Micellar Effects on Nucleophilicity - American Chemical Society

29 Micellar Effects on Nucleophilicity

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Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, CA 93106

Aqueous cationic micelles speed and anionic micelles inhibit bimolecular reactions of anionic nucleophiles. Both cationic and anionic micelles speed reactions of nonionic nucleophiles. Second -order rate constants in the micelles can be calculated by estimating the concentration of each reactant in the micelles, which are treated as a distinct reaction medium, that is, as a pseudophase. These second-order rate constants are similar to those in water, except for aromatic nucleophilic substitution by azide ion, which is much faster than predicted. Ionic micelles generally inhibit spontaneous hydrolyses. But a charge effect also occurs, and for hydrolyses of anhydrides, diaryl carbonates, chloroformates, and acyl and sulfonyl chlorides and S hydrolyses, reactions are faster in cationic than in anionic micelles if bond making is dominant. This behavior is also observed in water addition to carbocations. If bond breaking is dominant, the reaction is faster in anionic micelles. Zwitterionic sulfobetaine and cationic micelles behave similarly. N

SUBMICROSCOPIC, COLLOIDAL AGGREGATES can influence chemical reac

tivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behav ior of ionic and zwitterionic micelles and their effects on chemical reactivity. Micelles can incorporate hydrophobic solutes, and by virtue of their charge, ionic micelles attract counterions and repel co-ions. Micelles influ ence reaction rates and products in various ways. They provide a reaction medium apparently distinct from the bulk solvent, and rate constants may be 0065-2393/87/0215-0425$06.00/0 © 1987 American Chemical Society

In Nucleophilicity; Harris, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

NUCLEOPHILICITY


426

different in aqueous and micellar pseudophases. In addition, micelles can speed bimolecular, nonsolvolytic reactions by bringing reactants together or inhibit reactions by keeping reactants apart. For example, cationic micelles speed attack of nucleophilic anions, and anionic micelles inhibit these reac tions, but depending upon the reaction type, spontaneous reactions may be speeded or retarded. Quantitative analysis of these rate effects requires estimation of the contributions of the reactions in the bulk, aqueous medium and in the micellar pseudophase. This separation can be made provided that the reactant concentrations in each pseudophase can be estimated by direct meas urement or by calculation (4-16). The bulk of the experiments described were carried out by using surfactants of four different types: cationic [cetyltrimethylammonium salts, C H 3 N ( C H 3 ) X (CTAX), X = halide, mesylate, azide, or hydroxide]; anionic [sodium dodecyl sulfate, C ^ H ^ O S O g N a (SDS)]; zwitterionic [N,Ndimethyl-N-dodecylglycine, C H N ( C H ) C H C 0 - (Bl-12), or sulfobetaine, C H N ( C H ) ( C H ) S 0 - (SB3-16)]; and nonionic. The following discussion covers the effects of these surfactants upon spontaneous reactions, but for bimolecular, nonsolvolytic reactions, only cationic and anionic micelles are discussed. Water is the bulk solvent in all the experiments described here, al though normal micelles form in a variety of three-dimensional associated solvents including 1,2-diols, formamide, and 100% sulfuric acid (17-19), and some kinetic work has been done on micelles in aqueous 1,2-diols (20). 16

3

3

+

1 2

2 5

3

2

2

2

+

1 6

3 3

3

2

2

3

3

Quantitative Treatment The relation between rate constant and surfactant concentration is simple for spontaneous, or micellar-inhibited, nonsolvolytic reactions for which the distribution of only one reagent has to be considered (11, 12). D i s t r i b u t i o n of substrate, S, b e t w e e n aqueous and m i c e l l a r pseudophases (denoted by subscripts W and Μ, respectively) is written in terms of equation 1 where [D] — cmc is the concentration of micellized surfactant and K is the binding constant of S to micellized surfactant (12): s

[ S J = K [ S ] ( [ D ] - cmc) S

The first-order rate constant for the overall reaction, *

Φ

= [k'

w

+ k' K ([O] M

s

(1)

W

is given by

- cmc)]/[l + K ([D] - cmc)] S

(2)

where k' and k' are respectively the first-order rate constants in the aqueous and micellar pseudophases. Equation 2 does not fit rate-surfactant profiles for micellar-assisted, bimolecular, nonsolvolytic reactions, because the distribution of both reacw

M


29. BUNTON

Micellar Effects on Nucleophilicity

427

tants must be considered (4-10). In addition, the question of the appropriate measure of concentration in the micelles must be answered. For reaction of a nucleophile, Y, the first-order rate constants in equa tion 2 can be written as (5, 9) k'w — k [Y ] w

k'

= k m s = * [ Y J / ( [ D ] - cmc)

M


(3)

w

M

Y

(4)

M

and k has the dimensions of reciprocal time, because mole ratios are dimensionless, so that M

*ψ = ( M Y w l + k K [Y ])/[l M

s

M

+ K ([D] - cmc)]

(5)

S

Application of equation 5 requires estimation of [ Y ] . For some organic nucleophiles, the distribution of Y between water and micelles can be determined experimentally, and this approach has been used for reactions of imidazoles, oximes, amines, and phenoxide and thiophenoxide ions (6-8,13, 14) (Table I). M

Table I.

Micellar Effects on Reactivity of Organic Nucleophiles Reaction

C H C0 C H -4-N0 6

13

2

6

4

C H C0 C H -4-N0 6

13

2

6

4

CH C0 C H -4-N0 3

2

6

4

2

Kel

2

+ imidazoles

a

~10-2fc

2

+ imidazolide ions

a

-10*

+ C H S" 6

5

(C H 0) P0 C H -4-N0 + C H 0 6

5

2

2

6

4

2

6

5

l-fluoro-2,4-dinitrobenzene + C H N H l-fluoro-2,4-dinitrobenzene + C H N H N O T E : In C T A B r unless specified. Not determined. Reference 6. Reference 8. Reference 14. Reference 31. / I n SDS.

6

5

2

6

5

2

m

k

2

69

0.42 c

3000

0.53^

3-8 ~3

0.17* 0.12*/

is the second-order rate constant in the micelles.

a

h

c

d

e

The problem is more complex for bimolecular reactions of inorganic anions unless the ionic concentration in micelles (or water) can be measured directly (15, 16). Generally, the reaction solution contains both the reactive ion and the inert counterion of the surfactant, which compete for the micellar surface; Romsted showed (4, 5) how this competition can be de scribed by equations similar to those applied to binding to ion-exchange resins, and his treatment has been applied to many micellar-assisted reac tions. Table II gives some examples of reactions of nucleophilic anions in CTABr.


428

NUCLEOPHILICITY

Table II.

Micellar Effects on Reactions of Hydrophilic Anions

CH C0 C H N0 (4) + O H -

13

k m/k κ /K 0.14*

C H C0 C H N0 (4) + O H -

13

0.11*

nc_h

13

3.4*

25

0.14*

Reaction 3

2

7

6

15

4

2

K

2

6

4

2

n+CioH21 + 0H


(Q)

H C — ^ Q ^ - S 0 C H O S 0 - H (g)_Cl 3

2

1, involves bimolecular nucleophilic attack by water in the transition state:

H 0

+ CH 0 SC H ^[H 0-CH3-03SC H ]^ CH OH + C H S0 H

2

3

3

6

5

2

6

5

3

6

5

3

(8)

The data also suggest that bond breaking plays a major role in most S hydrolyses in aqueous micelles except for a methyl substrate. Bond making is clearly dominant in spontaneous hydolyses of carboxylic anhydrides and diaryl carbonates and here k /k~ > 1 (Table IV). These values of k /k~ are not related in any obvious way to the reactivity or hydrophobicities of the substrates, although hydrophobicity seems to affect the overall micellar inhibition, probably because the more hydrophobic substrates penetrate the micelles and are shielded from water molecules. N

+

+

Table IV.

Hydrolyses of Anhydrides and Diaryl Carbonates io3k'

Substrate (4-0 NC H CO) 0 2

6

4

2

5

3.4

0.33

0.02

0.06

2.2

0.21

0.01

0.02

1.8

0.10

0.54*

5.0

[4-i-(CH ) CC H CO] 0 3

6

4

2

0.45

(4-0 NC H CO) CO 2

6

4

2

N O T E : Reference 5 4 . In the sulfobetaine ( S B 3 - 1 6 ) , k

a

rel

=

k+/k-

Kel 0.20

2

3

(s-η 0.06

(C H CO) 0 6

w

26.60

0.62.


434

NUCLEOPHILICITY

Acyl triazoles and carboxylic anhydrides have similar mechanisms of hydrolysis, and consistently ionic micelles inhibit hydrolysis of triazoles with k+/k~ > 1 (52).


Hydrolysis of Acid Chlorides Hydrolysis rates show a striking dependence upon micellar charge. Micellar inhibition is generally observed, but for a series of benzoyl chlorides there is a relationship between values of k+lk~ and electronic effects of para substituents. Strongly electron-withdrawing substituents, for example, N 0 , CI, or Br, lead to values of k /k~ > 1, whereas a strongly electron-donating substituent, for example, O C H , leads to k /k~ < 1 (Table V). These changes in k /k~ can be related to changes in the relative importance of bond making and breaking. 2

+

+

3

+

Table V.

Substrate 3,5-(0 N) C H COCl 4-0 NC H COCl 4-BrC H COCl 4-ClC H COCl C H COCl 4-CH C H COCl 4-CH OC H COCl C H OCOCl 4-0 NC H COCl 2

2

6

2

6

6

4

6

4

6

4

5

3

6

3

6

4

6

4

5

2

6

4

3

Hydrolysis of Acyl Chlorides and Chloroformâtes

i0^k' (s-i) w

SDS

CTAX

SB3-16

200 53 190 214 1410 ~3 x 103 ~7 x 103 14 76

0.30 0.12 0.01 0.01 0.01 0.00 0.01 0.18 0.32

>2 ~2 0.50 0.05 0.008" 0.00 0.00 0.3° 1.7-

>7 -2.70 0.07 0.02 0.00 0.48 2.20

Wk-16 5.0 5.0 0.6 0.1 0.1 1.7 5.5

S O U R C E : Reference 54 and unpublished data of M . M . Mhala and J. R. Moffatt. N O T E : X = C I unless specified. X = Br.

A

Hydrolyses of acyl halides are sometimes described in terms of the S 1 - S 2 duality of the mechanism, or variants of it (56, 57), but these descriptions are unsatisfactory because they neglect the possibility of rehybridization of the carbonyl group in the course of reaction. Strongly electron withdrawing substituents favor nucleophilic addition by water to acyl centers, with assistance by a second water molecule acting as a general base (58-60), and good evidence for this mechanism exists in hydrolyses of carboxylic anhydrides and diaryl carbonates. This addition step should be followed by very rapid conversion of an anionic covalent intermediate into products, and the intermediate should have only a transient existence, at most, in polar, nucleophilic solvents. At the other mechanistic extreme, hydrolysis of 4-methoxybenzoyl chlo ride, breaking of the C - C l bond has probably made considerable progress in N

N


29.

BUNTON


435


the transition state, and nucleophilic participation by water has made corre spondingly less progress. However, little evidence exists for acyl cations as intermediates in hydrolyses of this, and similar, acyl chlorides, in water-rich solvents (61). Scheme I illustrates these mechanistic pathways at the simplest level of description.

RCOX

Scheme +

Values of k lk~ (Table V) give an indication of the relative importance of bond making and breaking in hydrolyses of acyl chlorides, and the electronic effects upon these steps can be rationalized in terms of three-dimensional free-energy diagrams that consider reactions involving either prior addition or prior ionization as the mechanistic extremes (62-64). Aryl chloroformâtes should be similar to nitrobenzoyl chlorides in their hydrolytic mechanisms because aryloxy groups are strongly electron withdrawing and as expected k /k~ > 1 (Table V). Most spontaneous deacylations are inhibited by aqueous ionic micelles, and the only exceptions to date are hydrolyses of 4-nitrophenyl chloroformate and nitrobenzoyl chlorides. These hydrolyses are faster in cationic micelles than in water, although anionic micelles of SDS inhibit reaction, and nu cleophilic addition should be most dominant for these reactions. Micellar effects upon these hydrolyses can be compared with those upon water addition to a preformed carbocation. The choice of carbocation is critical because, if it is too hydrophilic, it will not bind to a cationic micelle and, if it is too stable, reaction does not go to completion. +

Addition to Carbocations The 2,2',4,4',4"-pentamethoxytriphenyl methyl cation (I) is a suitable sub strate for this work. Anionic micelles of SDS have little effect on the rate of water addition, but cationic micelles of CTAC1 and CTABr speed the reaction (41). This micellar effect of k /k~ ~ 5 is as predicted for a reaction in which water addition is dominant (Scheme II). +


436

NUCLEOPHILICITY

J C—®—OMe

ilVleO^^ OMe

—

H O / (

\ j

MeO-^ OMe

2

0

H

C-®-OMe

2

Scheme II


Hydrolysis of Benzenesulfonyl

Chlorides

Most of the experiments have involved nucleophilic reactions at a carbon center, but the general principles can be applied to reactions at heteroatoms although only hydrolysis of arenesulfonyl chlorides has been examined to date (65). Spontaneous hydrolyses of sulfonyl chlorides are believed to involve nucleophilic attack in the rate-limiting step although questions arise as to the timing of the bond-making and -breaking steps, because attack of water and loss of C I " could be concerted or stepwise (66-68). The rate sequence for reactions of para-substituted benzenesulfonyl chlorides in water is H C O > C H > H > Br < N 0 . Rate extremes with systematic variation of substituents are often consid ered to be evidence of changes in the molecularity of a reaction, but for hydrolyses of arenesulfonyl chlorides, rate extremes are more reasonably ascribed to variations in the extents of S - O bond making and S - C l bond breaking. Variations in k /k~ support this hypothesis (Table VI), and as for hydrolyses of acid chlorides (Table V), bond making seems to be important but introduction of electron-donating groups increases the importance of bond breaking in the transition state. 3

3

2

+

Table VI.

Substituent

Hydrolysis of Benzenesulfonyl Chlorides

I03k'

4-N0 4-Br 4-H 4-CH 4-CH 0

2.45 1.96 3.07 3.86 6.10

2

3

3

N O T E : At 2 5 . 0

w

(8-0

SDS 0.04 0.04 0.01 0.01 0.01

CTACl SB3-16 0.89 0.86 0.17 0.21 0.05 0.05 0.03 0.03 0.01 0.01

k+/k"

21 5 3 3 1.1

°C.

S O U R C E : Reference 6 5 .

Betaine Surfactants Thus far, only ionic micelles have been discussed, but some results were obtained for reactions in zwitterionic micelles of the betaine (Bl-12) and the sulfobetaine (SB3-16). These surfactants differ in that the carboxylate moiety of B l - 1 2 could react nucleophilically, and this behavior is observed in



29. BUNTON

437


deacylation and in hydrolyses of acid chlorides (65). In these reactions, the overall effect is due to nucleophilic attack, which opposes an inhibition due to the medium effect of the micelle. The balance between these effects depends largely upon the susceptibility of the substrate to nucleophilic attack. The sulfonate moiety is only weakly nucleophilic in water, and the effects of micellized SB3-16 upon hydrolyses are generally very similar to those of cationic micelles (Tables IV-VI). This behavior suggests that in both systems the substrates bind close to the quaternary ammonium centers and that the sulfonate moiety, like a micellar-bound counteranion, is in the water and therefore not interacting strongly with the substrate (65). The only exceptions to this generalization were observed with hydrolyses of some acid chlorides (Table V). In these reactions, the balance between bond making and breaking seems to be very sensitive to the reaction medium (58). Duality of Mechanism in Spontaneous Reactions The molecularity of nonsolvolytic substitutions is given by the kinetic order, although second-order, bimolecular reactions may be concerted or stepwise. Other tests have to be used for spontaneous reactions, and many tests have been developed especially for reactions at saturated carbon (69-71). Compel ling evidence suggests that ionization occurs in solvolyses of alkyl halides or arenesulfonates where a carbocation or its ion pair can be trapped in the course of reaction, and nucleophilic attack on methyl groups is all important, so that in these cases a clear distinction exists between the so-called S 1 and S 2 mechanisms. The concept of the duality of mechanism that developed from these reactions has been applied to a wide variety of organic and inorganic reactions and has tended to dominate thinking about reaction mechanisms in solution (69). Even here problems arise in the mechanistic description because nucleophilic protic solvents can not only participate at the electrophilic center nucleophilically (70) or by dipole-dipole interactions but can also solvate a leaving group (72). Despite earlier controversy, the weight of evidence suggests that the S 1 and S 2 models are extremes in a mechanistic spectrum. Many sol volytic reactions at saturated carbon seem to involve linked bond making and breaking, as well as stabilization of the transition state by hydrogen bonding to a leaving anion, and the relative importance of these steps depends on substrate structure and on nucleophilicity of the solvent and its interaction with leaving groups. Water is not an especially nucleophilic solvent, but water effectively solvates anionic leaving groups so that in water considerable ionization in the transition state should occur (72). The surfaces of micelles are water-rich (34, 35) and values of k /k~ for hydrolyses of alkyl halides and arenesulfonates suggest that nucleophilic participation is dominant in reactions at methyl N

N

N

N

+


438

NUCLEOPHILICITY

groups but that increased alkyl and especially aryl substitution leads to bond breaking becoming dominant. The situation is more complex for spontaneous hydrolyses of acyl deriva tives, and values of k /k~ depend upon the ease of departure of the leaving group and attack of water and, for a given leaving group, upon electronic substituent effects. For relatively poor leaving groups, for example, R C 0 ~ or A r O " where bond making is all important, a carbonyl addition mechanism can be written that is equivalent to the B 2 mechanism of ester hydrolysis. The situation is more complicated when a good leaving group is present, as with acid chlorides or chloroformâtes. If strongly electron withdrawing groups are present, nucleophilic addition is dominant, and for hydrolysis of aryl chlo roformâtes and nitrobenzoyl chlorides, the mechanism is similar to that of anhydride hydrolysis (Tables IV and V). But electron-donating groups favor bond breaking, and the mechanism of hydrolysis of benzoyl chloride and its 4 - C H and 4 - C H 0 derivatives has considerable S 2-like character, with extensive bond breaking in the transition state, which will have an open structure with extended bonding to the entering and leaving groups (54, 61). +


2

Ac

3

3

N

The relative importance of bond making and breaking in the transition state depends not only upon the substrate structure but also upon the reaction medium. Electronic effects upon solvolyses of benzoyl chlorides are very sensitive to changes in the solvent (58). In formic acid, a weakly nucleophilic solvent, but one that should effectively solvate leaving anions, a plot of log k against Hammett s p parameter has a slope of p = - 4.4, but in a poorly ionizing solvent (95% acetone-5% H 0 ) , p ~ 2, but a shallow mini mum occurs at ~ p —0.2 for strongly electron denoting groups (e.g., C H and O C H ) . In 50% acetone-50% H 0 , the corresponding plot has a very well defined minimum at ~ 0.2. For reaction in water, p = —3.4, based on hydrolyses of benzoyl chloride and its 4-C1, — C H , and — O C H deriva tives, but a minimum occurs at ~ 0.5. This behavior is understandable because water is a much better nucleophile than formic acid and a much better ionizing medium than aqueous acetone (71). 2

3

3

2

3

3

Whether changes in the relative importance of bond making and break ing, as in solvolyses of acyl chlorides, are to be regarded as changes in reaction mechanism is a matter of opinion, but clearly micelles, like any other reaction medium, can influence transition-state structure. Therefore, although values of k /k~ can be considered as indicative of "mechanism", the conclusions apply only to reactions taking place at micellar surfaces. How ever, these surfaces are water-rich, so the transition-state structures are expected to be similar to those in water. A l l the evidence to date fits the hypothesis that micellar charge effects are related to mechanism, but the results are not so easy to explain. In a reaction dominated by bond breaking, positive charge developing at the reaction center should interact favorably with an anionic head group and the +



29. BUNTON

439


leaving anion will interact with water molecules adjacent to the micellar surface. For a spontaneous reaction, dominated by bond making, positive charge develops on the attacking water molecule but is distributed into nearby water molecules by hydrogen bonding. At the same time, negative charge will develop at, or adjacent to, the reaction center and will interact favorably with a micellar cationic head group. A preformed carbocation interacts unfavorably with a cationic head group, and this interaction will decrease as water adds to the carbocation, and for this reaction and those of strongly electrophilic acid chlorides, cationic micelles speed reaction, de spite their general inhibitory effect upon spontaneous hydrolyses. Acknowledgmen ts Support of this work by the National Science Foundation (Chemical Dynam ics Program) and the Army Office of Research is gratefully acknowledged. The efforts of my co-workers who are cited here are very much appreciated. List of Symbols 1

k k' k k' k

Overall first-order rate constant, s" First-order rate constant in water, s Second-order rate constant in water, M s First-order rate constant in the micelles, s" Second-order rate constant in the micelles, s , with concentration written as a mole ratio k Second-order rate constant in the micelles, M s ; with concentra tion written as a molarity k Ratio of the overall rate constants in the presence and absence of micelles K Binding constant of solute to the micelles K Ion-exchange constant for ions Y and X , given by [ Y ] [ X ] / [ Y ] [ X ] m Mole ratio of micellar-bound Y to micellized surfactant cmc Critical micelle concentration [D] Stoichiometric concentration of surfactant (detergent) [ D J Concentration of micellized surfactant V Volume element of reaction, sometimes identified with partial molar volume of a micelle W Aqueous pseudophase when used as a subscript M Micellar pseudophase when used as a subscript lJi

- 1

w

_ 1

_ 1

w

1

M

-1

M

m

-

1

-1

2

re]

s

Y

X

W

M

M

W

s

y

M

Literature Cited 1. Fendler, J. H . ; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. 2. Fendler, J. H . Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982.



440

NUCLEOPHILICITY

3. Cordes, Ε. Η.; Gitler, C. Prog. Bioorg. Chem. 1973, 2, 1. 4. Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, p 509. 5. Romsted, L. S. In Surfactants in Solution; Mittal, K. L.; Lindman, B., Eds. Plenum: New York, 1984; Vol. 2, p 1015. 6. Martinek, K.; Yatsimirski, A. K.; Levashov, Α. V.; Berezin, I. V. In Micellization, Solubilization and Microemulsions; Mittal, K. L . , Ed.; Plenum: New York, 1977; Vol. 2, p 489. 7. Sudholter, E. J. R.; van der Langkruis, G. B.; Engberts, J. B. F. N . Recl. Trav. Chim. Pays-Bas, 1980, 99, 73. 8. Cuccovia, I. M . ; Schroter, E. M . , Monteiro, P. M . ; Chaimovich, H . J. Org. Chem. 1978, 43, 2248. 9. Bunton, C. A. Catal. Rev.—Sci. Eng. 1979, 20, 1. 10. Chaimovich, H . ; Bonilha, J. B. S.; Politi, M . J.; Quina, F. H . J. Phys. Chem. 1979, 83, 1851. 11. Bunton, C. Α.; Fendler, E. J.; Sepulveda, L.; Yang, K.-U. J. Am. Chem. Soc. 1968, 90, 5512. 12. Menger, F. M . ; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. 13. Epstein, J.; Kaminski, J. J.; Bodor, N . ; Enever, R.; Sowa, J.; Higuchi, T. J. Org. Chem. 1978, 43, 2816. 14. Bunton, C. Α.; Cerichelli, G.; Ihara, Y.; Sepulveda, L. J. Am. Chem. Soc. 1979, 101, 2429. 15. Larsen, J. W.; Tepley, L. B. J. Colloid Interface Sci. 1974, 49, 113. 16. Bunton, C. Α.; Ohmenzetter, K.; Sepulveda, L. J. Phys. Chem. 1977, 81, 2000. 17. Ray, Α.; Nemethy, G. J. Phys. Chem. 1971, 75, 809. 18. Ionescu, L. G.; Fung, D. S. J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2907. 19. Menger, F. M . ; Jerkunica, J. M . J. Am. Chem. Soc. 1979, 101, 1896. 20. Bunton, C. Α.; Gan, L . - H . ; Hamed, F. H . ; Moffatt, J. R. J. Phys. Chem. 1983, 87, 336. 21. Bunton, C. Α.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979, 101, 1253. 22. Bunton, C. Α.; Romsted, L. S.; Thamavit, C.J.Am. Chem. Soc. 1980, 102, 3900. 23. Bunton, C. Α.; Gan, L . - H . ; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981, 85, 4118. 24. Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881. 25. Stadler, E.; Zanette, D.; Rezende, M . C.; Nome, F. J. Phys. Chem. 1984, 88, 1892. 26. Bell, G. M . ; Dunning, A. J. Trans. Faraday Soc. 1970, 66, 500. 27. Mille, M . ; Vanderkooi, G. J. Colloid Interface Sci. 1977, 59, 211. 28. Gunnarsson, G.; Jonsson, B.; Wennerstrom, H . J. Phys. Chem. 1980, 84, 3114. 29. Bunton, C. Α.; Moffatt, J. R. J. Phys. Chem. 1985, 89, 4166. 30. Bunton, C. Α.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. 31. Bunton, C. Α.; Carrasco, N . ; Huang, S. K.; Paik, C.; Romsted, L. S. J. Am. Chem. Soc. 1978, 100, 5420. 32. Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1, p 153. 33. Ramachandran, C.; Pyter, R. Α.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198. 34. Menger, F. M . Acc. Chem. Res. 1979, 12, 111. 35. Whitten, D. G., Bonilha, J. B. S.; Schanze, K. S.; Winkle, J. R. In Surfactants in Solution; Mittal, K. L . ; Lindman, B.; Eds. Plenum: New York, 1984; Vol. 1, p 585. 36. Hicks, J. R.; Reinsborough, V. C. Aust. J. Chem. 1982, 35, 15. 37. Bunton, C. Α.; Moffatt, J. R.; Rodenas, E. J. Am. Chem. Soc. 1982, 104, 2653.



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