PHYSICO-CHEMICAL STUDIES ON THE INTERACTION OF CLAYS WITH SURFACTANTS

Abstract

The clay minerals comprise a complex series of aluminosilicate structures in which a number of basic types shade almost imperceptibly into each other. Basically the aluminosilicate, 'backbone' of the clays, is composed of alternating, parallel, two dimen sional layers formed from silicate tetrahedra and aluminate octahedral., The disposition of these layers and the extent and nature of isomorphous substitution within them determine to a great extent the chemical and physical properties of the material. Ideally we recognise three basic types of clay minerals. Montmorlllonite clays These are three layer minerals (Fig.la) which are based on a repeating unit consisting of one aluminate layer sandwiched between two silicate layers. In such type of clays a tetravalent silica is replaced by trivalent aluminium or in the octahedral sheet aluminium is partly replaced by divalent magnesium without the third vacant position being filled. Such substitutions by elements of lower valence results in an excess of negative charge on the lattice. The adsorption of cations, both on the interior and the exterior surfaces of the stack takes place to compensate the net negative charge on the lattice. These compensating cations may be exchanged for other cations and are known as exchangeable cations of the clay. T T TT T T T TTTTTT T T '[WOO WOOWO 0 1OOWOOWOOW i JTTTTTTT TTTTTTTTT. ++ Woter , M N ^TTTTTTTTTTT1 ^owo0wwoQwQo TTTTTTTTTTTT{ C-oxis Water, M N* ITTTTTTTTTTTTi qWoowwoqwwo01 ^TtTtTtTtTtTtI + . . + •* Woter, M N TTTTT T TTT. (a) TTV TTTVTT V T T TTT TTV 0 O O O 0 Oi 0 0 0 0 0 T T V T V V T T T T .+ ..++ Water, M N iTVTVTTVTTT' 0 rt 0 _ 0 „ 0 0 01 ooooo |TTVTT'VTTTT Water, M+ N TTVTTVTTTT Oo0o°o0o0n0o| VTTVTTTT VTTI Water, M+ N*' tttvtt¥t (b) j T T T T T JT T T T T T ooooo • o ooo o: Water ItItLtItTj 0o0o0o0o0o°< Water tiJj_Il1lt 0o0o0o0oOqJ Water JTTTTTTL (c) Schematic diagram illustrating the three principal types of clay minerals: (a) Montmorillonite, (b) Illite; (e) Kaolinite. T = Tetrahedral Silicate 1 , 0 * Octahedral aluminate layers, V= Isomorphous substitution at tetrahedral sites, W= Isomorphous subs- + + + tituiou at octahedral sites. M , N ,balancing cations. FIG.1 2 The most typical property of this clay Is the phenomenon of interlayer swelling with water, Water penetrates between the unit layers and pushes them apart a distance equivalent to 1-4 monomolecular layers of water, which increases the C spacing from 10 A0 to 12.5 - 20 A°. Illltes These clays have the same basic structure (Fig.lb) as montmorillonites. The total amount of lattice substitution is larger than in the case of montmorill onites and is predominantly that of silica by aluminium In the tetrahedral sheet. The striking feature of these clays is that the compensating cations are mainly potassium ions. These minerals do not show interlayer swelling which is attributed to the strong electrostatic attraction between the potassium ions and the two charged unit layer on each side. In the absence of interlayer swelling these cations are not available for exchange. The basal spacing of these minerals is about 10A°. Kaolinites It is the simplest type (Fig.lc) and is found in China clay deposites. In this type the basic unit is a double layer consisting of one silicate and one aluminate layer with negligible isomorphous substitution in either of them. Units such as these are stacked together along the c-axis of the crystal, which is perpendicular to the layer 3 planes, at the edges and corners of which there will normally be free hydroxyl groups. The free hydroxyl groups may be exchanged with anions when immersed in salt solution. Since they are weakly acid, the hydrogen in them may also exchange with cations. As a result, the material shows a small exchange capacity which is approximately the same for both anions and cations. Phenomenon of ion-exchange The clay minerals have the property of sorbing certain cations and anions and retaining them in an exchangeable position. These ions are held around the outside of the silica alumina clay mineral structural unit, and the exchange reaction generally does not affect the structure of the silica alumina packet. The exchange reaction is stoichiometeric and the exchange capacity is measured in terms of milliequivalents per 100 gms-of clay. The importance of this property is shown below. In agricultural soils, plant foods are frequently held in the soils as exchangeable ions, and consequently their persistance in the soil and their availability for Plant growth depends on exchange reactions. The retention and availability of potash added in fertilizers depends on cation exchange between the potassium salt and the clay mineral of the soil. The replacement of the Na by another ion, usually Ca will make the soil more suitable for agriculture.