Determination of the Saturation Adsorption Density of Paramagnetic Ion Complexes on Colloidal Silica by NMR Field-Cycling Relaxation Spectroscopy
✍ Scribed by Patrice Roose; Jean Van Craen; Chellapah Pathmamanoharan; Henri Eisendrath
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 110 KB
- Volume
- 188
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
electronic magnetic moment of the ion (time-dependent hy-For paramagnetic ions with a long electron spin relaxation time, perfine interaction). Therefore, the observed relaxation time such as Mn 2/ , the measurement of the magnetic field dependence mainly reflects the relaxation of the coordinated protons. By of the proton spin-lattice NMR relaxation time (referred to as measuring the magnetic field dependence of the spin-lattice nuclear magnetic relaxation dispersion) provides an indirect way relaxation time T 1 (referred to as nuclear magnetic relaxation to determine the number of ions, either free in solution or adsorbed dispersion, NMRD), the power spectra of these time-depenonto the surface, in aqueous colloidal silica suspensions. At pH dent interactions are directly monitored.
values higher than the pH of precipitation of the metal-hydroxide,
In previous work, the adsorption of Mn 2/ ions on colloidal the proton relaxation enhancement results from adsorbed ions silica particles suspended in water was studied by measuring only. By adding sufficient ions, saturation of the accessible silica surface was obtained and a maximum coverage of 0.43 Mn 2/ ions the proton NMRD curves as a function of pH, temperature, per nm 2 was found. For Ni 2/ , a fast relaxing paramagnetic ion, particle size, and ion/silica concentration ratio (1). The the proton NMRD curves do not allow us to distinguish between paramagnetic relaxation enhancement was interpreted within free and adsorbed ions. It appears that in NiCl 2 solutions the the framework of the Solomon-Bloembergen-Morgan theproton spin-lattice relaxation time substantially reflects the chemory (2-4) which showed to be very appropriate provided ical exchange time of the protons complexed to Ni 2/ . Under these that the relaxation of the paramagnetic ion spin was slow. circumstances, one can evaluate the rate constants of the uncata-The adsorption density of Mn 2/ ions was evaluated as a lyzed and the acid-base catalyzed proton exchange in Ni 2/ aquofunction of pH from the NMRD data. In those samples, complexes from the pH dependence of the proton relaxation rate.
saturation of the accessible silica surface by Mn 2/ was not