Die Hydratation von Carboxyl-Gruppen in Ionenaustauscherharzen Eine dielektrische Untersuchung / The Hydration of Carboxylic Acid Groups in Ion Exchange Resins A Dielectric Investigation

The hydration of carboxylic acid groups in ion exchange resins was studied by means of dielectric measurements. For three resins carrying carboxylic groups with quite different pK-values the dielectric constant and the dielectric loss were measured as a function of frequency, water content, and temperature. A comparison of their behaviour showed that the observed dielectric dispersion is due to a Maxwell-Wagner mechanism. In the course of the water adsorption the resin with very weak carboxylic acid groups exhibited a discontinuity of the dielectric constant, if one molecule of water was attached to two carboxylic groups. This can be explained by assuming that the carboxylic groups which are connected by hydrogen bonds in the dry state begin to dissociate at the above mentioned water content. As a consequence, the formation of H5O2⊕-complexes becomes possible. The corresponding increase of the proton mobility leads to the observed Maxwell-Wagner effects. Hydration structures of the carboxylic acid groups below and above this critical water content are proposed.In the case of the two other resins with higher acidity of the carboxyl groups no discontinuity during water sorption could be observed. The mobility of the proton and hence the Maxwell-Wagner effects could thus be observed at much lower water contents.The appreciable differences in the shapes of the corresponding wateradsorption isotherms are in agreement with conclusions from the dielectric measurements.