Factors that influence the design of modified clays - or how knowing your clay can save your day

Full text: Smectites vary greatly in their permanent layer charge characteristics, including total charge, distribution of charge between tetrahedral and octahedral sheets and heterogeneity of charge from flake to flake. Smectites and vermiculites are different from the micaceous layer silicates in their ability to swell by the uptake of cations and polar and non polar solvents. Vermiculites differ from the smectites predominantly in the large contribution of tetrahedrally located charge relative to their total layer charge density. Understanding of the complex relations between layer charge and interlayer space of clay mineral surfaces can be applied toward the design of optimal organically modified clays suitable for environmental and industrial uses. In general, it is known that smectite charge density dictates the total amount of modifying organic cation that can be added to a particular clay, the orientation that the organic cation adopts within the interlayer spaces with respect to the siloxane surfaces of the clay and ultimately, the capacity of specific, organically modified clay to imbibe contaminants or other compounds. These same properties are dependent on the size and configuration of the modifying organic cation(s) as well as the percentage of the exchange capacity utilised, and thus, the amount of specific surface of the clay that is covered by the modifying organic cation. All these factors must be kept in mind in the design of inexpensive and useful modified clays. This paper reports on the application of polarised FT-IR and X-ray diffraction methods to the observation that layer charge density governs the orientation of trimethylphenylammonium (TMPA) cations in the interlayer space of smectites and vermiculites. The TMPA exchanged forms of several smectites and vermiculites were studied, whose layer charges ranged between X=0.37 and X=0.95 e- per formula unit and in which the location of charge varied with respect to the octahedral and tetrahedral sheets. Solvents (water, ethanol and toluene) were used to modify the basal spacings of the TMPA-clays, and changes to the orientation of TMPA in the interlayers were monitored with polarised FT-IR. The TMPA cation takes a different orientation in the interlayers of vermiculites than in those of smectites. For hydrated TMPA-vermiculites (layer charge X>0.7 e- per formula unit, predominantly tetrahedral), X-ray diffraction analysis indicates that the TMPA cation is orientated such that its A1 symmetry axis is normal to the siloxane surface. In contrast, for hydrated TMPA smectites (X<0.55), polarised IR analyses indicate that the TMPA cation orientates with its A1 symmetry axis parallel to the siloxane surface, but the plane of phenyl-ring (B2 axis) varies from near normal (X∼0.55) to near parallel (X∼0.4) to the siloxane surface. However, clay type and solvent uptake also influence the orientation of organic cations where sufficient interlayer space exists. Hydration of the lower charged TMPA smectites with water or ethanol increased the d-spacing and causes the A1 axis of TMPA to adopt a more inclined angle to the siloxane surface. Uptake of toluene only significantly increases the observed d-spacing of the lowest charged smectites, but results in the TMPA to orient with its B2 axis parallel to the siloxane surface. Observed and calculated charge-space relations indicate that a layer charge density for smectites of Xμ0.7 e- per formula unit is required for TMPA to orientate in the manner observed for vermiculites. This value is the layer charge by which the smectites are classified separately from vermiculites