Micelles solvent effects

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

Very large solvent effects arc also observed for systems where the monomers can aggregate either with themselves or another species. For example, the apparent kp for polymerizable surfactants, such as certain vinyl pyridinium salts and alkyl salts of dimethylaminoalkyl methacrylates, in aqueous solution above the critical micelle concentration (cmc) are dramatically higher than they are below the cmc in water or in non-aqueous media.77 This docs not mean that the value for the kp is higher. The heterogeneity of the medium needs to be considered. In the micellar system, the effective concentration of double bonds in the vicinity of the... 

Surfactants, not surprisingly, exert a highly significant influence on the fluorescence of FBAs in solution. This effect is associated with the critical micelle concentration of the surfactant and may be regarded as a special type of solvent effect. Anionic surfactants have almost no influence on the performance of anionic FBAs on cotton, but nonionic surfactants may exert either positive or negative effects on the whiteness of the treated substrate [33]. Cationic surfactants would be expected to have a negative influence, but this is not always so [34]. No general rule can be formulated and each case has to be considered separately. 

Kitchens, C.L., McLeod, M.C. and Roberts, C.B. (2003) Solvent effects on the growth and steric stabilization of copper metallic nanoparticles in AOT reverse micelle systems. Journal of Physical Chemistry B, 107 (41), 11331-11338. 

The quantitative treatment of micellar rate effects upon spontaneous reactions is simple in that the overall effect can be accounted for in terms of distribution of the substrate between water and the micelles and the first-order rate constants in each pseudophase (Scheme 2). The micelles behave as a submicroscopic solvent and to a large extent their effects can be related to known kinetic solvent effects upon spontaneous reactions. It will be convenient first to consider unimolecular reactions and to relate micellar effects to mechanism. 

The effect of micelles on these spontaneous hydrolyses is difficult to explain in terms of kinetic solvent effects on these reactions. Mukerjee and his coworkers have refined earlier methods for estimating apparent dielectric constants or effective polarities at micellar surfaces. For cationic and zwitterionic betaine sulfonate micelles Def is lower by ca 15 from the value in anionic dodecyl sulfate micelles (Ramachandran et al., 1982). We do not know whether there is a direct connection between these differences in effective dielectric constant and the relation between reaction rates and micellar charge, but the possibility is intriguing. 

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. 

We will present mechanistic aspects of the Diels-Alder reaction, its selectivity and reactivity in order to explain solvent effects on the one hand, and the effects of Lewis acids on the other. Other catalytic systems like micelles will also be addressed. Some of the explanations may seem trivial or are well-known but, as we will use these in later sections, a clear terminology is desirable. 

SOLVENT EFFECTS ZWiTTERGENTS DETERGENTS MICELLE ZWITTERION ZYMOGEN Zymogen activation,... 

The effect of low concentrations of urea (2M) on the large dihydroxy bile salt micelles is striking, while similar concentrations have no effect on the small trihydroxy or dihydroxy micelles. The effects of urea on micelle formation and aggregate size are undoubtedly complicated (10) and involve changes in solvent structure and thus hydrophobic bonding and hydration of polar groups. For large micelles of dihydroxy bile salt... 

Figure 2 Solvent effect on the quenching rate constant of 3-methylindole. Comparison between experimentally determined k.Q values in homogeneous solvents and those calculated using Eq. (17) (adapted from Ref. 14). The experimentally determined value of kap in SDS micelles ( ) and the value of k(j estimated from Eq. (21) (A) have been included.

Also Fryar and Kaufman8 studied the solvent effect on the stability of barium dinonylnaphthalene sulfonate in toluene, toluene/methanol, and methanol solutions by ultracentrifugation and viscometry. The aggregation number of the micelles reduced from about 10 in toluene to about 4 when the mole fraction of free methanol in the solvent mixture was approximately 0.03. In pure methanol BaDNNS micelles did not exist. 

Of the frequency shifts reported in this work, the largest is the change in the v, CH2 band noted for the monomer to micelle transition of CgAO. The frequency of the o, CH2 band in the spectrum of a surfactant molecule incorporated in a micelle is lower than that of the same surfactant as an unassociated monomer in solution (27), due primarily to the large decrease in the contact between the aqueous solvent and the hydrophobic methylene chain. This solvent effect was also observed in this study for CgAO, as shown in Figure 6 and Table 4. 

Local concentration effects result from the high concentration of solutes which may exist in the small volume of the micelle. This effect is commonly observed for bimolecular reactions between hydrophobic solutes. In some cases, bimolecular photoreactions can occur in micellar solutions where the same total macroscopic concentration in organic solvents would yield no reactions such phenomena may result simply from a higher local concentration in the micelle. 

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). 

An examination of solvent effects on reverse micelles without added water provides even greater insight into the results for C11-14 EO5. The solvent effect on the aggregation number of dry alkali dinonylnaphthalenesulfonates was studied in a classic paper by Little and Singleterry (2). For sodium as the counterion, the... 

While transient phenoxyl radicals for the above resonance Raman measurements were produced by radiolysis, other investigators used photolysis to produce phenoxyl radicals for Raman studies. Such studies were carried out with several tocopherols in various organic solvents and in miceUar solutions and phospholipid bilayers . From the solvent effect on the Raman frequencies and the spectra observed in sodium dodecyl sulfate micelles it was concluded that the chromanoxyl group of tocopherol was located in a highly polar environment. However, the spectra in neutral and positively charged micelles and in the membranes suggested that the chromanoxyl group is in an environment of intermediate polarity. 

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