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The magnetic-field dependence of the water-proton spin–lattice relaxation time (referred to as nuclear-magnetic-relaxation dispersion) has been measured in colloidal silica suspensions after addition of calibrated MnCl2solutions. The NMRD curves always show the diamagnetic dispersion normally observed in a paramagnetic ion-free silica sol but, above 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 concentration until a contribution attributed to free Mn2+aquo-ions is observed as well. The ion adsorption density[formula]is temperature and pH sensitive; e.g., no adsorption is observed at pH 2.4. The paramagnetic contribution to the experimental NMRD curves, due to adsorption of Mn2+aquo complexes onto the silica, is satisfactorily described by the Solomon–Bloembergen–Morgan equations. The estimated parameters, resulting from least-squares comparisons, are consistent with the assumption that the primary hydration shell of the adsorbed ion is retained. |
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