Continued discussion of the article by V.A. Shaposhnik “Electrostatic theory of ion exchanger selectivity”

Abstract

The discussion paper by R. Kh. Khamizov is devoted to the materials presented by Professor
V.A. Shaposhnik in the journal “Sorption and Chromatographic Processes” (2021, V.21, No 1). The article by R.Kh. Khamizov and A.M. Dolgonosov “Focus on the article by V.A. Shaposhnik “Electrostatic Theory of Selective Ion Exchangers”” considers approaches to the issue, including the one presented in the model by J. Pauley used to determine the equilibrium constants of ion exchange.

The model of G. Eisenman appears to be a more correct selectivity model, even though it only acts as a qualitative characteristic. The appeal of the Eisenman's model stems from the fact that it has not been adjusted. For his calculations Eisenman used table values of crystal-chemical radii assuming that there are no water molecules between a fixed ion and a counterion.

Critical remarks about V. A. Shaposhnik's approach to the theory of selectivity were made in connection with the fact that when a hypothesis is developed, it is necessary to use not only the first steps of your predecessors, but also their subsequent (or most recognised) achievements. Therefore, a disagreement was expressed with the position of V. A. Shaposhnik who chose to ignore the dehydration energy. A failure to take into account the energy of dehydration when considering the selectivity of ion exchangers leads to very serious errors. It should be noted that D. Reichenberg successfully used the qualitative model based on the energy of the Coulomb interaction to generate quantitative estimates by introducing the therms for the energies of short-range interactions, including dispersion interactions into the corresponding relations.

The author of this article believes that it is impractical to correct the calculations using fractional charges in the Coulomb model (which does not provide for delocalisation). Considering the applicability of Poisson's equation for ion exchangers, the author of this article recommends reading his works (in collaboration with N. A. Tikhonov), in which the authors solved very complex problems for strongly basic anion-exchangers and graphene-based hydrogels.