Aggregations crystalline solids

A. Hasmy and R. Jullien Sol-Gel Process Simulation by Cluster-Cluster Aggregation. J. Non-Crystalline Solids 186, 352 (1995). 

When crystallized from an acetone-water mixture, benzoic acid is obtained as the crystalline needles shown in Figure 1 A. On the other hand, a mixture of needles and irregular crystalline solid is obtained from methanol-water mixtures (Figure IB). From pure water, aggregates of monoclinic crystals are obtained (Figure 1C). 

Metastable amorphous solids can in general be prepared from stable phases by bringing in excess free energy [5]. In the case of water, amorphous solids have been prepared from stable phases in all three aggregate states from the gas, the liquid, and the crystalline solid [131]. 

An important point in the science of conjugated polymers is the dependence of their optical properties as a response to their physical state (solution, aggregated, liquid crystalline, solid state, thermal treatment) [50]. Processable PPEs, particularly dialkoxy-PPEs, have been known since the early 1990s [1], and their spectroscopic behavior has been studied both in... 

The most important compound is lithium aluminum hydride, LiAlHt, a nonvolatile crystalline solid, stable below 120°C, that is explosively hydrolyzed by water. In the crystal there are tetrahedral AlHf ions with an average A1H distance of 1.55 A. The Li+ ions each have four near hydrogen neighbors (1.88 - 2.00 A) and a fifth that is more remote (2.16 A). Lithium aluminum hydride is soluble in diethyl and other ethers and can be solubilized in benzene by crown ethers. In ethers, the Li+, Na+, and R4N+ salts of A1H and GaH4 tend to form three types of species depending on the concentration and on the solvent, namely, either loosely or tightly bound aggregates or ion pairs. Thus LiAlHt is extensively associated in diethyl ether, but at low concentrations in THF there are ion pairs. Sodium aluminum hydride (NaAlH) is insoluble in diethyl ether. 

Crystalline solids need not be single crystals. Usually they are composed of an aggregate of crystals which can be distinguished as separate entities under the microscope. They can also be composed of crystallites, that is, of crystals in which the pattern repeats itself a few times in each direction. In this case, they cannot be resolved by ordinary microscopic examination. Crystallite size is extremely variable and not easily measurable. However, sometimes crystallites are so nearly parallel to each other that the solid as a whole can be called a single crystal (L4). 

Surfactant molecules by definition have polar groups such as ions or ethylene oxide chains and nonpolar groups such as hydrocarbon or fluorocarbon chains. When they are added to water, aggregation normally occurs at fairly low concentrations to minimize the area of contact between the nonpolar groups and water. For low temperatures and molecules with long, straight hydrocarbon chains, separation into a crystalline solid phase and a dilute aqueous solution of molecularly... 

The ordered, mesoporous materials can be prepared by many synthetic routes. This is not surprising since the lack of crystallinity does not place as many demands on the assembly process as with zeolites. By following the assembly process with in situ NMR, we were able to show that the high temperature construction of MCM-41 involved the formation of organic aggregates that subsequently ordered silica to form the final composite [18]. At other conditions, the assembly process can be different and involve layered phases [19]. Since it is now established that layered materials can be transformed into crystalline solids, e.g., MCM-22 [20], FER [21], VPI-5 [22], ERB-1 [23], the lack of crystalline mesoporous materials is not likely due to the inability to form layered intermediates. 

Polymeric association of different molecules through multiple hydrogen bonding has been used for the formation of non-liquid-crystalline bulk solids [12], fibrous solids for gelation in solvents [119], and as monolayers [120], Simpler H-bonding such as the interaction between carboxylic acid and pyridine has also been shown to be useful for one- or two-dimensional aggregates in solid states [121-124], For example, a one-dimensional polymeric complex from an A-B type monomer is formed in crystalline solids [121],... 

Secondary bonding forces, as we saw earlier, are responsible for intermolecular bonding in polymers. You will recall also that these forces are effective only at very short molecular distances. Therefore, to maximize the effect of these forces in the process of aggregation of molecules to form a crystalline solid mass, the molecules must come as close together as possible. The tendency for a polymer to crystallize, therefore, depends on the magnitude of the inherent intermolecular bonding forces as well as its structural features. Let us now discuss these in further detail. 

Chain flexibility — In the process of aggregation to form a crystalline solid, polymer molecules are opposed by thermal agitation, which induces segmental rotational and vibrational motion. Polymers with flexible chains are more susceptible to this agitation than those with stiff backbones. Consequently, chain flexibility reduces the tendency for crystallization. 

The field of supramolecular chemistry is concerned with a large number of systems ranging from simple host-guest complexes to more complicated solution assemblies, as well as two-dimensional (organized monolayers) and three-dimensional assemblies (crystalline solids). Nonco-valent interactions play an important role in the kinetic assembly and thermodynamic stabilization of all these systems and constitute their most distinctive feature. Electron-transfer reactions can obviously be affected by supramolecular structures, but the reverse is also true. It is possible to alter the structure and the thermodynamic stability of supramolecular assemblies using electrochemical (redox) conversions. In other words, electron-transfer reactions can be utilized to exert some degree of control on supramolecular aggregates. Provided in this article is an overview of the interplay between supramolecular structure and electron-transfer reactions. 

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