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

Abstract

We present the basic formulae for the analysis of IR dichroism data in terms of molecular order parameters of lamellar systems considering the special case of the Attenuated Total Reflection experiment on thick films. The infrared order parameter is expressed as the sum of five molecular order parameters which characterise the longitudinal and transverse ordering within the lamellae. The formulae are applied to polymethylene chains which are packed in a crystalline subcell. The information about transverse ordering is averaged out in the orthorhombic perpendicular packing mode of the hydrocarbon chains. The IR linear dichroism of vibrations of the methylene, methyl and carboxyl groups of polycrystalline stearic acid can be consistently explained by means of this formalism assuming a mixture of C- and E-type crystals. The mean tilt (35°) and azimuth (40°–50°) of the polymethylene chain axes were determined from the IR order parameters of the split methylene bending and rocking modes. Mixtures of protiated and deuterated stearic acid were investigated to vary the degree of intermolecular vibrational coupling, and thus, to modulate the influence of Fermi resonance on the dichroism of the CH 2 stretching bands. The disturbing effect of Fermi resonance is, however, small if the corresponding IR order parameters are determined from the absorbance which is integrated over the range of the band maximum. The dichroism of the C–O stretching vibration depends strongly on the degree of intramolecular vibrational coupling with the methylene wagging modes. The systematic error of neglecting birefringence has been estimated and discussed. A method is presented which checks the validity of Harrick's thick film approximation used for data analysis.