Analysis of the application of optical two-wavelength techniques to measurement of the Soret coefficients in ternary mixtures

Abstract

We discuss the application of two-wavelength optical methods for the determination of Soret coefficient in ternary mixtures. Though the principle of such an approach was formulated in the literature earlier, we have found a significant obstacle to its implementation and suggested a way to circumvent that obstacle. The main target of our approach is the analysis of the compatibility of the matrices of contrast factors (the derivatives of refractive indices with respect to concentrations) with available light sources used for measuring the Soret effect in ternary mixtures. We report on the development of a simple mathematical approach to verify the applicability of the chosen wavelengths for laser diodes to measure transport coefficients for a certain class of ternary mixtures. The approach has been applied to a number of aqueous ternary mixtures: water–ethanol–k-ethyleneglycol, k = mono, di, tri, and water–t-butanol–DMSO) and light sources in the visible λ₁ = 670 nm and infrared λ₂ = 925 nm spectrum. The regions of feasibility/infeasibility of the methods have been revealed for these mixtures.

Keywords:

• diffusion
• Soret effect
• refractive index
• condition number

Acknowledgements

This work is supported by the PRODEX programme of the Belgian Federal Science Policy Office and ESA. The authors are indebted to V. Yasnou (ULB, Brussels) for help in conducting the experiments and Dr. A. Mialdun (ULB, Brussels) for useful discussions.