Comparison of ternary diffusion coefficients obtained from dynamic light scattering and Taylor dispersion

Abstract

Over the last decade dynamic light scattering (DLS) has developed into a powerful technique for the determination of transport properties in liquids and liquid mixtures. Its advantague is based on the fact that microscopic fluctuations in concentration and temperature are used to characterize the transport behavior and no external macroscopic gradients are needed. All classical methods of Fick's diffusion coefficient determination, like Taylor dispersion (TD) or interferometric methods, however, use macroscopic concentration gradients and give a diffusion coefficient matrix.In the present paper we report results of diffusion coefficient measurements in ternary liquid mixtures of glycerol(0)–acetone(1)–water(2) at 298.15K. Taylor dispersion technique is used to determine the mutual diffusion coefficients Dij along a path of constant mole fraction of water x2=0.4200 from the binary edge towards the critical solution point in the ternary mixture. These results are compared with dynamic light scattering data in the vicinity of the critical solution point and along the very same path of constant x2. The aim of the comparison is to identify the physical character of the mass diffusion mode Dm obtained in our previous DLS measurements. We found that in this ternary system the diffusion mode Dm is not related to any of the diffusion coefficients Dij but we can clearly show that it coincides with the lowest eigenvalue of Fick's diffusion coefficient matrix. There are theoretical arguments which, at least in the vicinity of the critical solution point, lead to the conjection that this behavior might be general for ternary systems with molecules of similar size.