Neutron scattering studies of hydrated layer silicates

Slow-neutron scattering spectrometry has been used to investigate the dynamics of interlamellar water molecules in layered silicate clay minerals. Experiments on hydrated lithium vermiculite give information in three regions. At energy transfers between 200 and 800 cm–1, vibrational bands associated with hydrogen motions in the silicate lattice and in the water are distinguished. In the low-frequency region, ca. 15–150 cm–1, a broad band of scattering characteristic of water emerges as the water content of the clay increases. Line-broadening effects arising from small energy transfers are indicative of diffusional processes in the water layers, but improved technique is required to make this certain for the thinnest films. The data are analyzed with particular reference to the thickness of the interlamellar water layers, and indicate significant changes in the water structure for water thicknesses of ca. 1–2 molecular layers. Similar results obtain in experiments with sodium vermiculite, and with lithium and sodium montmorillonite.