Solubilization in micellar solutions

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 approach to the thermodynamics of solubilization in micellar solutions is based on the determination of a given partial molar property of the solute (volume, enthalpy, heat capacity, compressibility) as a function of the surfactant content. The simplest approach is to use the pseudophase model. The partial molar quantity, L will thus be an average value of Y in the micellar and aqueous phases, as described by... 

Solubilization in Micellar Solutions (of Organic Water Insoluble Molecules) in Micelles... 

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]. 

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. 

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. 

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).

Pennell, K. D. Adinolfi, A. M. Abriola, L. M. Diallo, M. S. "Solubilization of Dodecane, Tetrachloroethylene, and 1,2-Dichlorobenzene in Micellar Solutions of Ethoxylated Nonionic Surfactants." Environ. Sci. Technol. 1997,31, 1382-1389. 

Pennell, K.D., Adinolfi, A.M., Abriola, L.M., and Diallo, M.S. (1997). Solubilization of dodecane, tetrachloroethylene, and 1,2-dichlorobenzene in micellar solutions of ethoxylated nonionic surfactants. Environ. Sci, Technol, 31, 1382-1389. 

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. 

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. 

This review will focus on a variety of photochemical reactions which have been studied in micellar solutions, and will show how the micelle affects the outcome of these reactions. Photochemical reactions can be altered by solubilization into micellar solutions, such that the products or the relative yield of products can change relative to homogeneous solution. Furthermore, an increase or decrease in the dynamics or the efficiencies of photochemical reactions may also occur upon solubilization in micelles. 

This report gives results of uilibrium studies of the solubilization and metal-ion complexation behavior of three amphiphilic triamine ligands in micellar solutions. The separation selectivity has been evaluated using results of studies of solutions... 

The process of enhanced solubility in micellar solutions is normally referred to as solubilization, or in the words of McBain solubilization is the term given to a particular mode of bringing into solution substances that are otherwise insoluble in a given medium. Similar definitions were proposed later, with the only significant change that solubilization includes increased solubility caused by the presence of micelles. Over the years a considerable amount of empirical information relating to solubihzation has been pubhshed. The early studies have been reviewed by McBain, Klevens, McBain and Hutchinson, and by Elworthy et ah Later developments have been described in several reviews, the most comprehensive is the book edited by Christian and Scamehom. ... 

Gao, Z., Wasylishen, R.E., and Kwak, J.C.T., An NMR paramagnetic relaxation method to determine distribution coefficients of solubilization in micellar systems, J. Phys. Chem., 93, 2190, 1989. Treiner, C., The partitioning of neutral solutes between micelles and water as deduced from critical micelle concentration determinations, in Solubilization in Surfactant Aggregates, Christian, S.D. and Scamehorn, J.R, Eds., Marcel Dekker, New York, 1995, chap. 12. 

In micellar solutions of ionic surfactants the mechanism for solubilization of the small polar molecules appears to involve, initially at least, ion-dipole interaction between the solubilizate and the counterion of the surfactant present in the interior of the micelle, possibly followed by weaker interaction (e.g., via hydrogen bonding) between the solubilizate and the surfactant ion (Kaufman, 1964 Kitahara, 1969, Kon-no, 1971a). In solutions of nonionic POE surfactants, solubilization of polar molecules appears to be by interaction with the ether oxygens of the POE chain. 

The reactions of organic compounds can be catalyzed markedly in micellar solution. Catalysis by both normal micelles in aqueous medium and by reversed micelles in nonpolar solvents is possible (Fendler and Fendler, 1975 Kitahara, 1980). In normal micelles in aqueous medium, enhanced reaction of the solubilized substrate generally, but not always, occurs at the micelle-aqueous solution interface in reversed micelles in nonaqueous medium, this reaction occurs deep in the inner core of the micelle. 

M. Aihara, M. Arai, and T. Taketatsu, Flow Injection Spectrofluorimetric Determination of Europium(III) Based on Solubilizing Its Ternary Complex with Thenoyltrifluoroacetone and Trioctylphosphine Oxide in Micellar Solution. Analyst, 111 (1986) 641. 

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