Effects of the Exchange Capacity and Cross-Linking Degree on the Hydration States of Anions in Quantitative Loading onto Strongly Basic Anion-Exchange Resins

摘要

The water content was determined for five strongly basic anion-exchange resins (trimethyammonium type having different exchange capacities and cross-linking degrees by divinylbenzene) in definite anionic forms (ten singly, three doubly, one triply, and one quadruply charged) dried at 25 deg C and at a relative humidity of 50percent. Incorporation of the results of the previous research on the conventional resins by X-ray absorption fine structure and diffraction methods indicated that the present method gave the number of intrinsic water molecules strongly interacting with an anion. The hydration numbers of weakly hydrating anions (Cl~(-), Br~(-), and ClO_(4)~(-)) and a small anion (F~(-)) were independent of the exchange capacity and slightly decreased with an increase in cross-linking, especially at 8percent. The small and strongly hydrating ion F~(-) kept the in-water hydration structure to form a water-separated ion pair in the resins, while the other weakly hydrating ions were appreciably dehydrated to form a contact ion pair. The hydration number of a strongly hydrating ion, H_(2)PO_(4)~(-), appreciably decreased with increases in both the exchange capacity and cross-linking degree accompanied by intermolecular hydrogen bonding between the anions. This may be related to other characteristics of the H_(2)PO_(4)~(-) form resin, such as a higher concentration required for quantitative exchange, a systematic change in infrared spectra on the degree of exchange, and facile thermal dehydration, giving H_(2)P_(2)O_(7)~(2-). In contrast, multivalent anions were exchanged without dehydration, due to the larger space allowed for in the resins and the stronger interaction with water compared to those of monovalent anions.