Deuteron relaxation dispersion in aqueous colloidal silica

The magnetic relaxation dispersion from the water deuteron resonance in aqu eous colloidal silica sols has been measured with the field-cycling techniq ue in the frequency range from 1.5 kHz to 7.7 MHz. Dispersion profiles were recorded as a function of pL, L stands for a general hydrogen atom, in the range 2-11 for two sols with different particle size and at two temperatur es. The profiles were well described by a Lorentzian dispersion function wi th an amplitude of beta and an apparent correlation time of tau. The near i nvariance of t with particle size, temperature, and pL demonstrates that th e usual motional narrowing theory of spin relaxation is not applicable. A m ore general, nonperturbative theory, however, can quantitatively rationaliz e the data and yields, through a global fit, physically meaningful values o f the microscopic parameters in the model. The analysis shows that the disp ersion is partly due to long-lived water molecules and partly to silanol de uterons in rapid exchange with water. The silanol contribution is about 50% at pL 5, increasing to 90% at pL 8-10. Over most of the pL range, tau is e ssentially a measure of the residual quadrupole frequency of water and sila nol deuterons and, hence, is not directly related to a motion in the system . The long-lived water molecules contributing to the dispersion have a resi dence time distribution spanning the microsecond range and are presumably t rapped in micropores at the silica surface. The surface density of such tra pped water molecules is found to be higher for silica particles of the more porous Stober variety. The relaxation data also yield the surface density and orientational order parameter of silanol deuterons, as well as the rate constants for acid- and base-catalyzed silanol hydrogen exchange.