G. Lagaly

Interaction of alkylamines with different types of layered compounds

Abstract Long chain alkylamines C n H 2 n +1 NH 2 can be successfully used as guest molecules to test an intracrystalline reactivity of layered materials. A large variety of host compounds consisting of neutral layers intercalate alkylamines. Generally the alkylamines between the layers aggregate to paraffin-type structures. If the successive layers are negatively charged and separated by an interlayer region containing exchangeable cations, alkylammonium ions can be bound by cation exchange. The alkylammonium ions between the solid surfaces aggregate in a diversity of structures. Besides paraffin-type structures of the chains in all-trans conformation (exemplified by KniAsO 4 , aggregates of chain containing gauche-conformations are commonly formed. Typical are gauche-block structures as in alkylammonium silver decamolybdate. The exchangeable cations in the interlayer region of many non-silicatic compounds are not quantitatively exchanged. The exchanged proceeds to such an extent that distinct types of alkyl chain arrays can be formed. In many cases, the required packing density of the chains is obtained by an additional uptake of alkylamine during exchange of the alkylammonium ions. Under very distinct conditions crystals of several alkylammonium derivatives disintegrate into individual layers or thin packets of a few of such layers, so that colloidal dispersions form.

Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions

The transition between stable colloidal dispersions and coagulated or flocculated systems is a decisive process in practical applications of million of tons of bentonites (containing the clay mineral montmorillonite). Dispersion into the colloidal state requires the transformation of the original bentonite into the sodium form, for instance by soda activation. Therefore, we review here the coagulation of sodium montmorillonite dispersions by inorganic and organic cations and the influence of compounds of practical interest such as phosphates, cationic and anionic surfactants, alcohols, betaine-like molecules and polymers like polyphosphates, tannates, polyethylene oxides with cationic and anionic end groups, and carboxy methylcellulose. Typical properties of the sodium montmorillonite dispersions are the very low critical coagulation concentrations, the specific adsorption of counterions on the clay mineral surface, and the dependence of the cK values on the montmorillonite content in the dispersion. In most cases coagulation occurs between the negative edges and the negative face. The phosphates Na2HPO4, NaH2PO4 and Na4P2O7 increase the edge charge density and change the type of coagulation from edge (−)/face (−) to face (−)/face (−) with distinctly higher cK values. Polyanions like polyphosphate and tannate stabilize in the same way. Carboxy methylcellulose causes steric stabilization. Montmorillonite particles with adsorbed betaine-like molecules provide an example of lyosphere stabilization.

Principles of flow of kaolin and bentonite dispersions

Flow of kaolin and bentonite dispersions is decisively determined by edge(+)/face(−) contacts (card-houses) in an acidic medium and face(−)/face(−) contacts (band-like structures) in an alkaline medium. Formation of the different networks depends on pH and CaNa ratio. Calcium ions promote face(−)/face(−) contacts and stabilize band-like structures. In alkaline dispersions of homoionic sodium smectites and at low salt concentration, sodium ions cause disintegration of the particles into thinner lamellae and stacks of silicate layers which, at low solid content (about <5%), move independently under applied stress (Newtonian flow). These dispersions are sensitive to the CaNa ratio. Some amounts of calcium ions link the lamellae and stacks of layers to form band-like networks, and the consistency of the dispersion increases considerably. Admixed crystalline or non-crystalline materials affect the flow of clay dispersions when they interact with the clay minerals. An example is the influence of iron oxides. Organic compounds can stabilize or destabilize networks, which is demonstrated for surface active agents and kaolinite.

Clay mineral-organic interactions

Abstract Organic compounds can interact with clay minerals by i) adsorption at the external surfaces, ii) adsorption at the external and internal surfaces, iii) by exchange of exchangeable ions at the external surfaces, iv) by exchange of exchangeable ions at the external and internal surfaces, and v) by grafting reactions with silanol and aluminol groups leading to covalent bonds. Kaolin minerals intercalate only are a limited number of compounds whereas the reactions of 2:1 clay minerals, in particular smectites and vermiculites, are very manifold. Special attention is given to the interaction with neutral organic molecules such as alcohols, fatty acids, amines, amino acids, aromatic compounds, macrocyclic compounds, and nuclein bases. The interaction with complexes and dyes also provides the basis of advanced applications of clay minerals. Binding of long chain alkylammonium ions is a fundamental reaction for hydrophobising clay mineral particles as needed in many applications. The interaction of clay minerals with polymers including proteins is not only an actual field of research but also of practical importance. Organo-clay minerals are used as effective adsorbents. As these materials also adsorb solvent molecules together with the adsorptive, the adsorption process must be considered as adsorption from binary solution which, therefore, is also described in this chapter.

Sol-gel transitions of sodium montmorillonite dispersions

Abstract The flow behavior of sodium montmorillonite dispersions at salt concentrations below the critical coagulation concentration is determined by the influence of the ionic double layers on the mobility of the particles (secondary electroviscous effect). At solid contents above 3% the dispersions became gel-like with the appearance of a yield value and viscoelastic properties. Increasing salt concentration reduced the thickness of the diffuse ionic layers and the immobilization of the particles. As a consequence, the yield value and the viscosity decreased to a minimum at about 2–20 mmol/l NaCl (depending on the montmorillonite). This behavior was virtually independent on the type of salt. Above the electroviscous minimum the values of the flow properties increased with the salt concentration. Again a gel formed because the interaction between the edges(−) and faces(−) and, at somewhat higher salt concentration, between the faces(−) became attractive. High yield values and storage moduli were observed. The reversible part of the compliance reached 60–70%. The gel-like dispersion showed pronounced thixotropy. At salt concentrations above 400 mmol/l NaCl and solid contents below 2–3% (depending on the montmorillonite), viscosity, yield value, storage modulus, and reversible compliance decreased again because the gel transformed into a sediment. The cause is the contraction of the network into distinct particles when the attraction between the silicate layers is too strong. Formation and properties of the attractive gel were influenced by the type of salt. Potassium and cesium ions enhanced the elasticity of the gel. Sulphate anions reduced the yield value and storage modulus. This effect was very strong with diphosphate which liquefied the gel to a sol. The different states of sodium montmorillonite dispersions: sol, repulsive and attractive gel, sediment, are represented in phase diagrams which were constructed on the basis of compliance (creeping) measurements.

Clay-organic interactions

Clay minerals interact with organic materials by adsorption, intercalation and cation exchange. Basic principles of intercalation reactions were obtained with kaolinite which intercalates a limited number of neutral organic compounds. The interaction of neutral organic compounds with mica-type layer silicates (2/1 clay minerals) is of quite different type. As illustrated for the interaction with nuclein bases, the adsorption can be strikingly dependent on the layer charge and the concentration of salts and co-adsorption phenomena can occur. Various organic materials are bound by cation exchange. Besides some other examples, the reaction with alkylammonium ions is of interest because of widespread practical applications. From a more scientific point of view, the interactions of alkylammonium ions with clays provide models for studying surfactant aggregations on solid surfaces and possible conformational changes in aggregates of long chain compounds (mono- and bimolecular films, as in biomembranes). Negatively charged organic ions can also be bound by clays. The main mechanisms are binding by positive edge charges or exchanging structural OH-groups.

Intercalation and exchange reactions of clay minerals and non-clay layer compounds

AbstractPresently, a large variety of layered materials are synthesized that are able to intercalate neutral guest molecules or to exchange inorganic and organic ions for interlayer ions. Several of these materials are also found as minerals.The intracrystalline reactivity of a few selected compounds will be described and compared to clay minerals:- intercalation into crystalline silicic acids;- reactions of phosphates, arsenates, and sulfates;- reactions of titanates, niobates, and molybdates with long chain alkylammonium ions, and- anion exchange properties of double hydroxides. A general conclusion is that the non-clay minerals in many ways behave like clay minerals, but there is no doubt that the reactivity of clay minerals and the variety of their reactions cannot be exceeded by any other material.

LAYER CHARGE HETEROGENEITY IN VERMICULITES

The broad charge heterogeneity typical of nearly all smectites is not necessarily characteristic of vermiculites. In addition to vermiculites with pronounced heterogeneity, minerals with no or only limited charge heterogeneities are known. Layer charge and charge heterogeneity of 25 vermiculites were determined by alkylammonium ion exchange. The comparison of experimental basal spacings with dL/n-plots provided a simple determination of the average charge density. The spacings of high-charged vermiculites (≥0.8 eq/(Si,Al)4O10) with paraffin-type interlayers follow a straight line in the dL/n-plots. Lower-charged vermiculites were recognized by stepwise increasing spacings due to mono-, two-, or three-layer chain packings. Charge heterogeneity produced a superposition of the dL/n-curves for different charges, and the basal reflections of some of the alkylammonium derivatives became nonintegral.РезюмеШирокая разнородность заряда, типичная для почти всех смектитов, не является характерной для вермикулитов. В дополнение к вермикулитом со значительной разнородностью известны также минералы с нулевой или ограниченной разнородностью заряда. Слоевой заряд и разнородность заряда 25 вермикулитов определялись путем обмена ионов алкиламмония. Сравнение величин экспериментальных промежутков с графиками dL/n позволило определить очень просто среднюю плотность заряда. Расстояния сильно заряженных вермикулитов (.-0,8 экв/(Si,Al4O10) с внутренными слоями типа парафина составляют прямую линию на графиках дь/п. Слабее-заряженные вермикулиты были распознаны на основании увеличивающихся шагами промежутков в результате моно-, двух-, или трех-слойных цепных упаковок. Разнородность заряда вызывала наложение кривых dL/n для разных зарядов и основные отражения некоторых дериватов алкиламмония становились неинтегральными. [E.C.]ResümeeIm Gegensatz zu den Smectiten, für die eine mehr oder weniger breite Ladungsverteilung typisch ist, kommen neben Vermiculiten mit ausgeprägter Ladungsheterogenität auch Vermiculite mit recht gleichmäßig verteilten Ladungen vor. Schichtladung und Ladungsverteilung yon 25 Vermiculiten wurden durch die Alkylammonium-Methode bestimmt. Der Vergleich der Schichtabstände mit theoretischen dLn-Diagrammen bietet eine sehr einfache Möglichkeit zur Schichtladungsbestimmung. Hochgeladene Vermiculite (≥0.8 eq/(Si,Al)4O10) mit paraffinartigen Zwischenschichtstrukturen sind an dem linearen Anstieg des Schichtabstandes mit der Alkylkettenlänge zu erkennen. Bei niedriger geladenen Vermiculiten ändert sich der Schichtabstand infolge der Anordnung der Alkylammoniumionen in mono-, bi-, und pseudotrimolekularen Schichten stufenweise. Ladungsheterogeneität führt zur Überlagerung der für homogene Vermiculite geltenden dL/n-Kurven und dem Auftreten nicht-integraler Basisreflexe bei bestimmten Kettenlängen.RésuméLa grande hétérogénéité de charge typique de presque toutes les smectites n’est pas nécessairement caractéristique des vermiculites. En plus des vermiculites à hétérogénéité prononcée, des minéraux n’ayant aucune hétérogénéité de charge, ou une hétérogénéité de charge limitée sont connus. La charge de couche et l’hétérogénéité de charge de 25 vermiculites ont été déterminées par échange des ions alkylammonium. La comparaison d’espacements de base expérimentaux avec des diagrammes dL/n a permis une détermination simple de la densité de charge moyenne. Les espacements de vermiculites à charge élevée (≥0,8 eq/(Si/Al)4O10) avec des interfeuillets de type paraffine suivent une droite dans les diagrammes dL/n. Les vermiculites à charge plus basse ont été reconnues par des espacements croissant par étape à cause de structures en chaines à une, deux, ou trois couches. L’hétérogénéité de charge a produit une superposition des courbes dL/n pour différentes charges, et les réflections de base de certains dérivatifs alkylammonium sont devenues nonintegrales. [D.J.]

General Introduction: Clays, Clay Minerals, and Clay Science

Abstract Clays and clay minerals are recognized as the materials of the twenty-first century. Chapter 1 provides a general introduction into clay science, illustrates the classification of the clay minerals (planar and non-planar 1:1 and 2:1 clay minerals), shows the idealized formulae of some representative clay minerals, lists the current names of clays, and reports the important properties of clay minerals. As yet, there is no uniform nomenclature in clay science, a unifying terminology is proposed that should be acceptable to all disciplines, users, and producers. This mainly concerns the terms “clay and clay mineral“, “associated minerals and associated phases“, “particles and aggregates“, “swelling“, and “delamination and “exfoliation“. Finally, in addition to belonging to the class of silicates, three alternative concepts of clay minerals are proposed to extend the benefit to a wider scientific audience.

Rheological properties of sodium montmorillonite dispersions

The rheological behaviour of dispersions of montmorillonite in water is highly pH- and salt-dependent. This is exemplified for 4% (wt/wt) dispersions of montmorillonite from Wyoming and its homoionic sodium form in water, 0.01 M NaCl and 0.1 M NaCl solutions, at different pH and at temperatures from 20°C to 60°C. Generally, the shear stress (for instance, at shear rates of about 100 s−1) decreases sharply in an acidic medium to a minimum around pH 6 (pH 7.5 for the homoionic sodium form) and then increases again very strongly. Thixotropic and antithixotropic behaviour depend on pH, salt concentration and temperature. The presence of Ca2+ ions (molar ratio Na/Ca about 6.5/1) in the “pristine” montmorillonite affects the type of flow very sensitively. Ca2+ ions convert diffuse ionic layers into quasicrystalline structures with a central layer of gegen ions. Attractive potentials are then created in the contact regions between the particles, even in diluted salt solutions. Formation of band-like structures (as in the “Bandermodell” of A. Weiss) is promoted. Further, results are reported on the influence of particle size (fractions < 0.06 μm to 2 μm) on the flow behaviour at different concentrations of homoionic sodium montmorillonites from Amory and Cameron.