QUANTUM CHEMICAL EVIDENCE OF A FUNDAMENTAL DIFFERENCE BETWEEN HYDRATIONS AND ION EXCHANGE SELECTIVITIES OF SODIUM AND POTASSIUM IONS ON CARBOXYLIC AND SULFONIC ACID CATION EXCHANGERS

Abstract

Structural and electronic characteristics of molecular models of swollen polyacrylic and sulfostyrene-type cation exchangers in sodium and potassium ionic forms are calculated within the non-empirical HF/basis set method using the MINI Huzinaga basis set and the Firefly program. The obtained data are compared with those for water clusters of a comparable size. The molecular models include four and two monomeric units of corresponding polymers with ten water molecules per functional group; the water clusters contain 20 and 40 water molecules. The following characteristics of the model structure are obtained: the number and the types of the nearest neighbors of cations and water molecules; the distances between the interacting ions and the atoms; the orders of interionic and ion-molecular bonds; binding energy distribution of water molecules in the systems; number and length distributions of hydrogen bonds; the number and character of $$\text{RCO}_{2}^{-}\cdots {{\text{I}}^{+}},\ \text{RSO}_{3}^{-}\cdots {{\text{I}}^{+}}\ \text{and}\ {{\text{H}}_{\text{2}}}\text{O}\cdots {{\text{I}}^{+}}$$ bonds. The lengths of hydrogen bonds and the total number of intermolecular and ion-molecular bonds per water molecule in water clusters are established to be similar to those in Na+ and K+ molecular models of ion exchangers. The first coordination layer around both ions is filled with oxygen atoms of water molecules and fixed anions. The coordination numbers of Na+ and K+ are almost equal to those in aqueous solutions. The calculations indicate that direct interaction of Na+ and K+ with the anionic groups is responsible for the difference between their ion exchange selectivities on carboxylic and sulfostyrene ion exchangers. The Na+ ion forms direct largely covalent chemical bonds with the fixed carboxylate anion. The K+ ion forms long electrostatic bonds with the carboxylate group. This accounts for a higher selectivity of Na+ on carboxylic ion exchangers than that of K+. The K+ ion forms long direct electrostatic bonds with the fixed sulfonate anion. The Na+ ion does not form direct bonds with the sulfonate due to strong hydration of the former. This accounts for a higher selectivity of K+ on sulfonic acid resins that that of Na+.