Micelles cationic

The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyl-trimethylammonium bromide (TTABr) and sodium dodecyl sulfate (SDS) on the rates of alkaline hydrolysis of securinine (223) were studied at a constant [HO ] (0.05 m). An increase in the total concentrations of CTABr, TTABr and SDS from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, and 7, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelles. Cationic micelles of CTABr speed attack of hydroxide ion upon coumarin (224) twofold owing to a concentration effect. ... 

Another example of catalysis involving a non-micellizing cation comes from deacylation studies by Kunitake and coworkers who used tri- -octylmethylam-monium chloride, (5) [123]. This ion in low concentration speeded deacylation of... 

Catalysis by micelles of the hydroxide-ion catalysed hydrolysis of substrates appears to be qualitatively understood on the basis of a concentration effect of reactant on, or around, the micelle surface and need not necessarily involve a dilference in the free energies of activation in the micelle and bulk phase. That is not to say that the cationic micelles could not and do not cause electrostatic stabilisation of the transition state. The cationic micelle surface can act as an electrostatic sink for the anionic intermediate leading to its stabilisation, but a rate enhancement requires preferential stabilisation of this intermediate compared with the reactant. The small rate enhancement of the micelle catalysed reaction, about 50-fold, is equally well explained by considering that the increased concentration of reactants at the micelle surface leads to a higher observed rate. Incorporation of the reactants into a limited volume decreases the entropy loss that is associated with bringing reactants together in the transition state and this leads to an increase in the pseudo first-order rate constants in the presence of surfactant micelles. Cationic micelles of CTAB have also been shown to facilitate the alkaline hydrolysis of the cephalosporin, cephalexin (Yatsuhara el al., 1977). 

It is interesting to note that for different types of micelles - cationic, anionic and non-ionic we have obtained the same dependence of log vs. p - The existence of the... 

Surfactants Form micelles, cationic ones reverse charge on capillary wall... 

FT-EPR spectra of the ZnTPPS/DQ system in a solution of cetyltriinethylaininonium chloride (CTAC), a cationic surfactant, are shown in figme BE 16.21. As in the TX100 solution, both donor and acceptor are associated with the micelles in the CTAC solution. The spectra of DQ at delays after the laser flash of less than 5 ps clearly show polarization from the SCRP mechanism. While SCRPs were too short-lived to be observed in TXlOO solution, they clearly have a long lifetime in this case. Van Willigen and co-workers... 

In this section the influence of micelles of cetyltrimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS) and dodecyl heptaoxyethylene ether (C12E7) on the Diels-Alder reaction of 5.1a-g with 5.2 in the absence of Lewis-add catalysts is described (see Scheme 5.1). Note that the dienophiles can be divided into nonionic (5.1a-e), anionic (5.If) and cationic (5.1g) species. A comparison of the effect of nonionic (C12E7), anionic (SDS) and cationic (CTAB) micelles on the rates of their reaction with 5.2 will assess of the importance of electrostatic interactions in micellar catalysis or inhibition. 

The effect of micelles of SDS, CTAB and C12E7 on the apparent second-order rate constants of the Diels-Alder reaction between nonionic 5.1a, anionic 5.1 f and cationic 5.1g with 5.2 is reported in Table 5.1. These apparent rate constants are calculated from the observed pseudo-first-order rate constants by dividing the latter by the overall concentration of 5.2. 

Fonnation of a complex with a copper cation only further stimulates this behaviour. As a result, S.lg is almost completely bound to the micelles, even at low concentrations of Cu(DS)2. By contrast, the reaction of 5.1 f still benefits from an increasing surfactant concentration at 10 mM of Cu(DS)2. In fact, it is surprising that the reaction of this anionic compound is catalysed at all by an anionic surfactant. Probably it is the copper complex of 5.If, being overall cationic, that binds to the micelle. Not surprisingly, the neutral substrate S.lc shows intermediate behaviour. 

The Diels-Alder reaction of dienophiles 5.1a-e, containing neutral, cationic or anionic substituents, with diene 5.2 in the absence of Lewis acids is retarded by micelles of CTAB, SDS and C12E7. In the situation where the dienophile does not bind to the micelle, the reaction is inhibited because uptake of... 

FIGURE 19 6 Space filling model of a micelle formed by association of car boxylate ions derived from a long chain carboxylic acid The hydrocarbon chains tend to be on the inside and the carboxylate ions on the surface where they are in contact with water mole cules and metal cations... 

Asphalt emulsions are dispersioas of asphalt ia water that are stabilized iato micelles with either an anionic or cationic surfactant. To manufacture an emulsion, hot asphalt is mixed with water and surfactant ia a coUoid mill that produces very small particles of asphalt oa the order of 3 p.m. These small particles of asphalt are preveated from agglomerating iato larger particles by a coatiag of water that is held ia place by the surfactant. If the asphalt particles agglomerate, they could settle out of the emulsion. The decision on whether a cationic or anionic surfactant is used depends on the appHcation. Cationic stabilized emulsions are broken, ie, have the asphalt settle out, by contact with metal or siHcate materials as weU as by evaporation of the water. Siace most rocks are siHcate-based materials, cationic emulsions are commonly used for subbase stabilization and other similar appHcations. In contrast, anionic emulsions only set or break by water evaporation thus an anionic emulsion would be used to make a cold patch compound. 

There are cationic, anionic, and non-ionic micelles. Divalent metal ions having positive charges are highly hydrophilic and cannot be incorporated into cationic micelles. Anionic micelles tend to form water insoluble salts with divalent metal ions. Interactions of non-ionic micelles with divalent metal ions appear to be small. Thus incorporation of a divalent metal ion into a micelle to form a catalytic center... 

Table 10 indicates the results obtained in non-ionic micelles and may be compared with Table 9 of cationic micelles. 

In the latter function, the reagent behaves as a surfactant and forms a cationic micelle at a concentration above the critical micelle concentration (1 x 10 4M for CTMB). The complexation reactions occurring on the surface of the micelles differ from those in simple aqueous solution and result in the formation of a complex of higher ligand to metal ratio than in the simple aqueous system this effect is usually accompanied by a substantial increase in molar absorptivity of the metal complex. 

Micellar catalysis of azo coupling reactions was first studied by Poindexter and McKay (1972). They investigated the reaction of a 4-nitrobenzenediazonium salt with 2-naphthol-6-sulfonic and 2-naphthol-3,6-disulfonic acid in the presence of sodium dodecylsulfate or hexadecyltrimethylammonium bromide. With both the anionic and cationic additives an inhibition (up to 15-fold) was observed. This result was to be expected on the basis of the principles of micellar catalysis, since the charges of the two reacting species are opposite. This is due to the fact that either of the reagents will, for electrostatic reasons, be excluded from the micelle. 

If the coupling component is not ionic, however, more dramatic effects occur, as found by Hashida et al. (1979) and by Tentorio et al. (1985). Hashida used N,N-bis(2-hydroxyethyl)aniline, while Tentorio and coworkers took 1-naphthylamine and l-amino-2-methylnaphthalene as coupling components. With cationic arenediazo-nium salts and addition of sodium dodecyl sulfate (SDS), rate increases up to 1100-fold were measured in cases where the surfactant concentration was higher than the critical micelle concentration (cmc). Under the same conditions the reaction... 

A positive value of ME means that the insertion of a hetero atom or group makes the molecule more lipophilic. If ME is negative, the hetero surfactant is more hydrophilic. In general, hetero atom insertion hydrophilizes the surfactant molecule as does the shift of the hetero group to the middle of the carbon chain [71]. ME values are temperature-dependent. / and ME values can also be useful to take into account the influence of various cations on the critical micelle concentration. 

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