2832
Langmuir 1993,9, 2832-2835
A Quantitative Treatment of Micellar Effects upon Dephosphorylation by the Hydroperoxide Anion Clifford A. Bunton. and Houshang J. Foroudian Department of Chemistry, University of California, Santa Barbara, California 93106 Received March 8,1993. I n Final Form: July 8,199P Dephosphorylation of p-nitrophenyl diphenyl phosphate by the hydroperoxide ion ( H o d is speeded by micelles of cetyltrimethylammoniumchlorideand mesylate (CTACland CTAOMs). Added C1- competes with HOz- at the micellar surface and almost completelyeliminates the micellar rate enhancement. The micellar and salt effects are fitted by a pseudophase model in which the concentration of HOz- at the micellar surface is calculated by solving the Poisson-Boltzmann equation in spherical symmetry. The calculated second-order rate constant at the micellar surface is lower than that in water by a factor of ca. 5. Peroxyanionsare a-effect nucleophileswhose reactivity is higher than that predicted by relations between nucleophilicityand basicity.'Y2 However,their high reactivity is understandable if nucleophilicityis related to ionization or oxidation p ~ t e n t i a l s .Cationic ~ ~ ~ micelles and microemulsions increase rates of dephosphorylation of p-nitrophenyl diphenyl phosphate (pNPDPP) by peroxyanions.5 Micelles and similar colloidal assembliesgenerally increase rates of bimolecular reactions of hydrophobic substrates and counterions by concentratingboth reactants at the colloidal surface, and for many reactions secondorder rate constants in this region are similar to, or even lower than, those in water.6 A qualitative treatment of dephosphorylation of pNPDPP by peroxyphthalate dianion or m-chloroperoxybenzoate ion in micellized cetyltrimethylammonium chloride (CTAC1)gave second-order rate constants at the micellar surface higher than those in water by factors of 2-4.5b The pKa of the hydroperoxy group in peroxycarboxylic acids is ca. 8,' but PKa = 11.65 for hydrogen peroxide in watel.8 so that HO2- is formed only in solutions of relatively high pH. If the micellar reaction of pNPDPP and HO2- is followedunder conditions in which H202 is only partially deprotonated, e.g., with dilute OH- in excess over H202, micelles will affect both deprotonation of H202 and reactions involving H02-.6,9 The use of buffers or relatively high concentrations of OH- complicatesthe kineticsand may introduceadditional chemical reactions. We avoided these problems by taking H202 in excess over OH- so that [H02-] is approximately given by the concentration of added OH-, and the e Abstract published in Advance ACS Abstracts, September 15, 1993. (1) (a) Davies, D. M.; Deary, M. E.J. Chem. Soe., Perkin Trans. 2 1992, 559. (b) Buncel, E.;Wilson, H.; Chuaqui, B. J. Am. Chem. SOC.
1982,104,4896. (2) Jencks, W. P. Catalysis in Chemistry and Enzymology; McGraw Hilk New York, 1969; Chapter 2. (3) (a) Pross, A. Ace. Chem. Res. 1985,14212. (b) Hoz,S. J., J. Org. Chem. 1982,47,3545. (c) Shaik, S. S . Acta Chem. Scand. 1990,44,205. (4) (a) Ritchie, C. D. J . Am. Chem. Soe. 1983,105,7313. (b)Buncel, E.; Shaik,S. S.; Um, LH.; Wolfe, S. J. Am. Chem. Soe. 1988,47,3545. (5) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R.; Monarres, D.; Savelli, G. J . Org. Chem. 1984,49,426. (b)Bunton, C. A.; Mbala, M. M.; Moffatt, J. R. J . Phys. Org. Chem. 1990, 3, 390. (6) (a) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991,24,357. (b) Bunton, C. A.; Savelli, G . Adv. Phys. Org. Chem. 1986,22, 213. (7) Koubek, E.;Haggett, M. L.; Battaglia, C. J.; Ibne-Rasa, K. M.; Pyun, H. Y.; Edwards,J. 0. J. Am. Chem. SOC.1963,85,2263. (8) Everett, A. J.; Minkoff, G. J. Trans Faraday SOC.1953,49, 410. (9) Romsted, L. S. J . Phys. Chem. 1985,89,5107, 5113.
equilibrium between H202 and HO2- strongly favors the lattel.8 H202+ OH- e HO;
+ H,O
Micelles may affect deprotonation of H202 but their effect will be negligibleprovided that H202 almost quantitatively
protonates OH-, which will be the situation if [H2021 >> KwlKa.
Results and Discussion Reactions were followed over a range of [CTAClI with initial concentrations of H2Oz in the range 0.1-0.5 M and of OH- in the range 8 X lV to 2 X 103 M, where H202 decomposes readily, so we brought solutions of CTACl + NaOH to temperature equilibrium in the cuvette, then added H202 and immediately afterward started the reaction by adding pNPDPP in MeCN. Reaction was fast, t1/2 < 10 s in solutions of CTAC1, and bubbles caused no problem; however, we could not use a stopped-flow spectrometer, so we were restricted to conditions in which reactions could be followed in a diode-array spectrophotometer (Experimental Section). Rate-surfactant profiles for reactions in solutions of CTACl and cetyltrhethylammonium mesylate (CTAOMs) (Figures 1 and 2) are typical of bimolecular micellarassisted reactions.'j Observed first-order rate constants, k+, increase very sharply with increasing [CTACll, as for reactions of very hydrophobicsubstrates such as pNPDPP, and then d e ~ r e a s e . ~There ? ~ is scatter in the data for [CTAClI corresponding to the rate maximum where the reaction is fast (tip = 0.5 s) and only the final part of it could be followed. However, an increase of [H2021 from 0.2 to 0.5 M does not significantlyaffect k+ (Figure 1).An increase of added NaOH from 103 to 2 X 103 M markedly increases k+ and reaction is then too fast to be followed in dilute CTAC1. Added KC1stronglyinhibits the reaction by competing with HO2- at the micellar s u r f a ~ e . ~Rate *~ surfactant profiles in solutions of CTAOMs and CTACl are very similar (Figures 1 and 2). Quantitative Treatment Micellar rate effectsare generallyfitted by pseudophase models which regard water and micellesas distinct reaction regions.6 The overallrate is the s u m of rates in eachregion, which depend on the second-orderrate constants in water and at the micellar surface (kwand kZm, respectively),and reactant concentrations in the two pseudophases.
0743-7463/93/2409-2832$04.00/00 1993 American Chemical Society
Langmuir, Vol. 9,No. 11, 1993 2833
Dephosphorylation by Hydroperoxide Anion
to
-
Ionic distributions between the micellar surface and cell wall are calculated by solving the Poisson-Boltzmann equation (PBE), with r the distance from the micellar center.
0.46
1.0-1
In
I*
*
*
02
01
03
04
05
[KCI], M
_I I I I
S
In eq 2,4 is the reduced potential, 4 = e$/kT, where $ is the electrostatic potential, T the absolute temperature, e the electrostatic charge (esu), t the dielectric constant of water, k Boltzmann's constant, Zi the ionic valency of ion i, and ni the number concentration (ions cm-9. Boundary conditions arelOJ1
0.5-1 I I
I I I
1
5 I
I
0.01
002
4
I
0.03
0.04
[ CTACI] , M
005"O.l
tp=d4/dr=O
Figure 1. Reaction of pNPDPP in solutions of CTAC1: 0 , O . l
M H2Oz + 8 X l(r M KOH; O,0.2 M HzOz + 10-9 M KOH 0 , 0.5 M H202 + 10-9 M KOH; W, 0.5 M Ha02 + 2 X 10-9 M KOH; insert, 0.02 CTACl, 0.5 M HzOz, 10-9 M KOH, and added KCl. The solid lines are theoretical.
6 exp(-f/(l -IN[X;l (5) 1+ 6 exp(-f/(l- f , ) [&-I where 6 is a specificity parameter for the inert ion, X-. We take 6 = 0 for HO2- and 15 and 20 M-l for C1- and OMS-, respectively.lobvCAn alternative approach is to assume that values of 6 are the same for HO2- and C1- and we consider the conse uence of that assumption later. We take a = 21 and N = 80 or 85 for CTAC1, and a = 23 A and N = 85 for CTAOMs, in dilute electrolyte and A = 2.4 as used earlier in fitting data for reactions of OH-,lobtcand we assume that N increases on addition of KCl. In water the conversion of OH- into HO2- is approximately99%,98%,and 96.5% complete in 0.5,0.2,and 0.1 M H202, respectively? and we make this correction based on stoichiometric [OH-] of 8 X 10-4,lW, and 2 X 109 M. The reaction of pNPDPP consumes 2 equiv of HO2- (or OH-) and because, under most conditions, we only follow the final part of the faster reactions, we deduct the equivalent of concentration of pNPDPP (2X 106M). Our calculations are based on concentrations of HO2- which are lower than the nominal concentrations (of stoichiometric OH-) by 2-8%. We assume that micellar binding of HO2- is Coulombically controlled and dependsmarkedly upon the micellar surface charge density, i.e., upon a and N.l0J1 We kept a and N constantlobtcexcept that we allowed Nto increase with [KC11 (Experimental Section). The fits for rate data in CTACl would improve, for constant kzm, if N increased modestly with increasing [CTACD by ca. 10% in 0.1 M CTAC1. We see no way of distinguishing between a small increase of N or a small increase in kzm with increasing [CTACl]. For example,variations of k, with [CTAClI in 0.2-0.5 M H2Oz with added 8 X 10-4 or 1W M OH- can be fitted with a = 21 A, N = 75, and kzm = 2.2 M-l s-l (data not shown). The rate data with 0.5 M H202 and added 2 X 103 M OH- were fitted best with N = 85 and kzm = 2.2 M-l s-l and that fit is shown in Figure 1. Although we assume that HO2- interacts only Coulombically with the micelles, we tested simulations on the assumption that C1- and HO2- have the same specificity parameter, 6. In this situation we useda Langmuir, instead of a Volmer, isotherm to estimate variations of the fractional micellar coverage, f, because of its arithmetical simplicity.1k For reactions in CTACl predicted rate constants then fitted observed values provided that we
1
1 I I
1
0.01
0.02
0.03
0.04
f
0.05
[CTAOMa], M
Figure 2. Reaction of pNPDPP in solutions of CTAOMs with 0.5 M H202 + 10-9 M KOH. The solid line is theoretical.
The first-order rate constant for overall reaction is given by
k, =
kw[H02w-l+ k2mK8([CTAX] - C~C)[HO,-]A (1) 1 K, ([CTAXI - cmc)
+
In eq 1 concentrations of HO2- are in water (subscript w) and in a shell of thickness, A, at the micellar surface, and K, is the binding constant of pNPDPP in terms of the concentration of micellized surfactant, [CTAXI - cmc, where the critical micelle concentration, cmc, is that under kinetic conditions.'j The value of KBis ca. lo4M-l so that under most conditions pNPDPP is largely micellar-bound and reaction in the aqueous pseudophase is negligible.6a We estimate [ H o z - ] ~on the assumption that HO2- is very hydrophilic so that its interaction with the micelle is wholly Coulombic and can be treated in terms of classical electrostatic models.'&12 We assume that with C1- and OMS- there is also an ion-specific term and that these specifically-bound ions neutralize the charge of an equivalent number of micellar cationic head group^.^^^^^ The solution is assumed to be made up of identical, sphericalcells, radius& containinga micelle, radius, a.l0J1 (10) (a) Bunton, C. A.;Moffatt, J. R. J. Phys. Chem. 1985,89,4166. (b)Bunton,C. A.; Moffatt, J. R. J.Phys. Chem. 1986,90,638. (c)Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988,92, 2896. (11) (a) Mille, M.; Vanderkooi, G. J. Colloid Interface Sci. 1977,59, 211. (b) Gunnarseon, G.; Joneson, B.;Wennerstrom, H.J. Phys. Chem. 1980,84,3114. (c)Ortega, F.;Rodenas,E.J. Phys. Chem. 1987,91,837. (12) Blask6, A.;Bunton, C. A.;Hong, Y.S.; Mhala, M. M.; Moffatt, J. R.; Wright, S. J. Phys. Org. Chem. 1991,4, 618.
(3)
d41dr = (1 -f, Ne2/ca2kT at r = a (4) In eq 4, N is the micellar aggregation number and the factor 1- f , where f is fractional coverage by counterions, describes charge neutralization of the micellar head groups. We write f in terms of a Volmer isotherm:
f=
I
atr=R
2834 Langmuir, Vol. 9,No. 11,1993 took k2" = 1.8 M-l s-1 with the specificity parameter for C1- used earlier.l& Small variations in the fitting parameters have only small effects upon estimated values of kzm. The values of N,a, 6, and A used to simulate the rate data are similar to those used to fit micellar effects of CTACl on anionic reactions10b*cJ2 and also on reactions of H30+ which are inhibited by CTACl under all conditions,13so the treatment is self-consistent, at least for dilute ionic reactants. Fits are poor in dilute surfactant (Figures1and 2)where the reaction is inconveniently fast (t1p ET 0.5 s), so only the last part of it was followed and approximations in the simulation treatment are least satisfactory. This problem is general for reactions of very hydrophobic substrates which may complex with monomeric surfactant or form submicellar complexes, or promote surfactant micellization,6J4invalidating the assumption that the cmc gives the concentration of monomeric surfactant and our model neglecta micellar polydispersityor the decrease of micellar charge density due to incorporation of the substrate. Most such treatments neglect perturbation of micellar structure by reactants, cf., ref 6 and 14a, which is most serious with very hydrophobic substrates at [surfactant] close to the cmc. Significance of Micellar Rate Enhancements Our estimated value of kzm ET 2 M-l s-1 for reaction of HO2- with pNPDPP in the micellar pseudophase is lower than that of the second-order rate constant in water? k, = 11 M-l s-l. The value of k P / k , ET 0.2 for reactions of H02- is very similar to that of ca. 0.14 for reaction of OHwith pNPDPP in aqueous and cationic micellar pseudophases.lobIc Rate enhancements (at the apparent rate maxima) are by factors of ca. 140 with M HO2- (based on k , = 11 M-' 8).If comparison is based on the reaction of OH-, in water and without H202, the rate enhancement is by a factor of ca. 3 X lo3. The difference between these values is due to introduction of reaction of HO2- which is a better nucleophilethan OH-. For reaction of OH- with pNPDPP, rate enhancements are by factors of ca. 25 in 0.01 M OHand ca. 3.5 in 0.5 M OH-.6aJobJ6These differencesare not mechanistically significant because ionic concentrations at micellar surfaces are not linearly related to those in ~ a t e r . ~With J ~ ~dilute electrolyte there is a very large counterionic gradient between micellar surfaces and bulk water, and it decreases as total electrolyte concentration is increased because counterionic concentration at the micellar surface increases much more slowlythan the bulk concentration.6J0J1J6 Estimates of counterionic concentrations at micellar surfaces depend upon the method of calculation, but concentrations are considered to be ca. 4-6 M.6J4a Therefore, in 10-3 M electrolyte, ionic concentrations in the aqueous and micellar pseudophases differ by a factor of ca. 5 X 103, but they will be essentially the same in 5 M electrolyte. The similarities of values of kz"/k, for reactions of pNPDPP with OH- and HO2- are consistent with micellar effects on other reactions of a-effect nucleophiles, e.g., oximate, hydroxamate, and imidazolide ions, where sec(13) Blaak6, A.; Bunton, C. A.;Armstrong, C.; Gotham, W.; He, 2.-M.; Nikles, J.; Romsted, L. S. J . Phys. Chem. 1991,95, 6747. (14) (a) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., E%.; Plenum Press: New York, 1984, Vol. 2, p 1015. (b) Dutta, R. K.; Bhat, S. N. BuZI. Chem. SOC.Jpn. 1992,65,1089. (15) Bunton, C. A.; Moffatt, J. R. Langmuir 1992,8, 2130. (16) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851.
Bunton and Foroudian
ond-order rate constants are similar at micellar surfaces and in water.sbJ7 The factors that control micellar rate enhancements of bimolecular reactions should be essentially the same for a-effect and other nucleophiles. We note recent evidence that correlates nucleophilicity with ionization potentials rather than with basicitya3p4This evidenceremovesthe justificationfor postulatingexistence of an a-effect in nucleophiles that have atoms with unshared electron pairs adjacent to the reaction center, because they have low ionization potentials relative to their basicities. In our experiments with H202 in large excessover added OH-, overall rate constants, and values of kzm (Table I), depend on the concentration of HOz-, not on [H2021. Micelles interact only weakly with H202,18 and it should have little effect on their structure. Micellar effects upon reactions of nucleophilic peroxyanions are very similar to those upon reactions of other hydrophilic nucleophilic anions.6*6p20*21 The situation is very different for oxidations by peroxyanions, e.g., HSOs, where values of k2" are very muchlower at cationicmicellar surfacesthan in water. The differencesin transition state structures for nucleophilic and electrophilic attack are responsible for these striking differences in micellar rate effects.lg Micellized alkylperoxy anions, e.g., anions of a-cumyl hydroperoxidemand of a hydroperoxysurfactant21are very effective deacylating agents and overall reaction rates are higher than those in water by several orders of magnitude. These experiments were made at pH 8.020or 9.45,2l where there is only partial deprotonation of the hydroperoxy group.8 The large rate enhancements are due to increased deprotonation and high reactant Concentrations in the micellar pseudophaseand to introduction of a new reaction path. We do not have the information needed to separate the various contributions to these rate enhancements. Experimental Section Materials. The preparation and purification of pNPDPP, CTAOMe, and CTACl have been described6J0and 30% H202 (Aldrich)was used. Reactionswere followed in distilleddeionized water. Kinetics. Boiling water was flushed with N2 to remove C02 and just before reaction a weak vacuum was applied to solutions of NaOH to remove dissolved gases. Solutions of NaOH + surfactant in stoppered cuvettes were brought to 25.0 "C in an HP8461 diode array spectrometer. A solutionof Ha02 was added with a Hamilton syringeto a well-stirredsolution and immediately afterward a solution of pNPDPP in MeCN was added with a spring-loadedsyringe to start the reaction. We could only follow the laat part of the faster reactions, but the diode array spectrometer has a very good dynamic range, so the absorbance change was sufficient for estimation of rate constants. It is very difficult to establish accurate surfactant-rate profiies at [surfactant] close to the rate maxima,because with very hydrophobic substratessuch as pNPDPP profiles are very sharp and positions of the rate maxima are ill-defined. Conditions of our present experiments differ from those used earlier, and rate constants were then measured by using conventionalspectrometerswith stripchartrecorders whom response (17) (a) Fomasier, R.; Tonellato, U. J. Chem. SOC.,Faraday Trans. 1 1980, 76, 1301. (b) Bunton, C. A.; Hamed, F. H.; Romsted, L. S. Tetrahedron Lett. 1980, 1217. (18) Encinas, M. V.; Lissi, E. A. J . Photochem. Photobiol. 1988,97, 251. (19) (a) Bacaloglu, R.;Blask6,A.; Bunton, C. A.; Foroudian,H. J. Phys. Org. Chem. 1992,5,171. (b) Blaak6, A.; Bunton,C. A,;Wright, S. J. Phy8. Chem. 1998,97,5435. (20) Brown, J. M.; Darwent, J. R. J . Chem. Soc., Chem. Commun. 1979,169. (21) Moss, R. A.; Alwis, K. W. Tetrahedron Lett. 1980,21, 1303.
Langmuir, Vol. 9, No. 11,1993 2836
Dephosphorylation by Hydroperoxide Anion Table I. Fitting Parameters and Reaction Conditionr. [OH-], mM nominal actualb [HzOzl, M N (I,14 k p ,M-ls-l 21 2.0 0.1 80 0.8 0.74 1.0 1.0 1.0 2.0 1.0
0.95 0.95
0.95 1.95
0.95
0.2 0.5 0.5 0.5 0.5
80 80 85c 85 85-1ood
21 21 23c 21 21d
2.0 2.0 2.w 2.2 2.od
A t 25.0 "C with 2 X 1V M pNPDPP. Corrected for neutralization byH2O2andreactionproducts,seetextt.InCTAOMs. With 0.02 M CTACl and added KC1, N = 85 in up to 0.03 M KCl and then N increases linearly to 100 in 0.5 M KCI.
was probably too slow for the faster reactions.b" Decomposition of H202 in solutions containing an insufficiencyof OH- precludes use of a stopped-flow spectrometer but is not a problem for rapid
reactions followed in cuvettes in a diode-array spectrometer. The absorbance was followed at 402 mm and 25.0 OC. Simulation#. The fitting procedure includes contributions of reaction in the aqueous and micellar pseudophases but, except in very dilute surfactant, essentially all the reaction is in the micellar pseudophase. We do not include a contribution of reaction of OH- with pNPDPP because OH- should be almost completely protonated under our experimental conditions. The PBE was solved by numerical integration on desk-top PC microcomputers with the parameters given in Table I. The cmc was taken as 1.3 X lW M, except with added KCl, where it was negleded. Values of k, = 11 M-l s-l and K,= 104 M-l were used in all conditions.h We assumed that N increased slightly with increasing [OH-] and [HzOzl, and more strongly on addition of KC1, but a was constant.
Acknowledgment. Support of this work by the US Army Research Office is gratefully acknowledged.
