Solubilization in Aqueous Micellar Solutions

This surfactant possesses a CMC value of 3 x 1(T4 M. The micellar aggregates formed are relatively large, the molecular weight being 6.3 x 106 as determined by quasielastic light scattering technique. A variety of sensitizers, when solubilized in aqueous micellar solutions of II were shown to be oxidatively bleached under simultaneous formation of Ag°. Consider, for example, the cyanine dye ... 

Given the following chemical structures, predict the probable location of each of the following compounds if solubilized in aqueous micellar solutions of (a) sodium dodecyl sulfate, (b) n-hexadecyl-trimethyl ammonium bromide, (c) polyoxyethylene(7)dodecylphenol ... 

Coreta-Gomes et have employed NMR to develop a method of quantification of cholesterol solubilization in aqueous micellar solutions such as bile salts the obtained results were confirmed by dynamic light scattering. Jam beck and Lyubartsev proposed the use of NMR order parameters to validate a refined all-atom force field for phosphatidylcholine lipids, such as l,2-diauroyl-OT-glycero-3-phospocholine (DPLC), DMPC and DPPC. 

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... 

Solubilizing Capacity of Surfactant Micelles and Distribution Coefficient of Timobesome Acetate in Aqueous Micellar Solutions at 25°C Solubilizing Capacity Distribution ... 

The investigation of the kinetics of the solvolysis of p-nitrophenyl bis(chloromethyl) phosphinate 12 in the direct micelles of sodium dodecylsulfate (SDS) in ethylene glycol have demonstrated that the ratio of the factors Fc and Fn, in this system differ from that in aqueous micelles, namely, the positive predominant contribution of the factor of microenvironment to the micellar rate effect is observed (Table 15.1). In the solution of ethylene glycol the micellization is much less effective as compared to the aqueous solution (cmc of SDS in ethylene glycol is equal to 0.18M, while in water it is 0.0085 M) and the micelles formed have low aggregation numbers, a loose structure and a weak solubilization capacity (the binding constants of substrates are lower by two orders of magnitude as compared to those in aqueous micellar solutions). ... 

There would appear then to be only limited evidence that oils which exhibit antifoam effects, when present as emulsified bulk phase, can also produce antifoam effects when present only as solubilizates in aqueous micellar solutions of surfactants. In many instances, alternative explanations for supposed observations of the latter are possible, which do invoke the presence of the oils as bulk phase. However some of the observations described here are difficult to dismiss. Of particular interest in this context are the findings of Koczo et al. [15], Lobo et al. [21], and Binks et al. [16] concerning the effect of solubilized alkanes on the foam stability of aqueous micellar solutions of various surfactants. Attempts to explain such effects by recourse to dynamic surface tension behavior after the manner of Ross and Haak [11] would appear to be unconvincing (see reference [22]). It is, however, possible that it may concern the effect of the solubilized oil on the relevant disjoining pressure isotherm. Wasan and coworkers [15,21] have suggested that the phenomenon is a consequence of the effect of solubilization of alkanes on intermicellar interactions. Lobo et al. [21] find that the instability of the foams formed from certain ethoxyl-ated alcohols in the presence of solubilized alkanes depends on the magnitude of the micellar second virial coefficient describing those interactions. Reduction of the... 

Obviously, water, aqueous solutions of salts, and mixtures of highly hydrophilic solvents have also been found to be solubilized in the micellar core [13,44]. The maximum amount of such solubilizates that can be dissolved in reversed micelles varies widely, strongly depending on the nature of the surfactant and the apolar solvent, on the concentrations of surfactant and of additives, and on temperature [24,45-47]. 

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],... 

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. 

In aqueous surfactant solutions, either by circumstance or design, non—surface active organic species may be present. Examples are oil recovery, where crude oil is present, or micellar—enhanced ultrafiltration, where micelles are being used to effect a separation of dissolved organic pollutants from water. The ability of mixed micelles to solubilize organic solutes has received relatively little study. In addition, the solubilization of these compounds by micelles may change the monomer—micelle equilibrium compositions. 

Winsor (17) describes how electrical conductivity varies during addition of an alcohol to an aqueous micellar solution containing some solubilized oil. Conductivity initially decreases as mixed (and probably larger) micelles containing both surfactant and alcohol are formed. When liquid crystal (presumably having a lamellar structure) starts to appear in equilibrium with the micellar solution, conductivity decreases even faster. As more alcohol is added, the aqueous solution disappears, only liquid crystal is present, and the conductivity reaches a minimum. Addition of still more alcohol results in the appearance of an oil-continuous micellar solution and an increase in conductivity. Eventually all liquid crystal disappears, the increase in conductivity ceases, and conductivity... 

The present report describes new results for benzene at temperatures in the range 15 to 45°C, solubilized in aqueous solutions of sodium dodecylsulfate (SDS) and 1-hexadecylpyridinium chloride (referred to as cetylpyridinium chloride or CPC). The solute activity vs. concentration data provide insight into the nature of chemical and structural effects responsible for the solubilization of benzene by aqueous micellar systems in addition, the results find direct use in predicting the performance of MEUF in removing dissolved benzene from aqueous streams. 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

The totality of micelles represents a colloidal phase, into which a substance is dissolved in the aqueous phase partitions. The capacity of the micellar phase to solubilize a solute can therefore be expressed as a partition coefficient A n,. Hence, a linear relationship can be expected between the concentration of substance solubilized by micelles and the concentration of the surfactant Cs in the system. Because only micelles contribute to the solubilizing effect but not the monomeric surfactant molecules, the critical micelle concentration Ccmc must be subtracted from the total of the surfactant concentration. The resulting total concentration of solute in the micellar solution is then ... 

The method is based on the observation that chromophore-containing solutes undergo significant shifts in their UV spectra upon solubilization in micelles. The experimentally determined molar absorbance of a solute at a given wavelength in a micellar solution, E, will be an average value of the molar absorbance of the molecules in the micellar and the aqueous phase. 

We begin with a relatively simple model, which was suggested some years ago by Mukeqee and which provides considerable insight on solubilization of small amounts of solutes in spherical micelles. Suppose that an aqueous micellar solution has reached its solubilization limit and is in equilibrium with an excess liquid phase of a pure hydrocarbon or some other compound of low polarity. Equating the chemical potentials j,g and of the solute in the bulk organic phase and in the micelles, we have... 

A recent and exciting area of research is the solubilization of enzymes in nonaqueous solvents. One way solubilization is achieved is through noncovalent complexes of lipid (surfactant) and protein, to be referred to here as enzyme-lipid aggregates, or ELAs. Such complexes are reported to be highly active and stable. Moreover, the activity of ELAs can be significantly higher than free, suspended enzyme (in the absence or presence of surfactant), enzymes solubilized in aqueous-organic biphasic systems, or reverse micellar solutions, and can approach catalytic rates in... 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

Consistent with the above, in aqueous solutions of two different surfactants that interact strongly with each other (Chapter 11, Table 11-1), mixed micelle formation is unfavorable for the solubilization of polar solubilizates that are solubilized in the palisade layer and favorable for the solubilization of nonpolar ones that are solubilized in the micellar inner core. This is due to the reduction of a() and the sphere-to-cylindrical micelle transition and the increase in aggregation number resulting from the interaction (Treiner, 1990). 

The presence of micelles can also result in the formation of different reaction products. A diazonium salt, in an aqueous micellar solution of sodium dodecyl sulfate, yielded the corresponding phenol from reaction with OH- in the bulk phase but the corresponding hydrocarbon from material solubilized in the micelles (Abe, 1983). 

In our discussion of the effect of temperature on solubilization capacity (Chapter 4, Section 1B7), we mentioned that when the temperature of an aqueous micellar solution Wo of a POE nonionic surfactant is increased, its solubilization of nonpolar material O increases due to the increased dehydration of the POE chains, which increases the lipophilic character of the surfactant. If this occurs for POE nonionics of the proper structure in the presence of excess nonpolar material, the volume of the aqueous phase Wo increases and that of the nonpolar phase oil decreases as the temperature increases (Figure 5-5a,b). This is accompanied by a decrease in the tension yow at the O/W interface. With further increase in temperature, the POE chains become more and more dehydrated, the surfactant becomes more lipophilic, and more and more nonpolar material oil is solubilized into the increasingly asymmetric micelles. When the vicinity of the cloud point of the nonionic is reached the surfactant micelles, together with solubilized material, will start to separate from Wo as a separate phase D. If excess oil is still present, the system now contains three phases (Shinoda, 1968) excess oil a phase D, the... 

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