The recent determination of the three-dimensional crystal structure of a dickite:formamide intercalate allows insight into the clay:organic bonding schemes of amides and other small organic molecule intercalates of the kaolin minerals. It is demonstrated that the observed basal spacings of intercalates with these molecules are consistent with hydrogen bonding schemes in which, if possible, triple hydrogen bonds from the clay hydroxyls to 0=C< are formed. Variations in basal spacing within a series of amides can be explained by considering the maximization of hydrogen bonding while avoiding close van der Waals contacts.

Intercalates of Georgia well-crystallized kaolinite with formamide, N-methylformamide (NMF), and dimethylsulfoxide (DMSO) were prepared at room temperature by dispersing the clay in the organic liquid. Several physical and chemical properties of the intercalated organic molecules and the clay, while intercalated and after de-intercalation, were examined using nuclear magnetic resonance (NMR), infrared (IR), and electron paramagnetic resonance spectroscopy (EPR), and specific heat (Cp) measurements. The chemical bonding between the inner-surface hydroxyls and the organic molecules, as indicated by IR, was strongest for DMSO and weakest for formamide. The distortion of the kaolinite layer, as shown by EPR, also was greatest for DMSO and least for formamide. NMR T1 measurements indicated a relatively strong DMSO-kaolinite surface interaction that slowed down the methyl group reorientation comparable to that in bulk solid DMSO. T1 measurements indicated a weaker interaction for NMF. De-intercalation by mild heating did not return the kaolinite to its original structural state as shown by EPR and Cp. The greatest disorder was found for the DMSO de-intercalate and the least for the formamide de-intercalate. These experiments show that for sufficiently strong bonding between the clay inner surface and the intercalating molecule, the structure of the clay is capable of distortion, which is partly temporary and partly permanent. The permanent changes probably involve the introduction of stacking faults.

13C and 29Si nuclear magnetic resonance spectroscopy with magic-angle spinning bas been used to study the short-range ordering and bonding in the structures of intercalates of kaolinite with formamide, hydrazine, dimethyl sulfoxide (DMSO), and pyridine-N-oxide (PNO). The 29Si chemical shift indicated decreasing levels of bonding interaction between the silicate layer and the intercalate in the order: kaolinite: formamide (δ) = -91.9, ppm relative to tetramethylsilane), kaolinite: hydrazine (-92.0), kaolinite: DMSO (-93.1). The 29Si signal of the kaolinite:PNO intercalate (-92.1) was unexpectedly deshielded, possibly due to the aromatic nature of PNO. The degree of three-dimensional ordering of the structures was inferred from the 29Si signal width, with the kaolinite: DMSO intercalate displaying the greatest ordering and kaolinite: hydrazine the least. 13C resonances of intercalating organic molecules were shifted downfield by as much as 3 ppm in response to increased hydrogen bonding after intercalation, and in the kaolinite: DMSO intercalate the two methyl-carbon chemical environments were non-equivalent (δ = 43.7 and 42.5).

Stable, three-dimensionally ordered complexes were formed from synthetically hydrated, highly ordered kaolinite (d(001) = 8.4 Å) and several organic compounds. Removal of the intercalated organic compound by drying or by water washing recovered the 8.4-Å hydrate with its ordered layer stacking essentially unchanged. Some of the complexes were stable for less than a day, whereas others appeared to be stable indefinitely. The compounds that formed ordered complexes were dimethylsulfoxide, formamide, hydrazine-hydrate, 1,1-dimethylhydrazine, ethylene glycol, glycerol, and pyridine. Clay-organic complexes that were prepared from methanol, ethanol, 1- and 2-propanol, acetone, acetic acid, propionic acid, acetaldehyde, N-methylformamide, methylethyl ketone, tetrahydrofuran, and K-acetate were stable only if they were immersed in the intercalating medium and had little or no stacking order.

Many of the organic compounds intercalated by the 8.4-Å hydrate are not known to be intercalated by non-hydrated kaolinite either directly or indirectly. Isolated water molecules appear to be keyed into the ditrigonal holes formed by the basal oxygen of the silicate tetrahedra of the 8.4-Å hydrate. These water molecules, referred to as “hole water,” and fluorine ions that had replaced ~20% of the inner-surface hydroxyls of the 8.4-Å phase sufficiently altered the interlayer bonding to allow an expansion of the inner-layer spaces by a variety of guest molecules. The presence of these guest molecules between the clay layers not only changed the basal spacing and perturbed the infrared (IR) bands arising from the inner-surface hydroxyls, it also shifted the position of the IR band arising from the inner hydroxyl.

The adsorption and degradation on montmorillonite of s-triazine, the parent compound of a major group of triazine-based herbicides, were studied by Fourier-transform infrared spectroscopy. The original s-triazine appeared to hydrolyze with residual bound water in montmorillonite at 20°C to produce formamide, which formed an intercalate with the clay. A mechanism for the conversion of s-triazine to formamide was proposed which does not support earlier reports that formic acid and ammonia are the reaction products. The infrared spectrum of the formamide-Na-montmorillonite intercalate suggests that formamide reacted with the exchangeable Na cation and that the NH2 group hydrogen bonded with the clay. Assuming the intercalated formamide molecule to be planar, the molecular plane was found to be inclined at ~33° with respect to the plane of the silicate sheet. C-N and C-O bonds were also found to be tilted at the same angle with respect to the clay surface. The proposed orientation is in good agreement with the measured value of d(001) for the formamide-Na-montmorillonite complex. The proposed model of the formamide-montmorillonite complex serves as a basis for comparison of other clay-amide complexes that are important in protein-clay interactions.

The intercalation of formamide, potassium acetate, and hydrazine by halloysite and/or ka-olinite-rich samples, with and without subsequent displacement of the interlayer species by water or glycerol/water, has been investigated. Halloysite, as such, or in mixtures with kaolinite is completely expanded by all the treatments used, thereby enabling halloysite concentrations to be determined from the basal X-ray powder diffraction (XRD) peak ratios of the appropriate complexes. The values so obtained are usually proportional to the abundance of tubes, laths, and spherules in the transmission electron micrographs of the samples. The analysis of kaolin samples (halloysite plus kaolinite) by intercalation methods is less straight forward because a proportion of the kaolinite component in the system may not expand, even after lengthy (≥ 18 days) contact of the sample with the intercalating agent. Only prolonged immersion in hydrazine produces complete or nearly complete expansion of this component. When allowance is made for the presence of non-clay mineral components, kaolin-mineral percentages estimated from XRD peak intensity ratios of the hydrazine complexes generally agree with values derived from differential thermal analysis to within ±10%. Kaolinite in mixtures with halloysite cannot be directly determined by intercalation procedures inasmuch as treatments which result in complex formation with kaolinite also expand halloysite. In such systems, kaolinite can be estimated by difference between the concentration of kaolin minerals and halloysite.

Hydrated kaolinite (d(001) = 10 Å) can be synthesized by mild heating of a kaolinite-organic suspension, allowing time for the clay to be intercalated by the organic solvent, and then dissolving a fluoride salt in the liquid. After mild heating of the suspension, the salt and organic solvent are removed by repeated water washings. The kaolinite retains interlayer water in the form of a 10-Å kaolinite hydrate. The influence of the intercalating agent, type of salt, concentration of salt, and the time of treatment on the synthesis of 10-Å hydrate was examined for several kaolinites. The most effective salt is NH4F; much smaller yields were obtained using KF and RbF. Not all organic molecules gave high yields of the hydrate; dimethyl sulfoxide, formamide, and hydrazine worked well but N-methyl formamide did not. The reaction between clay and salt resulted in the replacement of some hydroxyls by fluoride. This replacement was rapid; after 1 min of fluoride treatment a substantial yield of hydrate was obtained. The intercalation step separated the layers and also disordered the kaolinite, facilitating the F for OH replacement at or near crystallite edges. This replacement weakens the interlayer bonding at the edges and thereby reduces the possibility of layer collapse and attendant dehydration.

A rapid and simple test to distinguish halloysite from kaolinite in mineral mixtures has been developed based on differences in the rate and extent of formamide intercalation. With halloysite, complex formation was both rapid (<1 hr) and complete, whereas no significant intercalation occurred with kaolinite until at least 4 hr after contact with formamide, and then the process may not have been complete. Unheated halloysite formed complete complexes with formamide regardless of the interlayer hydration state of the mineral. The test, however, was inconclusive for halloysite that had been oven-dried at 110°C, although some water may still have been present in the interlayer space. The extent of formamide intercalation by kaolinite was likely influenced by sample crystallinity, and the rate of complex formation was enhanced by the addition of up to 10% v/v water to the system. Nevertheless, the formamide test unambiguously differentiated halloysite from kaolinite. N-methylformamide, which yields complexes with a basal spacing of 10.9 Å, could be substituted for formamide (basal spacing = 10.4 Å) for samples containing appreciable amounts of illite-mica.

A new osmotic-swelling system for montmorillonite with formamide (FA) is described. Water and a variety of polar organic liquids were used to swell FA-montmorillonite complexes. The post-swelling behavior of the complex is interpreted in terms of the bi-functionality of FA as an electron donor/acceptor and the magnitude of the donor number (DN) of the polar liquid relative to that of FA. The FA-montmorillonite complex shows limited swelling of d(001) <2.3 nm for polar liquids with DNs larger than that of FA. In contrast, the FA-montmorillonite complex shows osmotic swelling when the DN of the polar liquid is smaller than that of FA.

A new technique utilizing Raman microscopy and Fourier transform infrared (FTIR) microsacopy is described. This technique uses thin films of oriented clay aggregates on glass slides suitable also for X-ray diffraction (XRD). Raman microscopy proved the most useful technique providing both better resolution of the OH-stretching bands and greater spectral resolution. Kaolinites from Washington County, Georgia, with varying defect structures involving layer stacking were intercalated with formamide and additional Raman bands were observed at 3610 and 3627 cm−1. A concomitant decrease in the inner-surface OH band intensities at 3695 and 3685 cm−1 occurred. These bands are attributed to the inner-surface OH hydrogen bonded to the formamide molecule through the C=O group. The 3627 cm−1 band is sharp with a half width of 7.5 cm−1 and comprises 11% of the total normalized band area. When two additional OH bands are observed at 3610 and 3627 cm−1 two C=O bands at 1674 and 1658 cm−1 are observed also. The two additional Raman inner-surface OH bands were not observed in the IR spectra. However, a band of low intensity was observed at 3590 cm−1. Models for the intercalation of formamide in kaolinites are proposed.

Factors influencing the degree of intercalation (‘reactivity’) of kaolinite group minerals have been investigated using six kaolin minerals, with variable crystallinity and particle size. Tests were performed to measure the degree of intercalation with potassium acetate, formamide, dimethylsulphoxide and hydrazine.

These experiments indicate that intercalation degree depends on a number of factors, including particle size, degree of ordering, type of guest molecules, method of intercalation, and presence and types of impurities. Anew method for determination of the degree of intercalation is proposed, which is not dependent on the intensity of complex reflections. The results indicate that formamide is the most sensitive reagent to distinguish differently ordered kaolin minerals whereas hydrazine appears as the most appropriate reagent to differentiate kaolin minerals from other 7 Å phases.