Water spin relaxation in colloidal systems. Part 3.—Interpretation of the low-frequency dispersion

Abstract

The contribution to the water ¹⁷O spin relaxation from slow molecular processes in colloidal solutions has been interpreted in terms of a general theory of spin relaxation in locally ordered fluids, coupled with a dynamic model which describes the translational motion of water molecules by a diffusion equation with an anisotropic and position-dependent diffusivity tensor. This model can explain all the experimental observations provided that the lateral and radial diffusivities of water molecules within ca. 1 nm of the colloid surface are reduced by 1–2 and 2–3 orders of magnitude, respectively, compared with bulk water. The dynamic coupling between water motion and rotational diffusion of colloidal particles does not affect water ¹⁷O spin relaxation for particle radii > ca. 10 nm.