Reversed phases micelles

A) P-2 nickel boride catalyst B) preformed P-2 nickel boride added to a micelle suspension C) nickel boride formed in the presence of a reversed phase micelle medium. (Redrawn using data from Ref 46.)... 

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap Hquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

The product distribution resulting from P-carotene oxidization by 02 was studied by Stratton et al. (1993) using reverse-phase HPLC, UV-vis spectrophotometry, and mass spectrometry. The oxidation products were identified as [3-ionone, P-apo-14 -carotenal, (i-apo-IO -carotenal, P-apo-8 -carotenal, and P-carotene-5,8-endoperoxide. The formation of 5,8-endoperoxide derivative by a [4+2] Diels-Alder addition mechanism was also reported in the 02-mediated oxidation of P-carotene in reverse micelles (Montenegro et al. 2002), P-ionone (Borsarelli et al. 2007), and of the A1E retinoid derivative (Jockusch et al. 2004). 

Most HPLC applications are performed with non-polar columns, thus in the reversed-phase mode (RPLC), since it allows simple and versatile conditions. Another advantage is that in general the applied mobile phase is an aqueous buffer. Moreover in RPLC chemical equilibria such as ion suppression, ion-pair formation, metal complexation, and micelle formation can easily be exploited to optimize separation selectivity. This explains the large number of commercially available non-polar HPLC columns. " ... 

Often used in the past for problematical compounds but with gradual improvement of reverse phases increasingly less used. Useful for chromatography of very lipophilic compounds such as in the separation of different classes of lipids and in the analysis of surfactants, which tend to form micelles under the conditions used for reverse-phase chromatography A moderately polar phase often used for the analysis of sugars and surfactants... 

Khaledi, M. G., J. K. Strasters, A. H. Rodgers, and E. D. Breyer. 1990. Simultaneous enhancement of separation selectivity and solvent strength in reversed-phase liquid chromatography using micelles in hydro-organic solvents nal. Chem62 130-136. 

Kwon et al. (1997) applied reversed-phase HPLC to determine doxorubicin (DOX) loading in PEO-bPBLA micelles. Samples of 20 mL diluted to jk /mL DOX with 0.10 M sodium phosphate buffer, pH 7.4, were separated af4Dat a low rate of 1.0 mL/min. The mobile phase was a linear gradient mixture of an aqueous solution of 1 % acetic acid and ACN (15% v/vto 85% v/v). Detection of DOX was done by measuring its UV absorbance at 485 nm. 

Fig. 2.20. Phase diagram (at 25 °C) from the work by Ekwall and co-workers (cf. Refs.8 86)) for the three-component system hexadecyltrimethylammonium bromide (CTAB) - hexanol -water. Li denotes a region with water-rich solutions L2 a region with hexanol-rich solutions D and E are lamellar and hexagonal liquid crystalline phases, respectively. In the figure are also schematically indicated the structures of normal (Lj region) and reversed (L2) micelles as well as of the liquid crystalline phases. (From Ref.9Sb...

The popularity of reversed-phase chromatography can be explained by its unmatched simplicity, versatility, and scope.12 Although reversed-phase chromatography is used routinely for separating non-polar, non-ionic compounds, it is also possible and practical to separate ionic compounds on standard reversed-phase stationary-phase materials by using secondary equilibria, such as ion suppression, ion-pair formation, metal complexation, and micelle formation. To take advantage of these secondary equilibria,... 

One of the major differences between micellar chromatography and standard reversed-phase chromatography is the selectivity of the separation. As the micelle concentration is increased, solute retention decreases as a result of increased solute-micelle interactions in the mobile phase. The rate of decrease varies from solute to solute, however, since different solutes will have a different affinity for the micelles thus, inversions in retention orders are produced.34... 

Micellar electrokinetic capillary chromatography (MECC) is a mode of CE similar to CZE, in which surfactants (micelles) are added to the buffer system. Micellar solutions can be used to solubilize hydrophobic compounds that would otherwise be insoluble in water. In MECC the micelles are used to provide a reversed-phase character to the separation mechanism. Although MECC was originally developed for the separation of neutral species by capillary electrophoresis, it has also been shown to enhance resolution in the analysis of a variety of charged species.16... 

It is commonly assumed that transfer processes can be modeled by bulk phase thermodynamics and that surface or interfacial effects are negligible. These assumptions may, in the case of partitioning into amphiphilic structures formed by micelles or bilayer membranes, not always be appropriate. These interfacial solvents have a large surface to volume ratio, similar to interfacial solvents used in reversed-phase liquid chromatography. The partitioning into such phases is the basis of the chromatographic separation. 

Micellar liquid chromatography (MLC) is another variation on reversed-phase and ion-pair chromatography. In this mode, the counter ion is a surfactant of high concentration and a long-chain hydrocarbon. Micelles form when the concentration of the surfactant is increased to the point at which aggregation occurs and spherical particles are formed. The hydrophilic parts of the long-chain hydrocarbon are oriented toward the outside of the sphere with the hydrophobic end in the center of the sphere. A mixture of compounds of varying polarity partitions between the... 

Micelle solutions were originally characterized with a bulk aqueous phase where the hydrophobic carbon chains were turned inward to help stabilize the oil phase. Later, reverse micelles were also characterized, where the conditions were reversed. A bulk oil phase was used with the hydrophilic head groups turned inward to help stabilize the aqueous phase. Micelles require very stringent conditions, dictated by the molar proportions of oil, water, and surfactant. However, the formation of micelle solutions is driven by the differences in the polarity of the two groups any factor that affects the polarity, such as temperature,... 

Addition of micelle-forming agents to the buffers can help improve selectivity. The agents can be either cationic (e.g., CTAB) or anionic (e.g., SDS). As mentioned before, this technique is called MECC. It is similar to reversed-phase HPLC in that an analyte partitions between a mobile phase,... 

Just as the interiors of micelles in water provide a hydrophobic environment to solubilize chlorobiphenyls and then photodegrade them by reductive dechlorination, so do octadecyl-functionalized silica gel [89], normally used as reverse phase packing in HPLC and for solid-phase extraction, and the hquid-semisohd polydimethylsiloxane [-OSi(CH3)2 -] [90,91], also used for solid-phase extraction. In both media, reductive dechlorination is the primary photochemical pathway. 

Normal aqueous micellar media can also be employed to extract and purify components from solid matrices. Proteins have been extracted from wheat kernals using aqueous NaLS (399). This same surfactant system has been employed in an improved method for the extraction of filth from cheese (417). In another application, aqueous solutions of Brij-35 micelles have been employed to extract components (i.e. vanillin and ethylvanillin) from smoking tobacco (106). In a similar manner, various phenolic compounds have been extracted from herbal/plant leaves using nonionic Triton X-100, Brij-35, or octyl glucoside (0G) (393). In both of these latter examples, the indicated compounds could be identified and quantitated by reversed phase HPLC using as mobile phase the same micellar solutions (refer... 

Micellar Liquid Chromatography (MLC) uses surfactant solutions as mobile phases for reversed phase liquid chromatography. The two main properties of surfactant molecules, as related to chromatography, are micelle formation and adsorption at interfaces. The micelles play the role of the organic modifier, so their influence on retention has been extensively studied (1). At surfactant concentrations above the critical micellar concentration (CMC), micelles are present and the amount of free surfactant is essentially... 

When a selective solvent is used to solubilize block copolymers, a reversible assembly may occur in order to minimize energetically unfavorable solvophobic interactions. Micelle formation requires the presence of two opposing forces, i.e., an attractive force between the insoluble blocks which leads to aggregation, and a repulsive force between the soluble blocks which prevents unlimited growth of the micelle into a distinct macroscopic phase. Micelles are stabilized in the solution due to the interaction of the soluble blocks and the solvent [23]. 

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