Molecular orientation in thin Langmuir–Blodgett films of dipalmitoylphosphatidylcholine as studied by FTIR transmission and reflection–absorption spectroscopy

Abstract

Langmuir–Blodgett (LB) films of DPPC were prepared by the horizontal lifting technique at various surface pressures encompassing the plateau region of its surface pressure (π) – area (A) isotherm. Molecular orientation in the LB films was quantitatively evaluated by a method recently proposed by Umemura et al. (J. Phys. Chem. 94, 62 (1990)) using Fourier transform infrared (FTIR) transmission and reflection–absorption (RA) spectra. From the results obtained, it is apparent that (i) in the LB films deposited at higher surface pressures above the plateau, the hydrocarbon chains are of all-trans conformation and well oriented at the tilting angle of ca. 20° from the surface normal; (ii) at lower surface pressures below the plateau, the chain segments are almost randomly oriented (like in a liquid-crystalline state) with an increased amount of the gauche conformers; and (iii) in the films deposited within the plateau, the hydrocarbon chains are in an intermediate state between the above two states. It is therefore concluded that the plateau corresponds to a process of the reorientation of the hydrocarbon chains from an almost randomly-oriented state to a highly-oriented one with the increase in the number of the all-trans conformers. Key words: dipalmitoylphosphatidylcholine, Langmuir–Blodgett film, molecular orientation, infrared reflection–absorption spectra, surface pressure–area isotherm.