I. Depolarized Light Scattering Studies of Rotational-Translational Coupling in Liquids Composed of Small Anisotropic Molecules. II. Investigation of the Coupling Between Reorientation and Longitudinal Modes in the Brillouin Spectra of Liquids Composed of Anisotropic Molecules

Citation

O'Steen, Byron Lance (1982) I. Depolarized Light Scattering Studies of Rotational-Translational Coupling in Liquids Composed of Small Anisotropic Molecules. II. Investigation of the Coupling Between Reorientation and Longitudinal Modes in the Brillouin Spectra of Liquids Composed of Anisotropic Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wda1-bj76. https://resolver.caltech.edu/CaltechETD:etd-09142006-110153

Abstract

Part I

In an attempt to better understand the molecular interactions governing the behaviour of the coupling parameter R measured in light scattering experiments, the depolarized (l_VH) spectra for a series of liquids composed of small aromatic molecules, very similar in size and shape, have been measured. The molecules studied here were generally monosubstituted benzene and pyridine derivatives. All were found to exhibit the doublet structure indicative of dynamic coupling between molecular reorientation and shear modes, or more simply, rotational-translational coupling. The degree of this coupling is measured by a parameter R (0 ≤ R ≤ 1) which is often thought of as the fraction of the shear viscosity attributable to reorientational motion. From the depolarized spectra the coupling parameter R, collective reorientation frequency, and low frequency shear viscosity were determined.

The values of R were found to vary from 0.24 to 0.55 for the liquids studied here. This range is nearly as broad as that observed in all previous studies, which have included such diverse molecules as carbon disulfide, tri-phenyl phosphite, and the highly anisotropic liquid crystal MBBA. This suggests that size and shape considerations, or steric forces, are not the primary factor in determining the degree of rotational-translational coupling as measured by light scattering. If this is indeed the case then other non-steric interactions must be producing the observed variation in R. With this in mind, we have examined possible electrostatic interactions. A simple correlation with dipole moment was not found to exist. Instead it appears that the variation in R can only be understood by consideration of the detailed molecular charge distribution. This is determined to a large extent by resonance interactions with the aromatic ring which are generally reflected in the change of dipole moment from the aliphatic compound (CH₃-R) to its aromatic analog (C₆H₅-R).

Based upon resonance structure arguments and our own experimental results, it is concluded that the anomolously large R values found for pyridine from the l_HH depolarized spectrum might well be correct and the more orthodox l_VH measurements in error. If this assertion is indeed correct, then the reliability of rotational-translational coupling measurements by l_VH depolarized light scattering must be considered questionable for weakly scattering molecules such as pyridine.

Part II

The coupling of reorientational motion to longitudinal modes was investigated by studying the Brillouin spectra of aniline and p-anisaldehyde over a wide range of scattering angles. The primary goal of these studies was to attempt to confirm the microscopic theory of orientational relaxation in the polarized spectrum. Theoretically it has been shown that for symmetric tops the coupling between longitudinal modes and reorientation in the polarized spectrum is identical to that between shear modes and reorientation in the depolarized spectrum. Thus the Brillouin scattering studies performed here also provide an internal check on the consistency of the 2-variable molecular theory for depolarized scattering and the corresponding theory of orientational relaxation in the Brillouin spectrum. Aniline and p-anisaldehyde were chosen for this investigation since their depolarized spectra at the temperature of interest are well described by the simple 2-variable molecular theory coupling shear modes to orientation. The collective reorientation frequency for both liquids could easily be varied in the low gigahertz range by operating at temperatures near the freezing point. Thus the effects of orientational relaxation in the propagation of longitudinal waves (1 - 10 GHz) should be observable in an angular study of the Brillouin spectra. Moderate viscosites and large rotational-translational couplings also made these liquids attractive from an experimental standpoint since these quantities determine the relaxation strength.

Our results for attenuation and velocity of the longitudinal waves demonstrate that there is indeed relaxation in the shear viscosity as predicted by theory. However, shear relaxation in addition to that due to reorientational motion is indicated by the k-dependence of the attenuation results. This stands as an apparent contradiction to the depolarized results which suggest that only orientational relaxation should be important under these conditions. Since the different relaxation effects cannot be unambiguously separated here, a detailed confirmation of orientational relaxation theory could not be obtained.

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