NMR-STUDY OF SPIN-LATTICE RELAXATION OF WATER PROTONS BY MN2+ ADSORBED ONTO COLLOIDAL SILICA

The magnetic-held dependence of the water-proton spin-lattice relaxati on time (referred to as nuclear-magnetic-relaxation dispersion) has be en measured in colloidal silica suspensions after addition of calibrat ed MnCl2 solutions. The NMRD curves always show the diamagnetic disper sion normally observed in a paramagnetic ion-free silica sol brit, abo ve pH 7, also reveal a supplementary relaxation peak at high magnetic fields typical for paramagnetic ion complexes adsorbed onto the silica . The paramagnetic contribution increases linearly with ion concentrat ion until a contribution attributed to free Mn2+ aquo-ions is observed as well. The ion adsorption density Gamma(Mn2+) is temperature and pH sensitive; e.g., no adsorption is observed at pH 2.4. The paramagneti c contribution to the experimental NMRD curves, due to adsorption of M n2+ aquo complexes onto the silica, is satisfactorily described by the Solomon-Bloembergen-Morgan equations. The estimated parameters, resul ting from least-squares comparisons, are consistent with the assumptio n that the primary hydration shell of the adsorbed ion is retained. (C ) 1996 Academic Press, Inc.