J . Phys. Chem. 1989, 93, 7851-7856
7851
Reactions of Anionic Nucleophiles in Anionic Micelles. A Quantitative Treatment Clifford A. Bunton,* Marutirao M. Mhala, and John R . Moffatt’ Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: December 9, 1988; In Final Form: May 26, 1989)
Anionic micelles of sodium dodecyl sulfate (SDS) inhibit reactions of OH- with p-nitrophenyl diphenyl phosphinate (pNPDP) and of OH-, Br-, SCN-, and with methyl naphthalene-2-sulfonate (MeONs). However, first-order rate constants for reactions of OH-, CNS-, and SO,” reach limiting values at high [SDS] under conditions in which the bulk of the substrate is in the micellar pseudophase. Added inert salt speeds reactions of OH-, SCN-, and S032-at high [SDS]. The increase is larger with tetramethylammonium than with sodium salts and added cations decrease the repulsion between anions and SDS micelles. A Coulombic model, based on solution of the Poisson-Boltzmann equation in spherical symmetry, predicts that co-ions should not be completely excluded from micellar surfaces. An increase of [electrolyte] reduces the micellar surface electrical potential and increases co-ion concentration at that surface. Cations that interact specifically with the anionic micelle, e.g., Me4N+,reduce its surface charge density and are especially effective at increasing anion concentrations at the micellar surface. The overall micellar effects on rate can be treated quantitatively in terms of this pseudophase model, and it can also be applied to deacylations of hydrophobic carboxylic esters by OH- in SDS with and without added salt.
Aqueous ionic micelles assist bimolecular reactions of apolar substrates with hydrophilic counterions by bringing reactants together at the micellar surface and inhibit reactions of hydrophilic co-ions by excluding them from the surface.2 For example, anionic micelles sharply inhibit reactions of anionic nucleophiles and rate constants of overall reaction are very much lower at high [surfactant] than in water. The inhibition can be treated quantitatively in terms of a pseudophase model that treats micelles and water as distinct reaction media so that the overall reaction rate is the sum of those in the two p ~ e u d o p h a s e s . ~ .For ~ reactions of OH- with hydrophobic esters in anionic surfactants there is a small, but finite, rate even when the substrate is fully micellar bound, so that OHis not completely excluded from the micellar ~ u r f a c e .This ~ finite rate was explained in terms of Romsted’s ion-exchange model,2a which had initially been applied to counterion reactions, because for reaction in sodium dodecyl sulfate (SDS),for example, at high [SDS] sodium ions will compete with hydrogen ions at the micellar surface. The postulated decrease in hydrogen ion concentration will increase OH- concentration at the micellar surface and assist reaction, and added NaCl assisted r e a ~ t i o n . ~ Added salts decrease the inhibition by SDS of the reaction of CN- with ~ e t o g l a u c i nand , ~ it was suggested that cations of the salts were increasing the concentration of CN- at the micellar surface by competing with H+.2a Acid-catalyzed decompositions of bound Meisenhiemer complexes6 and of acetals’ are inhibited, but not completely suppressed, by cationic micelles, and added salts speed acetal hydrolysis. The pseudophase ion-exchange (PIE) model assumes that reaction occurs in a region of uniform composition at the micellar surface that is usually identified with the Stern and second-order rate constants can be calculated for reaction in this region .*
An alternative treatment of interactions of ions and ionic micelles assumes that electrostatic interactions will bring counterions toward the micellar surface and repel co-ions. These distributions can be estimated by solving the Poisson-Boltzmann equation in the appropriate ~ymmetry.~-l’However, a simple point-charge model is inadequate, because there are specific micelle-ion interactions that depend on ionic properties,2 and allowance for them can be included in the treatment.’@12 This model differs from the PIE model in that it predicts a smooth variation of ion concentration with distance from the micellar surface, whereas the PIE model is described in terms of a sharp break in concentration at the boundary of the Stern layer. The two models make similar predictions for reactions of counterions, because the PBE predicts a high concentration of these ions close to the micellar surface. However, the PBE predicts that co-ion concentration will be very low a t the micellar surface but will increase sharply, but smoothly, with distance from the surface because of Coulombic repulsions, but added ions will decrease the micellar electrical surface potential and reduce this co-ion repulsion. This decrease will affect interactions of all co-ions. The PIE, with its emphasis on competition between cations, e.g., H + and Na+ in SDS, is restricted to strongly basic anions whose concentration at the micellar surface will be governed by an acid-base equilibrium, e.g., by the autoprotolysis of water.4 In the present work we examined the effect of SDS upon reactions of OH- and of several weakly basic nucleophilic anions. The PIE model, with its emphasis on competition between H + and other cations, e.g., Na+, predicts that added salts should not assist reactions of weakly basic anions with fully micellar-bound substrates. W e also attempted to fit our kinetic data by using the PBE to estimate the distribution of coions in the vicinity of an SDS micelle. We used substrates that bind readily to micelles and examined SN2 reactions of methyl naphthalene-2-sulfonate (MeONs) with hydroxide, bromide, thiocyanate, and sulfite ions.
( I ) Present address: Hewlett-Packard, Ink-jet Components Operation, Corvallis, OR 97330. (2) (a) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B.. Eds.: Plenum: New York. 1984: Vol. 2. D 1015. (b) Bunton. C. A.; Savelli. G. Ado. Phys. Org. Chem. 1986, 22, 213. IC) QuinaiF. H.; Chaimovich, H. J. Phys. Chem. 1979,83, 1844. ( 3 ) (a) Fendler, J . H . Membrane Mimefic Chemistry; Wiley-Interscience: New York, 1982. (b) Menger, F. M.; Portnoy, C. E. J. Am. Chem. SOC.1967, 89, 4698. (4) Chaimovich, H.; Aleixo, R. M. V.; Cuccovia, 1. M.; Zanette, D.; Quina, F. H. In Solution Behavior ofSurfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum. New York, 1982; Vol. 2, p 949. (5) Srivastava, S. K.; Katiyar, S. S. Eer. Bunsen-Ges. Phys. Chem. 1980, 84, 1214. (6) Lelievre, J.; Gaboriaud, R. J. Chem. Soc., Faraday Trans. 1 1985,8/, 335. ( 7 ) Armstrong, C.; Gotham, W.; Jennings, P.; Nickles, J.; Romsted, L. S.; Versace, M.; Waidlich, J., unpublished results. (8) Cordes, E. H . ; Gitler, C. Prog. Eioorg. Chem. 1973, 2, I .
0022-365418912093-7851$01SO10
MeONs NU-= O H ,C N S , SO:(9) (a) Bell, C. M.; Dunning, A. J. Trans Faraday Soc. 1970,66, 500. (b) Mille, M.; Vanderkooi, G. J. Colloid Interface Sci. 1977, 59, 21 I . (c) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1980,84,3114. (10) (a) Bunton, C . A.; Moffatt, J. R. J . Phys. Chem. 1985,89,4166. (b) Bunton, C. A.; Moffatt, J. R. J . Phys. Chem. 1986, 90, 538. (c) Bunton, C. A,; Moffatt, J. R. J. Phys. Chem. 1988, 92, 2896. (11) Rodenas, E.; Ortega, F. J. Phys. Chem. 1987, 91, 837. (12) Rathman, J. F.; Scamehorn, J. F.J . Phys. Chem. 1984, 88, 5807; Langmuir 1987, 3, 372.
b 1989 American Chemical Societv
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7852
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
Bunton et al.
TABLE I: Reaction of Methyl Naphthalene-2-sulfonate in SDS in the Presence and Absence of Bromide ion" IO'[SDS], M 5 IO 30 60 I 06k$, s-I 19.1 (11.2) 7.01 (7.88) 5.28 (4.18) 3.88 (4.21)
180 3.42
90 (3.76)
" A t 25.0 OC with 0.3 M Br' unless specified, in the absence of SDS 106k$ = 2190 s-I: values in parentheses are in the absence of Br- and in water. 106klL = 12.0 SK' in the absence of Br-.
TABLE 11: Reactions of MeONs in SDS'
[SDSI, M 0.005 0.01 0.02 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24
0.3 M OH27.5 (26.3) 13.6 (12.5) 5.67 (4.88)
0.1 M SOX2235 (234)
0.05 M 118 (117) 12.1 (11.6)
2.17 (1.75) 1.86 (1.44) 1.21 (0.83) 1.13 (0.75) 1.08 (0.70) 0.97 (0.59)
3.64 (2.2) 2.75 (2.37) 1.65 (1.27) 1.46 (1.08)
5.80 (5.42) 3.08 (2.70) 3.04 (2.66)
0.3 M SOX2705 (704) 178 (177) 105 (104) 54.0 (53.5) 43.3 (42.9) 24.1 (23.7) 14.4 (14.0) 9.84 (9.46)
1.65 (1.27)
0.2 M SCN13.2 (12.0)
0.3 M S C N 19.2 (18.0) 6.88 (5.77) 5.16 (4.37)
I .84 (1.42) 1.22 (0.84)
2.36 (1.94) 1.94 (1.52) 1.56 (1.18)
1,12 (0.74) 1.25 (0.87) 1.17 (0.69)
1.29 (0.87) 1.26 (0.88) 1.29 (0.87)
aValues of I O 5 k$, s-l, at 25.0 'C; values in parentheses are kC$, corrected for the contribution of the reaction with water.
TABLE 111: Salt Effects upon the Reaction of OH- with p-Nitrophenyl 0.18 no salt 0.03 M NaBr [salt], M 4.50 (4.7) I 03k$, s-l 3.8 0.3 M NaCl [salt]. M 0.2 M NaCIO, 6.94 (7.8) 8.56 (9.1) 103k$. s - ~ [salt], M 103kq. s-i
0.1 M NaCl 5.74 (7.2)
no salt 4.4
0.24 M SDS 0.1 M Me4NClb 9.36 (8.6)
@ I n0.03 M NaOH at 25.0 OC; values in parentheses are predicted.
Some of these reactions had been examined in cationic and zwitterionic m i c e I I e ~ . ' ~ % ' ~ We also examined nucleophilic attack by OH- upon p-nitrophenyl diphenyl phosphinate (pNPDP) Ph2PO.OC6H4NO2-p pNPDP We used the more reactive pNPDP rather than p-nitrophenyl diphenyl phosphate (pNPDPP), whose reactions had been followed in SDS,I4 because they are inconveniently slow. We also analyzed data for reactions of p-nitrophenyl octanoate, decanoate, and dodecanoate (NPO, NPDe, and NPDo, re~pectively).~
Results Reactions of MeONs with anionic nucleophiles, Br-, OH-, SCN-, and SO3*-,are inhibited by anionic micelles of SDS, but reaction is not suppressed even at high [SDS], in part because there is a spontaneous reaction with water which is not eliminated by S D S (Table I). Micellar effects upon this water reaction fit eq where k$ 1 2 3 3
k$ =
kW'
-k kM'Ks[DnI
I + K,P"I
(1)
is the observed first-order rate constant, kw' and kM' are respectively first-order rate constants in water and micelles, and K, is the substrate binding constant written in terms of the concentration of micellized surfactant (detergent), [D,]. Generally [D,] is taken as [D,o,al]- cmc, where the critical micelle concentration, cmc, gives the concentration of monomeric s ~ r f a c t a n t . ~ , ~ s-I from eq We estimate K, = IO3 M-I and kM, = 3.7 X 1 and the data for reaction with H 2 0 in Table I . In cationic micelles of cetyltrimethylammonium sulfate (CTA(S04), 2) K , = IO3 M-I and kM' = 4.42 X s-l and is larger than =
dw'
(13) Bunton, C. A.; Mhala, M. M.: Moffatt, J. R. J . Phys. Chem. 1989,
93, 854.
(14) Bunton, C. A.; Robinson, L . J . Org. Chem. 1969, 34. 773.
Diphenyl Phosphinate in SDS" M SDS 0.1 M NaCIO4 0.1 M Me4NClb 8.95 (8.0) 6.43 (6.5) 0.3 M NaBr 0.3 M NaCIO, 8.50 (9.1) 8.15 (9.1) 0.3 M NaCl 9.80 (10.1)
0.18M NaBr 7.10 (7.7) 0.3 M Me,NClb 13.7 (14.1) 0.3 M Me4NClb 14.0 (13.8)
With 6 = 3 M-l.
TABLE IV: Reaction of OH- with p-Nitrophenyl Diphenyl Phosphinate in Mixtures of SDS and NaCI" 0.03 M NaOH + 0.3 M NaClb [SDS], M 0.06 0.09 0.12 0.16 0.24 103k$, SKI10.5 (8.8) 8.30 (8.7) 8.80 (8.8) 9.20 (8.8) 9.15 (9.15) 0.15 M NaOHc [SDS], M 0.09 0.12 0.16 0.18 103k$. SKI34.0 (33) 28.0 (33) 33.7 (35) 34.2 (36)
0.15 M NaOH + 0.3 M NaClb [SDS], M 0.12 0.15 0.18 103k$, s-I 54.3 (54) 57.0 (56) 58.8 (58)
'At 25.0 "C, values in parentheses are predicted with a = 24 A. 'With S-', N = 120. ' k w = 8.7 M-' S-', N = 95.
kw = 3.65 M-'
1.2 X s-I.lk This favorable charge effect of cationic and unfavorable effect of anionic micelles has been seen in other SN2 reactions with water.I5 The reaction of Br- with MeONs is effectively suppressed by SDS, because at high [SDS] k$ i= kM' (Table I). However, reactions with OH-, SCN-, and SO,2- are not completely suppressed by SDS, although they are strongly inhibited (Table 11). The observed first-order rate constants, kiC/' = k$ - kHzO,where kHIOis the first-order rate constant for reaction with water in SDS solutions (Table I), tend toward limiting values at high [SDS]. These rate constants are higher than predicted by an equation analogous to eq 1 for a residual reaction of substrate and anionic nucleophile in the aqueous pseudophase, with no reaction in the micelle. Reaction of pNPDP with OH- is inhibited, but not suppressed, by SDS (Figure 1). The spontaneous water reaction is very slow, as with the phosphate, pNPDPP,I6 and its contribution can be (15) (a) AI-Lohedan, H.: Bunton, C. A,; Mhala, M. M . J . Am. Chem. SOC. 1982, 104, 6654. (b) Bunton, C. A,; Ljunggren, S. J . Chem. Soc., Perkin Trans. 2 1984, 355. (16) Bunton, C. A.; Fendler, E. J.; Humeres, E.; Yang, K.-U. J . Org. Chem. 1961, 32, 2806.
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7853
Reactions of Anionic Nucleophiles in Anionic Micelles TABLE V: Salt Effects on Reactions of MeONs in SDS' 0.1 M S0320.3 M SCN-b no salt 3.04 (3.08) 1.46 0.1 M NaCl (3.04) 0.2 M NaCl (2.96) 0.2 M Me4NCI 3.70 2.87 0.4 M Me4NCI 4.55 3.85
t
'Values of I05k+, s-I at 25.0 OC in 0.18 M SDS unless specified; values in parentheses are for 0.15 M SDS. 0.24 M SDS, 0.2 M SCN-, and 0.6 M Me4NCI I05kJ.= 3.1 s-I and with no Me4NC1 105k$ = 1.17 s-I.
n.3
TABLE VI: Salt Effects on Reactions of MeONs in WateP 0.1 M 0.2 M 0.3 M OH-b 0.3 M S 0 3 z - b SCN-C no salt 2.75 23.4 70.1 1.34 0.1 M NaCl 1.33 0.3 M NaCl 2.82 20.2 66.3 1.10 0.1 M Me4NCI 1.42 0.3 M Me4NCI 3.57 28.9 94.9 1.63
I-
'Values of 104k+, s-I at 25.0 OC. bReference 10b,c. CExperimental.
neglected. Reaction with OH- in S D S is speeded by added salts (Tables 111 and IV), as found earlier for reactions of OH- with carboxylic esters in S D S 4 Reactions of SCN- and S032-with MeONs in SDS are also assisted by Me4NCl, but NaCl slightly slows reaction of S032-(Table V). These salts effects depend upon the cation of the salt, rather than the anion (Table 111). They are not wholly due to residual reactions of the substrates with nucleophilic anions in the aqueous pseudophase, although they may be playing a minor role. In water, sodium salts generally slightly inhibit bimolecular reactions of nucleophilic anions and tetraalkyl ammonium salts speed them.'6J7 This behavior had been seen for reactions of OHwith pNPDPP,16 and for reactions of MeONs NaCI has only small salt effects, but Me4NCI assists reactions (Table VI). However, the rate enhancements by Me4NCI are larger in SDS than in water, consistent with the hypothesis that there is a contribution of reaction in the micelles. A hydrophilic salt, e.g., NaCI, could salt a substrate out of water and into the micelles18and thereby inhibit reaction. This effect is clearly unimportant in reactions of hydrophobic carboxylic esters4 and of pNPDP (Tables 111 and IV), but it may be responsible for the slight inhibition of the reaction of S032-with MeONs by added NaCl (Table V). (MeONs binds less strongly than the other substrates to micelles.) Discussion
Chaimovich, Quina, and their co-workers explained results on the reactions of OH- with p-nitrophenyl alkanoates very elegantly in terms of an ion exchange between Na+ and H+which affected the concentration of OH- at the surface of SDS micelle via the autoprotolysis of water.4 This explanation cannot be applied to reactions of MeONs in S D S with the weakly basic anions SCNand SO?- (Tables I1 and V). We therefore consider an alternative model in which there is a finite concentration of anion at the micellar surface, especially at high concentrations of cations. The distribution of ions around an ionic micelle can be calculated in terms of the cell model by solving the Poisson-Boltzmann equation (PBE) in spherical symmetry.+" The radius of the cells, R, is given by ( 4 / 3 ) r R 3 = 1 OOON/(NA[D,] )
(2)
where N is the micellar aggregation number and NAis Avogadro's number. (17) (a) Bunton, C. A.; Robinson, L. J . A m . Chem. SOC.1968, 90,5965. (b) Bunton, C. A.; Huang, S. K. /bid. 1972, 94, 3536. (c) Postle, M. J.; Wyatt, R. A. H. J . Chem. Soc., Perkin Trans. 2 1972,474. (d) Ride, J. N.; Wyatt, P. A. H. [bid. 1973, 746. (18) Bunton, C. A.; Gan, L-H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G . J . Phys. Chem. 1981,85, 41 18.
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O
