Quasielastic neutron scattering of two-dimensional water in a vermiculite clay

A well-characterized Na-vermiculite clay, containing zero, one, or two mole cular layers of water between the clay platelets, has been studied by quasi elastic neutron scattering (QENS). Experiments were carried out at a temper ature of 300 K in two different scattering geometries; the clay platelets b eing at 45 and 135 degrees angles to the incident beam in order to make the elastic Q-vector perpendicular and parallel, respectively, to the clay pla telets for a scattering angle of 90 degrees (Q approximate to 1.33 Angstrom (-1)). The resulting QENS spectra show that almost no hydrogen motion occur s perpendicular to the clay platelets on the experimental time scale (about 2-40 ps). The two-H2O layer vermiculite exhibits a planar rotational motio n of water molecules, forming hydration shells around the Na ions, and a ba sically two-dimensional translational jump-diffusion motion. The translatio nal motion was modeled using the Gaussian jump-length distribution model, r esulting in a mean jump length of 1.1 Angstrom and an average residence tim e of 2.3 ps. Using these values we obtain an effective diffusion coefficien t of 8.8* 10(-10) m(2)/s, which is only a factor 2-3 lower than for bulk wa ter. The correlation time of the rotational motion was estimated to approxi mately 27 ps. In the case of the one-H2O layer vermiculite we were only abl e to observe a planar rotational motion with a rotational correlation time of 16 ps, i.e., faster than in the two-H2O layer vermiculite. This suggests that a smaller number of water molecules are involved in the rotational pr ocess in the one-H2O layer vermiculite, and furthermore that the translatio nal motion, if existent, is too slow to be observed on the experimental tim e scale. (C) 2000 American Institute of Physics. [S0021-9606(00)70231-5].