On the validity of the Onsager reciprocal relations in multi-component diffusion

In this work the validity of the Onsager reciprocal relations (ORR) for the multi-component diffusion case in the presence of external forces is re-examined. It is shown that the ORR are necessarily fulfilled for this process even if the quasi-equilibrium postulate (Gibbs–Duhem theorem) is applied. Moreover, it is shown that the phenomenological coefficients for this case are uniquely defined. It is believed that this work could contribute to the further understanding of the Onsager reciprocal relations.