Role of the velocity frame of reference in thermodiffusion in liquid mixtures

The role and significance of the velocity frame of reference in the interpretation, modeling and formulation of thermodiffusion in multicomponent liquid mixtures were investigated, focused on the nonequilibrium thermodynamics modeling approach. The effect of the velocity frame of reference on the phenomenological equations and thermodiffusion coefficients and expressions is explored. Theoretically and also by the aid of representative calculations, it is shown that while in binary mixtures transformation from one frame to another does not affect the sign and magnitude of the thermodiffusion coefficients, in multicomponent mixtures (ternary and higher), even the sign of the thermodiffusion coefficients may change when an alternative frame is used. This implies that in multicomponent mixtures for either experimental data or model estimations, the employed velocity frame for the thermodiffusion coefficients plays an important role in calculations. The Soret coefficients and the thermodiffusion factors are independent of the frame of reference.