Effective electric surface susceptibility tensor as a probe of the thermal behavior of Langmuir-Blodgett films

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 degrees C. The results are discussed in terms of the imaginary part of the effective electric susceptibility tensor [gamma] 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 [gamma], Im(gamma(t)) and Im(gamma(n)) (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(gamma(t)) and Im(gamma(n)) 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(gamma(t)) and Im(gamma(n)) 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.