IONIC MOBILITY AND CONTACT ION-PAIRING STUDY BY NA-23, CL-35, AND K-39 NUCLEAR-MAGNETIC-RESONANCE IN A POLY[METHACRYLIC ACID-CO-(N,N-DIMETHYLAMINO)ETHYL METHACRYLATE] HYDROGEL

Na-23, Cl-35, and K-39 line widths and chemical shifts were studied for amphoteric copolymer hydrogel systems as a function of water content and 1 N NaCl and 1 N KCl solution contents. A rapid decrease of the Na-23 line width and a downfield shift of the resonance appeared in the range between 1 and 6 water molecules per sodium ion. These results show that the counterions in the sodium methacrylate-co-(N,N-dimethylamino)ethyl methacrylate (MAANa-co-DMAMA) hydrogel establish a contact ion pairing with the polyions. The data suggest the presence of 5 or 6 water molecules in the first hydration sphere of sodium and potassium as counterions. The line shape of Na-23 and K-39 NMR between 6 and 15 water molecules per counterion shows the non-Lorentzian character. Also the chemical shift amove down field, and the correlation time remains at a constant value. The results in the range show that the relaxation rate and chemical shift of counterions in the MAA-co-DMAMA hydrogel mainly depend on the change of electric field gradients occurring at the anisotropic hydrated counterion around the polyion and the hydrogen bonding between the -COO- groups and the hydrated ion species, respectively. The Cl-35 relaxation rate of the systems MAANa-co-DMAMA. HCI and MAAK-co-DMAMA.HCl in a 1 N NaCl solution is markedly decreased when free water is produced. This result shows that the Cl-35 relaxation is strongly affected by the mobility of water molecules in this system.