The dynamics of water in heterogeneous systems

Abstract

A model for the dynamics of water molecules in heterogeneous systems proposed earlier and involving dynamically oriented water molecules at interfaces exchanging with others remote from interfaces is developed further with respect to the spin-relaxation properties of the hydrogen isotopes in the water molecules. A single-crystal system is considered in which all interfaces make the same angle with respect to an external axis. Expressions are derived for the contributions of both exchange of water molecules between surface and bulk sites and intermolecular proton exchange to proton relaxation at low frequencies for a system in which the observed proton spectrum is a single line. A density matrix treatment is used to describe the dependence on pulse separation of the deuterium transverse relaxation rate in such a system, measured using a 90° _x′ (τ-90° _y′-τ) _n spin-echo sequence which removes the residual static quadrupole interaction, the source of the doublet deuteron spectrum. It is shown that a study of this τ dependence together with measurement of the deuteron T ₁ allows the independent determination of the spectral densities of the fluctuating electric quadrupole interaction at the frequencies 0, ω₀ and 2ω₀. Measurements on the magnetically orientable lamellar phase of the caesium perfluoro-octanoate/water system are used to illustrate the various features of the theoretical results.