Secondary equilibria separation

In many forms of secondary equilibria separations, the concentration of the equilibrant, or the mobile phase component which participates in the secondary equilibria, controls, at least partially, the strength and selectivity of the mobile phase. In micellar chromatography the concentration of micelles plays this role, which means that for all separations carried out with micellar mobile phases, the strength of the mobile phase can be changed while maintaining an unchanging bulk solvent composition. This unique aspect of micellar mobile phases does indeed allow the solution to "problems that cannot be solved by other means . 

Figure 224. Separation process for enhancement of energy media transformation, (a) Schematic of the process, (A) an original equilibrium, (B) separation of hydrogen, (C) secondary equilibrium, (b) Relationship between separation process and reaction equilibrium line...

The ability to separate analytes on the bases of their electrophoretic mobility, in absence of secondary equilibrium in solution and interactions with the capillary wall, is expressed by the inherent selectivity, which is given by the ratio of the motilities of two analytes (1 and 2) migrating as adjacent peaks ... 

When ideal retention dependencies prevail in the RPC separations of peptides, i.e. no secondary equilibrium effects occur, these empirical dependencies reduce to the familiar form given by equations 9 and 10. 

The position of the secondary minimum of the total free energy plus the thickness of a platelet (1 nm) was considered as the equilibrium separation between layers. The surface charge density was assumed to be constant. The Kuhn segment length was evaluated using the expression... 

Analytical chromatographic options, based on linear and nonlinear elution optimization approaches, have a number of features in common with the preparative methods of biopolymer purification. In particular, both analytical and preparative HPLC methods involve an interplay of secondary equilibrium and within the time scale of the separation nonequilibrium processes. The consequences of this plural behavior are that retention and band-broadening phenomena rarely (if ever) exhibit ideal linear elution behavior over a wide range of experimental conditions. First-order dependencies, as predicted from chromatographic theory based on near-equilibrium assumptions with low molecular weight compounds, are observed only within a relatively narrow range of conditions for polypeptides and proteins. 

Just as we equated Xeq, the position of the secondary minimum predicted by DLVO theory, with the equilibrium separation d in Chapter 1, so now we equate Xmin, the position of the secondary minimum predicted here, with the interlayer d-value. Equation 2.21... 

If two solutes are inseparable based on pH alone, secondary equilibrium can be employed to effect a separation. The following equilibrium expressions can be written [5]. 

In this case, with the P-phase semipermeable to mass transfer, the thermodynamic simulation was carried out as follows. Once a and P-phases were generated, kinetic equations were solved independently for both phases. After a differential time, the a-phase was driven to equilibrium segregating a differential amount of material rich in modifier that was incorporated in the P-phase. At this time, the P-phase, modified both by the material received from the a-phase and the evolution of species through the continuation of the polymerization, was driven to equilibrium. Under these conditions, a secondary phase separation (i.e. a phase separation inside the P-phase) took place, as shown in Fig. 24. The Y-phase (dispersed phase inside particles of the p-phase) is rich in epoxy-amine copolymer whereas the 5-phase (continuous matrix inside particles of the P-phase - also called submatrix), is rich in the modifier. While y- and 5-phases are always at equilibrium, a- and P-phases are not, due to the semipermeable character of the latter. It is observed that, as most of the phase separation takes place at conversions close to the cloud point, the P-phase keeps a significant proportion of epoxy-amine copolymer even at high overall conversions. This agrees with experimental estimations of the composition of dispersed-phase particles in rubber-modified epoxies [103,104]. 

Fig. 24. Coexistence curves when the P-phase is semipermeable to mass transfer. A secondary phase separation is produced inside the -phase leading to y and 5 phases at equilibrium (Reprint from Polymer, 35, C.C. Riccardi, J. Borrajo, R. J. J. Williams, Thermodynamic analysis of phase separation in rubber-modiiied thermosetting polymers influence of the reactive polymer polydispersity, SS41-SSS0, Copyright (1994), with kind permission from Butterworth-Heinemann journak, Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK)...

The basic mechanism of separation in MLC is fairly well understood and there is a reasonable theoretical foundation on which to build. MLC is a fascinating example of the use of a secondary chemical equilibrium in liquid chromatography. The primary equilibrium is the partitioning of the solute between bulk mobile phase and stationary phase, and the secondary equilibrium is the partitioning to micelles. This secondary equilibrium is affected by a great variety of parameters type and eoncentrations of surfactant and additives such as salts or organic modifiers (for instance, alcohols), and pH. The current knowledge on MLC interactions is exposed... 

Separation of heavy and transition metals with ion exchangers requires complexation of metal ions in the mobile phase to reduce their effective charge density. Because selectivity coefficients for heavy and transition metals of the same valency are so similar, a selectivity change is obtained only by the introduction of a secondary equilibrium such as a complexation equilibrium established by adding appropriate complexing agents to the mobile phase. 

Multiple solvent fronts are also observed with mobile phases containing solvents of different strength [8,27,41]. As the mobile phase rises through the layer it becomes depleted in the component with the greatest affinity for the stationary phase. Eventually a secondary front is formed that separates the equilibrium solvent... 

For all reversible secondary reactions, deliberately feeding BY PRODUCT to the reactor inhibits its formation at source by shifting the equilibrium of the secondary reaction. This is achieved in practice by separating and recycling BY PRODUCT rather than separating and disposing of it directly. 

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