Aluminum polymers, hydroxy

Rao GV, Rao KSJ. 1992. Evidence for a hydroxy-aluminum polymer(AL13) in synaptosomcs. FEBS Lett 311 49-50. 

Hugi-Cleary D, Helm L, Merbach AE (1985) Variable temperature and variable pressure NMR study of water exchange of hexaquaaluminum(III). Helv Chim Acta 68 545-554 Hunter D, Ross DS (1991) Evidence for a phytotoxic hydroxy-aluminum polymer in organic soil horizons. Science 251 1056-1058... 

Huang, P. M., and Kozak, L. M. (1970). Adsoiption of hydroxy-aluminum polymers by muscovite and biotite. Nature (London) 228, 1084-1085. 

Vermiculites. Vermiculites are 2 1 expanding minerals with a structure similar to micas (Table 7-4). They are considered to be derived from the alteration of micas (Douglas, 1977). Cation exchange capacity is high, as is the surface area. Potassium or NHj in solution tends to be strongly fixed by vermiculites. Upon fixation of these ions, the CEC decreases and the properties of vermiculite become like those of mica. In acid soil, hydroxy aluminum polymers can be fixed in the interlayer position to form an "island-like" structure (Jackson,... 

These hydroxy aluminum polymers dissolve under acidic conditions, such as those produced by acid rains. 

This report illustrates that and hydroxy-aluminum polymers react with phosphate differently. AP tends to form soluble complexes with phosphate, whereas the polymers can be precipitated by phosphate easily. Because a rapid equilibrium between AV and polymers does not exist, the reaction products will be dependent on the nature of aluminum species originally present in the solution. 

Mechanism of Precipitation. It has been proposed (JO) that hydroxy-aluminum polymers are composed of two closely packed planes... 

Hydroxy-Aluminum Polymers by Phosphate and Various NaH2P04/AlCl3 ratios ... 

Stumm and Morgan (15) have also emphasized the chemical aspect of precipitation, although they did not take into consideration the structure of hydroxy aluminum polymers. 

Figure i. Schematic diagram of four hydroxy-aluminum polymers, hexagonal ring is composed of 6 Al and 12 OH ions (10)... 

The aluminum was determined spectrophotometrically using 8-hydroxy-quinoline in a known quantity of polymer obtained from the xylene solution and previously purified. The absence of titanium from the polymeric product, isolated in this fashion, was checked by conventional analytical methods. 

Recently, systems have been developed where the aluminum alkoxide is covalently bonded to solid porous silica [73]. This system takes advantage of the exchange reaction between the alkoxide and the hydroxy-terminated free molecules to produce a catalytic process, i.e., to produce a larger number of polymer chains than aluminum complexes present. The initiator/catalyst used can easily be recovered by filtration and recycled. In addition, the polymers obtained are free from metal residues. 

Aluminum-hydroxy species are known to exhibit different biological activity. For xample, data have shown that some leafy plants are sensitive to Al3+ while others are mown to be sensitive to Al(OH) monomers. It has also been shown that polymeric duminum (more than one A1 atom per molecule) is toxic to some organisms. Alumi-lum is one of the cations most difficult to predict in the soil solution. This is because t has the ability to form complex ions such as sulfate pairs and hydroxy-AI monomers ind polymers. 

Because of the higher affinity for higher-valence cations by the interlayer siloxane cavity, relatively low pH values allow the interlayer to become loaded with Al-hydroxy polymers. Complete occupation of the interlayer spacing by octahedrally coordinated aluminum-hydroxy polymers produces chlorotized vermiculite or montmorillonite (Fig. 4.10). Such minerals are referred to as 2 2 clay minerals. Under these conditions, the interlayer space is limited to approximately 4 A (0.04 nm) and A1 is nearly permanently lodged in the interlayer, diminishing mineral CEC. However, in most cases chlorotization is partial. Thus, Al-hydroxy islands are formed (Fig. 4.11). In such cases, the interlayer gap is also held at approximately 4 A and many interlayer... 

Figure 4.11. Schematic showing the distribution of aluminum-hydroxy polymers in the interlayer space of 2 1 clay minerals. (I) A uniform distribution (II) an atol distribution (from Dixon and Jackson, 1962, with permission).

Figure 4.12. Schematic demonstration of the potential rote of an aluminum-hydroxy polymer on cation exchangeability (from Rich, 1968, with permission).

Commercial end functional polymers have been converted to alkoxyamines and used to prepare PEO-Woci-PS. The hydroxyl group of alkoxyamine 284 was used to initiate ring-opening polymerization of caprolactonc catalyzed by aluminum Iris(isopropoxide) and the product subsequently was used to initiate S polymerization by NMP thus forming polycaprolactone-Woc/r-PS. The alternate strategy of forming PS by NMP and using the hydroxy] chain end of the product to initiate polymerization of caprolactonc was also used. 

Reaction with Aluminum Solutions of High Basicity. Results. Three hydroxy-aluminum solutions whose basicities were 2.7, 2.55, and 2.4 were used. Aluminum is known to be present entirely as high hydroxy-polymers, with little free Al , in these solutions (JO). Both aluminum and phosphate were found to precipitate immediately and completely at the isoelectric point, which was defined as the point where the sum of H2P04" and OH" equals AP on an equivalent basis. As an example, the results for the solution of basicity of 2.55 are shown in Table I. 

A similar reasoning has been proposed to interpret the formation of amorphous and crystalline Al(OH)3 (JO). A distinction, however, must be made between the two systems. Hydroxyl ion is a part of the polymer structure. By continuing addition of OH" to the isoelectric point, crystalline Al(OH)3 is obtained in a matter of hours. Nevertheless, because of steric reasons, all the phosphate ion tetrahedra can only stay between polymers and twist them to different orientations. Therefore, hydroxy-aluminum phosphates obtained at the isoelectric points at room temperature have always been amorphous. The crystallization of such precipitates involves a drastic rearrangement of the structure, and a high activation energy may be required. 

An attempt to interpret these results with the prevailing concept of the hydrolysis of AV has not been successful. Because of indisputable evidence that polymers are present in hydroxy-aluminum solutions, many chemists suggested the following expression for the hydrolysis of AF (i,2). 

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