Dynamics of 2D-water as studied by quasi-elastic neutron scattering and neutron resonance spin-echo

We have investigated the dynamics of confined water in an Na-vermiculite cl ay by combining quasi-elastic neutron scattering (QENS) and neutron resonan ce spin-echo (NRSE) experiments in order to cover a large dynamical window. The QENS study of the fully hydrated Na-vermiculite clay gave indications of a planar rotational motion of water molecules, forming hydration shells around the Na ions, and a fast basically two-dimensional translational jump diffusion motion. However, the NRSE measurements, covering the correlation time range of 3-3000 ps, show that there is also slower dynamics of the co nfined water. This motion has a e-dependence similar to translational jump diffusion and is interpreted to arise from the diffusional motion of water molecules interacting with the clay surfaces, in contrast to the faster tra nslational motion observed by QENS, which is only a factor 3 slower than th e jump diffusion in bulk water and therefore attributed to the motion of wa ter molecules not interacting with the clay surfaces. The time evolution of the observed NRSE relaxation can be described either by the Kohlrausch-Wil liams-Watts stretched exponential function or with two exponential decay fu nctions. (C) 2001 Elsevier Science B.V. All rights reserved.