Nuclear magnetic relaxation and structure in aqueous heterogenous systems

Abstract

The relaxation of water in aqueous heterogeneous systems is described in terms of the effects of the substrate structure and order on the motions and arrangements of the water molecules. The general observation that T ₁ is long compared to T ₂ indicates that at least one of the correlation times (τ_c) is long. A long τ_c can result from tight binding of some molecules or from the combination of preferential orientation and substrate order. Since doublet splitting characteristic of preferential orientation has been observed for water on a wide variety of organic and inorganic substrate materials, the latter effect might be of general importance in the relaxation phenomena of heterogeneous systems, including biological tissue. For this effect, the conventional 90°–180° T ₂ relaxation curve and the solid-echo curve are calculated for random transitions in a powder sample and for transitions in a gaussian distribution of doublet splitting constants. The results are applied to measurements on clay-water systems to illustrate some effects of structure on transverse relaxation. This application indicates that (a) samples made from oriented clays contain domains characterized by different water contents and different orientations, and (b) the rigidity and domain size and order for powder samples increase with particle size decrease.