Investigation of the Molecular Organization in Langmuir−Blodgett Films Using Polarized Infrared Spectra:  Comparison of Two Methods

Abstract

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.

Second-Harmonic Probe of Pressure-Induced Structure Ordering within Long-Chain Fluorescein Monolayer at the Air/Water Interface

“…The dye molecules are indeed forced into some anisotropic configurations with certain symmetry of packing. For simplicity, the monolayer at the onset of the phase transition could be treated as a set of nearly randomly oriented domains, each of which is represented by a regular array of chromophores with an identical polarizability and each situated at an identical site according to C 2 v packing symmetry. All of the transition dipole moments within a domain are oriented in the same direction: the θ , φ direction, where θ is the tilt angle of the transition moment from the interface normal and φ is the azimuthal angle.…”

“…A change in the slope of the π−A isotherm reveals the phase transition, but the diagram does not directly yield microscopic information. The in-plane organization of the monolayer was often considered totally isotropic until unequivocal evidence showed that some molecules take a specific in-plane orientation based on polarized linear and nonlinear spectroscopy. − The observed in-plane orientation was attributed to anisotropic interactions induced and directed by compression, causing these molecules to present an anisotropic orientation within the plane of interface. ,, Therefore, even though anisotropic orientations have been found in various cases, little is known concerning the mechanism through which in-plane anisotropic interactions achieve ordering.…”

Molecular Organization of LB Multilayers of Phospholipid and Mixed Phospholipid/Viologen By FTIR Spectroscopy

“…The high sensitivity, together with the nondestructive character of this tool, allows the analysis of both of the groups forming the molecules and their orientation within the monolayer. Also, important information on the interactions of the components of the monolayer can be obtained, and monolayer-substrate as well. ,, The transmission, − reflection−absorption (RA), − and attenuated total reflection (ATR), − with and without polarized light, methods point out the modern available techniques in order to determine the molecular orientation of LB films, although a combination of them is often used. − …”

“…Also, important information on the interactions of the components of the monolayer can be obtained, and monolayer-substrate as well. 2,7,8 The transmission, [9][10][11][12][13] reflection-absorption (RA), [14][15][16] and attenuated total reflection (ATR), [17][18][19][20][21][22] with and without polarized light, methods point out the modern available techniques in order to determine the molecular orientation of LB films, although a combination of them is often used. [23][24][25][26][27][28][29][30][31][32] In previous papers, 32,33 mixed stable monolayers of Ntetradecyl-N′-methyl viologen (TDMV) and L-R-dimyristoylphosphatidic acid (DMPA) in a molar ratio 1:2, were formed at the air-water interface, and subsequently one mixed monolayer was transferred onto a solid support and characterized by cyclic voltammetry 33 and FTIR spectroscopy.…”

Evanescent Electric Field Amplitudes in Thin Lipid Films for Internal Reflection Infrared Spectroscopy

“…Between the germanium support and the membrane, there is likely to be a much thicker layer of germanium oxide with its own distinct refractive index. One should not be surprised that the membrane itself, being highly ordered and inhomogeneous with respect to its optical properties, does not exhibit the optical properties predicted for a homogeneous and isotropic phase. For films with these dimensions and this complexity, it is essential that theory be tested against an unambiguous reference standard for molecular orientation, and it should not be surprising to find a discrepancy between reference standard and theory.…”