Functional Micellar Systems

These are distinguished from simple surfactants by the fact that the micelle itself participates in the redox events. One can visualise functional micellar assemblies constituted in various ways 

PHOTOINDUCED ELECTRON TRANSFER IN FUNCTIONAL SURFACTANT SVSTEMS 

The significance of this functional organization becomes evident also when the back reaction of Cu+ and DI is considered. Previous studies have shown that the intramicellar electron transfer from Cu to DI , though thermodynamically highly favourable, cannot compete kinetically with the escape of the cuprous ion from its native 

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Sutton, D. Nasongkla, N. Blanco, E. Gao, J. M., Functionalized micellar systems for cancer targeted drug delivery. Pharmaceutical Research 2007, 24, (6), 1029-1046. 

The above enantioselectivities are obviously complex functions of many factors, perhaps even more complex than in natural enzymes. Complexity is partly due to the present co-micellar system in which it is difficult to analyze separately the interaction of the substrate with the achiral micelle, and that of the substrate with the catalyst complex. 

Another example of rapid turnover in a micellar system is the cleavage of carboxylic and phosphate esters by o-iodosobenzoate in cationic micelles. This reaction was not studied with a functional micelle, but it is useful to note it in this context (Moss et al., 1983, 1986). 

CTAC1 and hydroxy- and imidazole-functionalized micelles. Rates compared in micellar systems... 

In a more general way, the two major driving forces for the design of novel micellar systems are the control over morphology (spheres, vesicles, rods, tubules etc. with controlled size) and function (stimulus-responsive materials, biological functions). Both of these aspects are intimately related since a given morphology can induce a specific function. 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

Cnrrent work on dephosphorylation includes the use of different micellar systems. Scheme 10 illustrates dephosphorylation via novel functional detergents which contain an imidazole ring snbstitnted with an oxime or hydroxamic acid moiety. ... 

For systems, which cannot be marked easily, the displacement method is an alternative, in particular for low concentration ranges (36). This is a modification of isotachophoresis, because current flow is not constant with time and field strength is a function of the position along the capillary. Instead of analyte peaks, profiles are obtained (Fig. 6a) as injection and separation are carried out in one step. Although this method is not suitable for all micellar systems, one outstanding advantage is the higher UV sensitivity (which is important for most surface-active compounds). Because of the reliance of the method on displacement, the micellar phase is not diluted. 

Dekker et al. [170] studied the extraction process of a-amylase in a TOMAC/isooctane reverse micellar system in terms of the distribution coefficients, mass transfer coefficient, inactivation rate constants, phase ratio, and residence time during the forward and backward extractions. They derived different equations for the concentration of active enzyme in all phases as a function of time. It was also shown that the inactivation took place predominantly in the first aqueous phase due to complex formation between enzyme and surfactant. In order to minimize the extent of enzyme inactivation, the steady state enzyme concentration should be kept as low as possible in the first aqueous phase. This can be achieved by a high mass transfer rate and a high distribution coefficient of the enzyme between reverse micellar and aqueous phases. The effect of mass transfer coefficient during forward extraction on the recovery of a-amylase was simulated for two values of the distribution coefficient. These model predictions were verified experimentally by changing the distribution coefficient (by adding... 

Rahaman and Hatton [152] developed a thermodynamic model for the prediction of the sizes of the protein filled and unfilled RMs as a function of system parameters such as ionic strength, protein charge, and size, Wq and protein concentration for both phase transfer and injection techniques. The important assumptions considered include (i) reverse micellar population is bidisperse, (ii) charge distribution is uniform, (iii) electrostatic interactions within a micelle and between a protein and micellar interface are represented by nonlinear Poisson-Boltzmann equation, (iv) the equilibrium micellar radii are assumed to be those that minimize the system free energy, and (v) water transferred between the two phases is too small to change chemical potential. 

Tween 85 is used extensively for RME [84]. Russell and coworkers [234] used Tween 85/isopropanol microemulsions in hexane to solubilize proteins and not only showed >80% solubilization of cytochrome C at optimum conditions, but also proved that Tween 85 does not have a detrimental effect on the structure, function, and stability of subtilisin and cytochrome C. There are other reports available on the extraction and purification of proteins using Tween 85-RMs and also on the stability of protein activity in these systems [234]. It has also been shown that Tween 85-RMs can solubilize larger amounts of protein and water than AOT. Tween 85 has an HLB of 11, which indicates that it is soluble in organic solvents. In addition, it is biodegradable and can be successfully used as an additive in fertihzers [235,236]. Pfammatter et al. [35] have demonstrated that RMs made of Tween 85 and Span 80 can be successfully used for the solubilization and growth of whole cells. Recently, Hossain et al. [84] showed an enhanced enzymatic activity of Chromobacterium viscosum Hpase in AOT/Tween 85 mixed reverse micellar systems when compared to that in classical AOT-RMs. This is due to the modification of the interface in AOT-RMs caused by the co-adsorption of Tween 85, and increased availability of the oHve oil molecules (substrate) to the enzyme. 

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

There are a number of books [168-173] and reviews [68,174,175] that provide good starting points. Refs. [168,176-178] discuss methods for the determination of anionic surfactants, which are probably the most commonly encountered in industry. Most of these latter methods are applicable only to the determination of sulfate-and sulfonate-functional surfactants. Probably the most common analysis method for anionic surfactants is Epton s two-phase titration method [179,180] or one of its variations [171,172,181-183]. Additional references are provided elsewhere [163], Aqueous surfactant micellar systems have also been utilized successfully in virtually every area of analytical chemistry (for example, [184—186]). 

In this area, recent unrelated efforts of the groups of Bhattacharya and Fife toward the development of new aggregate and polymer-based DAAP catalysts deserve mention. Bhattacharya and Snehalatha [22] report the micellar catalysis in mixtures of cetyl trimethyl ammonium bromide (CTAB) with synthetic anionic, cationic, nonionic, and zwitterionic 4,4 -(dialkylamino)pyridine functional surfactant systems, lb-c and 2a-b. Mixed micelles of these functional surfactants in CTAB effectively catalyze cleavage of various alkanoate and phosphotriester substrates. Interestingly these catalysts also conform to the Michaelis-Menten model often used to characterize the efficiency of natural enzymes. These systems also demonstrate superior catalytic activity as compared to the ones previously developed by Katritzky and co-workers (3 and 4). 

Figure 7. Propylviologen radical, PVS, formation as a function of light adsorbed, monitored by the increase of absorbance at = 602 nm (e = 12,500 M cm ). Key in A, Ru(bipy)s as sensitizer a, SiOt system b, homogeneous system c, micellar system d, NaLS micellar system with 0.7 M NaCl and in B, Zn-TMPyP arid Zn-TPPS as sensitizers a, SiOt system with Zn- TMPyP b, homogeneous system with Zn-ThiPyP "" c, SiOg system with Zn-TPPS d, homo-...

Figure 10. Reduction of KsFe(CN)t as a function of light adsorbed. Key a, SiOt system including PVS° b, SiO, system arrow, time of PVS° addition c, homogeneous system and d, NaLS micellar system (20).

Figure 11. Quantum yield for propylviologen, PVS-, formation as a function of the surface potential of negatively charged interfaces. Key O, SiO, system and , NaLS micellar system (22).

Hydrolysis of Esters in Functional Micellar and Related Systems... 

Neutralizing these polyesters with different hydroxyamines made it possible to obtain stable micellar systems. The size of these micelles was shown to essentially be a function of the structure and the polyester molecular weight. The smallest micelles are obtained with diacid polyester of lower molecular weight. A model for the chain conformation of polyester in a micelle was proposed, taking into account the area occupied by one end group located at the micelle surface. Furthermore, such micelles were demonstrated to be able to solubilize non-neutralized polyester chains, as well as monomers like styrene or butylmethacrylate (BMA). 

An example of exchange with an oxygen-containing functional group is given which utilises a micellar system. Cationic micelles of cetyltrimethylammonium chloride and bromide, and tetradecyltrimethylammonium chloride and bromide, accelerate the reaction of 2-(4-nitro-phenoxy)quinoxaline with hydroxide ion to give quinoxalin-2(l/f)-one. ... 

Mukherjee P, Crank J A, Sharma P S, et al. Dynamic solvation in phosphonium ionic hquids Comparison of bulk and micellar systems and considerations for the constmction of the solvation correlation function, C(t). J. Phys. Chem. B. 2008. 112, 3390-3396. 

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