An investigation of the motion of liquid short chain molecules using Rayleigh-Brillouin spectroscopy
Dynamic laser light scattering spectroscopy is used to study
the motion of short chain flexible molecules in the liquid state.
Depolarised Rayleigh-Brillouin scattering is used to investigate
the isomers of the homologous alkane series n-pentane to n-octane. In
addition to the low frequency Lorentzian component, associated with
molecular reorientation, a second Lorentzian component is found in the
spectra from the liquids 2,2,4-trimethylpentane, 2-methylhexane,
3-methylhexane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,2,3-
trimethylbutane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane,
n-pentane and 2-methylbutane. The contribution to the spectra of this
second Lorentzian, which is broader than the Lorentzian associated with
molecular reorientation and distinct from the interaction induced
component, reduces as the molecules become more anisotropic and flexible.
The origin of this component is associated with local translational order.
The correlation times obtained from molecular reorientation are
andlysed in terms of a modified Stokes-Einstein-Debye relation. The
Stoke~;-;':instein-Debye volumes obtained are interpreted as the quantity
(g2/J2)apv, where (g2/J2) is associated with molecular correlation, P
and a with molecular shape and V is the molecular volume.
Values of a, the stick-slip coefficient, obtained for the nalkanes
indicate that the average molecular conformation is approximately
midway between a sphere and an extended chain. For all the isomers a is
feound to dUCt'(_~ii~;(; de; the molecule be cornea mor-e rigid and more spherical.
The liquid 2,2-dimethylbutane is analysed in terms of the theory
of Madden [Mol Phys, ~, 365 (1978)J; dipole-induced dipole interactions
between pairs of density fluctuations cause intermolecular optical anisotropy.
The results indicate that molecular reorientation may not be the
predominant relaxation mechanism.
Oleic acid was investigated using polarised Rayleigh-Brillouin
spectroscopy. The temperature gradient of the hypersonic frequency
shift and elastic modulus show anomalous behaviour in the range 14 - 18°C.
This suggests that oleic acid undergoes a change of phase to form a
locally nematic structure about 5 °C above the melting point. Complementary
measurements of viscosity, density and refractive index support