Solubilizers hydrophobes

The formation of a microphase structure can be sensitively detected by using hydrophobic fluorescent probes. Hydrophobic microdomains tend to solubilize hydrophobic small molecules present together in aqueous solution. For example, diphenylhexatriene (DHT) is hydrophobically bound to the St aggregates in ASt-x in aqueous solution and, as a result, the fluorescence intensity is greatly enhanced. Figure 9 shows the fluorescence intensity of DHT in the presence of ASt-x relative to the intensity in its absence (I/I0) as a function of the ASt-x concentration [29],... 

The rates of intermolecular Diels-Alder reactions of hydrophobic dienes and dienophiles are significantly increased when the cycloadditions are performed in pure ethylene glycol (EG) [49a]. Some examples are illustrated in Scheme 6.30. This performance is due to the fact that the EG (i) forms extensive hydrogen bonding, (ii) is able to solubilize hydrophobic dienes and dienophiles, and (hi) forms molecular aggregations with the reactants. 

Dendrimers can be designed to have a hydrophobic interior and a hydrophilic periphery. This gives them properties that are similar to those of conventional surfactants, and they can solubilize hydrophobic substances such as pyridine in aqueous solution by including them as guest molecules. They are therefore effectively mimolecular micelles. 

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. 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

Neutral pentaethylene glycol side chains have previously shown the ability to solubilize hydrophobic cores in water [94]. A similar approach was used to bring about solubility of the mPE backbone in water, where hexaethylene... 

Thayumanavan s group reported dendrimers containing polar and apolar groups at every repeat unit that inverts from a normal to inverted micellar-type conformation going from aqueous to organic media (Fig. 11.9 Basu et al. 2004 Aathimanikandan et al. 2005 Klaikherd et al. 2006). This solvent-sensitive conformational behavior affords a unique capability to solubilize hydrophobic molecules in water and hydrophilhc molecules in lipophilic media. 

Organized media including micellar solutions and cyclodextrins (CDs) have been the subject of continuous interest. These media are able to solubilize hydrophobic compounds in water and modify significantly their physicochemical properties. 

By maximizing the contact area between extraction solvent and surfactant-solubilized hydrophobic oil contaminant through the use of state-of-the-art hollow fiber membrane columns, we hypothesize that hydrophobic oil contaminants can be separated from surfactant solutions without macroemulsification. For this research we were interested in the partitioning of the hydrophobic oil from the hydrophobic environment of the micelle via its aqueous concentration into a more preferred extracting solvent. 

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... 

Micellar solutions have also been used as media for organic reactions. Reactions in micellar solutions have much in common with reactions in microemulsions but the capability to solubilize hydrophobic components is much smaller in micellar solutions than in microemulsions. Micellar solutions are therefore of limited value for preparative purposes. For both systems separation of the surfactant from the product may constitute a work-up problem. 

Hydrophilic-hydrophobic diblock copolymers exhibit amphiphilic behavior and form micelles with a core-shell architecture. These polymeric carriers have been used to solubilize hydrophobic drugs, to increase blood circulation time, to obtain favorable biodistribution, and to lower interactions with the reticuloen-... 

We have developed an oil-in-water and bicontinuous microemulsion inkjet ink composition comprising a solubilized hydrophobic dye which forms nanoparticles ("pigment-like") upon application on a substrate surface. The concept was demonstrated for direct patterning of water-insoluble organic molecules in the form of nanoparticles. The method is based on formation of thermodynamically stable oil-in-water microemulsions, in which volatile "oil" contains the dissolved organic molecules. As schematically illustrated in Fig. 3, the microemulsion droplets are converted into organic nanoparticles upon impact with the substrate surface due to evaporation of the volatile solvent. 

Additional substances (buffer additives) are often added to the buffer solution to alter selectivity and/or to improve efficiency, and the wall of the capillary may be treated to reduce adsorptive interactions with solute species. Organic solvents, surfactants, urea and chiral selectors are among the many additives that have been recommended (table 4-24). Many alter or even reverse the EOF by affecting the surface charge on the capillary wall, whilst some help to solubilize hydrophobic solutes, form ion-pairs, or minimize solute adsorption on the capillary wall. Chiral selectors enable racemic mixtures to be separated by differential interactions with the two enantiomers which affects their electrophoretic mobilities. Deactivation of the capillary wall to improve efficiency by minimizing internet ions. with solute species can be achieved by permanent chemical modification such as silylaytion or the... 

Cholesterol and triacylglycerols are transported in body fluids in the form of lipoprotein particles. Each particle consists of a core of hydrophobic lipids surrounded by a shell of more polar lipids and apoproteins. The protein components of these macromolecular aggregates have two roles they solubilize hydrophobic lipids and contain cell-targeting signals. Lipoprotein particles are classified according to increasing density (Table 26.1) chylomicrons, chylomicron remnants, very low density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Ten principal apoproteins have been isolated and characterized. They are synthesized and secreted by the liver and the intestine. 

Phospholipid polymers having a 2-methacryloylox-yethyl phosphorylcholine (MPC) were investigated as a solubilizer for paclitaxel. The paclitaxel solubility was observed to increase up to 5.0mg/ml in the presence of a copolymer of MPC and Ai-butyl methacrylate (BMA), poly(MPC-co-BMA), with 70mol% of the BMA unit. The MPC polymer forms a polymer aggregate with the diameter of 23 nm, called a polymeric lipid nanosphere, in aqueous media by hydrophobic interaction, which may solubilize hydrophobic drugs. 

Humic and fulvic adds are ill-defined and heterogeneous mixtures of naturally-occurring organic molecules that possess surface active properties The molecules that comprise this mixture are also known to form aggregates of colloidal dimensions Humic and fulvic adds are shown to be able to solubilize hydrophobic organic compounds (HOC) in a manner that is consistent with known, micelle-forming surfactants, but not at organic carbon concentrations that are environmentally relevant In addition, it is found that some HOCs are not solubilized to the same extent as other HOCs Some implications of the micellar nature of humic materials are briefly discussed ... 

Organic Solvents May increase solubility at low levels Solubilize hydrophobic residues Protein denaturation... 

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