Structural Characterization of Langmuir−Blodgett Films of Octadecyldimethylamine Oxide and Dioctadecyldimethylammonium Chloride. 2. Thickness Dependence of Thermal Behavior Investigated by Infrared Spectroscopy and Wetting Measurements

Infrared (IR) reflection−absorption (RA) spectra have been measured for one- and five-monolayer Langmuir−Blodgett (LB) films of octadecyldimethylamine oxide (C18DAO) and dioctadecyldimethylammonium chloride (2C18DAC) on gold- and silver-evaporated glass slides over a temperature range of 25−110 °C. Dependences of the thermal behavior of the LB films of C18DAO and 2C18DAC upon the substrates and the number of layers have been discussed. The LB films of C18DAO on the gold-evaporated glass slides are more thermally stable than those on the silver-evaporated glass slides. The interaction between the headgroup and the substrate may be stronger for the former than the latter, providing the films with the higher thermal stability. The one-monolayer LB film of C18DAO on the gold-evaporated glass slide shows a clear order−disorder transition around 70 °C. In contrast, the corresponding five-monolayer LB film gives a sharp transition at about 45 °C and a rather broad one around 80 °C. It seems that the accumulation of the upper layers yields independent transitions for the first layer and the rest of the layers and lowers the thermal stability of the first layer. The thermal behavior of the LB films of 2C18DAC and C18DAO is clearly different. For example, the 2C18DAC films show partial recovery after the annealing from 110 °C, but the C18DAO films undergo irreversible temperature-dependent changes. Probably, the difference in the thermal behavior results from the different nature of the substrate−headgroup interaction between them.