Effective Electric Surface Susceptibility Tensor as a Probe of the Thermal Behavior of Langmuir−Blodgett Films

Abstract

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.

A New Approach to the IR Spectroscopic Study of Molecular Orientation and Packing in Adsorbed Monolayers. Orientation and Packing of Long-Chain Primary Amines and Alcohols on Quartz

“…The "spectrum fitting" approach is reduced to determining the principle values of the permittivity tensor of the film by fitting the simulated spectra into the experimental ones, utilizing the matrix method, 12,13,21,25,29,30 the three-layer exact Fresnel formulas, 9,31,-34 or the thin-layer approximation. 13,24 In the "surface susceptibility" approach, 19,35 instead of the spectra of the imaginary and real parts of the complex refractive index, the spectra of the imaginary and real parts of the surface susceptibility of the film are obtained from the reflection spectra. The third ("dichroic ratio (DR) fitting") approach consists of extracting the average orientational angles by fitting the DR values.…”

Interpretation of IR Spectra of Ultrathin Films

“…However, studies concerning absorption of head groups, such as carboxylic and carboxylate groups of fatty acids [495][496][497], amide [498][499][500][501][502] and phosphate [503] groups of biophysical LB films (Section 7.7), and the CN groups of chromophoric molecules [504], have also been reported.…”

“…The νCH 2 temperature dependence has been used in determining the melting point of a wide range of LB films [503,[515][516][517][518][519] and SAMs [370,520]. The νCH 2 molecular area dependences have been used to study the mesophase transformations in L monolayers [508,[521][522][523][524], while the νCH 2 time dependences have been helpful in studies of the kinetics of aggregation (self-assembly) † Being negligibly small for near-normal orientation of the chains [506], the TO-LO shift will arise in p-polarized spectra with increasing the tilt angle.…”

“…For a crystalline-like film, the ratio of the intensities of the ν as CH 2 and ν s CH 2 bands (A as and A s , respectively) gives the twist angle directly [604][605][606]: Servant et al [503,607] have shown that the spectra of the imaginary parts of the surface susceptibility tensor [Eq. (1.89)], which are easily obtained from the polarized IR spectra with no measurement of the film thickness, take into account the domainlike composition typical of LB films.…”

“…In the spectroscopy of ultrathin films, the DR is defined as the ratio of the peak (or integrated) intensity of the absorption band in the s-polarized spectrum, A s , to that in the p-polarized spectrum, A p : DR = A s A p . ( 3.58) Similar to the surface susceptibility method [503,507], DR fitting allows one to exclude the film thickness and optical constants as input parameters in spectral simulations, a very important advantage in the case of adlayers when there is no way to determine the film thickness accurately [622]. As opposed to spectrum fitting, DR fitting is applicable to the cases when (1) properties of the film depend on the substrate (the general case), (2) preliminary information about the film is poor or absent, and (3) when the film can not be perfectly reproduced in a series of experiments.…”