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 (C(18)DAO) and dioctadecyldimethylammonium chloride (2C(18)DAC) on g old- and silver-evaporated glass slides over a temperature range of 25-110 degrees C. Dependences of the thermal behavior of the LB films of C(18)DAO and 2C(18)DAC upon the substrates and the number of layers have been discus sed. The LB films of C(18)DAO on the gold-evaporated glass slides are more thermally stable than those on the silver-evaporated glass slides. The inte raction between the headgroup and the substrate may be stronger for the for mer than the latter, providing the films with the higher thermal stability. The one-monolayer LB film of C(18)DAO on the gold-evaporated glass slide s hows a clear order-disorder transition around 70 degrees C. In contrast, th e corresponding five-monolayer LB film gives a sharp transition at about 45 degrees C and a rather broad one around 80 degrees C. It seems that the ac cumulation of the upper layers yields independent transitions for the first layer and the rest of the layers and lowers the thermal stability of the f irst layer. The thermal behavior of the LB films of 2C(18)DAC and C(18)DAO is clearly different. For example the 2C(18)DAC films show partial recovery after the annealing from 110 degrees C, but the C(18)DAO films undergo irr eversible temperature-dependent changes. Probably, the difference in the th ermal behavior results from the different nature of the substrate-headgroup interaction between them.