Mass diffusion: is $D_{AB} \neq D_{BA}$ at high pressures? If so, why?

Question

From [Takahashi][2], I validated some models I have for estimating diffusion coefficients (methods of Fuller modified by Riazi for high-pressures, all pulled from [Poling][3]). Here is the figure mixing experimental data and the models I was looking at.

The experimental data was measuring diffusion coefficients between CO₂ and C₂H₄ over a range of pressure. As expected, at low pressures, does not matter if we are looking at how a trace of CO₂ diffuses into C₂H₄ or if a trace of C₂H₄ diffuses in CO₂, the mass is being diffused at the same speed and thus the diffusion coefficients are the same. However, at high pressures, it is not the case. I've seen complex mathematical expressions modeling it however, and clearly, even my crude model picks up at least the trend, but I'm not sure I understand exactly why it happens. Is it the diffusion matrix that becomes non-symmetric? Or the diffusion process can no longer be modeled just by this diffusion coefficient and other terms appear? Either way, from a molecular point of view, what explains this change of behavior?

[2]: S. Takahashi and M. Hongo. Diffusion coefficients of gases at high pressures in the CO2-C2H4 system. Journal of Chemical Engineering Japan, 15:57–59, 1982.

[3]: B. E. Poling, J. M. Prausnitz, and J. P. O’Connell. The Properties of Gases and Liquids, Fifth Edition. McGraw-Hill, 2001.

Answer 1

If I were to hazard a guess, I would say that it's due to the intermolecular forces. The London dispersion force should increase with density (and hence pressure) making the gas molecules more attracted to each other.

Now, C₂H₄ is lighter than CO₂ and thus has less intermolecular force. So, when you have just a trace of CO₂ in C₂H₄ at high pressure, it should diffuse faster (less intermolecular force in the "bulk" gas) than when you have a trace of C₂H₄ in CO₂ (higher intermolecular force in the "bulk" gas). This is reflected by the higher CO₂-C₂H₄ coefficients.