Study of “Non-Fickian” Diffusion Anomalies through Time Lags. IV. More Complex Cases of Axial Distance-Dependent Anomalies

The time‐lag treatment of Paper II of this series is here extended by considering systems in which the thermodynamic diffusion ${\mathrm{(D}}_T)$ and solubility $\mathrm{(S)}$ coefficients depend both on activity $\mathrm{(a)}$ and distance in the direction of flow $\mathrm{(X)}$ in such a way that the variables $\mathrm{a,\; X}$ are not separable. The properties of the steady‐state permeation flux and of the time‐lag quantities defined in Papers I and II of this series are examined by numerical solution of the diffusion equation in a few carefully selected cases and compared with the results obtained in Paper II. It is shown that considerable information can be obtained from such permeation measurements about the nature of the diffusion system, including the question of separability of $\mathrm{a,\; X}\mathrm{in}{\mathrm{S(a,\; X),\; D}}_T\mathrm{(a,\; X)}$ .