Light scattering in liquid mixtures as a test of the critical point universality principle

The principle of critical point universality is thought to govern critical phenomena in systems as disparate as ferromagnets, super-fluids, superconductors, and binary liquid mixtures exhibiting a critical point of solution. Among these, the binary mixtures have solvent properties that can be exploited in order to search for critical effects in physicochemical systems, which have so far included solubility, adsorption, and ion exchange. In addition to these effects, strong light scattering, known as critical opalescence, can be observed at the critical point of solution. On the microscopic scale, light scattering has its origin in refractive index distortions caused by fluctuations in composition having spatial dimensions of the order of the wavelength of light. Using a diverse selection of mixtures and solutes, we show that turbidity expected in a binary mixture is quenched upon the addition of a completely soluble third component. We suggest that the third component serves to limit the spatial extent of the fluctuations. By combining statistical thermodynamics with an advanced version of the Gibbs phase rule, we show that this interpretation is completely consistent with the universality principle. We fit the temperature dependence of the dimensionless transmitted intensity (turbidity) to a rigorously derived expression having two adjustable scaling parameters and a critical exponent set equal to the Ising model value of 1.241.