J. Phys. Chem. 1986, 90, 5849-5853 laws with powers ranging from 0.25 to 0.40. The constancy of the driving force implies that we should find the transition from the smooth ("gravity") regime to the "porous regime" at about a constant value of the average thickness ( h ) , denoted by h*. We tabulate h*, calculated as for a spherical cap, i.e., h* = 2 f l / ( ~ R * in ~ )Table , 11. Of course R* is not obtained very accurately, so that h* is merely a rough indication of the transition rather than a precise criterion. Nevertheless, given that ( h ) varies, in this experiment, over 3 orders of magnitude, we might say that h* occurs, indeed, in a narrow range of values, thereby supporting our argument. (9) According to a model of flow along a rectangular duct (ref 4) and assuming that the scanning profiles of the various samples are homothetic (which seems actually to be the case), we would expect D yo, the maximal variation of profile. Clearly, the phenomena are more complicated, and two situations are observed (Table I). For yo k 5p, D yo indeed, but for smaller yo, D decreases much more rapidly. Thus the model is not immediately applicable in the present case.
-
-
5849
The final regime corresponds to the case where the liquid film at the edge becomes thinner than the characteristic length scale of the surface roughness. Then, the capillary force starts to decrease again and a corresponding slowing down is observed. We note in passing, that this characteristic thickness (and the corresponding transition) may vary, according to roughness between tens of microns and 0.2 pm, this lower bound being of the same order of magnitude as the famous macroscopic precursor film expected to appear when e,, becomes very small.' Acknowledgment. We thank the Optical Workshop of the Ecole SupErieure d'Optique at Orsay for their kind help in preparing and characterizing some of the depolished surfaces for this study, M. A. Guedeau for providing us with smooth hydrophobic glass surfaces, and A. Bouillault for carrying out part of the experimental work. We also acknowledge F. Brochard, P.-G. de Gennes, J. P. H u h , A. Libchaber, M. May, and M. VeyssiE for kind interest and helpful discussions. This work was partly supported by Exxon Research and Engineering Co.
Size vs. Reactivity in "Organized Assemblies": Deacyiation and Dephosphorylation in Functionalized Assemblies Cirma Biresaw*' and Clifford A. Bunton* Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: February 28, 1986; In Final Form: June 3, 1986)
Deacylation of p-nitrophenyl benzoate and dephosphorylation of p-nitrophenyl diphenyl phosphate were followed in 1:10 aggregates of tri-n-octyl(2-hydroxyiminoethy1)ammoniumchloride (lb) and tri-n-octylethylammonium mesylate (la) and in micelles of cetyldimethyl(2-hydroxyiminoethyl)ammonium chloride (2b) or 1:lO comicelles of 2b and cetyltrimethylammonium chloride (2a). Analysis of the rate data shows that second-order rate constants in aggregates and micelles are similar and are independent of the size of the assembly. The second-order rate constant in the functional micelle is similar to that of reaction of the model compound trimethyl(2-hydroxyiminoethy1)ammoniumchloride (3) in water.
Introduction Amphiphiles, in solutions of three-dimensional, associated solvents, form organized assemblies of varying size,2 including single- and multilayer vesicles, micelles, and aggregates of trin-octylalkylammonium 1. Micelles and aggregates of (n-CBH17)3N+RX-
SCHEME I
pNYYPDPP
la: R = Et; X = OMS lb: R CH,CHNOH, X C1 IC: R CH,CHNOH, X = OMS Id: R = CH2CH20H, X = OMS
2a: R
CH3, X C1 2b: R E CH,CHNOH, X CI 2c: R CH3, X Br 2d: R CH2CH20H,X Br 2e: R CHICHIOH, X = C1 (CHS)IN*CH&HNOH C1E
0
0
n-C16H33N+(CH,)2RX-
II
RR'2NtCH2CH=NO-CPh
II
RR'2N)CH2CH=NOP-(OPh)2
+
t
0-c~ H 4N02
0 - c H~,No2
1b.lc: R Z R ' ~ n - C ~ H 1 72 b : : Rgn-Cq6H33, R':CH3;
3: R I R ' E C H 3 .
0 p N P B E P - N O ~ C B H ~ Op~NP PD ~ P; P 5 p-N02CsH40P(OPh)2
3
1 self-associate, but energy, e.g., from sanification, is often required for vesicle formation. Aggregates of 1, like micelles of the cor(1) Present address: Alcoa Technical Center, A h a Center, PA 15069. (2) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. Romsted, L. S.In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 2, p 1015. Bunton, C. A. In The Chemistry of Enzyme Action; Page, M.I., Ed.; Elsevier: New York, 1984; p 461. (3) Okahata, Y.; Ando, R.; Kunitake, T. J. Am. Chem. SOC.1977, 99, 3067. (4) (a) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.;Plenum: New York, 1984; Vol. 2, p 1093. (b) Bunton, C. A.; Hong, Y. S.; Romsted, L. S.; Quan, C. J. Am. Chem. SOC.1981,103,5784,5788. (c) Bircsaw, G.; Bunton, C. A.; Quan, C.; Yang, Z.-Y. Ibid. 1984,106,7178. (d) Bunton, C. A.; Quan, C. J. Org. Chem. 1984,49, 5012. (e) Bunton, C. A.; Quan, C. Ibid. 1985, 50, 3230. (5) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.;Hirakawa, S. J. Am. Chem. SOC.1980, 102, 7877.
responding cationic surfactants 2, bind reactants and can speed uni-"~~-~ and bimolecularM reactions. However, unlike surfactants, 1 do not have a critical micelle concentration, cmc, and cooperativity may be important in their aggregation. Also, unlike micelles,aggregates 1 are probably highly polydisperse, have lower aggregation numbers, and are poorer at binding hydrophilic counterions. These differences must be considered in analyzing reaction rates in these assemblies!v6 We use the terms aggregates and micelles to denote assemblies of 1 and surfactants, respectively. Oximes are deprotonated at high pH giving oximate anions (Scheme I), which are effective nucleophiles in deacylation, dephosphorylation and other bimolecular rea~tions.~-'O (6) Biresaw, G.; Bunton, C. A., this issue. (7) (a) Bunton, C. A.; Ihara, Y. J. Org. Chem. 1977,42,2865. (b) Bunton, C. A.; Hamed, F.; Romsted, L. S. J. Phys. Chem. 1982, 86, 2103.
0022-3654/86/2090-5849%01 .50/0 Q 1986 American Chemical Society I
,
5850 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
We have studied deacylation of pnitrophenyl benzoate, pNPB, and dephosphorylation of p-nitrophenyl diphenyl phosphate, pNPDPP, in aqueous aggregates of tri-n-octyl(2-hydroxyiminoethy1)ammonium chloride ( l b ) or mesylate (IC) and in micelles of cetyldimethyl(2-hydroxyiminoethy1)ammonium chloride (2b). Because of solubility problems we used 1:lO coaggregates of l b l a , or 1:lO comicelles of 2b:2a. In some cases, small amounts of MeCN or MeOH had to be added to solubilize the amphiphiles. The rate data were analyzed by using the pseudophase model, with reactants distributed between water and the organized assembly,2" and reactivities in the micellar and nonmicellar aggregates were compared with that of the model compound trimethyl(2hydroxyiminoethy1)ammonium chloride (3) in water. In the accompanying paper we describe a stepwise self-association model and apply it to spontaneous and bimolecular reactions in solutions of 1: and here we compare reactions in normal aqueous micelles and in aggregates of 1. Experimental Section
Materials.I2 Trimethyl(2-hydroxyiminoethy1)ammonium chloride ( 3 ) was prepared by reaction of equimolar 2-chloroacetaldoxime13 and Me3N in 2-propanol at room temperature. After severql hours, the solvent was removed and the resulting solid was recrystallized (EtOH-Et20) to give an extremely hygroscopic product with the following NMR absorptions: 6(D20, external Me4Si) 3.13 (syn), 3.22 (anti), (s, 9 H, (CH3),); 4.06 (syn), 4.26 (anti) (d, 2 H, CH,), and 7.22 (anti), 7.73 (syn) (t, 1 H, CH) ppm. Anal. Calcd for C5Hl3N20C1:C, 39.35; H, 8.58; N , 18.35; C1, 23.23%. Found: C, 38.65; H, 8.60; N , 17.93; C1, 23.28%. Tri-n-octyl(2-hydroxyiminoethy1)ammoniumchloride (lb) was prepared by reaction of tri-n-octylamine with equimolar 2chloroacetaldoxime13in anhydrous EtOH at room temperature. Removal of the solvent and recrystallization (EtOAc-Et,O) gave a mostly syn product with the following N M R absorptions: 6(CDCl,, Me4Si) 0.6-3.5 (m, 45 H), 3.3 (m, 6 H, (CH2)3), 4.3 (d, 2 H, CH,), 7.5 (t, 1 H, CH), and 12.5 (b, 1 H, OH) ppm. Anal. Calcd for CZ6HS5N20C1:C, 69.83; H, 12.40; N , 6.26, C1, 7.93%. Found: C, 69.71; H, 12.44; N, 6.30; C1, 7.84%. Tri-n-octyl(Zhydroxyiminoethy1)ammoniummesylate (IC) was prepared by treating l b with silver mesylate14 in MeCN. The fine white precipitate was removed by filtration on Celite. The solvent was removed and the resulting solid was recrystallized (Et,O) to give mostly syn product with the following N M R absorptions: 6(CDC13,Me4Si) 0.4-2.2 (m, 45 H), 2.8 ($, 3 H, CH,), 3.2 (m, 6 H, N+(CHJ3), 4.1, 4.3 (d, 2 H, CH,), 7.0, 7.5 (t, 1 H, CH), and 12.2, 12.6 (b, 1 H , NOH) ppm. Anal. Calcd for C27HSBN204S: C, 63.99; H, 11.53; N , 5.53. Found: C, 64.10; H, 11.31; N , 5.57. Cetyldimethyl(2-hydroxyiminoethy1)ammoniumchloride (2b) was prepared by reacting equimolar 2-chloroacetaldoxime'3 and hexadecyldimethylamine in Et,O at room temperature. Solvent was removed in vacuo and the solid was recrystallized (EtOAcEtOH) to give mostly syn product having the following NMR absorptions: 6(CD30D, Me4Si) 0.5-2.2 (m, 33 H), 3.1 (s, 6 H (CH3),), 4.1, 4.3 (d, 2 H, CH,), and 7.2, 7.6 it, 1 H, CH) ppm. Anal. Calcd for C&143N20C1: C, 66.17; H, 11.94; N, 7.72; C1, 9.77%. Found: C, 66.28; H, 11.78; N, 7.89; C1, 9.67%. The preparation and purification of the other materials is described e l ~ e w h e r e . ~Solvents ~'~ were purified by standard methods. (8) Yatsimirski, A. K.; Martinek, K.; Berezin, I. Tetrahedron 1971, 27,
2855. (9) Kunitake, T.; Shinkai, S.; Okahata, Y. Bull. Cbem. SOC.Jpn. 1976, 49, 540.
(10) (a) Anoardi, L.; de Buzzacarini, F.; Fornasier, R.; Tonellato, U. Tetrahedron Lett. 1978, 3945. (b) Anoardi, L.;Fornasier, R.; Tonellato, U. J. Chem. Soc., Perkin Trans. 2 1981, 260. (11) Menger, F. M.; Portnoy, C. E. J. Am. Chem. SOC.1967.89, 4698. (12) NMR spectra were measured on Varian T-60 or EM-360 spectrometers with chemical shifts in parts per million dowpfield from Me4Si. Microanalysis was by Galbraith Laboratories, Inc., Knoxville, TN. (13) Meister, W. Ber. 1907, 40, 3442. (14) Emmons, W. D.; Ferris, P. F. J . Am. Chem. SOC.1953, 75, 2257.
Biresaw and Bunton TABLE I: Effect of Added Orgrnic Reagents on Deacylation of pNPB and Dephosphorylation of pNPDPP k,, M-I sW1 added solvent or reagent MeOH, 2.57r5.0 (v/v) 3, 0.5-1.0 mM (Me),N+CH2CH20HCI-
pNPB 3.0b 200c 18d 18c
pNPDPP
0.41f 2.0c 1S d 3.1e
"At 25.0 "C with 0.0014.005 M NaOH. bLiterature, kw = 3.2 M-' s-l, ref 4c. CBasedon pK, = 15.5 for MeOH, ref 15. dBased on pK, = 9.35 for 3. 'Reference 4c. fliterature, kw = 0.48 M-l s-l, ref 17.
Deprotonation Equilibrium. The acid dissociation constants of the oximes, K, (Scheme I), were determined spectrophotometrically in 0.01 M carbonate buffer from the UV absorbance of the monomeric oxime and oximate anions (total concentrations: [3] = 0.1 mM; and [IC] = [2b] = 33.3 pM). The solutions were scanned in the 350-200-nm range and the oximate anion had A, = 228 nm. No absorbance maximum was observed for the oxime. Values of K, were obtained from eq 1. where A and AT are
absorbances at 228 nm a t partial and complete deprotonation, and eHx and e, are molar absorptivity coefficients of the oxime and oximate, respectively. Linear plots of (AT - A)/[H+] vs. A gave the following pK, values: IC,9.72; 2b, 9.59; 3, 9.35. The pK, of l b (X C1-) was not determined but should be similar to that of IC (X = OMS-). Kinetics. Formation of p-nitrophenoxide ion a t 25.0 OC was followed spectrophotmetrically a t 405 nm. A Beckman or a Gilford spectrometer was used for the slower and a Durrum stop-flow spectrophotometer for the faster reactions. First-order rate constants are in reciprocal seconds. The low solubility of some of the amphiphila forced us to follow reactions in aqueous-organic solvents of high water content. Cosolvents when present were MeCN or MeOH. Solutions were made up in redistilled C0,-free water. A small amount of MeCN (0.08-0.17%, v/v) was introduced into the reaction solution with the substrate, and [substrate] was 2.5-5 pM. Results
Reactions in Water. Effect of Added Organic Solutes on Reaction of Hydroxide ion in Water. Some reactions in aggregates and micelles had to be followed in the presence of organic cosolvent, so we also examined the effect of the organic cosolvent on reactions in the absence of amphiphiles. Added MeOH speeded reactions of pNPB and of pNPDPP in water, due to a reaction with methoxide ion (Table I). Based on pK, = 15.5 for MeOH," the second-order rate constants for deacylation of pNPB and dephosphorylation of pNPDPP by methoxide ion are 200 and 2.0 M-' s-', respectively. Eior pNPB kMa/koH = 63 is similar to the value of 41 estimated for deacylation of p-nitrophenyl acetate.16 The second-order rate constant of 0.41 M-' s-* for the reaction of OH- with pNPDPP is slightly lower than the reported value of 0.48 M-I s-',I7 and MeO- is only slightly more reactive than OH-. Reaction of Trimethyl(2-hydroxyiminoethyl)ammonium Chloride ( 3 ) in Water. The second-order rate constants for deacylation of pNPB and dephosphorylation of pNPDPP by the model oxime 3 in water are 18 and 1.5 M-' s-I, respectively, based on pK, = 9.35. A value of 0.92 M-' s-l has been reported for dephosphorylation of pNPDPP in water by a similar but more hydrophobic oxime.% These values are fortuitously similar to those for the reaction of cholinate zwitterion with the same substrates in water, which are 18 and 3.1 M-' S - I . ~ Thus the oximate ion (15) Ballinger, P.; Long, F. A. J . Am. Chem. Soc. 1960, 82, 795. (16) Jencks, W. P.; Gilchrist, M. J . Am. Chem. SOC.1962, 84, 2910. (17) Bunton, C. A,; Robinson, L. J . Org. Chem. 1969, 34, 773.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5851
Size vs. Reactivity in Organized Assemblies
SCHEME I11
IO4 [U], M
Figure 1. Oximate reactions in 1:lO coaggregates of lb:la, 0.005 M NaOH, 5% (v/v) MeCN: 0 , pNPB; 0,pNPDPP. The lines are predicted by using the model.
is a much better nucleophile, relative to the basicities of the two reagents. Reactiom in Nonmicellar Aggregates. Reactions in Aggregates of l b . Deacylation ofpNPB was followed a t pH 10.8 (0.01 M carbonate buffer). Even with added MeCN, 0.4-0.7 vol %, lo4 M l b solutions were slightly turbid and only approximate rate constants were obtained. With 0.1, 0.2, 0.3, and 0.4 mM lb, approximate values of lo3 k, are 12 f 2 s-l for variation of [NaOH] between 0.8 and 1.6 mM. The pKa of the oxime mesylate salt, IC,is 9.7, so the oxime should be extensively deprotonated under our conditions. The turbidity of the solutions increased with increasing l b and the apparent increase in rate which was observed with higher [lb] is probably due to formation of globules of l b which bind substrate and assist reaction. These results were not treated quantitatively. Reactions in 1 : l O Coaggregates of 1 b : l a . To overcome solubility problems, the hydrophobic oxime l b was mixed with a tenfold excess of the more soluble la, and MeCN was added. The rate constants of deacylation of pNPB and dephosphorylation of pNPDPP in 1:lO coaggregates of lb:la, 5% (v/v) MeCN and 0.005 A4 NaOH are shown in Figure 1. The rate increases with increasing [la lb] toward a plateau, because of increased binding of the substrate to the coaggregate. Reactions were followed with [OH-] such that the oxime is fully deprotonated. The lines in Figure 1 are predicted by using a pseudophase model in which the aggregates are formed by a stepwise self-association process.6 In analyzing the data in Figure 1 we made the assumption that, because of similarities in the structures of l a and lb, the stepwise self-association of the mixture will be similar to that of pure la or lb. The original model, which was derived for a pure tri-n-octylalkylammonium salt: was modified as briefly described below. We assumed that coaggregates are formed by a stepwise self-association process (Scheme 11) in which the self-association constant, K,, is independent of the aggregation number,I8 and that only tetramers and higher aggregates promote reaction. We also Scheme I1 Kz K3 4 T, .., 2 T l e T2 + TI F! T3+ ... T,, + TI
TABLE II: Best-Fit Parameters for Bimolecular Reactions with Functional Aggregateso substrate (ammonium salt) pNPB (la
+
1b)
pNPDPP (la
K 2 = K3 =
... = K ,
=
... = constant
t , = [TI] = equilibrium monomer concentration
t, = [T,] = ( K t l ) 4 / K
+
5
5
1O d 20 1O d 10
10 10 20 30
b
1400 1500
1.5
600 1000 600 1500 250 200 20 180
3.3 4.2 1.25 0.97
1b)
pNPDPPE (la)
0.16 0.16 0.16 0.16
+
aNaBsmox 1 20 in l a lb. bFull deprotonation of l b is assumed under the reaction condition. Data from ref 6; Naggmar 1 30. Naggmax 1 20.
(Scheme 111). The observed rate is the sum of the rates in water and in the aggregates.*q6 We neglect reaction of OH- in the aggregates because they only weakly bind hydropilic ions. In Scheme 111, S, and S, are substrate in water and that bound to q-mers. It appears that the contribution to the overall rate of aggregates larger than N is insignificant and can be neglected. The equations to be usedTanalyzing bimolecular reactions are as follows6
+
+
103[NaOH], MeCN, KBaPP, K,, K, M 3' % (v/v) M M-' M-' k,, s-] 5 5 b 800 1500 9.5
k, =
kwoHIOHl + kwNINwl + kMmNS(Ks/K)Aq 1 + (Ks/K)A,
(2)
where NrrmU
A, =
C
q(Kt1)'; NaBma,= 10-40
(3)
4=4
Ktl =
1
+ 2Kt - (1 + 4Kt)'I2 2Kt
t = [la]
(4)
+ [lb]
and the other parameters are shown in Schemes I1 and I11 and kwoHand kWNare second-order rate constants (M-I s-l) for reactions of OH- and a nucleophile N in water, kMis the secondorder rate constant with respect to aggregate bound nucleophile whose concentration is written as mole ratio with respect to aggregate, mNs:b,c Equations 2-4 were combined by using a simple computer program to analyze the rate data, neglecting the contribution of the nucleophile reaction in water; i.e. kWN[N,]i= 0 The oxime was fully deprotonated; thus
e
t
= [TI = Cqt, ,-I
assume that binding of substrates by aggregates is directly proportional to their aggregation number, but that rate constants in the aggregates are independent of the aggregation number (18) The treatment is as detailed in ref 6 with the added simplification that the ratio lb/la is assumed to be the same in the monomer and the various aggregates.
The plots in Figure 1 were obtained by using this model, K = 1500 M-I, and K, values of 800 and 1400 M-I for pNPB and pNPDPP, respectively. The fits of bimolecular reactions in coaggregates of the tri-noctyl derivatives ( l a + lb) are not as good as for unimolecular reactions in aggregates of pure la or Id, and bimolecular reactions in pure ld,6 probably due to a perturbation of the self-association process due to mixing of functional and nonfunctional amphiphiles.
Biresaw and Bunton
The Journa: of Physical Chemistry, Vol. 90, No. 22, 1986
5852
.
V
I
1 1
I 2
I
to3[&],
M
Figure 2. Oximate reactions in micellar 2b, 0.005 M NaOH, and 2.5% (v/v) MeOH: 0 ,pNPB; 0,pNPDPP. The lines are predicted by using eq 5. 0
SCHEME IV
2
KD
S,,,
t 0, e S,
L
4
d[%l, y
1
Reactions of pNPB with lyate ion in micellar CTACI (2a) and 0.005 M NaOH. Solid line with 2.5% (v/v) MeOH: broken line without added MeOH is predicted, ref 22. Figure 3.
product
Our analysis neglects such perturbation, and the low solubility of l b is a problem. Table I1 compares the best-fit parameters in coaggregates of ( l a lb) with those for the dephosphorylation of pNPDPP in aggregates of tri-n-octylhydroxyethylammoniummesylate (ld).6 The values of K are consistent with the effect of added electrolyte (NaOH) or organic solutes (MeCN) on water structure and aggregation of amphiphiles in that electrolytes should favor, and hydrophilic solvents disfavor, aggregation. The values of Ksare consistent with a decrease of binding due to MeCN and with the known hydrophobicities of pNPB and pNPDPP. Micellar Reactions. Reactions in Micelles of 2b. We found the functional surfactant, 2b, to be almost insoluble in water, even with small amounts of added MeCN. The reactions were followed in 2.5% (v/v) aqueous MeOH, and the rate constants of deacylation of pNPB and dephosphorylation of pNPDPP in micelles of 2b and 0.005 M NaOH are given in Figure 2. The data were analyzed by using the psuedophase (Scheme IV), where the substrate is assumed to be distributed between water, S,, and micelles, SM,and Dn is the micellized surfactant. Monomeric 2b has pK, = 9.59 (experimental) and micellization should favor deprot~nation.'~-~' Thus, complete deprotonation of the oxime is assumed under the reaction conditions, resulting in a zwitterionic micellar surface which should not bind hydroxide or methoxide ions. Under these conditions, the rate follows binding of the substrate
+
(5) where kM is the second-order rate constant in the micelle expressed in reciprocal seconds, with the nucleophile concentration expressed as a mole ratio, and k,' is the first-order rate constant in the water. The lines in Figure 2 are those predicted by using eq 5 and reported values of K,." Effect of Added MeOH on Micellar Deacylation. To determine the effect of added MeOH in conditions where anion binding was possible, i.e., with a nonfunctional micelle, rates of deacylation of pNPB in micellar CTACl, 0.005 M NaOH, and 2.5% (v/v) MeOH were measured. Figure 3 compares results with the values (19) Hartley, G. S . Trans. Faraday SOC.1934, 30, 444. (20) (a) Bunton, C. A.; Romsted, L. S.; Sepulveda, L. J . Phys. Chem. 1980,84,2611. (b) Bunton, C. A.; Hong, Y.-S.;Romsted, L. S . In Solurion Behavior ofSurfacranrs; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 2, p 1137. (21) Romsted, L. S . J . Phys. Chem. 1985,89, 5107, 5113.
3
.. * ' 2 'v)
OC
I
I
I
io 15 io3 [g+ 3 1, E Figure 4. Effect of MeOH on reactions in 1: 10 oximate comicelles of 2b:2a A,pNPB in 0.01 M NaOH and 2.5% (v/v) MeOH; 0,pNPB in 0.005 M NaOH and 2.5% (v/v) MeOH; m, pNPB in 0.005 M NaOH without added MeOH; 0,pNPDPP in 0.005 M NaOH and 2.5% (v/v) MeOH; 0 ,pNPDPP in 0.005 M NaOH without added MeOH. Broken lines are predicted by using the empirical eq 6 . 0
5
in absence of MeOH.22 A fourfold increase in the rate maximum suggests that there is a significant reaction with methoxide ion in the micelle. Quantitative analysis of the rate data requires values for the micellar exchange constants of methoxide ion with chloride and hydroxide ions, but these values are not available. Qualitatively, the large increase in rate in presence of MeOH is consistent with the greater nucleophilicity of MeO- over OH(Table I) and also better binding of the less hydrophilic MeOto the micellar surface. Reactions in 1 : l O Comicelles of 26:2a. Mixtures of the surfactants 2b and tenfold excess 2a are soluble in water without added MeOH. Rate constants of deacylation of pNPB and dephosphorylation of pNPDPP in l :10 comicelles of 2b:2a, 0.005 M NaOH are shown in Figure 4. The rate constants in the comicelles go through maxima with increasing [surfactant]. Similar behavior has been observed in other comicelles and has (22) Biresaw, G.; Bunton, C . A. J . Org. Chem. 1986, 51, 2771.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5853
Size vs. Reactivity in Organized Assemblies been attributed to competition for the comicelles between hydroxide ion and inert counterions.2 Interionic competition in these systems is generally smaller than that observed in nonfunctional micelles and could not be fitted to the usual pseudophase, ionexchange m ~ d e l ? + but ~ J ~rate effects fitted the empirical equation ( 6 ) where y represents the sensitivity of the rate constant to the k+ =
KAkO (1 + KSD")(1 + Yr)
substrate pNPB
nucleophile -CHICH20-CH,CHNO-
(6)
inert counterion, r is the ratio of the total hydroxide ion to total inert counterion concentrations, and ko is the hypothetical rate constant in the comicelle having only OH- as the counterion. Evidence on the failure of the ion-exchange equation in functional micelles and the use of eq 6 is given elsewhere.22 The rate constants in comicelles of 2a + 2b were analyzed by using eq 6 and the predicted values are shown by the broken lines in Figure 4. The fits using independently estimated values of y = 0.6 and ko = 4.2 s-l for pNPB are good. These values, when corrected for dilution, are fortuitiously similar to that obtained in the hydroxyethyl functionalized system of 2e, which are ko = 42 s-l and y = 0.8.22The similarity in the reactivity of the two functional groups toward pNPB is also observed in the water reactions with the corresponding model compounds (Table I). The fit for pNPDPP is also shown in Figure 4 and was obtained by using y = 1.5 and ko = 1.1 s"'. The slightly higher y value might mean that the sensitivity of the reaction to the inert counterion is slightly substrate-dependent. Effect of Added MeOH on Reactions in 1 : l O Comicelles of 2b:2a. Added MeOH speeds deacylation of pNPB in the functional oximate comicelle and the effect follows increasing [OH-] (Figure 4). This result is similar to that observed for the same reaction in micellar CTACl (Figure 3) and is consistent with the higher hydrophobicity and nucleophilicity of methoxide over hydroxide ion. Added MeOH slightly inhibits dephosphorylation of pNPDPP in the functional oximate comicelle (Figure 4). In water, the rate constant of dephosphorylation of pNPDPP by methoxide ion is slightly higher than that by the model oximate, 3 (Table I); the order appears to be reversed in the comicelle. Although MeOis a much stronger nucleophile than OH- in deacylation, nucleophilicities are similar in dephosphorylation.
Discussion Comparison of Reactivities in Micelles and Nonmicellar Aggregates. The second-order rate constants, kM,for deacylation of pNPB by the oximate anions in nonmicellar coaggregates of ( l a lb) and micelles of 2b are 9.5 and 24 s-I, respectively, and the corresponding values for the dephosphorylation of pNPDPP are 1.5 and 3.4 s-l, respectively (Figures 1 and 2). Several studies have shown that the second-order rate constants, kM, in tri-noctylalkylammonium salts are similar to those in the corresponding cationic micelle^.^ Table I11 compares the second-order rate constants for reactions of various substrates in micelles and in aggregates of tri-n-octylalkylammonium salts. Aggregates of tri-n-octylalkylammonium salt are smaller than micelles and, hence, are less effective at binding substrates, but correction for the differences in binding shows that size differences do not
+
TABLE III: Comparison of Second-Order Rate Constants in Aggregates of 1 and Micelles of 2
pNPDPP
benzimidazolide ion
reaction medium
+ Id 2c + 2d l a + IC la
2b la 2c
-CH2CH2O-CH,CHNO-
la 2d la 2b
bis-2,4DNPPd
-CH2CH20-
2,4-DNCBe
-CH2CH20-
ld 2d Id 2d
+ Id
+ IC
kM,s-I 113" 264" 9.5 24 11-31' 76
3-4"se 3.7-5.6" 1.5 3.4 5.7 x 10-3c (3.3-3.6) X lo-' 0.19c 0.3"
" Reference 4c. Reference 4b. Reference 6. Bis-2,4-dinitrophenyl phosphate, ref 4d. e 2,4-Dinitrochlorobenzene. markedly alter second-order rate constants in the different colloidal species, based on the mole ratio of nucleophile to quaternary ammonium ion. Comparison of Reactivities in Water and in Organized Assemblies. The second-order rate constants for reaction of pNPB and of pNPDPP with various nucleophiles in aqueous solvents are summarized in Table I. These rate constants cannot be compared directly with those in organized assemblies because of differences in dimensions. Comparison can be made, however, if a value for the reaction volume in the organized assembly can be estimated. For aggregates of tri-n-octylalkylammonium salts, no estimation of a reaction volume has been made, because there is no information on their geometries. Micelles are approximately spherical, and reactions are assumed to take place in the Stern layer, whose molar volume has been estimated as between 0.14 and 0.37 L mol-1.2*8910~23 Based on the lower value, the second-order rate constant in the micelle, in units of M-l s-I, is given by eq 7. Values
k2" = 0.14kM of kzmfor pNPB and pNpdPP in micellar 2b of 3.4 and 0.48 M-' s-', respectively, are slightly lower than the corresponding values in water (Table I). This result is consistent with other observations that bimolecular rates in water and "organized assemblies" are usually similar and the apparent "catalysis" by micelles and other organized assemblies is largely a concentration effectz4 Acknowledgment. Support of this work by the National Science Foundation and the U.S. Army of Research is gratefully acknowledged. Registry No. lb, 103981-36-8; IC, 103981-38-0; 2b, 103981-39-1; 3, 103981-35-7; pNPB, 959-22-8; pNPDPP, 10359-36-1. (23) Buntoa, C. A.; Carrasco, N.; Huang, S . K.;Paik, C.; Romsted, L. S. J. Am. Chem. SOC.1978, 100, 5420. (24) We see no simple way to estimate the volume element of reaction in a nonmicellar aggregate, but values of kH are similar in these aggregates and in cationic micelles.
