DETERMINATION OF THE SATURATION ADSORPTION DENSITY OF PARAMAGNETIC ION COMPLEXES ON COLLOIDAL SILICA BY NMR FIELD-CYCLING RELAXATION SPECTROSCOPY

For paramagnetic ions with a long electron spin relaxation time, such as Mn2+, the measurement of the magnetic field dependence of the proto n spin-lattice NMR relaxation time (referred to as nuclear magnetic re laxation dispersion) provides an indirect way to determine the number of ions, either free in solution or adsorbed onto the surface, in aque ous colloidal silica suspensions. At pH values higher than the pH of p recipitation of the metal-hydroxide, the proton relaxation enhancement results from adsorbed ions only. By adding sufficient ions, saturatio n of the accessible silica surface was obtained and a maximum coverage of 0.43 Mn2+ ions per nm(2) was found. For Ni2+, a fast relaxing para magnetic ion, the proton NMRD curves do not allow us to distinguish be tween free and adsorbed ions. It appears that in NiCl2 solutions the p roton spin-lattice relaxation time substantially reflects the chemical exchange time of the protons complexed to Ni2+. Under these circumsta nces, one can evaluate the rate constants of the uncatalyzed and the a cid-base catalyzed proton exchange in Ni2+ aquo-complexes from the pH dependence of the proton relaxation rate. (C) 1997 Academic Press.