Discordance in Understanding of Isotope Solute Diffusion and Elements for Resolution

摘要

The literature is inconclusive on the modelling of isotopic co-diffusion, and notes the lack of a wide-enough experimental basis. Further, there is discordance amongst technical specialists on what diffusion is and what it engenders once molecules involving different isotopes of the same chemical element are involved. A dialogue is needed among chemists, physicists, and other specialists in order to come to a common view. This will require, ultimately, experimental work. In the field of radionuclide migration there is a view - captured in Fick's Law - according to which the individual molecules of any isotopic mix are to be considered as independent and uncorrelated with one another and that diffusion is simply a manifestation of Brownian motion (A-B interactions) and is a non-chemical phenomenon. Another view also exists that interactions amongst molecules of the same chemical solute (A-A interactions) are at the core of chemical behaviour - they are at the basis of such concepts as chemical potential, fugacity, chemical activity and solubility product - and these interactions should play a role also in diffusion of isotopic solutes. This contribution is codified, in mathematical terms, in models such as that expressed by the new Equation 3. It is suggested that the overall constitutive law for the diffusion of isotopic solutes should take account of both the A-B and A-A contributions. The proposed law, Equation 5, should help in the debate that is hoped will be opened on the topic of the diffusion of isotopic solutes and, eventually, in assessing the relevance of any correction for isotopic effects.