Forschungshistorie

The studies in the seventies were related to the hydrophobization of layered silicates and applications of such materials. It was shown that the reaction of layered silicates, in particular 2:1 clay minerals, with alkylammonium ions provides a sensitive and reliable method of identification of 2:1 clay minerals and interstratified materials and the determination of the layer charge, even in mixtures. The method also allows the detection of extremely small quantities of smectites and vermiculites in rocks, sands, or synthetic mixtures. The 2:1 clay minerals in soils can be identified and their quantities can be approximately determined.

An important result is the identification of the mixed layer character of most smectites and vermiculites. The density of the layer charges within the individual "crystals" varies from layer to layer between certain limits (heterogeneous charge distribution).

In 1971 a paper was published showing the delamination of smectitic layer silicates into the individual silicate layers and the reaggregation of these layers. As a consequence of the non-uniform charge distribution reaggregation can change the charge distribution. The colloidal aspects of these processes and the coagulation of differently charged layers were discussed within the frame of the DLVO theory. About ten years later, delamination and reaggregation were studied by P. Nadeau in connection with the concept of fundamental particles.

A central issue from 1970 to 1980 was related to the conformational changes of the alkyl chains in alkanol-alkylammonium films between inorganic layers (silicates, uranyl phosphates, uranyl vanadates). Formation of kinks and kink-blocks or gauche-blocks in cooperative reactions revealed the possible conformations of alkyl chains in the central bimolecular lipid structure of biomembranes. The importance of cis-unsaturated alkyl chains and their influence on the lipid bilayer structure could be explained. The bimolecular films were also considered as models for defects in crystalline and melted polymers.

Besides alkylamines and alkylammonium ions the reaction with different types of organic compounds such as aliphatic and aromatic hydrocarbons, alcohols, highly polar molecules (nitriles, nitrobenzene, dimethyl sulphoxide), codein, non-ionic and ionic surface active agents, poly(ethylene glycols), nuclein bases, ADP, ATP showed the diversity of bonding mechanisms with inorganic layers. The adsorption of poly(ionenes) on different clay minerals revealed the importance of charge pattern interactions.

Swelling processes of the hydrophobized silicates were studied under different aspects. As the volume available to the guest molecules is restricted, the mutual structural adjustment between the alkyl chains and the intercalated guest molecules is decisive. The clustering of water molecules between the alkyl chains was studied in several papers.

An important step in the knowledge of swelling processes was the study of adsorption from binary liquid mixtures in cooperation with Imre Dékány at University Szeged (Hungary). The first paper of a series of 15 contributions was published in 1982. Further information was obtained by calorimetric measurements of these systems which revealed the importance of the entropy changes.

A first report on crystalline silicic acids in 1973 introduced two new groups of interlamellar reactive layered compounds: the alkali silicates and crystalline silicic acids. Several of these alkali silicates are found as minerals (kanemite, makatite, magadiite, kenyaite and a few other compounds) but only synthetic products are considered for technical applications. The first synthesis of magadiite was reported in 1973. The crystalline silicic acids (we know about 20 different types of the composition SiO2 × x H2O) are obtained by cation exchange of the alkali silicates (and a few other silicates). Several crystalline silicic acids are interesting alternatives to silica. The crystal structure of most alkali silicates and silicic acids is unknown but the layer structure was clearly established by intercalation reactions. Structural information was also obtained from MAS-NMR measurements.

Another important group of compounds introduced in 1988 contains the layered double hydroxides (sometimes called "anionic clays" or "hydrotalcides" ). Presently, many groups study these materials because of their adsorptive and catalytic properties. The first paper on similar compounds, the hydroxy double salts, was published in 1993.

The intercalation behavior of many other inorganic layer compounds was studied under different aspects (1976-1987): the mica analogous compounds potassium niobate, nickel arsenate, and uranium micas, and other compounds such as uranium vanadates, zirconium phosphates and arsenates, niobium phosphate, chloro tin arsenate, manganese arsenate, silver molybdate.

The colloidal aspects of clay minerals were considered in a first paper in 1970 and in a few papers during the following years. A turn to thorough studies on colloid science is noticed in 1983. Main subjects are the stability of colloidal dispersions and the processes of aggregation of clay mineral layers, lamellae, and particles. This interest led to in-depth studies of the flow behavior of clay mineral dispersions. The colloidal properties are of outstanding importance in numerous applications of clays and clay minerals.

Combinations of "anionic clays" with smectitic clay minerals show useful colloidal properties and provide new types of fillers, thickening agents, and rheological additives.

Solid stabilized emulsions (Pickering emulsions) were studied since 1990. Combinations of smectites with harmless surface active agents such as di- and triglycerides represent effective stabilizers of oil/water emulsions. Extremely stable emulsions were prepared with combinations of bentonites and layered double hydroxides as solid stabilizers and without surfactants. The formation of shells composed of hydroxide and bentonite particles around the oil droplets and between them was visualized by X-ray microscopy.

Recent studies aim to the development of micro- and mesoporous materials from the alkylammonium derivatives of water glass solutions. An interesting possibility to obtain such materials is calcination and redispersion of bentonites and bentonite-latex composite materials.

In studies related to the environmental protection new types of pesticide formulations were developed in cooperation with Shlomo Nir and Baruch Rubin (The Hebrew University of Jerusalem). Herbicides were adsorbed by a diversity of modified clay minerals and porous materials to reduce leaching into the ground water and distribution in the environment. A interesting mechanism is entrapping the herbicide molecules in coagulating montmorillonite layers and lamellae. The successful application of several formulations was tested in Rehovot (Israel).

Sequence of the papers

year first paper on
1967 layer charge determination
1969 calorimetric measurements
1970 arrangement of alkylammonium ions
1970 delamination and reaggregation of smectites
1970 irregular-interstratified clay minerals
1971 conformational changes of interlamellar films
1971 identification of kinks and kink blocks
1973 alkali silicates: magadiite
1973 crystalline silicic acids
1973 non-silicate inorganic layered compounds
1975 NMR studies
1976 kaolinite intercalation
1982 adsorption from binary liquid mixtures
1984 general colloid chemistry
1984 rheology
1986 latex dispersions
1988 layered double hydroxides
1988 gas adsorption and porosity
1993 hydroxy double salts
1998 solid-stabilized emulsions
1999 X-ray microscopic studies
1999 ancient ceramics
2000 new pesticide formulations