LIGHT SCATTERING FROM BINARY SOLUTIONS

The spectrum of the light scattered by a binary solution is calculated from thermodynamic fluctuation theory and the linearized hydrodynamic equations appropriate to a two-component fluid. The spectrum consists of three peaks. Expressions are obtained for the positions and widths of the two-side, Brillouin peaks. In general the central, unshifted Rayleigh peak is found to consist of a superposition of two Lorentzians that involve the combined dynamical effects of heat conduction and diffusion. The condition is stated under which it is possible to separate the central peak simply into two contributions, one arising from diffusion and one from thermal conduction. For many binary systems this separation is justified. In these cases measurement of the spectrum of the scattered light should prove to be an attractive alternative means of measuring the diffusion coefficient of binary solutions.