Cationic clay structure modification

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

The indirect ion exchange method consists first of a modification of the PILC with ammonia. The reaction of the gas with the H+ ions present in the clay structure after calcination results in the formation of NH4+ ions, which can subsequently be exchanged for any other desired cation from a salt solution. These ion modifications result in the introduction of cations in the PILC pores and alter the adsorption properties of the substrate. 

In addition to MMT and other cationic clays, layered double hydroxides (LDHs) with the hydrotalcite-type structure are another family of layered materials [18] which, despite they have been used in a lesser extent than MMT (and structure-related materials) as nanocomposite polymer fillers, have shown enormous more potentially advantages than natiual clays, specially concerning their purity and crystallinity and particle size control in addition, they can be also easily functionalized in different ways with different agents, thus permitting a chemical modification of the layers environment to optimize their compatibility with the polymer [19,20]. 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

Smectite clay catalysts are potential alternative adsorbents, although some modifications of the natural mineral are necessary. Interlayer sites in smectite dehydrate at temperatures above 200°C, collapsing to an illitic structure. Since the ion-exchange capacity of smectite centres on the interlayer site, collapse must be prevented if clay catalysts are to be used in thermal treatments of chemical organic toxins. The intercalation of thermally stable cations, which act as molecular props or pillars, is one... 

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

Other research confirmed the influence of the modification of bentonite in their adsorptive properties. The adsorption isotherms were determined with solutions of phenol in concentrations 50 mg L-i to 1000 mg L L in pH 6.5, at 20°C and 24 h. Structural changes were carried out with tetra-decy 1-trimethyl-ammonium bromide (TDTAB) and hexa-decyl-trimethy 1-ammonium bromide (HDTAB), with changes in 25%, 50% and 100% of capacity exchange cation. The equilibrium time was approximately 7 h and kinetic results indicated the possible presence of heterogeneous regions on the surfaces of clays modified with 25% and 50% of its cation exchange capacity. The clays modified with TDTAB and HDTAB in 100% obtained the best results for the removal efficiencies (Yilmaz Yapar, 2004). 

The use of inorganic mineral products as oil sorbents requires in most cases their chemical or surface modification, in order to ameliorate their hydrophobic character and affinity for organic compoimds. For example, modification with CF3-(CH2)2-groups can be used, in order to limit the structure collapse of silica aerogels due to water adsorption, resulting in an excellent oil sorption capacity. Cationic surfactants, such as quaternary ammonium cations can be applied in order to ameliorate the oil sorption capacity of inorganic materials, such as zeolites, clays and fly ash [30-38],... 

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