Quantum chemical modeling of the interaction of Ca2+ and Ni2+ ions with carboxyl groups of cation exchanger

Abstract

The interaction of calcium and nickel cations with acetate, succinate and glutarate anions and different numbers of water molecules was studied. This study is related to previously established facts of unusual behavior of polyacrylic and polymethacrylic ion exchangers, such as achieving different stationary volumes of grains in the same solutions along different trajectories and manifestation of grain plasticity in solutions of salts of divalent ions. The results of quantum-chemical modeling showed that the energy-optimized structure of the Ca(CH3COO)₂·(H₂O)₄ complex is similar to the structure of the Ni(CH₃COO)₂·(H₂O)₄ complex. Both cations are in an octahedral environment of 6 oxygen atoms, two of which are oxygen atoms of carboxyl groups, two are oxygen atoms of water molecules not bound to acetate ions, and two are oxygen atoms of water molecules that form bridges between the cation and the oxygen atoms of the carboxyl groups. However, the distances between the calcium cation and the oxygen atoms in the Ca(CH₃COO)₂·(H₂O)₄ complex are significantly greater than the corresponding distances in the Ni(CH₃COO)₂·(H₂O)₄ complex. The addition of subsequent water molecules to the acetate complexes results in the displacement of the oxygen atoms of the carboxyl groups into the second coordination sphere in the case of the calcium complex, but in the nickel ion complex, the direct coordination of the oxygen atoms of the carboxyl groups to the cation is maintained. In the case of succinate and glutarate anions containing two carboxyl groups separated by two and three methylene groups, respectively, their carboxyl groups retain direct bonds with the central nickel and calcium cations through one of the two oxygen atoms even in the presence of 8 water molecules; the coordination numbers of 6 are retained for both central cations. The interatomic distances Ni–O and Ca–O in all the studied complexes remained practically the same as in the acetate complexes. The nickel ion forms stronger similar complexes with the acetate, succinate and glutarate anions compared to the calcium ion, which is consistent with the data on the stronger binding of nickel ions to the polymethacrylic cation exchanger. The modeling results allow us to assume that the interaction of the carboxyl groups of the polymethacrylic cation exchanger with the nickel and calcium counterions can also occur through direct coordination of the oxygen atoms of these groups to the counterion. Such a strong binding of polyelectrolyte chains by coordination bonds with nickel counterions leads to the formation of a rigid structure with a reduced swelling degree, which is not capable of quickly rearranging the conformational state in response to a change in external conditions or the imposition (or removal) of a load. At the same time, the carboxyl groups fixed on the polymer chain of the ion exchanger are conformationally in a more difficult position than in the anions studied. Therefore, it can be assumed that for some of the carboxyl groups of the polymethacrylic cation exchanger, direct coordination to metal cations is difficult. In the case of a more weakly binding calcium cation, the interaction occurs through water molecules. Therefore, a change in external conditions, such as a change in the concentration of a solution, a change in temperature, or the imposition (or removal) of a load, primarily causes changes in the number and mutual arrangement of rapidly migrating water molecules.