In nonideal systems an effective Fickian diffusivity cannot be calculated, and some values of the Fickian ternary diffusivities can be negative. The extension of the Maxwell-Stefan equations to nonideal ternary systems is straightforward. Equations (15-54) were the most general form of the Maxwell-Stefan equations for binary systems. For ternary systems these equations become
Expanding the chemical potential μ in terms of activity coefficients, these equations become
If we note ...