Micellar mobile phase concentration change

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 . 

The same model is vahd for hybrid micellar mobile phases at fixed concentration of organic solvent, although both constants, Kas and K m, decrease when the modifier concentration increases especially for non-polar solutes. An extended model, including the effect of changes in organic solvent concentration, has also been proposed ""... 

Examination of equations 5, 6, and 7 reveals that retention can be controlled by variation of the surfactant micelle concentration, variation of pH (for ionizable species), and by manipulation of the solute-micelle binding constant (K. ) which, in turn can be influenced by additives (salt, alcohol referto data on DDT, Table VI) or the type (charge and hydrophobicity) of micelle-forming surfactant employed (refer to data in Table VII for 1-pentanol). Table VIII summarizes some of the factors that influence retention for surfactant-containing mobile phases and compares the effect of changes in these factors upon the retention behavior observed in both micellar liquid and ion-pair chromatography (81). 

Sometimes other variables must be investigated such as the pH and/or the ionic strength of the buffer in the mobile phase or the concentration of additives in the mobile phase such as for instance tensio-active substances in micellar chromatography. In such a case the first step in an optimization is to screen these factors and to identify the most important ones for the subsequent optimization. The screening (Section 6.4.2) leads to a definition of the experimental domain in which the optimum is probably situated. This is somewhat similar to the retention optimization step. It is followed by an optimization step (Sections 6.4 and 6.7), in which the most important variables are changed, often according to an experimental design. Similar methods are used in capillary zone electrophoresis. 

Solute retention varies with the concentration of propanol, butanol, and pentanol in the mobile phase in the same way as CMC does. This means that the collateral effects which change the CMC in an organic-micellar system are, at least partially, those that induce shorter retention with hybrid mobile phases the modification of bulk water and micelle. As noted, another important factor that affects retention is the modification of the structure of the stationary phase. The analogous effects on both microenvironments (micelle and stationary phase) are evident in the parallel variation of solute-micelle and solute-stationary phase partition coefficients, as the concentration of organic solvent changes. 

To illustrate the use of Eqs. 2.17-2.19, let s suppose that 0.01 M HCl is added into one liter of the 1 M SDS solution previously mentioned. It should be pointed out that a 1 M SDS solution is a highly concentrated micellar solution. The aqueous pseudo-phase pH is 1.87 and not 2.0, the pH of a classical solution the pK value in 1 M SDS is 14.25, slightly higher than the value in pure water (14.00). The pH shift is only 0.13 unit (6.5%) with a (Pn, value of 24.6%. The pH shift is smaller than 0.05 in micellar media with cp values lower than 10%, such as those used as mobile phases in MLC. This pH change can, consequently, be ignored. However, when concentrated emulsions or microemulsions are used, the pH shifts must be taken into account. 

Solute retention varies with the concentration of propanol, butanol, and pentanol in the mobile phase, in the same way as CMC does. This means that the collateral effects which change the CMC in an organic-micellar... 

A H NMR relaxation study of di-block and tri-block copolymers of ethylene oxide and 1,2-butylene oxide aqueous solutions has shown a phase transition from a micelle to a gel in the relaxation time of the ethylene oxide block, consistent with gel formation by close packing of micelles. NOE shows that the blocks of ethylene oxide and 1,2-butylene oxide interpenetrate at the core-fringe boundary.307 Similar phenomena have been observed for the tri-block copolymer of ethylene oxide and propylene oxide. At the critical micellar temperature, a marked transition in the relaxation times of the hydrophobic propylene oxide block occurs, which is attributed to a change from well-solvated mobile chains below the critical micellar temperature to a more restricted concentrated micelle-core environment above this temperature. However, no transition in the properties of the hydrophilic block of ethylene oxide has been observed. NOE data indicate that in the micelles there is considerable interpenetration of the... 

For (12), it is assumed that the volume of the micellar phase is proportional to the tenside concentration and that the partial molar volume v remains constant. It is also assumed that the ionic mobility of the micellar phase does not change on taking up a solute (Mmc = constant). In contrast to high-performance liquid chromatography (HPLC), substances which have an infinitely high kp value, that is, which are completely dissolved in the micellar phase, can be detected. In this case, the sample molecule migrates with the mobility of the micelle. 

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