Focus on a discussion about the nature of selectivity of strong ion exchangers (A response to the commentary by V. A. Shaposhnik published in the journal “Sorption and Chromatographic Processes”, No 2, 2021)

Abstract

The article continues the discussion about the theory of selectivity of strong ion exchangers proposed by Professor V. A. Shaposhnik. The application of models using fractional charges lacks any physical meaning. A high quality physical theory should operate with integer values in relation to the elementary charge and the volatility of the values of the energy of Coulomb interaction can be explained by different distances between charges, screening effects, etc.

The article presents mathematical conclusions regarding the transport of ions in a dilute solution under the influence of a weak electric field. These conclusions are based on the Nernst-Einstein law which relates the speed of ions to their diffusion coefficients and temperature. According to the well-known model of counterion jumps between fixed charges during the motion in the ion exchanger phase the internal diffusion coefficients follow the rule which expresses them in terms of distribution coefficients and coefficients of ion diffusion in a liquid with good accuracy. The latter strongly depend on temperature, which must be taken into account in the calculations. The distribution coefficients are related to the excess Gibbs energy during sorption, which corresponds to the energy of the jump activation.

We compared the data obtained by us and V. A. Shaposhnik regarding the estimation of the contribution of the Coulomb interaction between fixed charges of ion exchangers and counterions to the sorption energy. The results showed (according to Kitaygorodsky) the effectiveness of the author's approach to the considered issue.