Selectivity factors

The selectivity factors for closely related organic compounds are generally relatively low, particularly in systems where the major differences between components are based on very small non-specific interactions, such as Van der Waals forces. If the pair to be separated have acichbase or other complexing capabilities, then manipulation of the experimental environment can increase a values. Such considerations have already been discussed for single stage separations as in Chapters 15 and 16. 

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The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

Three separate factors affect resolution (1) a column selectivity factor that varies with a, (2) a capacity factor that varies with k (taken usually as fej). and (3) an efficiency factor that depends on the theoretical plate number. 

The relative selectivity of a chromatographic column for a pair of solutes is given by the selectivity factor, a, which is defined as... 

The identities of the solutes are defined such that solute A always has the smaller retention time. Accordingly, the selectivity factor is equal to 1 when the solutes elute with identical retention times, and is greater than 1 when is greater than fr A-... 

In the same chromatographic analysis for low-molecular-weight acids considered in Example 12.2, the retention time for isobutyric acid is 5.98 min. What is the selectivity factor for isobutyric acid and butyric acid ... 

Using the data from Problem 1, calculate the resolution and selectivity factors for each pair of adjacent compounds. For resolution, use both equations 12.1 and 12.21, and compare your results. Discuss how you might improve the resolution between compounds B and C. The retention time for an unretained solute is 1.19 min. 

Ion-exchange isotherms assume different shapes depending on the selectivity factor and the variations in with the level of exchange The rational selectivity coefficient includes the ionic charge and is given by... 

Additional Selection Factors. System characteristics and physical and chemical properties can be principal considerations in reactor... 

Site Selection Factors that must be considered in evaluating potential solid-waste-disposal sites are summarized in Table 25-71. Final selection of a disposal site usually is based on the results of a preliminary site survey, results of engineering design and cost studies, and an environmental-impact assessment. 

Site Selection Factors that must be considered in evaluating potential sites for the disposal of hazardous waste are covered in state and federal regulations. In Cahfornia, landfills where hazardous wastes can be received are referred to as Class I disposal sites. To quahfy as a Class I site, it must be shown that ... 

As during diagnostics the major is a selectivity of the suggested devices, the electrode selectivity factors were defined by fixed preventing ion method. As preventing used one polymeric substance to another and synthetic washing means Lotus and OMO that contains cation SAS. The received values of selectivity factors varies in l,l-10 -2,2-10 interval. 

Aromatic solvents or polycyclic aromatic hydrocarbons (PAFI) in water, e.g. can be detected by QCM coated with bulk-imprinted polymer layers. Flere, the interaction sites are not confined to the surface of the sensitive material but are distributed within the entire bulk leading to very appreciable sensor responses. Additionally, these materials show high selectivity aromatic solvents e.g. can be distinguished both by the number of methyl groups on the ring (toluene vs. xylene, etc.) and by their respective position. Selectivity factors in this case reach values of up to 100. 

Goodrich, J.A., Tjian, R. TBP TAF complexes selectivity factors for eucaryotic transcription. Curr. Opin. Cell Biol. 6 403-409, 1994. 

The discussion of materials selection factors is naturally divided into three parts (1) overall factors pertinent to selection of the composite material itself, (2) factors governing the selection of the fibers, and (3) factors essential to selection of the matrix system. Those three types of selection trade-offs will be described, followed by summary remarks on the process of selecting a suitable composite material. 

The materials selection factors that might be considered are fairly obvious and are displayed in Figure 7-20. These factors are actually the same ones that would be used when choosing a composite material as opposed to a metal. J... 

The selection of a suitable matrix for a composite material involves many factors, and is especially important because the matrix is usually the weak and flexible link in all properties of a two-phase composite material. The matrix selection factors include ability of the matrix to wet the fiber (which affects the fiber-matrix interface strength), ease of processing, resulting laminate quality, and the temperature limit to which the matrix can be subjected. Other performance-related factors include strain-to-failure, environmental resistance, density, and cost. 

Those basic matrix selection factors are used as bases for comparing the four principal types of matrix materials, namely polymers, metals, carbons, and ceramics, listed in Table 7-1. Obviously, no single matrix material is best for all selection factors. However, if high temperatures and other extreme environmental conditions are not an issue, polymer-matrix materials are the most suitable constituents, and that is why so many current applications involve polymer matrices. In fact, those applications are the easiest and most straightforward for composite materials. Ceramic-matrix or carbon-matrix materials must be used in high-temperature applications or under severe environmental conditions. Metal-matrix materials are generally more suitable than polymers for moderately high-temperature applications or for modest environmental conditions other than elevated temperature. 

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