Uses of micellar phases

The first uses of micellar phases by Armstrong were done in GPC and thin layer chromatography (TLC). This was described in Chapter 3. TLCwas a useful tool for the determination of solute partition data in micellar systems. The micellar partition coefficient, Pv, the solute-stationary phase interaction coefficient, P s, and the micellar binding constant, Kd, could be obtained from the solute-Rf parameters with an equation very similar to eq. 13.5 [22]. A number of solutes were separated by micellar TLC such as phenols and dyes [22], indicators, caffeine, biphenyl, naphthol and benzamide [23], PAHs and amino acids [24], vitamins [25] or fluorescein derivatives [26]. The low cost, low toxicity, peculiar selectivity and ease of... 

The capillary electrophoresis (CE) technique can be used with micellar phases. The first use of such phases with CE was presented in 1984 by Terabe, who called the technique micellar electrokinetic chromatography (MEKC) [39]. Its success was tremendous because it opened the use of CE to noncharged molecules and species. CE became an essential separation tool especially in the field of biology. Today ten applications are published in MEKC for only one in MLC. Numerous books and review articles describe the CE technique including the use of micellar phases [40-43]. A simplified survey is presented here. 

In principle, non-ionic species interact with the micellar phase based on their respective hydrophobicities, whereas cationic and zwitterionic species interact with the charged Stern layer of the micelles. However, because the elution order of, for example, cationic catecholamines is based on the relative hydrophobicities of the solutes, the surface interaction does not appear to be the only aspect controlling solubilization [83, 84]. The same cationic catechols are difficult to resolve by zone electrophoresis alone, even in a system where 300 000 to 400 000 theoretical plates are generated [85]. Thus, the use of micellar phases is clearly a useful means for controlling separation selectivity in the electrophoretic system. 

Alain Berthod received his PhD in 1979 from the University of Lyon. He took an assistant professor s position at the French National Center for Scientific Research (CNRS) working in electrochemistry. In 1983 he was promoted as associate professor and in 1993 as research director. He focused on the developing and the use of micellar solutions and microemulsions in chromatography. His interests lie in the separation of chiral molecules and enantiorecognition mechanisms. He has contributed to the development of the countercurrent chromatography technique that uses a sup-port-free liquid stationary phase. He was member of the editorial board of major analytical chemistry and chromatography journals. He is editor-in-chief of Separation Purification Reviews (Taylor Francis, Philadelphia, Pennsylvania). 

In some cases even the use of micellar solvent phases can be avoided and the reactions can be carried out in an entirely aqueous medium. For water-soluble reagents, catalytic reactions such as hydrogenations and hydroformylations may be carried out homogeneously in the aqueous phase with water-soluble ligands such as triphenylphosphinotrisulfonate (Sheldon et al., 1998). 

Lastly, the use of micellar mobile phases allows a convenient means of studying micelle - solute interactions (i.e. determination of binding constants) (1,10 4,105) as well as determination of surfactant CMC values (from breaks in the log k gQ vs. log C, plots)... 

High-Performance Liquid Chromatography of Organic and Inorganic Anions Use of Micellar Mobile Phase... 

General regularities of molecular dynamics and local organization of micellar phase of polyelectrolytes complexes with ionic SAS [16-22, 26] were formulated for the solution of this problem spin probes were used. Formulas of some of the last ones are presented in Scheme 3. 

Two unusual approaches to TLC of amoxicillin and other penicillins are the use of silica gel impregnated with tricaprylmethylammonium chloride with a methanol/water mobile phase [111], and the use of micellar solutions as mobile phase [112]. The latter was reported to give better separation of penicillins and their degradation products than organic mobile phases. 

Using reversed micellar phase as a liquid membrane, Armstrong and Li (66) could perform continuous extraction from and stripping to aqueous solutions in either side of the membrane. However, improvements of kinetic mass transfer parameters, as influenced by membrane thickness and stability in conditions of appropriate hydrodynamics, remain necessary. 

The literature of QSRR with LSS is dominated by a specific SSD, the I ER solute parameters V, E, S, A, and B, as defined in Equation 15.2. An extraordinary amount of attention has been paid to predict retention (24,25) and to establish phase selectivity in MEKC using LSER (5, 7, 26-31). Attempts to classify and to contrast micellar phases with basis on the LSER coefficients have been pursued by many researchers (5,26,27,29). Interesting approaches comprise the classification of micellar phases by the combined use of LSER parameters and retention indexes (32), the clustering of micellar systems by principal component analysis (26), the use of LSER parameters to compose vectors for characterization of lipophilicity scales (33), and, more recently, the establishment of micellar selectivity triangles (34,35) in analogy to the solvent selectivity triangle introduced by Snyder to classify solvents and ultimately mobile phases in liquid chromatography. 

The earliest examples demonstrating the promise of LLC materials for accelerating chemical reactions involved the use of LLC phases of commercially available ionic or non-charged surfactants in water as nanoscale reaction media. In these systems, the surfactants and resulting LLC phases were not functionalized with any catalytic or reactive groups. All the reactants and catalytic entities were from external sources and solubilized in the LLC domains during reaction. Consequently, the rate acceleration effects observed in these systems can be attributed to the same types of confinement, solubilization, and electronic interactions found in micellar catalysis systems [98-100]. 

Examples of LLC phases for enzyme stabilization and bio catalysis include the micellar and LLC phases water (or glucose in water)/oclanol/octyl-/3-D-glucoside LLC system for accelerating /3-D-glucosidase-calalyzed hydrolysis of octyl-/ -n-glucoside to form glucose and octanol [108] and the use of LLC phases of numerous commercial surfactants to accelerate the (S)-hydroxynitrile lyase-catalyzed synthesis of (S)-mandelonitrile [109]. 

Other important biphasic concepts are based on the use of room-temperature ionic liquids (cf. Section 7.3) and, more recently, supercritical C02 (cf. Section 7.4) [31]. In addition, the second phase needs not necessarily to be another solvent. An amphiphilic approach can also be based on the use of micellar systems or vesicles formed by surfactants (cf. Section 4.5). A properly functionalized ligand itself can also function as the surfactant (cf. Section 3.2.4), which can even result in the formation of very stable aggregates [32]. 

Koenigbauer, M.J. Curtis, M.A. Use of micellar mobile phases and microbore column switching for the assay of drugs in physiological fluids. J.Chromatogr., 1988, 427, 277-285... 

Li, Y.-M. Chen, L.-R. Qu, Y. Use of micellar mobile phases and an HPLC column switching system for direct injection determination of urinary free cortisol. J.Liq.Chromatogr., 1993,16, 2583—2594 [column-switching direct injection urine LOD 1.2 ng/mL]... 

Molina, M., Perez-Bendito, D., and Silva, M., Multiresidue analysis of A-methylcarbamate pesticides and their hydrol)4ic metabolites in environmental waters by use of solid-phase extraction and micellar electrokinetic chromatography. Electrophoresis, 20, 3439-3449, 1999. 

Armstrong [58] has recently pioneered the use of micellar solutions as the mobile phase in liquid chromatography. This technique appears to be very powerful and, as applied to high-pressure liquid chromatography, allows the use of cheap aqueous solvents instead of expensive, and often hazardous, organic solvents. 

Another advantage of the use of micellar solutions as mobile phases is the solubilization of nonpolar molecules. The necessary low amount of organic solvent used in micellar phases is very positive. It reduces the toxicity, flammability, environmental impact and cost of these phases. 

In 1980, Armstrong moved to Georgetown University at Washington DC. He started to apply his ideas on the capabilities of micellar phases to HPLC. The first MLC article presented the separation of phenols and PAIfc. The first published MLC chromatogram is shown in Figure 3.4 [16]. The efficiency of the o-cresol peak (Peak 7) was only 800 plates for 30 cm ODS column (h.e.t.p = 375 pm or 38 particle diameters). This low efficiency was probably not pointed out because a flow gradient was used [16]. 

Most reported procedures for the determination of compounds in Micellar Liquid Chromatography (MLC) make use of micellar mobile phases containing an organic modifier, usually a short-chain alcohol or acetonitrile. These modifiers increase the elution strength, which is particularly important for the most hydrophobic solutes, and often improve the shape of the chromatographic peaks. The most hydrophilic alcohols do not penetrate the micelles, but butanol and pentanol can be inserted into the... 

The practical potential of nonionic MLC was demonstrated by the use of micellar solutions of Brij 35 in the analysis of tobacco [18], Samples of smoking tobacco were extracted with an ueous solution of 30% Brij 35, and an aliquot of the extract was chromatographically separated without further preparation, with a 6% Brij 35 mobile phase. Comparison with an aromatic aldehyde standard mixture enabled verification of vanillin and ethylvanillin as two of the extract components. Brij 35 was chosen for this study over other nonionic surfactants (such as Tritons , Spans , Igepals or Tweens ) on the basis of its commercial availability, high purity, low cost, low toxicity, high cloud temperature, and low background absorbance, compared to the other types of surfactants mentioned. Brij 35 does not possess a strong chromophore and its absorption is minimal. 

M.J. Koenigbauer and M.A. Curtis, Use of Micellar Mobile Phases and Micro-Bore Column Switching for the Assay of Drugs in Physiological Fluids, J. Chromatogr., 71 111 (1988). 

Y.M. Li, L.R. Chen and Y. Qu, Use of Micellar Mobile Phases and an HPLC Column-Switching System for Direct-Injection Determination of Urinary Free Cortisol, J. Liq. Chromatogr., 16 2583 (1993). 

Unfortunately, common mobile phases for RPLC utilize organic solvents, which may be detrimental to the ICPs analytical performance. A decrease in sensitivity can result due to excessive solvent loading of the plasma. Higher plasma instability, increased background at certain masses due to the formation of molecular ions and carbon deposition on the sampling cone of ICP have been reported. Use of mobile phases that do not utilize file conventional organic solvents is therefore worthwile. Micellar mobile phases have been proposed for RPLC/ICP-MS speciation... 

MLC uses micellar mobile phases with classical RPLC columns. This chapter expands the field to include some mobile phases that can be considered close to micellar phases, such as normal and reverse microemulsions, bile salt solutions, and surfactant solutions in supercritical fluids. Also, this chapter rapidly surveys the use of micellar mobile phases with non-RPLC stationary phases such as size exclusion or gel permeation polymer phases. Allied techniques using micellar phases such as ion-exchange chromatography and capillary electrophoresis are also briefly presented. 

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