Selectivity separable kinetics

The effectiveness of the separation (R ) in HPLC analysis is dependent on both thermodynamic factors (retention and selectivity) and kinetics factors (peak width and column efficiency)d° The relationship of resolution to other parameters can be expressed somewhat quantitatively in the resolution equation ... 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

For reactions that occur with different selectivities or kinetics on the induced sites, the implications are far more complex. Teichner has found that the selectivities for the isomerization of methylcyclopropane and for cracking of benzene and cyclohexadienes may be substantially different on activated silica or alumina than on the normal oxides (see above). In these cases, a separate rate expression may need to be included. Depending on the rate constants and relative kinetics, this can substantially change the reaction-rate expression. Further, differences in the activation energies may affect the contribution at different temperatures. If the reaction temperature is sufficient, the activation of the support may be able to occur during the course of the experiment. 

Miscible blends of poly(vinyl methyl ether) and polystyrene exhibit phase separation at temperatures above 100 C as a result of a lower critical solution temperature and have a well defined phase diagram ( ). This system has become a model blend for studying thermodynamics of mixing, and phase separation kinetics and resultant morphologies obtained by nucleation and growth and spinodal decomposition mechanisms. As a result of its accessible lower critical solution temperature, the PVME/PS system was selected to examine the effects of phase separation and morphology on the damping behavior of the blends and IPNs. 

In general, metal colloid sols are referred to as monodisperse when the particle size deviates by less than 15% from the average value, and histograms with a standard deviation a from the mean particle size of approximately 20% are described as showing a narrow size distribution. The kinetics of the particle nucleation from atomic units and of the subsequent growth process cannot be observed directly by current physical methods. The tools of the preparative chemist to control the particle size in practice are size-selective separation [50c,52] and size-selective synthesis [42,12a,50,53]. 

Another possibility is the separation of carbohydrates after complexation with boric acid [99], Although such separations are characterized by high selectivities, the kinetics for complex formation are very slow. Thus, low flow rates are required which again result in long analysis times. 

This approach is known as precombustion CO capture [4], Solvents for H S removal processes are often selected based on their ability to selectively separate H S from COj [1, 2], In chemical solvents, selectivity for H S can be achieved throng preferential reactions favoring rapid reaction of H S with amines but disfavoring its reaction with CO through steric hindrance or slow kinetics [1,2]. 

Like most other FI analytical processes, the separations are also almost always performed under non-equiiibrated conditions, and the phase transfer factors P are rarely higher than 0.3. usuall> being in the range 0.05-0.2. While this sometimes may have some unfavourable effects on sensitivity, they may be compensated for whenever necessary, by preconcentration measures during the gas-liquid separation. On the other hand, the non-equilibrium conditions may be exploited favourably to improve selectivity through kinetic discrimination (cf. Sec. 5.5.1 Tolerance of interferences in FI hydride generation systems). 

Gas separation is achieved by two alternative mechanisms kinetic separation and selective adsorption. Kinetic separation is based on the kinetically-controlled gas diffusion caused by the constrictions of the apertures of the pores. The diameters of the bottle-necked pores are in the same range as those of the adsorbed molecules. Thus, when a CMS is used in an air separation process, the oxygen molecules, which have a smaller diameter than the nitrogen molecules. 

Strength of extractant Stripping Cii/Fe selectivity Extraction kinetics Phase separation Stability Crud formation Entrainment losses... 

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