Infrared Study of the Molecular Orientation in Ultrathin Films of Behenic Acid Methyl Ester: Comparison between Single Langmuir-Blodgett Monolayers and Spin-Coated Multilayers

Single Langmuir-Blodgett (LB) monolayers of behenic acid methyl ester (BAME) were studied by infrared spectroscopy. Anisotropic optical constants of this system were calculated from a perpendicular-polarized attenuated total reflectance spectrum and a parallel-polarized reflection-absorption spectrum and were compared to those obtained, in a previous article, for spin-coated multilayers (Pelletier, I.; Bourque, H.; Buffeteau, T.; Blaudez, D.; Desbat, B.; Pezolet, M. J. Phys. Chem.B2002, 106, 1968). These optical constants were used to calculate the tilt angle of the alkyl chains in both types of films deposited on solid substrates and to simulate polarization modulation reflection-absorption spectra of the Langmuir film at the air/water interface. The results indicate that the molecular tilt angle is near 30° for spin-coated multilayers, while single LB monolayers have a tilt angle of 11±2°. A similar orientation of the alkyl chains is obtained on single monolayers deposited onto solid substrates or spread at the air/water interface (Langmuir films), showing that no change of molecular orientation results from the Langmuir-Blodgett transfer. The presence in the infrared spectra of several bands due to the methylene wagging and twisting modes and of the splitting of the band due to the methylene bending mode at 1463 and 1473 cm^-1 indicates that in all types of films BAME alkyl chains are in the all-trans conformation and packed in an orthorhombic subcell. However, under certain circumstances, the splitting of the methylene bending mode band was not observed. This phenomenon is associated with the roughness of the solid substrates supporting the LB film leading to the formation of gauche defects in the film and thus to a molecular disordering of the monolayer.