Single-phase gels

Single-phase gels and jellies can be described as three-dimensional networks formed by adding macromolecules such as proteins, polysaccharides, and synthetic macromolecules to appropriate liquids. In pharmaceutical applications, water and hydroalcoholic solutions are most common. Many polymer gels exhibit reversibility between the gel state and sol, which is the fluid phase containing the dispersed or dissolved macromolecule. However, formation of some polymer gels is irreversible because their chains are covalently bonded. The three-dimensional networks formed in two-phase gels and jellies are formed by... 

Mechanical Properties of Simple, Single-Phase Gels... 

One view of a gel is as a modified mbber. The properties of amorphous polymers change dramatically above the glass transition point where the chains become mobile. Since polymer chains are mobile in solution and we think of a simple gel as a cross-linked solution, we can regard a single phase gel as a dilute rubber. Dense polyacrylamide, for instance, is a glassy polymer. We do not want to compare the gel properties with this state but with the same polymer as a cross-linked mbber above its glass transition. 

A NMR study of water adsorbed on silica gel has been made by Zimmerman el al. 18). Transverse (Ta) and longitudinal (Ti) relaxation times of various amounts of water adsorbed at 25° have been obtained with the use of the spin-echo technique and a two-phase behavior of both Ta and T relaxation times has been observed as illustrated in Figs. 10a and b. Generally only one T value is obtained, as for a single phase, except for x/m g HaO/g solid) values in the vicinity ol x/m = 0.126. Two values of Ta... 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

EUROTS-1 was synthesized (150g) by hydrothermal treatment of a gel derived from ethyl orthosilicate and ethyl orthotitanate, in the presence of tetrapropyl-ammonium hydroxide as template [31]. Each participating laboratory performed its own calcination (3 h or 16 h at a maximum temperature of 823 K, other conditions being varible). The Ti content was 2.8%, and the calcined product consisted of a single phase of orthorhombic symmetry, with a BET surface area of 435 m2g . Its IR and diffuse reflectance spectra (DRS) have been recorded. The material had the form of small rounded cubes of about 0.15/im size. 

Figure 3. Gel permeation chromatogram of the trimethylsilylated derivatives of silicate species in the single phase solution containing (2-hydroxyethyl)trimethylammonium ions at the maximum temperature of ca. 61 °C.

It is a misinterpretation of the concept of a phase to consider these two possible sites for a counterion as separate phases. This confuses a macroscopic property of a well-defined phase (a solid) in equilibrium with another well-defined phase (a homogeneous electrolyte solution) with an internal property of an inhomogeneous single phase, the macroionic (or colloidal or gel) phase. It divides the macroionic phase into two regions that have no physical counterpart. 

Rare earth silicates exhibit potential applications as stable luminescent materials for phosphors, scintillators, and detectors. Silica and silicon substrates are frequently used for thin films fabrication, and their nanostructures including monodisperse sphere, NWs are also reliable templates and substrates. However, the composition, structure, and phase of rare earth silicates are rather complex, for example, there are many phases like silicate R2SiOs, disilicate R2Si207 (A-type, tetragonal), hexagonal Rx(Si04)602 oxyapatite, etc. The controlled synthesis of single-phase rare earth silicate nanomateriais can only be reached with precisely controlled experimental conditions. A number of heat treatment based routes, such as solid state reaction of rare earth oxides with silica/silicon substrate, sol-gel methods, and combustion method, as well as physical routes like pulsed laser ablation, have been applied to prepare various rare earth silicate powders and films. The optical properties of rare earth silicate nanocrystalline films and powders have been studied. 

Cherivin et al. developed a non-alkoxide sol-gel method for the preparation of homogeneous nanocrystalline powders of Lo.85Sro.i5Mn03 (Cherivin et al., 2006). Calcination of the dried gels resulted in the crystallization of single-phase products at 700 °C with discrete particles free of hard agglomeration (Figure 57). 

Le et al. (2006) employed "amorphous citrate" gel method to prepare single phase ceramic powders of LaNiOs with controlled grain size by thermal annealing. The target product was obtained at 650 °C with particle sizes from 30 to 65 ran at annealing temperatures from 650 to 750 °C, while the crystalline sized varied from 10 to 15 ran. Accordingly, a wide... 

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