Light-scattering in binary liquid mixtures.I. isotropic scattering

The isotropic light scattering in liquid mixtures of small nonelectrolyte molecules was studied from both the theoretical and the experimental point of view. The necessity of the cross-term in the isotropic Rayleigh ratio resulting from the correlation of fluctuations in density and concentration is discussed. A new expression for this term is derived and compared with the previously derived expression. Isotropic scattering of various pure liquids and liquid mixtures is determined at the wavelengths 546 and 436 nm and the reliability of experimental data is discussed in great detail. Mixtures of cyclohexane, n-heptane and carbon tetrachloride (component 1) with benzene and carbon disulfide (component 2) were examined. The possibility of the determination of molecular weights from light scattering experiments on such systems is discussed. The reliability of the excess free energy of mixing data obtained by light scattering is carefully considered. It is concluded that light scattering phenomena can be explained by thermodynamics, but the light scattering method is generally inferior to vapour pressure methods both in sensitivity and accuracy, and because of the complexity of the evaluation of data.