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 determ ination of the mutual diffusion coefficient and the viscosity of binary liq uid systems was studied near the critical consolute point. When seed partic les are added to the system, the particle diffusion coefficient is measured , and the viscosity is obtained using the Stokes-Einstein relation. Since t he 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 f rom mutual diffusivity superimposed. In contrast, particle aggregation prev ented the determination of the viscosity of a critical mixture of triethyla mine and water. Despite this difficulty, and an unidentified contribution i n the signals obtained, the mutual diffusion coefficient and the critical e xponent nu could be determined in this system without a noticeable influenc e from the addition of seed particles.