Rayleigh light scattering from pure liquids and liquid mixtures

Abstract

This thesis presents measurements of the total and partial Rayleigh ratios, depolarization ratios, and associated auxiliary parameters for a number of pure liquids and liquid mixtures. The work may be conveniently summarized in three sections.

Pure Liquids

The total Rayleigh ratio and depolarization ratio were measured as a function of temperature for a number of normal alkanes and other liquids. The isotropic contribution to the total light scattered was calculated by means of the Einstein–Smoluchowski fluctuation theory. These data were then combined with depolarization ratios, calculated from measurements of the optical Kerr constant, to yield calculated values of the Rayleigh ratio that are completely independent of any light-scattering measurement. Comparison of these calculated values with experimental light-scattering data demonstrates the validity of the fluctuation theory of light scattering.

The temperature coefficient of the anisotropic contribution to the total scattered light was obtained by application of the well-known Cabannes relationship and was analysed in terms of the correlation of molecular orientations in the liquids studied.

Liquid Mixtures

The Gibbs excess function, GE, was obtained from light-scattering measurements for the binary mixtures cyclohexane + benzene, acetone + chloroform, and benzene + hexafluorobenzene. The values obtained were compared with those derived from vapour-pressure studies. It was found that light scattering yields GE values comparable to those obtained by the conventional vapour-pressure technique only when a statistical density–concentration cross term is included in the analysis. In general, however, the light-scattering method lacks the precision and accuracy of the vapour-pressure technique.

In addition, GE’s values were estimated from light-scattering data for the binary mixtures cyclohexane + hexafluorobenzene, cyclohexane + carbon disulphide, benzene + carbon disulphide, and hexafluorobenzene + carbon disulphide. For these latter systems, vapour-pressure data were not available; hence, the reliability of the light-scattering-derived GE values could not be critically assessed.
A method of analysing anisotropic scattering data from binary mixtures composed of optically anisotropic molecules was also developed. When applied to the mixtures listed above, this treatment yielded information concerning the orientational ordering brought about by the anisotropy of the intermolecular pair potential. The results obtained showed encouraging qualitative agreement with recent perturbation theory calculations.

Laser Photometer

The final phase of this work involved the construction of a laser Rayleigh scattering photometer with photon-counting detection. This apparatus allows direct determination of the anisotropic Rayleigh ratio of relatively isotropic species with an accuracy of a few per cent.

The photometer was utilized in a study of the compositional dependence of the anisotropic component of light scattered from a series of isomeric octanes dissolved in the pseudo-spherical solvent cyclohexane. The orientational correlation function obtained from light scattering showed no correlation with the excess thermodynamic properties of these mixtures.

Date of Award	Oct 1976
Original language	English