Effect of a Solute on Critical Opalescence in a Binary Liquid Mixture

The fluctuations in refractive index that occur near the critical point of a binary liquid mixture with a miscibility gap are the source of Rayleigh scattering of the incident light, resulting in a phenomenon known as “critical opalescence”. The refractive index oscillations are caused by fluctuations in the composition of the mixture, which are exaggerated at the critical point due to the mathematical divergence of the thermodynamic derivative that determines mixing. According to the Principle of Critical Point Universality, which is thought to govern all critical phenomena, this derivative is expected to diverge under conditions where the liquid mixture is at equilibrium at a fixed temperature, fixed pressure, and at the fixed composition characteristic of the critical point. The addition of a completely soluble solute to the mixture has the effect of introducing a second fixed composition variable. As predicted by the universality principle, the thermodynamic derivative governing mixing remains finite under such conditions. This finite behavior has the effect of quenching the critical opalescence. We have successfully tested this principle by observing the quenching of critical opalescence in the case of a variety of binary liquid mixtures to which various soluble solutes have been added. Despite the chemical diversity of the substances chosen, we have observed the suppression of critical opalescence without any exceptions.