Hybrid micellar mobile phase

The most serious limitations of pure micellar solutions are their weak elution strength and poor efficiencies. As early as 1983, the addition of a small percentage of 1-propanol was found to enhance the efficiencies and decrease the asymmetries of chromatographic peaks. Later, the term hybrid micellar mobile phases was given to the ternary eluents of water/organic solvent/ micelles. Although 1-propanol is still the most frequently used additive, other alcohols (methanol, ethanol, 1-butanol, and 1-pentanol) and organic solvents common... 

The concentration of organic solvent should be low enough to make the existence of micelles possible. Such maximal amount depends on the type of surfactant and organic solvent, and is usually unknown. For SDS, the maximal volume fractions of acetonitrile, propanol, butanol, and pentanol that seem to guarantee the presence of micelles are 20%, 15%, 10%, and 7% (v/v), respectively. However, analytical reports where authors claim the use of hybrid micellar mobile phases and these maximal values are exceeded—micelles do not exist— are not unusual. In such conditions, the system bears closer resemblance to an aqueous-organic system, although the surfactant monomers still affect the retention and efficiencies. 

Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc. 2 Hybrid Micellar Mobile Phases... 

Fig. 1 Solute-micelle and solute-stationary phase interactions in hybrid micellar mobile phases (see text for meaning of equilibrium constants).

The same model is valid for hybrid micellar mobile phases at fixed concentration of organic solvent, although both constants, Aas and decrease when... 

The elution strength of hybrid micellar mobile phases was measured for a number of organic additives (alcohols, alkane diols, alkanes, alkylnitriles, and dipolar aprotic solvents, such as dimethyl sulfoxide and dioxane) added to micellar SDS, CTAC, and Brij-SS. Benzene and 2-ethylanthraquinone were used as probe compounds. The presence of alcohols, alkane diols, alkylnitriles, and dipolar aprotic solvents produced a diminution of the retention times, reaching remarkable levels for the most hydro-phobic compound (2-ethylanthraquinone). The observed elution strength order roughly paralleled the octanol-water partition coefficients of the additives, Rq/w (Fig- 2), or their ability to bind to micelles, am- In contrast, alkanes (pentane, hexane, and cyclohexane) had relatively little effect on the retention. 

Rapado-Martinez, I., Garcia-Alvarez-Coque, M. C. and Villanueva-Camanas, R. M. Liquid chromatographic procedure for the evaluation of /1-blockers in pharmaceuticals using hybrid micellar mobile phases. J. Chromatogr. A 765(2) 221—231, 1997. 

I. Rapado-Martinez, M.C. Garcia Alvarez-Coque and R.M. Villanueva Camanas, LC Procedure for the Evaluation ofP-Blockers in Pharmaceuticals using Hybrid Micellar Mobile Phases, J. Chromatogr. A. 765 221 (1997). 

As a result of the increasing use of modifiers in MLC, the need for an adequate description of the retention in hybrid micellar mobile phases appeared. Khaledi et al. [7, 8] were the first which intended to model the retention of solutes in these systems. They assumed that the linear relationship between log k and the volume fraction of organic modifier, (p, followed in conventional RPLC over a small range of values of (p, was also valid in hybrid MLC at constant micelle concentration ... 

The equations that describe the retention on hybrid micellar mobile phases were first derived on a pure empirical basis. A further concern was to find an interpretation of these equations, based on physico-chemical properties. This permitted the improvement of the descriptive models, and the evaluation of the parameters of interaction between the three environments involved in MLC stationary phase, bulk water, and micelles, according to equilibria 8.1 and 8.2 [ 18]. The coefficients in eq. 8.26 were related to several parameters of retention. From the reciprocal of this equation ... 

Instead of the organic solvent-water mixtures used conventionally in RPLC, the mobile phase in MLC consists of an aqueous solution of surfactant-forming micelles, or a ternary mixture of water, surfactant, and organic solvent (called hybrid micellar mobile phase). In the latter case, the concentration of organic solvent is maintained low enough to permit the formation of micelles the micellization process is, however, altered. 

Garcia-Alvarez-Coque MC and Torres-Lapasio JR (2002) Hybrid micellar mobile phases. In Cazes J (Ed.) Encyclopedia of Chromatography. New York Marcel Dekker. 

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 ""... 

In MLC, highly hydrophobic solutes are removed from the column more effectively with regard to conventional RPLC, transported by micelles. Also, when compounds in a wide range of polarities are eluted with hybrid micellar mobile phases at increasing volume fraction of organic... 

Improving the resolution and simultaneously reducing the analysis time are the two most important goals in optimization strategies. Several examples have been published where a hybrid micellar mobile phase was able to achieve an... 

The routine work with hybrid micellar mobile phases requires keeping some cautions usually not described in scientific reports or specialized manuals. The experimental procedure comprises two main stages the mobile phase preparation and the column conditioning. "... 

Pure micellar mobile phases are certainly attractive, considering the increasing restrictions in the use of organic solvents in laboratories. Hybrid micellar mobile phases were first belittled because some of MLC s appeal was considered to be lost. However, most reported analytical procedures in MLC utilize hybrid eluents. For most analytes, retention with pure micellar mobile phases is too high, which necessitates the addition of an organic solvent to achieve adequate retention times. This makes peak shape to improve as well. 

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