Binding to micelles

These results can be extended to other Diels-Alder reactions. In view of the stmctures of most dienes and dienophiles a spatial separation of these compounds upon binding to micelles can be expected for the majority of Diels-Alder reactions. This arrangement most likely explains the unexpectedly small influence of micelles on the rates of Diels-Alder reactions as reported in the literature. 

Provided that equilibrium is maintained between the aqueous and micellar pseudophases (designated by subscripts W and M) the overall reaction rate will be the sum of rates in water and the micelles and will therefore depend upon the distribution of reactants between each pseudophase and the appropriate rate constants in the two pseudophases. Early studies of reactivity in aqueous micelles showed the importance of substrate hydropho-bicity in determining the extent of substrate binding to micelles for example, reactions of a very hydrophilic substrate could be essentially unaffected by added surfactant, whereas large effects were observed with chemically similar, but hydrophobic substrates (Menger and Portnoy, 1967 Cordes and Gitler, 1973 Fendler and Fendler, 1975). 

The rate of attack of water upon the tri-/>-anisylmethyl cation is unaffected by binding of this cation to anionic micelles of sodium dodecyl sulfate (SDS) (Bunton and Huang, 1972) and equilibrium constants for aldehyde hydration are only slightly reduced by binding to micelles (Albrizzio and Cordes, 1979). These observations are also consistent with substrate binding at a wet micellar surface rather than in the interior of the micelle. 

Another problem is that different workers make their calculations of second-order rate constants in the micelle in different ways. For example, the surface potential of a micelle may be specifically included in the calculation in order to estimate ion binding, but there is a circularity in this argument because surface potentials are often estimated from micellar effects upon indicator acid-base equilibria which themselves depend upon ion-binding to micelles (Fernandez and Fromherz, 1977). 

The binding constants for the association of the solute with the micelle can be evaluated with the help of model developed by Sepulveda and coworkers. According to the model, solute binds to micelles with following equilibrium ... 

Because the majority of reactants appears to bind to micelles in the micellar Stern region (vide supra), we will focus on this zone. A number of approaches has been... 

Fundamental to our picture of counterion binding to micelles is a knowledge of whether counterions retain their hydration spheres or not. Mukeijee242 concluded from partial molar volume data that (for simple ions) it is the interaction of the hydrated ion with the micelle that is important and the same conclusion is drawn in a... 

An important observation was made in the de Haas laboratory in Utrecht, where the activities of the pancreatic phospholipase and its proenzyme on the same substrate have been tested (22). The phospholipase hydrolyzes substrates that are part of a micelle, but the prophospholipase is completely inactive against micelles. The phospholipase can also hydrolyze short chain phospholipids that are in monomolecular solution although at much slower rates. The prophospholipase is almost as active as the enzyme against such substrates. It must be concluded that the reactive site of the phospholipase is already completely assembled and active in the proenzyme. Tryptic activation forms a new site that binds to micelles. This site is called the anchoring site or recognition site (22). 

The elution strength of hybrid micellar mobile phases was measured for a number of organic additives (alcohols, alkane diols, alkanes, alkylnitriles, and dipolar aprotic solvents, such as dimethyl sulfoxide and dioxane) added to micellar SDS, CTAC, and Brij-SS. Benzene and 2-ethylanthraquinone were used as probe compounds. The presence of alcohols, alkane diols, alkylnitriles, and dipolar aprotic solvents produced a diminution of the retention times, reaching remarkable levels for the most hydro-phobic compound (2-ethylanthraquinone). The observed elution strength order roughly paralleled the octanol-water partition coefficients of the additives, Rq/w (Fig- 2), or their ability to bind to micelles, am- In contrast, alkanes (pentane, hexane, and cyclohexane) had relatively little effect on the retention. 

Specific ion electrodes have been used extensively to investigate counterion binding to micelles [49-51], but recently electrodes have been developed which are sensitive to monomeric surfactant, and can be used to study the aggregation of surfactants near the cmc [182]. 

Although binding to vesicles has been less extensively studied than binding to micelles, attempts will be made to rationalize why specific solute molecules bind to particular binding locations. 

As we intended to study the pH-dependent hydration of PEO in triblock copolymer micelles, we measured the solvent relaxation for patman embedded in PS-PEO micelles both in acidic (0.01 M HCl) and alkaUne (0.01 M NaOH) solutions for comparison. Because we found only marginal differences in the relaxation behavior, we can conclude that the dye itself does not exhibit any pH-dependent changes after binding to micelles and that the solvation of short PEO does not change much with pH (it is very important to emphasis that the PEO blocks are significantly shorter than those in the studied PS-PVP-PEO copolymer). 

Antibinding results from a compound being strongly excluded or repelled from the micelle. In fact, the solute is not only excluded from the micelle, but also from the double-layer around die micelle. The solute is thus forced onto the stationary phase. A ne tive water-to-micelle petition coefficient (Pwm < 0) has been assign to these compounds, which has in principle no physical meaning. However, as compounds that bind to micelles each have a characteristic coefficient, compounds that are excluded from micelles may have a characteristic negative parameter. This parameter may be useful, since in some cases it can be correlated to the degree of electrostatic repulsion between solute and micelles. 

D is influenced by binding to micelles, microemulsion droplets, or surfactant monolayers at o/w interfaces, as well... 

In the next part, we will focus our attention on nanosecond processes that occur in shells of self-assembled polymer micelle-like nanoparticles in aqueous media [56, 57]. Fluorescent probes that strongly bind to the nanoparticles have usually been employed to obtain information on the shell or on the immediate vicinity of nanoparticles. Suitable probes include amphiphilic fluorophores, i.e., fluorescent surfactants, such as prodan, laurdan, or patman (see chapter Huorescence Studies of Polymer Containing Systems , Fig. 2). They contain a fairly polar fluorescent head-group and a nonpolar aliphatic tail, which secures the favorable hydrophobic interaction and sorption on polymer nanoparticles. They bind to micelles [55, 56] and their localization depends on the polarity of the head-group and on the length of the tail. In the case of patman, the strongly polar head is usually located in the peripheric part of the solvated shell and the nonpolar tail is oriented towards the... 

Counterions bound to the micelles are only electrostatically attracted by the oppositely charged micelle surface. - For instance, alkali metal counterions bind to micelles with a... 

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