Structure of Water and Silica in Ion Exchange Resins

Abstract Dynamic and static structures of water sorbed in ion exchange resins have been investigated by 17O NMR and X-ray diffraction. Rotational motion of water molecules sorbed in ion exchange resins is significantly slowed according mainly to the amount of cross-linkage of polymer which decides the size of micropore surrounded by hydrophobic polymer networks, and not crucially affected by the difference of hydration properties of ion exchange groups. Static structure of water sorbed in anion exchange resin is revealed to be similar to that of pure water, which indicates a clathrate like structure enclosing a hydrophobic tetramethylammonium group. On the other hand the structure of water in cation exchange resin is quite different from that of pure water, but resembles the structure found in aqueous sulfuric acid solution, which is strongly influenced by the highly acidic atmosphere. The structure of silica adsorbed in anion exchange resin has also been observed by X-ray diffraction. Obtained radial distribution function is quite similar to that of bulk silica, which requires the existence of Si-O-Si interaction in quantitative analysis. The present results evidence that silica adsorption in anion exchange resin is not simple ion exchange but combined processes of ion exchange and micro silica gel growth.