Infrared spectroscopic study on thermal behavior of Langmuir-Blodgett films of octadecylammonium octadecanoate and octadecylammonium octadecanoate-d35

Langmuir-Blodgett (LB) films of octadecylammonium octadecanoate (C18H37NH3+C17H35COO−, ODASA) and octadecylammonium octadecanoate-d35 (C18H37NH3+C17D35COO−, ODASA-d35) were prepared and their thermal behaviors were investigated by variable-temperature Fourier transform infrared transmission spectroscopy. It was found that the two hydrocarbon chains of ODASA molecule in LB films are highly ordered while that protonated (H) chain in ODASA-d35 is partially disordered with some gauche conformers introduced at room temperature. The thermal stability of ODASA-d35 in LB films is also different from that of ODASA. The two hydrocarbon chains in ODASA LB films show one sharp order-disorder transition with temperature increasing from 85 to 90 °C. But in ODASA-d35 LB films, the protonated and deuterated chains show quite different thermal behavior, respectively. For example, the protonated chains undergo order-disorder transition between 80 and 90 °C, abruptly from 80 to 85 °C, while the phase transition of the deuterated ones undergoes slowly and persists long time from 70 to 85 °C.