Evidence of molecular interactions in liquids from anisotropic rayleigh light scattering

Abstract

Revision of methods currently in use for measuring depolarization factors and Rayleigh ratios for scattering of light by liquids includes use of a helium-neon cw laser as the source of incident light, a fused silica cell with dielectric reflecting coating for double-pass illumination of the sample, and clarification of samples by filtration through silver membrane or microporous porcelain discs of approximately 0.2 micron pore size. Values of the molar Rayleigh ratio for λ = 6328 Å, R TOTAL m (cm 2 mole −1 × 10 4) referred to the absolute value determined by D. J. Coumou for benzene at λ = 5460 Å, and values of D u, the depolarization factor for unpolarized incident light, were determined for nine liquids at 25° ± 2°C, as follows: acetone: 1.66, 0.163; acetonitrile: 1.40, 0.313; benzene: 7.66, 0.423; carbon disulfide: 27.49, 0.651; carbon tetrachloride: 2.91, 0.042; chloroform: 3.11, 0.215; cyclohexane: 2.76, 0.047; nitrobenzene: 35.22, 0.753; nitromethane: 1.40, 0.394. Separation of the total Rayleigh ratio into isotropic and anisotropic parts is accomplished by use of the Cabannes factor. While the fluctuation theory of Einstein and Smoluchowski gives an exact theoretical expression for liquid isotropic scattering, there is no generally accepted expression yet developed which, utilizing the principal polarizability axes as measured for the free molecule, can give an equally satisfactory account of the isotropic and anisotropic light scattering. An expression for the effective electric field at the scattering molecule, as afforded by the Onsager-Scholte model, yields corrections to give the expected values for the square of the mean polarizability α ̄ OS 2 and the anisotropy of the polarizability β OS 2. These are compared with experimental values from measurements on pure liquids and solutions in carbon tetrachloride.