Sieve effects, ion

The a-zirconium phosphate has a layered structure in which each layer consists of a plane of zirconium atoms coordinated octahedrally to oxygen. The free space in the sides of the layered structure is large enough to allow a spherical ion of 0.263 nm diameter to diffuse the cavity without any obstruction and is accessible to Li, Na" and K" " [139]. However, the size of the window is smaller than the ionic spheres of Rb (0.296 nm) and Cs (0.338 nm) and an ion sieve effect is encountered with these ions. 

Acid salts of Group IV phosphates and arsenates have layered structures with interlayer distances large enough to accommodate a number of ions. The size of the opening to the cavities of the acid salts with the same layered structure is dependent upon the particular composition. Table 5 lists the correlation between ion sieve effects and the interlayer distance of various acid salts [l40j. 

Table 5 Ion Sieve Effect for Alkali Metal Exhibited by Layered Acid Salts...

Unlike porous amorphous carbons, the high ratio of the external surface area to the total surface area of CNTs provides fast adsorption/desorption of electrolyte ions associated with the process of the formation of the electric double layer due to no ion-sieving effect occurring (Arulepp et al. 2006). Sorption of ions onto external surface area of CNTs makes the double-layer capacitance of CNT-based actuators less dependent on the ionic hquid species (ion dimensions) than the capacitance of amorphous carbon-based actuators, where the ion transport into the pores depends on the pore size and the size of electrolyte ions. The frequency dependence of generated strain has been attributed to the elecfrochemical kinetics different deflection amplitudes are the result of different ionic conductivities of EL species (Imaizumi et al. 2012). 

The effect of ion size on the rates of intercalation of NH J, CH3NH+, C2H5NH 3, n-C3H7NHt, i-C3H7NHt, (CH3)2NH , (CH3)3NH+, and (CH3) N+ into Z-4A zeolite has also been investigated using the pressure-jump technique (18). The results shown in Table V illustrate that Z-4A acts like an ion-sieve. For ions with a volume... 

Jhung, S.H., Lee, J.H., and Chang, J. (2008) Crystal size control of transition metal ion-incorporated aluminophos-phate molecular sieves effect of ramping rate in the syntheses. Micropor. Mesopor. Mater., 112, 178-185. 

Molecular sieve effects and their influence on catalytic selectivity offer important possibilities. Chen (48) showed that for a given reaction synthetic offretite, with its 12-membered rings of oxygen ions, exhibited no selectivity where the presence of small amounts of erionite (3%) resulted in an effective blocking of the large openings and the creation of selectivity. This emphasizes the possible influence of impurities on the practical uses of zeolite catalysts. 

The sieving effect of the carbon host was also demonstrated by measuring the capacitance values of an AC in a series of solvent-free ionic liquids (ILs) of increasing cation size [17], Since ions are not solvated in pure ILs, it was easy to interpret the electrochemical properties by comparing the nanoporous characteristics of carbon and the size of cations calculated by molecular modeling. It was found that the overall porosity of the carbon is noticeably underused, due to pores smaller than the effective size of the cations. The results with ILs confirm that the optimal pore size depends on the kind of electrolyte, i.e., the dimensions of pores and ions must match each other. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

Indirect detection techniques with UV-absorbing buffer components and non-absorbing analytes are widely applied in CZE of small molecules. To diminish electrodispersion, i.e. to achieve symmetrical peaks, the mobilities of analyte ions and background electrolyte should match closely [2]. In this case the sieving effect will dominate and peak broadening is only due to polydispersity. When mobility differences are large, electrodispersion rules... 

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