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Bagshaw, John Martin (1983) An experimental investigation of the motion of flexible molecules in the liquid state by light scattering. Doctoral thesis, City of London Polytechnic.
Depolarised light scattering is used to investigate the notion of flexible molecules in the liquid state. The spectra obtained are separated into a high frequency component associated with collisional motion, and a low frequency component associated with reorientation.
The high frequency component is separated from spectra for the n-alkanes. The time scale and collision induced anisotropy is such that this component is interpreted as being due to interactions between chain ends.
The low frequency component is examined for n-alkanes, both pure liquids and solutions in CC1(4), the n-alcohols, both pure liquids and solutions in CC1(4), the n-alcohols, and the isomer 224-trimethyl pentane. For the low viscosity liquids this component is Lorentzian. For the higher viscosity liquids the spectrum is modified by shear rotational coupling and exhibits a dip, the depth of which is related to the coupling parameter R. From spectral analysis one obtains molecular reorientation times and in addition for the higher viscosity liquids values of R.
The values of R are a weakly increasing function of chain length and almost independent of temperature. For pentadecane solutions in CC1(4), R is almost linear with concentration, in agreement with theory and observations for rigid molecules.
The reorientation times are interpreted using the Stokes Einstein relation. The Stokes Einstein volumes are interpreted as the quantity GapV where V is the molecular volume, alpha and p are associated with molecular shape and hydrodynamics and G with orientational correlation.
An attempt is made to separate Gop by solution measurements. The results indicate that a simple separation may not be possible for flexible molecules, G, alpha and p being independent functions of concentration and temperature.
For the shorter alkanes the results suggest that the more globular the molecule, the more it rotates within its cavity in the liquid, interacting less with surroundings, than hydrodynamics would suggest. This is substantially confirmed by the Stokes Einstein volume obtained for 224-trimethyl pentane.
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