Micellar solution solubilization

Some surfactants, as for instance the sodium diethylhex-ylsulfosuccinate (better known as AOT), are soluble in oil. These organic solutions may contain small surfactant aggregates. They are able to solubilize water, giving rise to reverse micelles that have an aqueous core. There is no clear or obvious difference between reverse micelles and water-in-oil microemulsions. As pointed out by Friberg there is continuity in phase diagrams between (direct or reverse) micellar solutions, solubilized systems, and microemulsions. 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

It is of particular interest to be able to correlate solubility and partitioning with the molecular stmcture of the surfactant and solute. Likes dissolve like is a well-wom plirase that appears applicable, as we see in microemulsion fonnation where reverse micelles solubilize water and nonnal micelles solubilize hydrocarbons. Surfactant interactions, geometrical factors and solute loading produce limitations, however. There appear to be no universal models for solubilization that are readily available and that rest on molecular stmcture. Correlations of homologous solutes in various micellar solutions have been reviewed by Nagarajan [52]. Some examples of solubilization, such as for polycyclic aromatics in dodecyl sulphonate micelles, are driven by hydrophobic... 

Ultrafiltration of micellar solutions combines the high permeate flows commonly found in ultrafiltration systems with the possibility of removing molecules independent of their size, since micelles can specifically solubilize or bind low molecular weight components. Characteristics of this separation technique, known as micellar-enhanced ultrafiltration (MEUF), are that micelles bind specific compounds and subsequent ultrafiltration separates the surrounding aqueous phase from the micelles [70]. The pore size of the UF membrane must be chosen such, that the micelles are retained but the unbound components can pass the membrane freely. Alternatively, proteins such as BSA have been used in stead of micelles to obtain similar enan-tioselective aggregates [71]. 

As a result of the micellar environment, enzymes and proteins acquire novel conformational and/or dynamic properties, which has led to an interesting research perspective from both the biophysical and the biotechnological points of view [173-175], From the comparison of some properties of catalase and horseradish peroxidase solubilized in wa-ter/AOT/n-heptane microemulsions with those in an aqueous solution of AOT it was ascertained that the secondary structure of catalase significantly changes in the presence of an aqueous micellar solution of AOT, whereas in AOT/n-heptane reverse micelles it does not change. On the other hand, AOT has no effect on horseradish peroxidase in aqueous solution, whereas slight changes in the secondary structure of horseradish peroxidase in AOT/n-heptane reverse micelles occur [176],... 

M. Baviere and T. Rouaud. Solubilization of hydrocarbons in micellar solutions Influence of structure and molecular weight (solubilisation des hydrocarbures dans les solutions micellaires influence de la structure et de la masse moleculaire). Rev Inst Franc Petrol, 45(5) 605-620, September-October 1990. 

Product recovery from reversed micellar solutions can often be attained by simple backextraction, by contacting with an aqueous solution having salt concentration and pH that disfavors protein solubilization, but this is not always a reliable method. Addition of cosolvents such as ethyl acetate or alcohols can lead to a disruption of the micelles and expulsion of the protein species, but this may also lead to protein denaturation. These additives must be removed by distillation, e.g., to enable reconstitution of the micellar phase. Temperature increases can similarly lead to product release as a concentrated aqueous solution. Removal of the water from the reversed micelles by molecular sieves or silica gel has also been found to cause a precipitation of the protein from the organic phase. 

A further possibility is the formation of liquid crystals on contact with body fluids at the site of application. The initially applied drug solution interacts with body fluids such as plasma, tears, or skin lipids and undergoes a phase transition into a mono-or multiphasic system of liquid crystals (Fig. 15). For example, oily solutions of reverse micellar solutions of phospholipids, which solubilize additional drug, trans-... 

Though CPE has many advantages [10], some problems remain to be solved such as (1) limited number of surfactants, (2) high cloud-point, and (3) strong hydrophobic nature. In an attempt to overcome some of these limitations, Tani et al. [281] proposed a new method that involves solubilization of hydrophobic membrane proteins into aqueous micellar solutions of alkylglucosides, followed by phase separation induced by the addition of a water-soluble polymer such as poly(ethylene)glycol (PEG) and dextran T-500. Using this approach they could carry out the whole procedure from solubilization to phase separation at 0 °C. 

Micellar and pre-micellar solutions of methanol in triolein were studied with three different surfactant systems using 2-octanol as a co-surfactant. Surfactants evaluated by viscosity, conductivity, density, refractive index and particle size data along with polarizing microscopic examinations were bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate. Data show phase equilibria regions of liquid crystalline phases as well as micellar solutions. All systems were effective for solubilizing methanol in triolein. The order of effectiveness for water tolerance is Tetradecyldimethylammonium linoleate>... 

When CTAC is solubilized in micellar solution with sulfide S2- ions, at low water contents (w < 10), the presence of CTAC induces a strong decrease in CdS nanocrystallite size. For a given water content, the absorption spectra are blue shifted when the syntheses are performed in the presence of CTAC compared to that obtained in its absence. The temporal evolution of absorption at 250 nm is approximated to nucleation rate of CdS. It slows down in the presence of CTAC. This blue shift is more pronounced at low water content and high CTAC concentration. Hence it is observed a decrease in the particle size by increasing CTAC concentration. This can be related to the decrease in the intermicellar potential in the presence of CTAC (64). 

When CTAC is solubilized in micellar solution with cadmium Cd2+ ions, a better resolution in the excitonic peak with increasing CTAC concentration is observed. The sharp peak is more intense for low water content and for high CTAC concentration. This clearly shows a narrow size distribution. 

The nucleation rate is slowed down with increasing CTAC concentration, notably at a water content w equal to 3. However, this phenomenon is less important compared to what is obtained previously by solubilizing CTAC in micellar solution with sulfide S2 ions. 

The irradiation of the micellar solution in the presence of dimethylaniline (DMA) which is also solubilized in the micell induced a charge separation to give RuC12B+ and DMA+ (Eq. (12)). 

These results can be rationalized by picturing the crystal violet carbonium ion as solubilized and oriented in the micelle, followed by attack by the aqueous hydroxide ion. The catalytic effect of the micellar solution has a twofold origin a concentration effect and an... 

In this study it is the homogeneous micellar phases that comprise the microemulsions. For this 5% surfactant system it is only over a relatively narrow range of temperatures that it is possible to have fairly extensive solubilization of either oil or water in the micellar solutions. 

Thus, C14MV2- quenches the excited state of [Ru(bipy)3]2+ with a rate constant fcq = 8 x 10s mol-1 dm3 s-1 and this is unaffected by cetyltrimethylammonium chloride (CTAC), up to concentrations of 5 x 10-2 mol dm-3, indicating that mixed CTAC/C14MV2- micelles are not formed.139 In the absence of CTAC, kb in this system is 4x 109 mol-1 dm3 s-1, but flash photolysis showed that this drops to kb s2x 107 mol-1 dm3 s-1 in micellar solution. Thus, the more hydro-phobic radical cation, C14MV+, is solubilized by CTAC micelles, which, having a positive surface, do not allow approach of the oxidized [Ru(bipy)3]3C This then gives an efficiency of 30% for the redox reaction. This study was extended by the removal of the CTAC from solution and the introduction of a Pt catalyst protected by a positively charged polysoap.138 This work is described in Section 61.5.4.7.2. 

So far it has not been possible to measure the chemical potentials of the components in the mesophases. This measurement is possible, however, in solutions which are in equilibrium with the mesophases. If pure water is taken as the standard state, the activity of water in equilibrium with the D and E phases in the system NaC8-decanol-water is more than 0.8 (4). From these activities in micellar solutions, the activity of the fatty acid salt has sometimes been calculated. The salt is incorrectly treated as a completely dissociated electrolyte. The activity of the fatty acid in solutions of short chain carboxylates has also been determined by gas chromatography from these determinations the carboxylate anion activity can be determined (18). Low CMC values for the carboxylate are obtained (15). The same method has shown that the activity of solubilized pentanol in octanoate solutions is still very low when the solution is in equilibrium with phase D (Figure 10) (15). 

Figure 10. The activity of n-pentanol in micellar solutions of sodium octanoate with solubilized pentanol and in equilibrium with the lamellar mesophase D. The abscissa indicates the mole fraction of sodium octanoate in the system (15).

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