We compare two methods for obtaining structural information on Langmuir-Blodgett films from polarized infrared spectroscopy. For sufficiently uniform films, we have already shown (refs 11 and 12) that all polarized spectroscopic properties could be characterized by a single quantity that we call the "electrical surface susceptibility tensor", γ . The imaginary parts of the susceptibility tensor could be readily obtained from reflectance measurements with the electric field parallel (Im(γ t)) and perpendicular (Im(γn)) to the plane of the film, independently of any specific properties of the film. This, in turn, could be related to the characteristics of individual molecules comprising the film, and their geometric disposition if the molecules are assumed to be interacting dipoles, which corrects, to a large extent, for the local field effects. We report the results of an infrared study performed on a dipalmitoylphosphatidylethanolamine monolayer. Attenuated total reflection spectra for both s and p polarization were recorded, and the spectra of Im(γ t) and Im(γn), as well as those of the imaginary part of the refractive index (kt and kn), are presented. Finally, the information deduced from the Im(γx) and kx (x ) s, p) are compared, and the molecular orientation is discussed both in terms of Im(γx) and kx. We show that by using this approach one obtains more truthworthy measurements of the disposition of oriented molecules at surfaces than those obtained from absorption coefficients.

The thermal behavior of Langmuir−Blodgett (LB) monolayers of dl-dipalmitoyl phosphatidylethanolamine (DPPE) deposited onto a ZnSe substrate was studied by polarized infrared attenuated total reflection (ATR), over the temperature range 25−140 °C. The results are discussed in terms of the imaginary part of the effective electric susceptibility tensor [γ] of the monolayer, a function that approximately corrects for the effect of the local field. We show that the spectra of the imaginary parts of the principal components of [γ], Im(γt) and Im(γn) (the subscripts t and n representing the directions tangent to and normal to the film, respectively), can be straightforwardly obtained from polarized reflection spectra. The results obtained using Im(γt) and Im(γn) are compared with those obtained from the extinction coefficients k t and k n, which were calculated from adapted Kramers−Kronig relations. We show that Im(γt) and Im(γn) provide a better account of the molecular processes occurring in the DPPE LB films as a function of temperature. No abrupt phase transition was observed. This is attributed to positional disorder in the headgroups of the monolayer as it was initially transferred onto the ATR prism.