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 analysed in terms of a modified Stokes-Einstein-Debye relation. The Stokes-Einstein-Debye volumes obtained are interpreted as the quantity (g2/J2)apv, where (g2/J2) is associated with molecular correlation, P and α with molecular shape and V is the molecular volume. Values of α, the stick-slip coefficient, obtained for the n-alkanes indicate that the average molecular conformation is approximately midway between a sphere and an extended chain. For all the isomers α is found to decrease as the molecule be cornea more rigid and more spherical. The liquid 2,2-dimethylbutane is analysed in terms of the theory of Madden [Mol Phys, 36, 365 (1978)]; 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 this interpretation.
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