The structure of interlayer water in vermiculite

Neutron diffraction, in conjuction with substitution of deuterium (D) for hydrogen (H), has been used to determine the structure of interlayer water in sodium- and nickel-substituted vermiculites. We have measured the intensities of the (00l) Bragg reflections as a function of relative humidity and H/D content, up to l=30. Difference analysis has then been used to obtain separate density profiles, ρ(z), for both the hydrogen atoms and the oxygen atoms plus the clay sheets. Ni–vermiculite was studied at 84% relative humidity, while Na–vermiculite was studied at both 88% and 30% relative humidity. At these values the layer spacings are 14.40, 14.96, and 11.78 Å, respectively. We find that each interlayer nickel ion is coordinated octahedrally to 6.0 water molecules. All of these water molecules are oriented to form a strong hydrogen bond to the adjacent clay surface. We also find that extra water is located close to the clay layers. This additional water is situated within the hexagonal rings of SiO4 and AlO−4 tetrahedra, which comprise the clay surfaces. In the 14.96 Å phase of Na–vermiculite there are an average of 4.9 interlayer water molecules per cation. About half of these water molecules are oriented to form a hydrogen bond to one of the clay surfaces. Additional water is found close to the clay surface, occupying the same hexagonal ring sites as in 14.40 Å Ni–vermiculite. In the 11.78 Å phase of Na–vermiculite there are an average of 2.1 water molecules per interlayer cation. The oxygen atoms of these interlayer water molecules are found close to halfway between the layers, while the hydrogen atoms are directed towards one of the adjacent clay sheets.