ONSAGERS RECIPROCAL RELATIONSHIPS WITH PHENOMENOLOGICAL COEFFICIENTS AND DIFFUSIVITY DEPENDING ON INTENSIVE PROPERTIES OF SYSTEM

It is demonstrated that application of Onsager's reciprocal relations, with coefficients dependent on the intensive characteristics of the system, to ordinary diffusion and to thermal diffusion still in the range of linearity of the phenomenological relations, enables diffusivity of the components to be expressed as a sum of two terms, one purely thermodynamic and the other also containing the hydrodynamic characteristics of the system. Combination of the two factors is explicit and very simple in the case of isothermal diffusion of dilute polymer solutions. When the thermodynamic factor is predominant there is a linear law relating diffusivity with concentration, coinciding with the equation confirmed experimentally in various polymer systems. In the limit, when the hydrodynamic factor reaches zero, Wagner's equation for the diffusion of solid PbCl in AgCl is obtained. © 1968, American Institute of Physics. All rights reserved.