Micelles proximity effects

For micelle-assisted bimolecular non-solvolytic reactions it is necessary to consider both medium and proximity effects. 

If MPT is solubilized in aqueous CTAB micelles together with a hydrophobic quencher, statistical and proximity effects will influence the efficiency and rate of the energy transfer in the same direction. Thus reaction 20 is enhanced in the confined space of a CTAB micelle by a factor of HO (19). 

Patients with ileal resection commonly develop vitamin B12 deficiency, necessitating therapy with parenteral cyanocobalamin. Most other water-soluble vitamins are absorbed in the proximal jejunum, and deficits of these vitamins are found only in more severe SBS. Small bowel bacterial overgrowth can contribute to diminished vitamin B12 because bacteria may metabolize the nutrient within the intestine, decreasing its availability for absorption. SBS patients with fat malabsorption can acquire deficiencies in vitamins A, D, E, and K. These fat-soluble vitamins depend on bile salt micelles for effective absorption, and malabsorption with depletion of the bile salt pool can lead to their deficits. ... 

The importance of proximity effects upon bimolecular reactions is evident from the example of micellar rate effects shown in Table 1. The volume of micelles in dilute aqueous surfactant solution is much less than that of the aqueous component, so that concentration of both reactants into the small volume of the micelles should increase the rate of a bimolecular reaction [1-6,25,61]. 

Mechanistic and product distribution differences between organic solvents, organized systems such as micelles, and water may be due to many causes, such as proximity effects due to differences in solvation, retardation or promotion of photoionization and other electron-transfer processes, concentration effects caused by increased solubility in one phase of a two-phase system, selective quenching or promotion effects by codissolved solutes, or other factors. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

The reaction involves the nucleophilic attack of a peracid anion on the unionized peracid giving a tetrahedral diperoxy intermediate that then eliminates oxygen giving the parent acids. The observed rate of the reaction depends on the initial concentration of the peracid as expected in a second-order process. The reaction also depends on the stmcture of the peracid (specifically whether the peracid can micellize) (4). MiceUization increases the effective second-order concentration of the peracid because of the proximity of one peracid to another. This effect can be mitigated by the addition of an appropriate surfactant, which when incorporated into the peracid micelle, effectively dilutes the peracid, reducing the rate of decomposition (4,90). 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

The micelles of this size and shape (i.e., in sufficient close proximity) must be concentrated sufficiently to result in the formation of the mesophase lattice at the prevailing temperature. At some higher temperature, this lattice will break down to give the amorphous liquid by the disintegrating effect of the increased thermal motion. 

The speed of AgO formation is so rapid that it cannot be resolved by ns-laser photolysis techniques. Obviously, this effect has to be attributed to the close proximity of photoactive donor and Ag" " sites on the surface of the micelle. The photoinduced reduction of Ag" can be performed with a number of other donor chromophores such as porphyrins. 

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