Interionic Interactions in Carboxylic Acid Cation Exchangers on the Base of Polyacrylic Acid. Ab Initio Calculations

Abstract

Ab initio calculations of the structure of representative fragments of carboxylic acid cation exchangers on the base of polyacrylic acid in forms of alkali, alkali earth, and some heavy metal ions (Cu2+, Ni2+, Co2+, Cd2+, Mn2+, Pb2+) have been made by the RHF SCF MO LCAO method. In the case Ni2+ and Cu2+ in the main valence state, the calculations were made by the ROHF SCF MO LCAO method. The calculated fragments contained up to nine repeating units of the polymer chain, the counterions and 0–10 water molecules per carboxylate group. The geometry and electronic characteristics of the Me–O bonds with carboxylic groups and water molecules in the hydration systems were analyzed. The ionic hydration in these systems was characterized by the number of water molecules directly bound only to the carboxylic groups, only to the counterion both to the carboxylate group and the counterion, and only to the other water molecules. It appeared that in all cases the counterions are strongly bound with carboxylate groups forming rigid structures in which one univalent counterion can be directly bound to two carboxylate groups. The bivalent cations are mainly bound to three carboxylate groups. The electroneutrality of the polymer fragment is provided through the group binding of the counterion and the carboxylic groups. The significant degree of covalence of the bonds between the cations and carboxylic group dependent on the atomic mass and number of water molecules in the system is mainly responsible for the selectivity series of the alkali and alkali earth metals. These series are opposite to those for sulfonic cation exchangers. The high selectivity of sorption of the heavy metal ions is due to the fact that the Me–O bonds with oxygen atoms of both carboxylate groups and water molecules have a high degree of covalence. These interactions are competitive and their balance is individual for any pair of fixed and counterion.