Opalescence and concentration fluctuations in binary liquid mixtures near the critical mixing point. - II. Theoretical

A theory of light scattering in fluids produced by local fluctuations of refractive index is developed from the theory of X-ray scattering in fluids which is applied to the phenomena of critical opalescence in one-component liquids due to density fluctuations and in two-component liquid mixtures due to concentration fluctuations. The dependence of the scattered light intensity on wave-length and scattering angle can be derived from the theoretical expressions when the correlation of the fluctuations in two volume elements as a function of the distance of these elements is known. The theory is used for determining the shape of the correlation function for a number of binary mixtures from the measured angular distribution of the scattered light intensity, as described in part I by means of numerical computation of Fourier integrals. It is found that the mixtures belonging to class I (as defined in part I) are characterized by a correlation function with a range of negative correlation, and those belonging to class II by a correlation function of nearly Gaussian shape. The absolute magnitude of the concentration fluctuations near the critical mixing point is also calculated from the observational material. A tentative qualitative explanation for the different shapes of the correlation function for the two classes of mixtures is presented.