HIGH-SENSITIVITY RAMAN-SPECTROSCOPIC STUDY ON SURFACE-PRESSURE DEPENDENCE OF THE STRUCTURE IN THIN LANGMUIR-BLODGETT-FILMS OF DIPALMITOYLPHOSPHATIDYLCHOLINE

By combining a high-sensitivity charge-coupled device detector with the total reflection technique, Raman spectra in the CH stretching region were recorded for thin Langmuir-Blodgett (LB) films of dipalmitoylphosphatidylcholine (DPPC) prepared at various surface pressures encompassing the plateau region of its surface pressure-area isotherm. Surface-pressure dependences of the lateral packing and the conformational order are discussed using the peak intensity ratio between the 2880 and 2850 cm-1 bands and that between the 2935 and 2850 cm-1 bands. The results indicate that at higher surface pressures above the plateau, the hydrocarbon chains of DPPC are tightly packed and highly oriented with the all-trans conformation. Below the plateau, however, the chains are disordered because of the increased number of the gauche conformers. The loosely packed hydrocarbon chains observed for the LB films deposited at lower surface pressures are gradually reoriented with time to form stable and highly ordered aggregates.