Observation and analysis of the depolarized Rayleigh doublet in isotropic MBBA and measurement of the de Gennes viscosity coefficients

The doublet in the depolarized (IVH) Rayleigh light scattering spectrum was studied in the isotropic phase of p-methoxybenzylidene- (p-n-butyl) aniline, (MBBA). All spectra were found to fit well to the Andersen–Pecora theory for the IVH spectrum in viscous liquids. The value of the rotational–translational coupling parameter R was determined to be 0.36±0.02 for MBBA, independent of temperature over the range 150–230 °C. A direct correspondence is made between the parameters of the Andersen–Pecora theory and the phenomenological theory of de Gennes. From this correspondence we obtain a microscopic equation for the de Gennes ’’a’’ parameter. Using this microscopic evaluation of the de Gennes parameter ’’a’’, we evaluate the de Gennes viscosity parameter ν for MBBA. Using the measured value of R and the values of ν and η we obtain values for the de Gennes viscosity parameter μ for MBBA. We show the value of ν for MBBA can be easily calculated from the slip model of rotational diffusion to good accuracy. Since we know R, the value of μ can also be obtained by the calculation. Finally values of μ are calculated for several liquids for which R is known and the relationship between μ and η is examined.