Reactions in non-micellar aggregates

The structure of vesicles formed from a given surfactant depends upon the extent of sonication, and over a period of time vesicles fuse and separation of phases occurs. The ease of fusion depends upon vesicular charge and the extent to which it is neutralized by added electrolyte. 

Vesicles are permeable to apolar non-ionic solutes, and, if ionic, can bind counterions at the inner and outer surfaces. Permeability to ions depends critically upon vesicle structure. 

Mechanistic studies of organic reactivities in vesicles have focused on two questions the first is the application of the pseudophase model to reactions in vesicles and the second that of reaction at the inner and outer vesicular surfaces. 

Chaimovich and coworkers have prepared large unilamellar vesicles of DODACl by a vaporization technique which gives vesicles of ca 0.5 pm diameter. These vesicles are much larger than those prepared by sonication, where the mean diameter is 30 nm, and their effects on chemical reactivity are very interesting. The reaction of p-nitrophenyl octanoate by thiolate ions is accelerated by a factor of almost 10 by DODACl vesicles (Table 2), but this unusually large effect is due almost completely to increased concentration of the very hydrophobic reactants in the small region of the vesicular surface and an increased extent of deprotonation of the thiol. There is uncertainty as to the volume element of reaction in these vesicles, but it seems that second-order rate constants at the vesicular surface are similar to those in cationic micelles or in water (Cuccovia et al., 1982b Chaimovich et al., 1984). 

Mizutani and Whitten, 1985). Vesicles prepared by sonication or vaporization are metastable, and it is necessary to use standard conditions to obtain reproducible kinetic data. 

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Fig. 6 Reaction of p-nitrophenyl diphenyl phosphate in non-micellar aggregates of tri-n-octyl ethylammonium mesylate (TEAMs) at pH 10.7 , 10 4M naphth-2,3-imidazole and O, 10-4 and 2 x KT4 M benzimidazole, respectively. (Reprinted with permission of the American Chemical Society)...

A very important monograph outlined the state of knowledge up to 1974, and also noted other associated species which could influence the rates of thermal, photochemical and radiation induced reactions [1]. The initial studies of micellar effects were made in water, but subsequently micelle-like aggregates were observed in non-aqueous solution. These aggregates can also influence chemical reactivity. In some respects the effects of micelles on reactivity are similar to those of cyclo-dextrins or synthetic polyelectrolytes. 

Most investigators of micellar and related phenomena have used water as a solvent. It is abundant, cheap, and easily purified, and because biological reactions occur in aqueous media we are naturally interested in reactions in water which model biological reactions. However, micelles, or micelle-like aggregates, can form in non-aqueous solvents, and it is useful to distinguish between the normal micelles which form in solvents which have three-dimensional structure [19,20], and the so-called reverse micelles which form in apolar solvents [1,126,127]. 

The kinetics of catalytic reduction of 4-BB in CTAB solutions involves micellar catalysis of the electron transfer (ET) between 9-PA anion radical and 4-BB (Equation 2) in a thick layer of surfactant at the electrode. The effective rate constant for this ET in 0.1 M CTAB increased more than three orders of magnitude compared (Table 1) to the same reaction in surfactant-free DMF. The rate-determining step in CTAB was not Equation 2 as in DMF. In CTAB decomposition of the 4-BB anion radical (Equation 3) became kinetically important. The major cause of the kinetic alterations was compartmentalization of the reactants in high concentrations in surfactant aggregates at the surface of the electrode. The same catalytic reaction was not as successful in non-ionic igepal micelles, which did not provide good stabilization for 9-PA anion radicals. 

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