Cationic micellization

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. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Fig. 5.5 The oligopeptide synthesis at cationic micelles using the condensation agent CDI leads to the intermediate (I), which is in equilibrium with an IV-carboxyanhydride (II). A free primary or secondary amino acid reacts with (II) and forms an amide linkage as well as a carbamide terminus. ...

HTAC cationic micelles also markedly enhance the CL intensity of fluorescein (FL) in the oxidation of hydrogen peroxide catalyzed by horseradish peroxidase (HRP) [39], However, no CL enhancement was observed when anionic micelles of sodium dodecyl sulphate (SDS) or nonionic micelles of polyoxyethylene (23) dodecanol (Brij-35) were used (Fig. 9). CL enhancement is attributed to the electrostatic interaction of the anionic fluorescein with the HTAC micelles. The local concentration of fluorescein on the surface of the micelle increases the efficiency of the energy transferred from the singlet oxygen (which is produced in the peroxidation catalyzed by the HRP) to fluorescein. This chemiluminescent enhancement was applied to the determination of traces of hydrogen peroxide. The detection limit was three times smaller than that obtained in aqueous solution. 

The CL enhancement of the lucigenin reaction with catecholamines in the presence of HTAH micelles was used for determination of dopamine, norepinephrine, and epinephrine [42], However, the presence of an anionic surfactant, SDS, inhibits the CL of the system. The aforementioned CL enhancement in the presence of HTAH can be explained in the following way the deprotonated forms of the catecholamines are expected to be the principal species present in aqueous alkaline solution due to the dissociation of the catechol hydroxyl groups, and to react with lucigenin to produce CL. The anionic form of the catecholamines and the hydroxide ion interact electrostatically with and bond to the cationic micelle, to which the lucigenin also bonds. Therefore, the effective concentration of the... 

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

Sukhan has used PTAB cationic micelles to enhance the CL reaction of 4-diethylaminophthalohydrazide with oxygen and Co(II) in the presence of fluorescein as sensitizer [48], This enhancement is mainly due to electron-excited energy transfer from the donor (4-diethylaminophthalohydrazide) to the acceptor (fluorescein). The addition of fluorescein combined with the presence of PTAB reduces the detection limit of Co(II) by a factor of 6. The method was successfully applied in the determination of Co in tap water samples. 

Menger and Portnoy (1967) developed a quantitative treatment which adequately described inhibition of ester saponification by anionic micelles. Micelles bound hydrophobic esters, and anionic micelles excluded hydroxide ion, and so inhibited the reaction, whereas cationic micelles speeded saponification by attracting hydroxide ion (Menger, 1979b). 

Equation (1) is generally used to estimate the rate constant, kin the micellar pseudophase, but for inhibited bimolecular reactions it provides an indirect method for estimation of otherwise inaccessible rate constants in water. Oxidation of a ferrocene to the corresponding ferricinium ion by Fe3 + is speeded by anionic micelles of SDS and inhibited by cationic micelles of cetyltrimethylammonium bromide or nitrate (Bunton and Cerichelli, 1980). The variation of the rate constants with [surfactant] fits the quantitative treatment described on p. 225. Oxidation of ferrocene by ferricyanide ion in water is too fast to be easily followed kinetically, but the reaction is strongly inhibited by anionic micelles of SDS which bind ferrocene, but exclude ferricyanide ion. Thus reaction occurs essentially quantitatively in the aqueous pseudophase, and the overall rate depends upon the rate constant in water and the distribution of ferrocene between water and the micelles. It is easy therefore to calculate the rate constant in water from this micellar inhibition. 

The pseudophase model is often applied to reactions of hydrophobic ionic substrates, e.g. pyridinium ion in solutions of cationic micelles (Tables 3 and 4) with the hydrophobic attraction between micelle and substrate over-... 

This hypothesis is satisfactory for nucleophilic reactions of cyanide and bromide ion in cationic micelles (Bunton et al., 1980a Bunton and Romsted, 1982) and of the hydronium ion in anionic micelles (Bunton et al., 1979). As predicted, the overall rate constant follows the uptake of the organic substrate and becomes constant once all the substrate is fully bound. Addition of the ionic reagent also has little effect upon the overall reaction rate, again as predicted. Under these conditions of complete substrate binding the first-order rate constant is given by (8), and, where comparisons have been made for reaction in a reactive-ion micelle and in solutions... 

It seems possible that a very hydrophilic anion such as OH- might not in fact penetrate the micellar surface (Scheme 1) so that its interaction with a cationic micelle would be non-specific, and it would exist in the diffuse, Gouy-Chapman layer adjacent to the micelle. In other words, OH" would not be bound in the Stem layer, although other less hydrophilic anions such as Br, CN or N 3 probably would bind specifically in this layer. In fact the distinction between micellar and aqueous pseudophases is partially lost for reactions of very hydrophilic anions. The distinction is, however, appropriate for micellar reactions of less hydrophilic ions. 

The problem may be a semantic one because OH- does not bind very strongly to cationic micelles (Romsted, 1984) and competes ineffectively with other ions for the Stern layer. But it will populate the diffuse Gouy-Chap-man layer where interactions are assumed to be coulombic and non-specific, and be just as effective as other anions in this respect. Thus the reaction may involve OH- which is in this diffuse layer but adjacent to substrate at the micellar surface. The concentration of OH- in this region will increase with increasing total concentration. This question is considered further in Section 6. 

The binding constants between the anionic substrates and cationic micelles are large because of the combination of coulombic and hydrophobic effects so rate enhancements may be large even with dilute surfactant. There is binding with non-ionic and zwitterionic micelles despite the absence of coulombic attraction (Bunton et al., 1975). 

Examples of this behaviour are shown in Table 7 where k+ is related to reaction of substrate fully bound to a CTAX micelle and k to reaction in an anionic micelle of SDS. The ratio k+/k is consistently larger than unity for hydrolyses of open chain anhydrides, diaryl carbonates and aryl chloroformates. In addition hydrolysis of 4-nitrophenyl chloroformate is slightly faster in cationic micelles than in water. Spontaneous hydrolyses of N-acyl triazoles are also inhibited less by cationic micelles of CTABr than anionic micelles of SDS (Fadnavis and Engberts, 1982). 

Zwitterionic micelles of the sulfobetaine C16H33N+Me2(CH2)3SO 3 have effects very similar to those of cationic micelles (Table 7). This result is understandable if the substrate binds close to the quaternary ammonium center and the anionic sulfate moiety extends into the aqueous region. 

The foregoing discussion of micellar charge effects has implicitly assumed that differences in water activity or substrate location in cationic and anionic micelles are not of major importance. If such differences were all important it would be difficult to explain the differences in k+/k for carbonyl addition and SN reactions, because increase of water content in an aqueous-organic solvent speeds all these reactions (Johnson, 1967 Ingold, 1969). As to substrate location, there is very extensive evidence that polar organic molecules bind close to the micelle-water interface in both anionic and cationic micelles, although the more hydrophobic the solute the more time it will spend in the less polar part of the micelle. Substrate hydrophobicity has a marked effect on the overall rate effects in both cationic and anionic micelles, but less so on values of k+/k. It seems impossible to explain all these charge effects in terms of differences in the location of substrates in cationic and anionic micelles. 

However, micelles do not always favor reactions of higher order. In dilute OH-, reaction of activated amides, for example (18), is typically second order in OH-, but the order decreases to one with increasing [OH-] because the tetrahedral intermediate is converted rapidly into products (Menger and Donohue, 1973 Cipiciani et al., 1979). These reactions are speeded by cationic micelles, but in the micelles they are always first order in OH-, even when the total concentration of OH- is low. This is simply because the micelles concentrate OH-, so that the tetrahedral intermediate in (18) is... 

Another example of rapid turnover in a micellar system is the cleavage of carboxylic and phosphate esters by o-iodosobenzoate in cationic micelles. This reaction was not studied with a functional micelle, but it is useful to note it in this context (Moss et al., 1983, 1986). 

Hartley showed that micellar effects upon acid-base indicator equilibria could be related to the ability of anionic micelles to attract, and cationic micelles to repel, hydrogen ions. More recently attempts have been made to quantify these ideas in terms of the behavior of a micelle as a submicroscopic solvent, together with an effect due to its surface potential (Fernandez and Fromherz, 1977). 

The basicity constants in water and micelles then have the same units (M 1), and values of K and Kb are not very different for arenimidazoles and nitroindoles under a variety of conditions (Table 10). The comparisons suggest that inherent basicities are not very different in water and cationic micelles, but, as with second-order rate constants of bimolecular reactions (Section 5), there is a limited degree of specificity because K /Kb is slightly larger for the nitroindoles than for the arenimidazoles, almost certainly because of interactions between the cationic micellar head groups and the indicator anions. 

There is also a problem in defining the volume element of reaction in a microemulsion droplet, but despite these uncertainties second-order rate constants in the droplet are similar to those in cationic micelles for reactions of anionic nucleophiles in alcohol-swollen droplets (Bunton et al., 1983b). Thus, the rate enhancements seem to be due to concentration of reactants in the droplet. 

Bimolecular E2 reactions involving OH " in aqueous solution are speeded by cationic and inhibited by anionic micelles (Minch et al., 1975) whereas spontaneous SN reactions are generally inhibited strongly by cationic micelles and less strongly by anionic micelles it is therefore relatively easy to observe micellar control of product formation. 

Sukenik and coworkers have used surfactants to change the relative extents of 1,2 and 1,4 reduction of enones by BH. Cationic micelles in water favpr 1,4-addition as does an alcohol of low polarity, e.g. 2-propanol, so that... 

N-Alkylhydroxamic acid hydrolysis Methyl Violet + OH" Cl C12H25S03Na + H30+, CTABr + OH". An attempt made to separate electronic and hydrophobic effects on the micellar reaction Anionic and cationic micelles. Effect of surfactant structure examined Berndt el at., 1984 Malaviya and Katiyar, 1984... 

Triphenylmethanedyes + OH" Effect of pressure on cationic micelles Taniguchi and Iguchi, 1983... 

Acyloximes hydrolysis and cyclization Cationic micelles inhibit hydrolysis but not cyclization Soto etal., 1981... 

E2 elimination of 3-bromo-3-phenylpropionate Cationic micelles assist base-mediated elimination Bun ton et al., 1974... 

---