Water spin relaxation in colloidal systems. Part 1.—17O and 2H relaxation in dispersions of colloidal silica

Abstract

Extensive water ¹⁷O and ²H spin-relaxation data from alkali-stabilized aqueous dispersions of spherical colloidal silica particles are presented. Longitudinal (R₁) and transverse (R₂) relaxation rates of both nuclei have been measured in samples of varying particle diameter (ca. 10–80 nm) as functions of resonance frequency, temperature, alkali content and particle concentration. Qualitative and quantitative differences in the results from the two nuclei demonstrate that surface silanol groups, exchanging deuterons very rapidly with bulk water, contribute to R₁(²H) and dominate R₂(²H). At the highest measured resonance frequencies, the enhancement of R₁ relative to bulk water is entirely due to rapid, local dynamics of water molecules (and, for ²H, silanol groups), whereas R₂ contains additional contributions from slow diffusive processes. The fast and slow dynamics are independent of particle concentration. The R₁ relaxation enhancement depends only on the ratio of silica surface area to water volume, independent of particle size, indicating that the structure of the particle surface is size invariant. In contrast, the slow dynamics contributing to R₂ has a systematic dependence on particle size.