Single Component or Exchange

The working capacity of a sorbent depends on fluid concentrations and temperatures. Graphical depiction of soration equilibrium for single component adsorption or binary ion exchange (monovariance) is usually in the form of isotherms [n = /i,(cd or at constant T] or isosteres = pi(T) at constant /ij. Representative forms are shown in Fig. I6-I. An important dimensionless group dependent on adsorption equihbrium is the partition ratio (see Eq. 16-125), which is a measure of the relative affinities of the sorbea and fluid phases for solute. 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

This assumption conveniently permits multicomponent cases to be treated as composites of single-component cases the behavior of each species can be calculated separately and the results be superimposed on one another. The assumption is quite acceptable for analytical chromatography at low concentrations and low degree of sorbent loading but becomes untenable at high concentrations or in ion exchange with high conversion, because the solute species then affect one another s sorption behavior as they compete for the limited number of available sorption sites. In equilibrium, the stationary-phase concentration of species i then depends on the mobile-phase concentrations of all species present rather than only on that of i ... 

A variety of sorbents have been used as the stationary phase in TLC, including silica gel, cellulose, alumina, polyamides, ion exchangers, chemically modified silica gel, and mixed layers of two or more materials, coated on a suitable support. Currently in the pharmaceutical industry, commercially precoated high-performance TLC (HPTLC) plates with fine particle layers are commonly used for fast, efficient, and reproducible separations. The choices of mobile phase range from single component solvent systems to multiple-component solvent systems with the latter being most common. The majority of TLC applications are normal phase, which is also a complementary feature to HPLC that uses mostly reverse-phase columns. 

In the trace case, the interactions between various solutes present can be neglected, provided not only that cA C0 (or ca ), but also that qa Q. The non-trace case has been derived only for a single solute, in simple adsorption or for a mixed feed of solute and eluting component, in exchange adsorption. 

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