SOME PHENOMENOLOGICAL AND THERMODYNAMIC ASPECTS OF DIFFUSION IN MULTICOMPONENT SYSTEMS

During the last twenty years diffusion coefficients have been primarily measured by light scattering and NMR techniques. Optical interferometric techniques, such as those of Gouy or Rayleigh (ref. 1), allowing direct observation of the time evolution of a diffusing boundary are not very popular at present. However, they are the only ones that give a reasonably accurate measurement of the set of (n-1)2 diffusion coefficients describing the brownian transport process in a multicomponent system. Experimental data on a variety of ternary systems indicate some aspects of diffusion in multicomponent systems: (a) The thermodynamic stability conditions: (i) D11+D22 > 0 and (ii) D11D22 - D12D21 greater-than-or-equal-to 0 have been verified experimentally and the relevant contribution of cross terms, which cannot be ignored in describing the transport process, has been pointed out. Furthermore, it was also experimentally verified that on approaching a critical mixing point the determinant (ii) approaches zero. (b) The main terms need not be necessarily positive; one of them may be negative. (c) The presence of a binding equilibrium between solutes 1 and 2 affects the experimentally measured values of the four diffusion coefficients. The equilibrium constant calculated from the experimental Dij's leads to values in very good agreement with those provided by direct thermodynamic techniques. The binding equilibrium promotes conditions leading to the transport of one component against its own concentration gradient, or its own chemical potential gradient (passive transport). (d) Diffusion measurements in three component systems provide a quantitative verification of the effect of the fluid-dynamics equations on the gravitational stability of diffusion boundaries or double diffusive convection, which is a convective transport process of great interest in several fields of pure and applied science.