Cations continued sizes

Precipitatioa (2,13—17) techniques employ a combination of nucleation and growth iaduced by adding a chemical precipitant, or by changing the temperature and/or pressure of the solution. Chemical homogeneity is controlled by controlling the rate of precipitation. FFeterogeneous precipitation iavolves the precipitation of a soHd of different composition from the solution, and the composition of the precipitate may change as precipitation continues. Coprecipitation iavolves the simultaneous precipitation of similar size cations ia a salt as a soHd solutioa. 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

A solid solution is a crystal structure in which two (or more) atom types are arranged at random over the sites normally occupied by one atom type alone. For example, in the comndum structure solid solution formed by Cr2C>3 and AI2O3, a random mixture of Cr3+ and Al3+ ions occupy the cation sites that are only occupied by one of these in the parent phases. The formula of the solid solution materials is written (Al i JCCrJC)203. In this example, x can vary continuously between 0 and 1.0. In some cases, especially when the atoms involved have different sizes, only partial solid solutions are found, characterized by a composition range in which the span of x is smaller than 1.0. Solid solutions are widely exploited as both the chemical and physical properties of the solid can be varied sensitively by changing the relative amounts of the components of the solid solution. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Schultz and Matijevic (16) prepared nanoparticles of palladium sulfide (PdS) by the continuous double-jet mixing of PdCl2 or Na2(PdCl4) and Na2S. They found that the particle size was 20-30 nm in mean diameter obtained in acidic media (pH = 2-3), but 2-5 nm in alkaline media, probably due to the high equilibrium concentration of sulfide ions S2- by dissociation of H2S and HS in the alkaline media (pH = 10-12). A cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), was found to be useful for stabilizing the small particles prepared in alkaline media. 

If opposite faces of the octahedra are used to continue their linking, the result is a strand of composition MX3 (Fig. 16.10). Strands of this kind occur among some trihalides of metals with an odd number of d electrons. Between pairs of adjacent octahedra metal-metal bonds occur /3-TiCl3, Zrl, (d ), MoBr, (d ), KuCI, KuBr, (d ). Anionic strands of the same kind are also known in compounds such as Cs[NiCl3] or Ba[NiO3] again, the comparable sizes of the cations Cs" " and Ba " " and the anions Cl and respectively, facilitate close packing. 

Crown ethers continue to be one of the most useful parts of supramolecular chemistry/91 From the beginning computations of metal ions complexes with synthetic ionophores/101 which have been aptly reviewed/111 emphasized the importance of including explicitly solvation in free energy calculations, also with ab initio calculations on calixarene complexes/121 Molecular dynamics simulations of 18-crown-6 ether complexes in aqueous solutions predict too low affinities, but at least correctly reproduce the sequence trend K+ > Rb+ > Cs+ > Na+. However, only the selection of K+ over Rb+ and Cs+ is ascribed to the cation size relative to that of the crown cavity, whereas K+ appears in these calculations to be selected over Na+ as consequence of the greater free energy penalty involved in displacing water molecules ftomNa/1131... 

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