Characterization of Montmorillonite Saturated with Short-Chain Amine Cations: 1. Interpretation of Basal Spacing Measurements

Abstract

AbstractBasal spacing measurements were made for a montmorillonite saturated with a series of small aliphatic primary, secondary, and tertiary amine and quaternary ammonium cations, using wet, oven-dry, and glycerol-treated specimens. The spacings ranged from about 12 to 14Å, depending on the cation, indicating that in each case a monolayer of cations is interleaved between adjacent montmorillonite layers. The observed spacings are consistent with the concept that the cations are oriented with their minimum thickness in the c-axis direction, but it is possible that the smallest cations have a long axis in this direction and are partially embedded in the clay surfaces.Montmorillonite saturated with cations containing up to four carbon atoms retained some sensitivity to water, as indicated by a slight contraction of the lattice on oven drying and rapid re-expansion on exposure to the humidity of the laboratory atmosphere.For cations having a layer thickness less than glycerol, limited lattice expansion occurred on glycerol treatment, usually to a spacing sufficient to accommodate a single layer of glycerol molecules. Apparently the cations function as pillars separating the montmorillonite layers, and the glycerol molecules may expand the lattice as necessary to enter the spaces between the pillars. However, exceptions occurred with methylamine-saturated montmorillonite, in which both one- and two-layer complexes of glycerol were present simultaneously in the same specimen, and with trimethylamine-saturated clay, in which two different one-layer complexes were present. It is suggested that the glycerol in one-layer montmorillonite complexes may exist in two orientations, one leading to a layer thickness of about 4.1 Å, the other to a thickness of about 4.6 Å.