Structural characterization of Langmuir-Blodgett films of octadecyldimethylamine oxide and dioctadecyldimethylammonium chloride. 1. Reorientation of molecular assemblies during the accumulation of upper layers studied by infrared spectroscopy

One- and multilayer Langmuir-Blodgett (LB) films of octadecyldimethylamine oxide (C(18)DAO) and dioctadecyldimethylammonium chloride (2C(18)DAC) were prepared on gold- and silver-evaporated glass slides. The first and the res t of the layers were deposited by vertical dipping and horizontal lifting m ethods, respectively. Infrared (IR) reflection-absorption (RA) spectra were measured for the LB films to investigate the dependence of molecular orien tation and structure on the number of layers and the substrates. X-ray diff raction and wetting properties were also measured for some of the films. Fo r both C(18)DAO and 2C(18)DAC, there are remarkable changes in the 3000-280 0 and 1700-1200 cm(-1) regions between spectra of the one-monolayer LB film s and those of the two-monolayer LB films irrespective of the substrates. I n the spectra of the LB films of C(18)DAO on gold-evaporated glass slides, bands due to CH2 stretching modes show a downward shift by 5-7 cm(-1) and t heir intensities become much weaker upon the deposition of the second layer on the first layer, suggesting that the hydrocarbon chain becomes ordered and perpendicular to the substrate surface. In general, the rearrangement o f the molecular assemblies takes place during the accumulation of the upper layers in the multilayer LB films of C(18)DAO and 2C(18)DAC on the gold-ev aporated glass slides.