Polar favors micelles

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

D19.6 The formation of micelles is favored by the interaction between hydrocarbon tails and is opposed by charge repulsion of the polar groups which are placed close together at the micelle surface. As salt concentration is increased, the repulsion of head groups is reduced because their charges are partly shielded by the ions of the salt. This favors micelle formation causing the micelles to be larger and the critical micelle concentration to be smaller. 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

The lower polarity of the mieellar Stern region (2) is also widely thought to make a signifieant eontribution to the rate retarding effects of micelles in fact, the transition state of the type of hydrolysis reactions commonly used to probe the micellar pseudophase is thought to be more polar than the reactants, indicating that a more polar medium would be favorable for reaction to occur. 

The first type has been most extensively studied, especially for mercury (33). If electron-donating ligands, such as halides or organic groups, are attached to the mercury atom, the rate of reaction with methylcobalamin slows down markedly, presumably due to the lowered electrophilic nature of the metal (69). Similarly, when the solvent is made less polar, the reaction rate also decreases (34, 54). Anything favoring the formation of base-off methylcobalamin will slow the transmethylation rate, hence the retardation of this reaction in the presence of micelles (34, 38). 

In nonaqueous solvents, the signiLcant energetic source of micelle formation is the reduction of unfavorable interactions between the ionic head group of the surfactant and the nonpolar solvent molecules. In these systems, small spherical micelles appearto be the most favored, especially wher the reduction of solvent/polar group interactions is important (Luisi et al., 1988 Huruguen et al., 1991). 

It is thus clear that a treatment of the micellization process of ionic amphiphiles must include a discussion of electrostatic effects. Furthermore, even for zwitterionic and nonionic surfactants, the electrostatic effects play a role. The favorable interaction between the polar groups of these amphiphiles and the solvent water is probably mainly of an electrostatic origin. 

The mechanism of the photoresponse was tentatively explained as follows. When azo units are in the planar, apolar, trans configuration, they merge into the hydrophobic core of the micelles, forcing the polypeptide chains to assume a coil conformation. Isomerization of the azo units to the skewed, polar, ds configuration inhibits hydrophobic interactions and causes the azo units to retreat from of the micelles, thus allowing the polypeptide chains to adopt the a-helix structure favored in the absence of micelles. In other words, the primary photochemical event is the trans-ds isomerization of the azobenzene... 

Organized media have also been used to influence the regiochemical outcome in the reactions. Photoaddition of 3-n-butylcyclopentenone with various terminal alkenes has shown a pronounced preference for the alignment of the enone and the alkenes with their polar groups toward the surface of the micelle. This effect is most pronounced in the case of 1-acetoxy-I-heptene, which gives exclusively the head-to-tail adduct in cyclohexane solvent but a 2.3 1 mixture in favor of the head-to-head isomer in the presence of potassium dodecyl sulfate (equation 13). 

What properties enable phospholipids to form membranes Membrane formation is a consequence of the amphipathic nature of the molecules. Their polar head groups favor contact with water, whereas their hydrocarbon tails interact with one another, in preference to water. How can molecules with these preferences arrange themselves in aqueous solutions One way is to form a micelle, a globular structure in which polar head groups are surrounded by water and hydrocarbon tails are sequestered inside, interacting with one another (Figure 12,9). 

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