With the current interest in the use of transition metal-exchanged phyllosilicates as catalysts for novel organic syntheses, investigations into the factors which affect the movement of reactant, product, and solvent molecules into and out of their interlamellar region are of considerable importance. Mixed organic-water intercalates of a Wyoming montmorillonite, exemplified by the Na-montmorillonitepyridine-water system which can form four different intercalates exhibiting basal spacing of 29.3, 23.3, 19.4, and 14.8 Å depending on the pyridine: water ratio, have been used as a model system. X-ray and neutron diffraction and quasielastic neutron scattering data relating to the interconversion of interlayer species indicate that access to and exit from the interlayer space is hindered at high partial pressures of water by a water-film diffusion barrier in the interparticulate voids which exist between the aggregated silicate layers. At lower water vapor pressures the rate-limiting step for interconversion from one intercalate to another is the rate of transport of reagents and products to and from the clay particles. Under conditions where these rates are fixed, the rate-limiting step is the rate of diffusion of the pyridine molecule in the lower-spacing intercalate. Processes which involve a change in basal spacing do not necessarily proceed via a single discrete step, but are also affected by the amount of water made available to the system. In organic reactions catalyzed in the interlamellar space of various cation-exchanged montmorillonites (e.g., the conversion of alk-1-enes to di-2,2’-alkyl ethers and the reaction of alcohols to form ethers), rate-determining steps similar to those found above are likely to be operative. In particular, for reactions carried out in the liquid phase, where mass transport is facile and where phase-transfer problems are avoided, such reactions are likely to be diffusion controlled.

A study was carried out on the influence of the sequence of addition of montmorillonite, hydroxy-Al ions, and tannic acid ([Al] = 0.015 M; 6 mmole Al/g of clay; tannic acid/Al molar ratio = 0.1) on the nature of organo-clay complexes formed at pH 4.5. A negligible amount of tannate was held on the clay surfaces in the absence of Al, whereas in the presence of Al, hydroxy-Al-tannate species were easily adsorbed on clay surfaces. Their distribution on the external surface and in the interlayer space of montmorillonite, however, was a consequence of how the components reacted with each other. The complexes showed broad X-ray powder diffraction peaks at 15.6 to 19.2 Å at room temperature. They also showed different behavior to preheating, ethylene glycol solvation, and chemical treatments. Whereas tannic acid showed two prominent exothermic peaks at 390° and 510°C, all hydroxy-Al-tannate-mont-morillonite complexes showed a broad exotherm at about 400°–450°C. Some complexes showed, in addition, a small inflection between 350° and 420°C. The complexes also showed distinct differences in cation-exchange capacity, carbon content, extractable Al, titratable acidity, and mode of aggregation after drying.

Emanation thermal analysis (ETA), based on radon release measurements from previously labeled samples, was used for the first time in the characterization of the thermal behavior of natural Mg2+-vermiculite (Santa Olalla, Huelva, Spain) and of Na+- and ${\text{NH}}_4^{+}$-exchanged vermiculite samples. In addition, vermiculite samples subjected to a chemical treatment with an aqueous solution of (NH4)2SiF6 and partially or totally re-saturated with Na+ ions were also investigated by ETA. The ETA results of natural Mg2+-vermiculite, Na+-vermiculite and ${\text{NH}}_4^{+}$-vermiculite gave supplementary information about microstructure changes of the samples observed under dynamic heating conditions. The method has proved to be very useful for characterization of microstructure changes due to modification in the interlayer space of samples during the heat treatment. The crystallization of vermiculite into new phases, such as enstatite (for ${\text{NH}}_4^{+}$-vermiculite and Mg2+-vermiculite) and forsterite (for Na+-vermiculite) was also observed by ETA.

When clay minerals, notably smectites, intercalate organic cations, their interlayer surfaces change from hydrophilic to hydrophobic. The resultant intercalates, known as organo-clays (OCs), have a large affinity for hydrophobic organic contaminants (HOCs). Organo-clays are used as sorbents of HOCs in wastewater treatment and as sorptive barriers in landfill liners. The structural and sorptive characteristics of OCs with respect to HOCs have been studied extensively, and a large volume of literature has accumulated over the past few decades. The interactions of OCs with HOCs and the various approaches to improving the sorption capacity of OCs are reviewed here, with particular reference to the application of novel analytical techniques, such as molecular modeling, to characterizing the OC—HOC interaction.