JOURNAL OF MAGNETIC RESONANCE, Series A 120, 206–213 (1996) ARTICLE NO. 0116 NMR Study of Spin–Lattice Relaxation of Water Protons by Mn 2/ Adsorbed onto Colloidal Silica P. ROOSE, J. VAN CRAEN, G. ANDRIESSENS, AND H. EISENDRATH Vrije Universiteit Brussel, Faculty of Sciences, Physics Department, Pleinlaan 2, B-1050 Brussels, Belgium Received December 6, 1995 The magnetic-field dependence of the water-proton spin–lattice of the relaxation interactions, and consequently the relax- relaxation time (referred to as nuclear-magnetic-relaxation disper- ation times themselves, are affected in a way different from sion) has been measured in colloidal silica suspensions after addi- that in the aquo-ion ( 9–12 ) . For this reason, the NMR relax- tion of calibrated MnCl 2 solutions. The NMRD curves always show ation times provide a convenient way to investigate ion ad- the diamagnetic dispersion normally observed in a paramagnetic sorption onto the silica surface. Among others, interaction ion-free silica sol but, above pH 7, also reveal a supplementary strengths and correlation times related to the electron–nu- relaxation peak at high magnetic fields typical for paramagnetic cleus spin interactions are obtained. ion complexes adsorbed onto the silica. The paramagnetic contri- In the present work we have measured the magnetic-field bution increases linearly with ion concentration until a contribu- dependence of the proton spin–lattice relaxation time T 1 tion attributed to free Mn 2/ aquo-ions is observed as well. The ion [ usually referred to as nuclear-magnetic-relaxation disper- adsorption density G Mn 2/ is temperature and pH sensitive; e.g., no sion (NMRD)] in aqueous colloidal silica containing para- adsorption is observed at pH 2.4. The paramagnetic contribution to the experimental NMRD curves, due to adsorption of Mn 2/ magnetic Mn 2/ ions. In particular we investigated the influ- aquo complexes onto the silica, is satisfactorily described by the ence of parameters such as the ion / silica concentration ratio, Solomon–Bloembergen–Morgan equations. The estimated param- pH, temperature, and particle size on the adsorption process. eters, resulting from least-squares comparisons, are consistent with The NMRD curves clearly demonstrate that ion adsorption the assumption that the primary hydration shell of the adsorbed only occurs in a narrow pH range close to the pH of precipi- ion is retained.  1996 Academic Press, Inc. tation of the metal hydroxide. An experimental water-proton NMRD curve in a colloidal silica sol containing paramag- netic ions is the sum of a diamagnetic and a paramagnetic INTRODUCTION relaxation contribution due to the additivity of the transition probabilities. The diamagnetic part, associated with the silica Silica (SiO 2 ), in its different forms, has numerous and sol in the absence of paramagnetic substances, shows a dis- wide-ranging applications in catalysis, separation science, persive magnetic-field dependence, which has been the sub- composite materials, microelectronics, and other technolo- ject of a previous study ( 16 ) . The paramagnetic contribu- gies. An aqueous colloidal silica sol consists of amorphous tion, on the other hand, is typical for an ion – macromolecular silica particles suspended in water. The particle surface is complex and was analyzed within the framework of the tradi- covered with hydroxyl groups ( silanol ) , and some are nega- tional Solomon–Bloembergen–Morgan (SBM) theory ( 2, tively ionized at alkaline pH, keeping the particles in suspen- 3, 9 ), which is briefly outlined in the next section. Several sion. A cloud of countercations surrounds the particles and authors have discussed the shortcomings of the SBM theory some are adsorbed onto the surface, forming the so-called in some circumstances and these are discussed when neces- ‘‘compact double layer’’ ( 1 ). The suspension is electrically sary. The results are also compared to other adsorption stud- neutral. ies ( e.g., potentiometric titration studies ) of hydrolyzable It is well established that the NMR relaxation times T 1 metal ions at oxide–water interfaces ( 1, 13–15 ). and T 2 of water protons are strongly enhanced in solutions containing paramagnetic ions ( e.g., ions possessing unpaired THEORETICAL BACKGROUND electrons ) . The paramagnetic relaxation enhancement is at- tributed to the time-dependent modulation of the hyperfine The nuclear-spin-relaxation theory of spin- 1 2 nuclei com- interaction between the nuclear spin of the liganded water protons and the electronic spin of the paramagnetic ion ( 2– plexed to a paramagnetic ion in dilute solutions was origi- nally described by Solomon, Bloembergen, and Morgan 9 ) . When a paramagnetic ion complex binds or adsorbs onto a large entity, such as a macromolecule, the time dependence ( 2, 3, 5, 9 ). They proposed the following equation to de- 206 1064-1858/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.