Surface and Colloid Chemistry of Clays S. LEE SWARTZEN-ALLEN and EGON MATIJEVIC* Institute of Colloid and Surface Science and Department of Chemistry. Clarkson College of Technology. Potsdam. New York Received April 20.
Contents I . The Structure of Dispersed Clay Minerals A. Crystal Structure and Charge Distribution B. The Electrical Double Layer C . Particle Association in Suspension
D. Electrokinetic Phenomena I I. Interactions at the Clay Surface A . Clay-Water Interface B. Inorganic Cations C. Anions
D. Organic Molecules E. Polymers and Polyelectrolytes I I I . Colloidal Stability of Clay Suspensions I V . References and Notes
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1. The Structure of Dispersed Clay Minerals
A. Crystal Structure and Charge Distribution Clay minerals are composed of small plate-like particles ranging in diameter from a few hundredths of a micron to several microns.’.’ Making up each such platelet are one or more unit layers, stacked like a deck of cards; the crystal structure of the unit layers consists of sheets of tetrahedrally coordinated silica in combination with sheets of octahedral alumina or magnesia. Three-layer clays such as montmorillonoidsZa and illites have a unit layer consisting of a sheet of octahedral alumina or magnesia sandwiched between two sheets of A . schematic representation of the tetrahedral ~ i l i c a . ~ ~ crys?al structure of the prototype silica-alumina mineral, pyrophyllite, is shown in Figure 1; an example of a clay mineral with similar structure is montmorillonite. This structure is termed dioctahedral, as only two out of every three octahedral sites are occupied. In contrast, illites and the montmorillonoids vermiculite and hectorite (and the synthetic hectorite-type clay, Laponite) are trioctahedral; the central sheet of the unit layer is predominantly magnesia, and ideally all octahedral sites are occupied. The prototype trioctahedral mineral is mica. The exposed basal faces of these three-layer clays consist of siloxane, that is Si-0-Si. In a two-layer clay, such as kaolin, the unit layer is composed of one sheet of silica and a sheet of alumina;3.4 the basal faces of these clays are thus half siloxane and half hydroxylated alumina. Electrokinetic studies of clay sols indicate that, at all pH values above two or three, clay particles carry a net negative charge which is compensated by the presence of positive counterions. The origins of this charge on the clay lattice are believed to be isomorphic substitution, lattice imperfections, broken bonds at the edges of the particles, and exposed structural hydroxyls. van O l ~ h e n suggests ~ ~ that the principal source of the observed negative charge on the clay particles is isomor-
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phic substitution. The latter implies that the constituent metal ions of the lattice are replaced isomorphically by cations of lower charge. For relatively highly charged clays (e.g., montmorillonite, vermiculite) chemical analyses reveal sufficient substitution (approximately 80-1 00 mequiv/100 g of clay) to account, for most of the observed cation exchange c a p a ~ i t y . I~n ~montmorillonite, ,~ most of the substitution is in the central alumina sheet: in vermiculite, large amounts of substitution also occur in the silica sheet, with up to 25% of the silicon being replaced by aluminum.’ In a clay with a low cation exchange capacity (such as kaolinite), isomorphic substitution does occur, although the small amount of substitution which would account for the negative charge on the lattice (1-10 mequiv/l00 g of clay) is difficult to measure a c c ~ r a t e l y . Hence, ~ ~ ? ~ it appears likely that isomorphic substitution is the chief source of negative charge in highly charged clays, and there is some evidence to indicate that it contributes to the negative charge on clays with low cation exchange capacities. The absence of a constituent metal atom from its place in the clay crystal sructure would result in a negative charge on the lattice. Such imperfections are difficult to measure, as the very small amount of material that need be missing to contribute significantly to the particle charge is analytically negligible compared to the total weight of the same metal in the clay. Other types of lattice imperfections which may contribute to the negative charge (e.g., dislocations, localized bond breakages) are also difficult to detect analytically. It has been suggested that the broken bonds at the edges of the particles may be in part responsible for the negative charge on the lattice. However, there is strong evidence to indicate that the edges of clay particles are positively charged in the neutral and acid pH rang e ~ . ~ ~ , Electron ~-’’ micrographs of a mixtl;re of kaolinite and a negative gold show the negative gold particles to be adsorbed along the edges of the kaolinite platelets. These positive charges are believed to arise from reactions of the type
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A number of studies indicate that the edges of clay particles are positively charged at pH
