Templates aqueous systems

The reaction of adenosine 5 -phosphoric imidazolide with adenosine analogues on a polyuridylic acid template is discussed in reference [158]. Another polymerization of oligoadenylates (pA)4 and (pA)6 on a polyuridylic acid template in an aqueous system... 

Over the last years the utilisation of supramolecular arrays of surfactant molecules as structure-directing templates [1] has been applied to the synthesis of numerous mesostructured aluminophosphates [2-11]. In most cases the preparations were carried out in aqueous systems under hydrothermal conditions, but tetraethylene glycol and/or unbranched primary alcohols were also used [2,4]. Several discussions have been made on the reaction mechanisms that are involved in the syntheses of mesostructured materials [1,12-15] and recently a number of in-situ investigations on the formation processes of mesostructured silica phases in aqueous media have been reported these studies employed small angle X-ray diffraction [16-19] as well as 2H, 13C, 29Si, and 8lBr NMR spectroscopy and polarised light optical microscopy [17]. 

The interaction of the lone pair on the nitrogen atom of an amine with a metal ion spans a considerable range from the very weak template assisted association of alkali and alkaline earth cations " to the very robust Co" and Ru" metal nitrogen bonds. In aqueous solution, most amines are basic (Chapter 4) and there will always be competition between the lone pairs on oxygen (OH" or HjO) and the lone pair on the nitrogen, for the metal center. This competition can be overcome to a certain extent by using non-aqueous systems or the free amine as a solvent." " ... 

Chitosan-coated CdSe quantum dots (CdSe/CS QDs) were synthesized through a polymer-template-assisted y-radiation route in aqueous system at room temperature under ambient pressure (Kang et al. 2008). No surfactants were used in any reaction process. The synthesized QDs were cubic zinc blende. The QDs exhibited an absorption peak at 460 nm and an emission peak at 535 nm. Under UV illumination, the QDs exhibit light green fluorescence in aqueous solution (Figure 23.4). These QDs might be useful for biological application. This method can be considered as an improvement to the conventional radiation route. 

These two kinds of surfactants are mutually insoluble in aqueous systems, and they can form separated hydrogenated surfactant micelles that coexist with fluorinated surfactant micelles. Consequently, mesoporous silica with a bimodal pore-size distribution can be obtained, because of the coexistence of these two different types of aggregates that template two different pore sizes. An example of bimodal mesoporous silica is shown in Figure 11.13. 

Currently, microelectronics relies on bulk sihca as an important dielectric material that is often used as an insulating template for further reactivity. On the nanoscale, sUica can be synthesized by polymerizing silicic acid in an aqueous system, or through hydrolysis and condensation of silicon alkoxides in the Stober synthesis [51]. The mechanism of these two methods is unique. The first method is dominated by monomers and tetra-functionalized species, such that the resultant sihcate sols are uniform, which means that they are fully hydrolyzed and grow by monomer addition. In contrast, for the second method, di- and tri-functionalized species are dominant for alkoxides. Regardless of the synthesis used, these particles induce a fractal interior with minimal morphological control due to their common template, ammonium hydroxide [51]. 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

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