Abnormal swelling in water of carboxylic ion exchanger in the forms of ammonium and tetraethylammonium ions

A fibrous carboxylic ion exchanger, obtained by post-radiation grafting of polyacrylic acid to polypropylene fiber, in the ammonium form has an abnormally low swelling in water, corresponding to 10 moles of water per equivalent, and in the form of tetraethylammonium ion (NEt₄⁺) – abnormally high 25 moles of water/eq. Considering that NH₄⁺ is a hydrophilic particle and NEt₄⁺ is hydrophobic, this fact seems paradoxical. The article attempts to explain this phenomenon using molecular modeling (ab initio calculations of the structure of hydrate complexes in the HF MO LCAO approximation with the 6-31G basis set) in combination with the Predominant Hydrates Model, which made it possible to calculate the theoretical water sorption isotherms of the ion exchanger and compare them with experimental data. The abnormally low swelling of the ion exchanger in the NH4+-form is caused by the formation of a strong bond between the carboxylate anion and ammonium with a significant degree of covalence due to the superposition of the Coulomb interaction of the ions and the formation of a hydrogen bond between them. The abnormally high swelling of NEt₄⁺-forms is caused by the absence of a strong interaction between cations and carboxylate groups due to the impossibility of their convergence due to steric reasons. The high swelling of the ion exchanger is caused by the absence of blocking of the hydrophilic carboxylate groups of the ion exchanger by hydrogen bonds with the cation. The ion exchanger matrix does not contain a cross agent and does not create a spatial restriction for the high swelling of the ion exchanger.

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