Static and dynamic light scattering measurements near the critical solution point of a ternary liquid mixture

Abstract

In the present paper we report results of static and dynamic light scattering measurements in the ternary liquid mixture glycerol–acetone–water (GAW) in the vicinity of its critical solution point. We determine the correlation length, osmotic susceptibility and mass diffusion coefficient over a temperature range from 290 to 306 K. Close to the critical solution point the experimental data can be well described by simple power laws with three-dimensional effective critical exponents for all three compositions of the system. The critical exponents ν_x, γ_x and ν*_x are obtained from the angular distribution of light scattering intensity, measured for three different compositions and over the temperature range near the liquid–liquid critical point. We obtain values of 0.71, 1.41 and 0.81, respectively. In the vicinity of the critical solution point the dynamic light scattering measurements in our system reveal two hydrodynamic relaxation modes with well-separated characteristic relaxation times. From the autocorrelation functions we can experimentally determine two effective diffusivities D₁ and D₂. As theoretically predicted by Anisimov et al. one of these two modes can be associated with mass diffusion and the other with thermal diffusion. In the special case of an incompressible liquid-mixture limit, D₁ and D₂ are decoupled, becoming the mutual diffusion coefficient D₁₂ and thermodiffusion coefficient D_T, respectively. Both the slow and fast mode have been measured as a function of temperature for all three investigated compositions. A possible physical meaning of D₁ and D₂ for a ternary mixture is discussed.