Infrared dichroism investigations on the acyl chain ordering in lamellar structures: I. Order parameter formalism and its application to polycrystalline stearic acid

We present the basic formulae for the analysis of IR dichroism data in term s of molecular order parameters of lamellar systems considering the special case of the Attenuated Total Reflection experiment on thick films. The inf rared order parameter is expressed as the sum of five molecular order param eters which characterise the longitudinal and transverse ordering within th e lamellae. The formulae are applied to polymethylene chains which are pack ed in a crystalline subcell. The information about transverse ordering is a veraged out in the orthorhombic perpendicular packing mode of the hydrocarb on chains. The IR linear dichroism of vibrations of the methylene, methyl a nd carboxyl groups of polycrystalline stearic acid can be consistently expl ained by means of this formalism assuming a mixture of C- and E-type crysta ls. The mean tilt (35 degrees) and azimuth (40 degrees-50 degrees) of the p olymethylene chain axes were determined from the IR order parameters of the split methylene bending and rocking modes. Mixtures of protiated and deute rated stearic acid were investigated to vary the degree of intermolecular v ibrational coupling, and thus, to modulate the influence of Fermi resonance on the dichroism of the CH, stretching bands. The disturbing effect of Fer mi resonance is, however, small if the corresponding IR order parameters ar e determined from the absorbance which is integrated over the range of the band maximum. The dichroism of the C-O stretching vibration depends strongl y on the degree of intramolecular vibrational coupling with the methylene w agging modes. The systematic error of neglecting birefringence has been est imated and discussed. A method is presented which checks the validity of Ha rrick's thick film approximation used for data analysis. (C) 1999 Elsevier Science B.V. All rights reserved.