Deuteron Relaxation Dispersion in Aqueous Colloidal Silica

The magnetic relaxation dispersion from the water deuteron resonance in aqueous colloidal silica sols has been measured with the field-cycling technique 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 temperatures. The profiles were well described by a Lorentzian dispersion function with an amplitude of ‚ and an apparent correlation time of U. The near invariance of U with particle size, temperature, and pL demonstrates that the usual motional narrowing theory of spin relaxation is not applicable. A more general, nonperturbative theory, however, can quantitatively rationalize the data and yields, through a global fit, physically meaningful values of the microscopic parameters in the model. The analysis shows that the dispersion is partly due to long-lived water molecules and partly to silanol deuterons 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, U is essentially a measure of the residual quadrupole frequency of water and silanol deuterons and, hence, is not directly related to a motion in the system. The long-lived water molecules contributing to the dispersion have a residence time distribution spanning the microsecond range and are presumably trapped in micropores at the silica surface. The surface density of such trapped water molecules