuir-Blodgett (LB) films of stearic acid with 1 to 9 monolayers. Difference in the spectral features observed among respective monolayers suggests that there are changes in structure, subcell packing, and molecular orientation as a function of the number of monolayers. (1) The hydrocarbon chains of stearic acid in the first monolayer are in the hexagonal subcell packing where each chain is freely rotated around its long axis oriented perpendicular to the surface. However, the molecules in the upper monolayers crystallize with the monoclinic form where the hydrocarbon chains are packed alternately and are inclined at the angle about 30° with the surface normal. (2) Stearic acid occurs as the cis configuration for the C=O and Cα-Cβ bonds in the 1- and 2-monolayer films, but the trans configuration starts to appear in the 3-monolayer film.
Band intensity enhancement of FTIR-ATR spectra of the stearic acid LB film is obtained by evaporation of thin Ag island layers. It is found that in the 1-monolayer LB film, all stearic acids react with evaporated Ag forming silver stearate and the symmetric COO- stretching band is enhanced a factor of 30. This phenomenon is ascribed to the electric field enhancement due to collective electron resonances associated with the island nature of the thin Ag layers.
