The literature on the complexes of n-alkylammonium ions with layer silicates is characterized by marked divergences, not only in the deductions and conclusions of the different investigators but also in their experimental results. An attempt is made to reconcile and rationalize the differences, and to form a self-consistent picture of these systems.

Given a saturating solution of sufficient concentration, the complexes tend during their formation to adopt an arrangement in which silicate layers are separated by double layers of ions oriented so that the alkyl chains are extended away from the silicate surfaces at a high angle. Water molecules, salt and free alkylamine may also be present in the interlayer spaces. When the excess material is removed by washing and the complexes dried, ideally the alkyl chains from opposing surfaces move over one another and interpenetrate so as to form a single layer of ions. During the contraction, however, the chains interact with one another and tend to stick at some point short of complete interpenetration. Weak attractive forces acting to bring the silicate layers together, and consisting mainly of electrostatic forces originating in the mineral layers and van der Waals interactions arising from increasing association of opposing alkyl chains with increasing interpenetration, are insufficient to overcome this tendency fully. As a result, an alkyl chain of given length can give rise to a series of complexes differing in their XRD characteristics.

Previous workers have noted the difficulty of achieving reproducible results and some have recommended grinding the complexes, but it appears that this treatment significantly alters the nature of the end-product.

A pyridine-vermiculite complex has been investigated by three dimensional X-ray diffraction techniques. The preparation of the complex was carried out at a pH ∼ 4 using a solution of pyridinium hydrochloride. Exsolved magnesium from the octahedral positions satisfied approximately one-half of the cation exchange capacity. The rest of the exchange capacity is satisfied by hydrogen ions and pyridinium ions which are statistically distributed over crystallographic sites so that less than three electrons per Å3 are present at any given point. Three dimensional difference electron density maps did not locate the carbon atoms except to indicate that the organic molecule occupies positions in the true and in the two pseudo mirror planes in the unit cell. Water molecules were present in the interlayer in octahedral arrangement around the cation as in the natural mineral.

Quantitative gas chromatographic analyses supplemented by X-ray diffraction studies of the adsorption of ethanol and acetone (as model polar organic compounds) on homoionic montmorillonite revealed marked variation in the number of molecules associated with each exchange cation. The results show increasing association in the order K+ <Na+<Ba2+<Ca2+. K+ and Na+ associate with two and three molecules, respectively, of either ethanol or acetone, and the resulting complexes expand to form a monolayer (∼13 Å). Ba2+ and Ca2+ form both monolayer complexes as well as double layer complexes. In the single layer complexes Ba2+ associates with either four molecules of ethanol or four molecules of acetone, Ca2+ associates with five molecules of ethanol or four molecules of acetone. In the double-layer complexes the observed cation-molecule ratios are 1 : 8 for both Ba2+-ethanol and Ba2+-acetone, 1 : 10 for Ca2+-ethanol, and 1 : 8 for Ca2+-acetone.

The striking dependence of ethanol and acetone adsorption on the nature of the exchangeable cation suggests that cation-dipole interactions play an important role in the adsorption process. Structural models of the organic complexes are presented.

Sorption of an acidic, amphoteric, sulphur containing and basic α-amino-acid (glutamic acid, glycine, methionine and lysine) by copper montmorillonite was studied by chemical and X-ray methods. With glutamic acid complex formation occurs only in solution but increasing basicity of the aminoacid favours complex formation in the clay interlayers.

The materials formed by dialysis of the precipitates formed by the reaction of NaOH with mixed Mg-Al solutions are described. In addition to aluminium and magnesium hydroxides, two hydrocarbonates having Mg/Al ratios of about 5 : 1 and 2 : 1 are formed, essentially pure, from solutions with Mg/Mg + Al molar ratios of 0.8 and 0.7, respectively. X-ray powder data shows that they have partially ordered hexagonal layer structures with unit layer dimensions a = 3.048 Å, layer thickness 7.60 Å for the Al-rich compound and a = 3.072 Å, layer thickness 7.92 Å for the Al-poor compound. X-ray and chemical data show that the new compounds resemble the minerals hydrotalcite and manasseite and also the compounds described as Mg-Al hydroxides by Feitknecht.

A structural scheme is proposed for the synthetic Mg-Al hydroxycarbonates described in Part I (Gastuche, Brown & Mortland, 1967) based on chemical and X-ray powder data. The structure consists of positively-charged brucite-like layers in which Al replaces Mg up to a maximum of about one in three sites. The positive charge is balanced by an interlayer sheet containing carbonate groups and water molecules. The idealized structural formula for the Al-rich material is:

[Mg4Al2(OH)12]2+ [CO3.3H2O]2−.

A similar structural scheme seems to apply to many other materials including the minerals of the pyroaurite group, the compound 4CaO.Al2O3.13H2O and the related mineral hydrocalumite, a nickeliferous magnesium hydroxide mineral described recently and to many synthetic products that have been referred to as double sheet structures.

Amorphous alumino-silicates of different compositions were aged at 175° C under acid and basic conditions. At high pH, crystallization of two zeolites, phillipsite and chabazite, was recorded. Under acid or neutral conditions, a demixing into boehmite and a more stable alumino-silicic gel has been observed.

These results are discussed in relation to the affinity of silica for alumina.

When phlogopites containing only a little iron are artificially altered with sodium tetraphenylboron solutions, the vermiculite-like products contain fewer interlayer cations than the original micas. Structural formulae of the products are calculated from the chemical analyses, together with infrared examination of the angular vibration band of water at 1650 cm−1 to distinguish between water of hydration and hydroxyl groups. The formulae obtained are consistent with the weight-loss curves and show that the products contain, more than 4(OH,F) per formula unit. The interpretation of these formulae is that the silicate layers lose net negative charge by protonation of structural oxygen anions to form new hydroxyl groups.

Addition of aluminium to the ferrous sulphate mother solution used to synthesize lepidocrocite, γ FeOOH, yielded a substituted, though excessively hydrous (Fe,Al)OOH structure. Apart from reducing X-crystallinity, the main effect of isomorphous incorporation of aluminium in the lepidocrocite lattice was a shift, to higher values, in the temperature at which the lepidocrocite dehydration to maghemite was complete. Increased thermal stability of this substituted lepidocrocite, deduced from measurements of magnetic attraction made after serial heating, was consistent with the presence of a boehmitic component in solid solution. Magnetic susceptibility data for an oxisol indicated that a similar ferri-aluminous complex had developed pedogenically in the upper solum. This constituent was, however, amorphous to X-rays.