Interpretation of proton N.M.R. spin lattice relaxation time minima in heterogeneous systems by the effects of bounded two dimensional diffusion

Abstract

The N.M.R. relaxation times of liquids in heterogeneous systems, where a molecule is either close to an interface, or has diffusive motions which are in some way geometrically bounded, often show features such as spin-lattice relaxation time (T ₁) minima resembling those found in bulk solids. These measurements have nearly always been interpreted using the theory of relaxation for bulk materials. This frequently leads to the conclusion that there is, as a result of proximity to the interface, a reduction in the molecular mobility of several orders of magnitude compared to the bulk liquid. This conclusion often conflicts with those derived from other measurements such as quasielastic neutron scattering. Another interpretation which takes into account the restricted nature of the diffusion is given. The effects that bounded two dimensional motion has on proton relaxation times are discussed in detail and the theory applied to a number of real cases. Calculations using the diffusion constants determined by neutron scattering give values of T ₁ and T ₁ minima in agreement with experiment. In our opinion these results place a doubt on the correctness of the conclusions of the majority of proton N.M.R. relaxation time measurements in heterogeneous systems.