A comprehensive light scattering study of a liquid composed of symmetric top molecules

Abstract

We have studied the $\mathrm{VH,}$ $\mathrm{HH,}$ and $\mathrm{VV}$ quasielastic light scattering spectra of the near‐symmetric top triphenylphosphite at several scattering angles and over a wide range of temperatures, such that ${\mathrm{(k\hspace{0.17em}}}^2{\mathrm{\eta /\rho }}_m\mathrm{\Gamma )}$ varied from 0.085 to 3600, where k is the scattering vector, η the shear viscosity, ${\rho }_m$ the mass density, and Γ the ’’reorientation’’ frequency. The data have been analyzed in a consistent manner in terms of a generalized hydrodynamic theory in which, in addition to the observed dielectric fluctuations $\mathrm{(\epsilon ̂)}$ and the conserved hydrodynamic collective variables $\mathrm{(n̂,}\mathbf{p̂}\mathrm{ê,),}$ unspecified ’’slow’’ variables $({\mathfrak{F̂}}^s\mathrm{,\hspace{0.17em}}{\mathfrak{F̂}}^t)$ are included, where, at $\text{k→0,}$ ${\mathfrak{F̂}}^s$ is a symmetric and ${\mathfrak{F̂}}^a$ an antisymmetric second rank tensor. Since the theory interrelates the relaxation processes, giving rise to the scattering at different polarizations, the spectra provide enough information to evaluate the considerable number of adjustable temperature‐dependent, but ω‐ and k‐independent, transport coefficients involved in the analysis. Of particular interest is our conclusion that three‐variable theories ${\mathrm{|\epsilon ̂}}_{\mathrm{xz}},$ ${\mathrm{p̂}}_z\mathrm{,\hspace{0.17em}}{\mathfrak{F̂}}_{\mathrm{xz}}\mathrm{|,}$ where ${\mathfrak{F̂}}_{\mathrm{xz}}={\mathrm{F̂}}_{\mathrm{xz}}^s,$ ${\mathfrak{F̂}}_{\mathrm{xz}}^a$ or ${\mathfrak{F̂}}_{\mathrm{xz}}^s+{\mathfrak{F̂}}_{\mathrm{xz}}^a,$ are incapable of explaining the angular dependence of the $\mathrm{VH}$ spectra or of providing parameters which can also fit the central features of the $\mathrm{HH}$ spectra. In the successful application of the four‐variable ${\mathrm{|\epsilon ̂}}_{\mathrm{xz}}{\mathrm{,\hspace{0.17em}p̂}}_z\mathrm{,\hspace{0.17em},\hspace{0.17em}}{\mathfrak{F̂}}_{\mathrm{xz}}^a|$ theory to $\mathrm{VH}$ scattering (and correspondingly more complex theories to $\mathrm{HH}$ and $\mathrm{VV}$ scattering), we focused attention on the transport coefficient C, where ${\mathrm{(Ck}}^2{\mathrm{\eta /\rho }}_m)$ is the autodecay frequency of the curl of the flow momentum. At high temperatures, in the hydrodynamic regime, we found $\text{C≈0,}$ which enabled us to identify ${\mathfrak{F̂}}_{\mathrm{xz}}^s$ and ${\mathfrak{F̂}}_{\mathrm{xz}}^a$ with the symmetric ${\mathrm{(\sigma ̂}}_{\mathrm{xz}}^s)$ and antisymmetric ${\mathrm{(\sigma ̂}}_{\mathrm{xzs}}^s)$ components, respectively, of the stress tensor, i.e., with the flux of a conserved quantity. However, in the viscoelastic regime we found $\text{C≠0,}$ small, and varying as ${\eta }^{\text{−1}};$ in this case we were, therefore, not able to identify ${\mathfrak{F̂}}_{\mathrm{xz}}^s$ and

Keywords

• LIGHT SCATTERING
• LIQUIDS
• VISCOSITY
• TEMPERATURE DEPENDENCE
• ORGANIC PHOSPHORUS COMPOUNDS

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