Neutron diffraction measurements for a preferentially oriented aggregate slab sample of deu- terated Na-montmorillonite from Upton, Wyoming, are described for a series of clay-water contents rang- ing from 0 to 500 mg/g. A neutron wavelength of 2.39 A was used with extended detectors to collect much of the "out of plane" component of the diffraction peak intensities. The diffraction pattern intensities from the 00~ e planes of the clay, corresponding to a reflection geometry, are a strong function of sample water content and show a variation in basal spacing from 9.8 to 19.0/~. The hk reflections from transmission geometry measurements show, however, that the lattice a and b axes are constant within experimental uncertainty (0.02/~) over the range in water content and their intensities vary only by a few percent. In this geometry, a broad, water-like diffraction pattern was noted as a background under the usual hk peak intensity series. This underlying water-like pattern varies in proportion to the sample water content. Data reduction steps included consideration of background removal, multiple scattering, flux normali- zation, and attenuation of scattering due to sample thickness. Analysis of the reduced data revealed that the clay-water has a "liquid-like" ordering, with a density increase of approximately 5% over bulk water. An association between a few interlayer water molecules and the silicate superstructure is indicated by the slight change in the hk band intensities, but this change seems to be complete at water contents below 100 mg/g. Fourier analysis of the basal peak series from the dry clay shows that the hydrogens of the lattice hydroxyl groups lie in the same basal plane as their associated oxygen atoms.

Interfacial Water Structure in Montmorillonite from Neutron Diffraction Experiments

The state and dynamics of water and cations in pure and mixed Na-Cs-montmorillonite as a function of the interlayer water content were investigated in the present study, using Monte Carlo and classical, molecular-dynamics methods. While highly idealized, the simulations showed that the swelling behavior of hetero-ionic Na-Cs-montmorillonite is comparable to the swelling of a homo-ionic Na- or Cs-montmorillonite. The mixed Na-Cs-montmorillonite is characterized by intermediate interlayer distances compared to homo-ionic Na- and Cs-montmorillonites. Dry, hetero-ionic Na-Cs-montmorillonite is characterized by a symmetric sheet configuration, as is homo-ionic Cs-montmorillonite. We found that at low degrees of hydration the absolute diffusion coefficient of Cs+ is less than for Na+, whereas at greater hydration states the diffusion coefficient of Cs+ is greaterthan for Na+. An analysis of the relative diffusion coefficients (the ratio between the diffusion coefficient of an ion in the interlayer and its diffusion coefficient in bulk water) revealed that water and Na+ are always less retarded than Cs+. With large interlayer water contents, tetralayer or more, Na+ ions preferentially form outer-sphere complexes. The mobility perpendicular to the clay surface is limited and the diffusion is equivalent to two-dimensional diffusion in bulk water. In contrast, Cs+ ions preferentially form ‘inner-sphere complexes’ at all hydration states and their two-dimensional diffusion coefficient is less than in bulk water. The question remains unanswered as to why experimentally derived relative diffusion coefficients of Cs+ in the interlayer of clays are about 20 times less than those we obtained by classical molecular dynamics studies.

Diffusion of Na and Cs in montmorillonite

Adsorption and deformation phenomena at the interaction of CO2 and a microporous carbon adsorbent.

The arrangement of paraquat bound by vermiculite and montmorillonite

X-ray superlattice reflections, infrared spectroscopy, and chemical analyses have established that cetylpyri- dinium bromide (CPB) is highly ordered when adsorbed on vermiculite. The molecules, which stand at about 57 ~ to the silicate surface, form close-packed arrays. Full surface coverage is achieved only for the most highly charged vermiculites. The packing within the arrays accounted for the superlattice observed and each adsorbed molecule had an area of 18.4 /~2 at the surface. The implications of these findings for the CPB method used in soil surface area studies are discussed.

The ordering of cetylpyridinium bromide on vermiculite

An X-ray diffraction analysis of a barium-vermiculite shows it to have a triclinic unit cell with the following dimensions a = 533, b = 926, c = 1247 A, α = 10075°, s = 935° and γ = 90° The proposed structure, adjusted in space group Cl, has the exchangeable cations approximately over the ditrigonal holes of the silicate surface and on either side of a hexagonal network of water molecules extending over the middle of the interlamellar region The interlayer material imposes a relative displacement of ± b/4 on adjacent silicate sheets The importance of these displacements, not previously found in layer silicates, is emphasized

X-ray study of the interlayer region of a barium-vermiculite

Fourier syntheses have been used to establish the arrangement of organic molecules in the interlamellar regions of a 16.10 A phase of ornithine-vermiculite and a 16.92 A phase of 6-aminohexanoic acid-vermiculite. The organic cations form two layers parallel to the silicate surfaces in ornithine-vermiculite and in 6-aminohexanoic acid-vermiculite the organic cations form ascending and descending “stairs” from the silicate sheets.

M. I. Telleria

Papers

X-ray study of the interlayer region of a barium-vermiculite

The structures of ornithine-vermiculite and 6-aminohexanoic acid-vermiculite