The temperature dependence of 29Si nuclear magnetic resonance linewidths in aqueous silicate solutions and their effect on exchange rate determinations

Abstract

The temperature-dependent ²⁹Si n.m.r. spectra of a range of alkali silicate solutions are reported. Linewidth studies of the silicate monomer resonance give an indication of the rate of exchange with other solution species. However, it is shown that the spin–spin relaxation time (T₂) surprisingly decreases with increasing temperature so that exchange rates determined from linewidth alone are invalid. For a 1.0 mol dm^–3 solution of sodium silicate with SiO₂: Na₂O = 1.0 the true monomer exchange rate is calculated to be 2.8 s^–1 up to 320 K. A similar solution having SiO₂: Na₂O = 0.33 gave values of 24 s^–1 at 277 K and 25–44 s^–1 at 320 K. These rates are lower than those measured by linewidth alone but higher than related values obtained by other workers using selective inversion recovery.