Microstructure and ion exchange in stearic acid Langmuir-Blodgett films studied by Fourier transform infrared-attenuated total reflection spectroscopy

Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectra h ave been recorded of 11-layer Langmuir-Blodgett (LB) films of stearic acid deposited at various surface pressures (0.1, 15, and 35 mN/m), and the mole cular orientation angles were evaluated quantitatively, which supplied insi ght into the molecular order with the alkyl chains tightly packed like crys tal in the LB films deposited at the zero and higher surface pressures, The se experimental results indicate that, in the Langmuir film as the precurso r of LB films, stearic acid molecules self-aggregate to form two-dimensiona l crystalline domains already even at the zero surface pressure, which resu lts in the inhomogeneity of monolayer. The analysis of dependence of v(C=O) intensity on the surface pressure, surface density, and subphase temperatu re leads to the conclusion that the defects in LB films originate from the Langmuir film and be conserved upon deposition. Annealing below 50 degreesC and cooling could improve the monolayer homogeneity, and thus a defect-fre e or low-defect LB films can be deposited. Furthermore, ion exchange conduc ted in the LB films, on the other hand, confirms the existence of structure defects in LB films of stearic acid. The polar plane microstructure, later al transport along the polar planes and the coordination types of stearic a cid/cation system may be the rate-limiting process. The results have implic ation on the possible uses of stearic acid LB films as ion-exchange materia ls or sensors. (C) 2001 Academic Press.