Diffusion and Soret in Ternary Mixtures. Preparation of the DCMIX2 Experiment on the ISS

We report on ground-based studies in course of preparation of the experiment DCMIX₂ (Diffusion coefficient in mixtures) to be performed on the International Space Station (ISS). In microgravity experiment the diffusion and thermodiffusion coefficients will be measured at six points with different compositions of the ternary mixture Toluene-Methanol-Cyclohexane. This mixture attracts attention of the researchers, as it has a miscibility gap and presumably large region with negative Soret coefficients. By using Optical Digital Interferometry we have measured the variations of refractive index Δn in the full parameter space of concentrations. The regions of stable and unstable behavior of system were determined from the time-dependent behavior of Δn. The system is hydrodynamically unstable in a wide region of compositions where Soret measurement should be done in microgravity. We present the results of the study of the compatibility of this mixture with often used sealing materials:Viton (R), Chemraz (R) and different types of Kalrez. To facilitate the expected theoretical studies we have measured density and thermal expansion in the points of the interest.