Mutual Diffusion Coefficient and Dynamic Viscosity Near the Critical Consolute Point Probed by Dynamic Light Scattering

The possibility of applying dynamic light scattering to simultaneous determination of the mutual diffusion coefficient and the viscosity of binary liquid systems was studied near the critical consolute point. When seed particles are added to the system, the particle diffusion coefficient is measured, and the viscosity is obtained using the Stokes-Einstein relation. Since the amplitude of light scattered from concentration fluctuations is low in a mixture with a small difference between the refractive indices of the pure components, this approach allowed the determination of the viscosity of a critical mixture of nitroethane and isooctane, without a signal component from mutual diffusivity superimposed. In contrast, particle aggregation preveInted the determination of the viscosity of a critical mixture of triethylamine and water. Despite this difficulty, and an unidentified contribution in the signals obtained, the mutual diffusion coefficient and the critical exponent v could be determined in this system without a noticeable influence from the addition of seed particles.