LONGITUDINAL SI-29 NUCLEAR MAGNETIC-RELAXATION IN AQUEOUS ALKALI-METAL SILICATE SOLUTIONS REVISITED

We report the results of a wide ranging Si-29 longitudinal (spin-latti ce) relaxation study of aqueous alkalimetal silicate solutions. Measur ement and interpretation of relaxation rates for the many chemically d istinct Si centers found in these solutions is complicated by (a) Si-S i chemical exchange during the inversion-recovery pulse sequences, (b) persistent paramagnetic impurities, and (c), in the case of moderatel y concentrated low-temperature solutions, the inapplicability of the e xtreme narrowing condition. The situation is therefore complex but non etheless amenable to analysis. Our findings indicate that the primary causes of Si-29 longitudinal relaxation are interactions (dipolar and contact) with adventitious paramagnetic ions, dipole-dipole relaxation by solvent protons, and an apparent spin rotation interaction, The re lative importance of these mechanisms at the individual Si centers is a function of solution composition and purity, along with the size and structure of the corresponding silicate anions. Analysis of the Si-29 -H-1 dipole-dipole contribution to relaxation for the hydrated silicat e monomer anion yielded an activation energy for isotropic tumbling (b etween 275 and 375 K) of 18 +/- 3 kJ mol(-1). Contrary to an earlier s uggestion, there was no measurable contribution from dipole-dipole int eractions with alkali-metal nuclei.