Hydrophobic molecules, separations

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

Micelles forming above the c.m.c. incorporate hydrophobic molecules in addition to those dissolved in the aqueous phase, which results in apparently increased aqueous concentrations. It has to be noted, however, that a micelle-solubilised chemical is not truly water-dissolved, and, as a consequence, is differently bioavailable than a water-dissolved chemical. The bioavailability of hydrophobic organic compounds was, for instance, reduced by the addition of surfactant micelles when no excess separate phase compound was present and water-dissolved molecules became solubilised by the micelles [69], In these experiments, bacterial uptake rates were a function of the truly water-dissolved substrate concentration. It seems therefore that micellar solubilisation increases bioavailability only when it transfers additional separate phase substrate into the aqueous phase, e.g. by increasing the rates of desorption or dissolution, and when micelle-solubilised substrate is efficiently transferred to the microorganisms. Theoretically, this transfer can occur exclusively via the water phase, involving release of substrate molecules from micelles, molecular diffusion through the aqueous phase and microbial uptake of water-dissolved molecules. This was obviously the case, when bacterial uptake rates of naphthalene and phenanthrene responded directly to micelle-mediated lowered truly water-dissolved concentrations of these chemicals [69]. These authors concluded from their experiments that micellar naphthalene and phenanthrene had to leave the micellar phase and diffuse through the water phase to become... 

Liquid-liquid extraction is a form of solvent extraction in which the solvents produce two immiscible liquid phases. The separation of analytes from the liquid matrix occurs when the analyte partitions from the matrix-liquid phase to the other. The partition of analytes between the two phases is based on their solubilities when equilibrium is reached. Usually, one of the phases is aqueous and the other is an immiscible organic solvent. Large, bulky hydrophobic molecules like to partition into an organic solvent, while polar and/or ionic compounds prefer the aqueous phase. 

There are three main types of CDs a-cyclodextrin (a-CD), -cyclodexlrin (p-CD), and y-cyclodextrin (y-CD), which are macrocycles formed by six, seven, and eight sugar ring molecules, respectively. The spatial structure of p-CD is shown on Fig. 3. Review [19] generalizes data on the synthesis, modification, physicochemical and theoretical investigations of CDs, and certain applications particularly for enantio-separation and pharmaceutical applications. CDs are able to form host-guest complexes (pseudorotaxanes) with hydrophobic molecules such as aza-dyes... 

Extraction and separation of plant phenolics [272], corticosteroids [268], verapamil [273], hydrophobic molecules [274], and ingredients of a medicine for common cold [275] were recently performed using this technique. Seifar et al. [276] gave the mechanisms involved in such separations. 

Because of the complex nature of most biological samples, a single fractionation technique may not be adequate for the separation of the wide range of molecules present. Better resolution of some molecules is obtainal when properties other than differences in size are exploited. These include differences in ionic characteristics, affinity for other molecules and hydrophobicity. In separations that involve any one or more of these properties, the sample constituents interact with the column material and are then eluted with a suitable eluant. As a consequence of this interaction, and the use of eluants, whose properties may not closely resemble those of the medium found in vivo, the metal may dissociate from the ligand. In addition, as the complexity of the sample increases it is difficult to predict the behaviour of the various constituents. Undesirable effects leading to irreversible interaction between some molecules in the sample and the column packing material, degradation and decomposition of some constituents may result. Furthermore, it may be difficult to rid the column of certain trace metal contamination. 

Membrane distillation is considered a promising separation method applicable primarily in environmental technologies. In membrane distillation a microporous and hydrophobic membrane separates aqueous solutions at different temperatures and compositions, as shown in Figure 9. The temperature difference existing across the membrane results in a vapor pressure difference. The molecules are transported through the pores of the membrane from the high-vapor-pressure side to the low-vapor-pressure side. At least one side of the membrane remains in contact with the liquid phase. Benefits offered by membrane distillation include (202) ... 

As compared with water, ammonia s increased ability to dissolve hydrophobic organic molecules suggests an increased difficulty in using the hydrophobic effect to generate compartmentalization in ammonia, relative to water. This in turn implies that the liposome, a compartment that works in water, generally will not work in liquid ammonia. Hydrophobic phase separation is possible in ammonia, however, albeit at lower temperatures. For example, Brunner reported that liquid ammonia and hydrocarbons form two phases, where the hydrocarbon chain contains from 1 to 36 CH2 units.5 Different hydrocarbons become miscible with ammonia at different temperatures and pressures. Thus, formation of ammonia-phobic and ammonia-philic phases, analogous to the hydrophobic and hydrophilic phases in water, useful for isolation would be conceivable in liquid ammonia at temperatures well below its boiling point at standard pressures. 

When ion-pairing agents (surfactants, tetraalkylammonium salts, organic amines) are introduced into the system, their effects resemble those observed with ion-pairing chromatography. Urea (2-6 M) is known to increase the solubility in water of hydrophobic molecules in MEKC separation by very lipophilic compounds was reported improved by highly concentrated urea. 

A characteristic property of surfactant molecules is their tendeney to aggregate at interfaces. Examples are adsorptions onto solids and monolayer formation at an air-water interface. Surfactants sometimes ereate their own interface by forming very small aggregates like mieelles or vesieles to remove a portion of their structure from direct contact with a solvent. In ease of a mieelle formed with a surfactant such as Triton X-IOO, the hydroearbon ehains are in closer contact in the center and form a hydrophobic microenvironment. The ethylene oxide moieties are exposed to water with mueh greater frequeney. If a hydrophobic species is added into this micellar system, there will be a tendeney for the hydrophobic molecules to be concentrated inside a mieelle. At low concentration, the micelle system and the added hydrophobic additives ean reach a thermodynamic equilibrium, which is often called microemulsion system. At high concentration, the hydrophobic additives form their own separate phase and the surfactant molecules serve only as a decorative layer... 

In normal-phase chromatography, polar stationary phases are employed and solutes become less retained as the polarity of the mobile-phase system increases. Retention in normal-phase chromatography is predominately based upon an adsorption mechanism. Planar surface interactions determine successful use of NPC in separation of isomers. The nonaqueous mobile-phase system used in NPC has found numerous applications for extremely hydrophobic molecules, analytes prone to hydrolysis, carbohydrates, and sat-urated/unsaturated compounds. In the future, with the advent of new stationary phases being developed, one should expect to see increasingly more interesting applications in the pharmaceutical industry. 

LI Separations in the reversed-phase mode — chiral recognition mechanisms and structural features of selectands. The primary mechanism of interaction between the macrocyclic selectors and the selectands in the reversed-pha.se mode (employing aqueous buffered mobile phases) is (partial) inclusion of hydrophobic molecules or parts of the molecules, such as (substituted) aromatic rings, into the apolar cavity of the CD. It is clear that the dimensions of the CD cavity play a dominant role to facilitate this... 

The ability of micelles to enhance photoionization yields of hydrophobic molecules was demonstrated in the early 1970s. Thus, the photoionization yields of pyrene [59], phenothiazine [60] and tetramethylbenzidine [61] cations increased when these molecules were encapsulated in anionic micelles. The effect was attributed to efficient escape of electrons from the geminate charge-separated species formed within the micelle, which is accelerated by the anionic interface. The negative micellar surface imposes an electrostatic barrier between the cations, which remain with the micelle, and the aqueous electron in the bulk water phase, thus increasing the lifetimes of the photoredox products. 

The primary structure of the cell membrane, shown in Fig. 3 is a 5-nm thick bimolecular lipid film that separates intracellular and extracellular fluids. The lipid is composed mainly of the phospholipids phos-phatidylserine and phosphatidylinositol, and contains saturated and unsaturated fatty acids and sterols. The bilayer exhibits high permeability to hydrophobic molecules and low permeability to hydrophilic molecules. 

Micellar electrokinetic capillary chromatography (MEKC) is used, often, for separating neutral and hydrophobic molecules. The surfactants in MEKC have the added advantage of solubilizing proteins. This can eliminate the need for extraction or deproteinization, allowing direct sample injection. The effect of sample matrix in MEKC is less dramatic than that in CZE. A... 

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