Cation layer silicates

Once a layer-silicate clay forms, it does not necessarily remain in the soil forever. As conditions change it too may weather and a new mineral may form that is more in equilibrium with the new conditions. For example, it is common in young soils for the concentrations of cations such as K, Ca, or Mg in the soil solution to be high, but as primary minerals are weathered and disappear, cation concentrations will decrease. With a decrease in solution cations, a layer-silicate such as vermiculite will no longer be stable and can weather. In its place. 

Three main types of composites can be formed when the layered clay is incorporated with a polymer, as shown in Figure 2 (Alexandre and Dubois, 2000). Types of composites formed mostly depend on the nature of the components used (layered silicate, organic cation and polymer matrix) and the method of preparation. 

The sheets consisting of tetrahedron-octahedron-tetrahedron layers in cation-poor sheet silicates are completely planar due to the symmetrical environment of the cation layer. If the sheets are electrically neutral as in talc, the attractive forces between them are weak as a consequence, the crystals are soft and easy to cleave. The use of talc as powder, lubricating agent, polishing material and filling material for paper is due to these properties. 

In characterizing layered silicate, including layered titanate (HTO), the surface charge density is particularly important because it determines the interlayer structure of the intercalants as well as the cation exchange capacity (CEC). Lagaly proposed a method of calculation consisting of total elemental analysis and the dimensions of the unit cell [15] ... 

Fig. 3. Possible arrangements of long-chain quaternary ammonium cations (here tetradecyltrimethylammonium) in the interlayer space of expanding 2 1 layer silicates. After Lagaly (1982) and Jaynes and Boyd (1991a).

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

Xu SH, Boyd SA (1995) Cationic surfactant adsorption by swelling and nonswelling layer silicates. Langmuir 1 2508-2514... 

Recently, a detailed study was reported for a molecularly ordered layered silicate surfactant mesophase [125, 126]. A subtle change in the surfactant which is used for the synthesis of such mesophases appears to induce dramatic modifications in the silica ordering. When cetyltrimethylammonium (C16NMe3+) cations are employed, then only two 29Si NMR resonances at —102 ppm (Q3) and... 

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

Electron spin resonance (ESR) is a useful technique for investigating the mobility and orientation of exchange cations at the surface of layer silicate clays in various states of hydration. Using Cu2+ and the charged nitroxide spin probe, TEMPAMINE+... 

Figure 1. Cross-sectional diagram of an expanding 2 1 layer silicate showing the octahedral layer, tetrahedral layer, and hydrated exchange cations in the interlayer.

Layer silicates, if initially in the dry state, can expand in water by hydration of the exchangeable cation and the surface. 

For layer silicates with low structural charge (i. . smectites), this expansion is limited to about four molecular layers of water if the exchangeable cation has a charge of +2. Since the silicate platelet is about 0.96 nm thick, the repeat spacing along the c-axis is then approximately 0.96 + (4x.26) = 2.0 nm. 

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

However, when protonated TEMPAMINE adsorbs by cation exchange on fully hydrated layer silicate clays (10, 11), the spectrum becomes less symmetrical as shown in Figure 5. The beidellite and montmorillonite spectra have line shapes typical for nitroxide molecules with rotational frequencies on the order of 10 Hz (17). 

The g-values and A values of Table IV reveal that the particular layer silicate has more effect on ESR parameters of adsorbed Cu " - than saturation of exchange sites with different cations such as Na+ and Ca +. Also, the smectites as a group have lower g and higher A values than vermiculite. From the perspective of molecular orbital theory, low g and high A values correspond to more covalent bonds between Cu + and the ligand (19). Thus,... 

The layer silicates comprise tetrahedral sheets of silica and octahedral sheets of aluminium and magnesium hydroxide, with varying amounts of the Si, Al and Mg replaced by cations of lower valence giving the lattice a net negative charge. Two basic combinations occur 1 tetrahedral sheet with 1 octahedral (e.g. kaoUnite, halloysite), and 2 tetrahedral with 1 octahedral (e.g. smectite, vermiculite, illite). 

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