J. Phys. Chem. 1985,89, 2345-2354
2345
Ground- and Excited-State Proton Transfers in Reversed Micelles. Polarity Restrlctions and Isotope Effects Mario J. POW,Ogden Brandt, and Janos H. Fendler* Department of Chemistry and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam. New York 13676 (Received: September 20, 1984) Ground- and excited-state proton transfers have been investigated with 8-hydroxy-l,3,6-pyrenetrisulfonate, POH, in sodium bis(2-ethylhexyl) sulfosuccinate, AOT,reversed micelle solubilized water pools in isooctane. Since POH is a much stronger acid in the singlet excited state, (POH)*, than in the ground state (pK, = 7.2, pK,* = O S ) , excitation of POH by 1-5-mJ, 8-ns,353-nm laser pulses, at pH values such that pK,* < pH < pK,, results in the dissociation of POH, governed by koff*. PO- reprotonation rates, k, values, have been determined by laser flash photolysis. In reversed micelles k,, values were found to depend on the water-to-AOT ratios (w values). In the absence of added base, no proton ejection from AOT reversed micelles could be observed at w < 7. The increase in the apparent k,, values in AOT solubilized w = 7 and 12 water pools with increasing pH has been discussed in terms of altered water activity, ionic strengths, dielectric constant, and pH. Deuterium solvent isotope effects of 1.3 and 2.2 have been determined for k,,/k,,,(D20) in w = 33 and 12 AOT solubilized reversed micelles in isooctane. Combining these values with the pK, of POH led to isotope effects of 7.8and 8.4 on k& in the corresponding solutions. An isotope effect of 2.2 has also been determined for the laser pH jump initiated bromocresol green deprotonation rate in AOT solubilized pools. Steady-state and subnanosecond time-resolved fluorescence measurements have been utilized for assessing POH excited-state deprotonation, governed by kOff*(Dz0), and reprotonation, governed by k,,*(D20),in pure D 2 0 and in AOT entrapped water and AOT-d entrapped DzO pools in isooctane. In bulk solvents isotope effects of 1.O and 5.6 have been obtained for kon*/kon*(D20) and koff*/koff*(DzO), respectively. In very small AOT solubilized water pools (w = 0.34) only (POH)" could be observed. Increasing the size of the water pools resulted in the increased dissociation of (POH)*. In w = 8.2 AOT solubilized pools an isotope effect of 3.2 has been found for kOff*/koff*(D20). SCHEME I
introduction The recognized importance of proton-transfer reactions1-' has prompted our investigations of these processes in organized surfactant assemblies.8-'2 Ground-state proton transfers have been and followed by transient initiated by laser-induced pH (1) Bell, R. P. "The Proton in Chemistry", 2nd ed.;Methuen Inc.: New York, 1973. (2)Caldin, E. F.; Gold, V. 'Proton-Transfer Reactions"; Methuen Inc.: New York, 1975. (3)Mitchell, P. 'Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation"; Glynn Research, Bcdmin: Cornwall, U.K., 1966; 'Chemiosmotic Coupling and Energy Transduction"; Glynn Research, Bodmin: Cornwall, U.K., 1968. (4)Boyer, P. D.;Chance, B.; Ernster, L.; Mitchell, P.; Racker, E.; Slater, E. C. Annu. Rev. Biochem. 1977,46,955-1026. (5) Nicholls, D.G. 'Bioenergetics. An Introduction to the Chemiosmotic Theory"; Academic Press: New York, 1982. (6)Kresge, A. J. Acc. Chem. Res. 1975,8,354-360. (7) Buncel, E.;Lee, C. C. 'Isotopes in Chemistry"; Elsevier: New York, 1976. (8)Escabi-Perez, J. R.; Fendler, J. H. J . Am. Chem. SOC.1978, 100, 2234-2236. (9)Politi, M.; Fendler, J. H. J . Am. Chem. SOC.1984, 106,265-273. (10) Fendler, J. H. 'Membrane Mimetic Chemistry"; Wiley-Interscience: New York, 1982. (1 1) Fendler, J. H.; Fendler, E. J. 'Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975. (12)Kano, K.;Fendler, J. H. Biochim. Biophys. Acta 1978,504,289-299. (13) Gutman, M.; Nachliel, E.; Gershon, E.; Giniger, R.; Pines,E. J . Am. Chem. SOC.1983,105,2210-2216. (14)Smith, K. K.; Kaufmann, K. J.; Huppert, D.; Gutman, M. Chem. P h p . Lett. 1979,64,522-527. (15) Campillo, A. J.; Clark, J. H.; Shapiro, S. L.; Winn, K. R. In 'Picosecond Phenomena, Proceedings of the 1st International Conference on Picosecond Phenomena, May 1978";Shank, C. V., Ippen, E. P., Shapiro, S. L.,Eds.; Springer-Verlag: West Berlin, 1978;Springer Ser. Chem. Phys. No. 4.. rDD 319-326. r (16) Clark, J. H.; Shapiro, S. L.;Campillo, A. J.; Winn, K. R. J . Am. Chem. Soc. 1979,101,746-748. (17) Gutman, M.; Huppert, D. J . Biochem. Biophys. Methods 1979,I, ~
-l%(ktff/k&)
(POH )*
+
A*, = 445 rn .
= pK' = O J
(POT k t
t
Ht
AI, = 510 run
I
f
- l ~ ~ , ( k ~ ~ *~ pK / k ~= n 7 2)
POH Ah,
E
400 nm
koff
\
% r .
Po-
t
"on
absorption spectroscopy. Excited-state deprotonation and reprotonation rates have been determined by picosecond time-resolved fluorescence spectroscopy.8J8~20~25-30 Attention in the present work is focused on the dynamics of ground- and excited-state proton transfers in surfactant entrapped water pools in isooctane. Surfactant entrapped water pools, reversed micelles, provide unique microenvironments for interactions and r e a c t i o n ~ . ' ~ - ~At~ relatively , ~ ~ - ~ low water-to-surfactant ratios,
~
. ._.
9-1 Q
(18)Huppert, D.; Gutman, M.; Kaufmann, K. J. Adu. Chem. Phys. 1981, 47,643-679. (19)Gutman, M.; Huppert, D.; Pines, E. J . Am. Chem. SOC.1981,103, 3709-3713. (20)Huppert, D.;Kolodney, E. Chem. Phys. 1981,63,401-410. (21)Gutman, M.; Huppert, D.; Pines, E.; Nachliel, E. Biochim. Biophys. Acra 1981, 642, 15-26. (22)Gutman, M.; Huppert, D.; Nachliel, E. Eur. J . Biochem. 1982,121, 637442. ._. -.
(23)Huppert, D.; Kolodney, E.; Gutman, M.; Nachliel, E. J . Am. Chem. SOC.1982,104,6949-6953.
Pines, E.; Huppert, D. J . Phys. Chem. 1983,8?,4471-4478. Forster, Th.; Volker, S. Chem. Phys. Lett. 1975,34, 1-6. Marzzacca, C. J.; Deckey, G.; Halpern, A. M. J . Phys. Chem. 1982, 86,4937-4941. (27) Klein, V. K. A.; Hauser, M. Z . Phys. Chem. (Wiesbaden) 1975,96, 139-146. (28) Khuanga, V.; McDonald, R.; Selinger, B. K. Z . Phys. Chem. (Wiesbaden) 1976,101,209-224. (29)Selinger, B. K.; Weller, A. Aust. J. Chem. 1977,30, 2377-2381. (30)Selinger, K. Aust. J. Chem. 1977,30, 2087-2090. (31) Fendler, J. H.Acc. Chem. Res. 1976,9, 153-161. (32)Luisi, P. L.; Straub, B. E. "Reversed Micelles. Biological and Technological Relevance of Amphiphilic Structures in Apolar Media"; Plenum Press: New York, 1984.
0022-3654/85/2089-2345.$01.50/00 1985 American Chemical Society
2346
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985
w values, all of the water molecules are tightly bound to the surfactant head groups at the polar cores of reversed micelles. These water molecules resemble the hydrophilic pockets of enz y m e and ~ ~have ~ ~high ~ viscosities, ~ low mobilities, and polarities. Altered dissociation constants and/or acidities of surfactant bound ~ a t e r ~ are ~ vimportant, ~ ~ - ~ ~and yet incompletely understood, properties of reversed micelles. Much information has been obtained by the recent measurements of excited-state proton-transfer reactions by phase fl~orometry.~' Advantage has been taken of the beneficial properties of 8hydroxy- 1,3,6-pyrenetrisulfonate(pyranine, POH).99121'3
PO H
Scheme I shows that excitation of 3 X M POH by a 1-5-m.J, 353-nm laser pulse (hv) results in the formation of ca. 3 X 10" M (POH)*. (POH)* may decay to its ground state by fluorescence (k,) emission at 445 nm and by nonradiative transition ( k q ) . Alternatively, (POH)* may dissociate to (PO-)* by a process governed by k,,*. Similarly, the excited-state anion, (PO-)*, may dissipate its energy by fluorescence emission at 510 nm (kf), nonradiative transition (k,,J,or reprotonation, k,*. At a pH above 5 the hydrogen ion concentration (bulk H+ H+ generated by the laser pulse) is insufficient to reprotonate (PO-)* within its lifetime (ca. 5 ns). The observable net result of POH excitation by a fast laser pulse when pKa* < pH < pKa is, therefore, the formation of PO- and H+. This, in turn, allows the determination of k,, according to Scheme I by laser flash photolysis. Alternatively, the laser pH jump generated proton can be transferred to an appropriate dye acceptor (bromocresol green, for e ~ a m p l e ) . ~ Under suitable conditions, rate constants for both the dye protonation, k,,(A), and deprotonation, kOff(A),can be determined by laser flash photolysis. A substantial isotope effect is reported in this work for bromocresol green deprotonation, governed by koff(A), both in bulk and in sodium bis(2-ethylhexyl) sulfosuccinate, AOT, solubilized water pools in isooctane. Steady-state and dynamic fluorescence measurements have been used to provide values for koff*,kOff*(D2O),Icon*, and k,,*(D20) in AOT solubilized H 2 0 and D 2 0 water pools in isooctane. For the sake of comparison, values for koa*(D20) and k,,*(D20) have also been obtained in pure D 2 0 .
+
Experimental Section Trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate,POH (Eastman), was recrystallized three times from aqueous acetone (5:95, v/v) with c h a r ~ o a l . 'Thin-layer ~ chromatography on Merck G-F 254 plates showed only one spot (n-C4H90H:H20 = 6:1, v/v, as POD, eluent). Trisodium 8-deuteriooxy-1,3,6-pyrenetrisulfonate, (33) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85-145. (34) Tsujii, K.; Sunamoto, J.; Fendler, J. H. Bull. Chem. SOC.Jpn. 1983, 56, 2889-2893. (35) Martinek, K.; Levashov, A. V.; Khrnelnitsky, Yu. L.; Klyachko, N . L.; Berezin, I. V. Science 1982, 26, 889-891. (36) Konco, H.; Miwa, I.; Sunamoto, J. J . Phys. Chem. 1982, 86, 4826-483 1. (37) Fujii, H.; Kawai, T.; Nishikawa, H. Bull. Chem. SOC.Jpn. 1979, 52, 2051-2055. (38) Fujii, H. Kawai, H.; Nishikawa, H.; Ebert, G. Colloid Polymer Sci. 1982, 260, 697-701. (39) El Seoud, 0. A,; Chinelatto, A. M.; Shimizu, M. J . Colloid Interface Sci. 1982, 88, 420-427. (40) El Seoud, 0. A.; Chinelatto, A. M. J . Colloid Interface Sci. 1983, 95, 163-171. (41) Bardez, E.; Goguillon, B. T.; Keh, E.; Valeur, B. J. Phys. Chem. 1984, 88, 1909-1913. We are grateful to Dr.Valeur for allowing us to use his prepubiication results.
Politi et al. was obtained by incubating purified POH in D 2 0 , rotary evaporating the solvent, and repeating the process three times. The absorption and emission maxima of POD were identical with those of POH. Sodium bis(2-ethylhexyl) sulfosuccinate, AOT (Fisher), was purified by two different methods. In the first method, due to E i ~ k e 100 , ~ ~g of AOT was mixed with 30 g of activated carbon and 1 L of MeOH. This mixture was stirred during 24 h and separated from the activated carbon by filtration with sintered glass filters No. 4 and 5. The MeOH was evaporated by means of a rotary evaporator ( T I40 "C), and the AOT was dissolved in 750 mL of petroleum ether. This solution was washed with 2 X 200 mL of HzO. After phase separation the organic phase was evaporated to a gel which was dissolved in 500 mL of MeOH. This solution was washed three times with portions of 300/ 100/100 mL petroleum ether. The MeOH phase was evaporated to dryness. The residue was dried in vacuo. Afterward the AOT was dissolved in ethyl ether, and the solution was evaporated to dryness and dried again in vacuo at 0.02 torr. In the second method, 100 g of AOT was dissolved in 1000 mL of methanol and 10 g of activated charcoal was added. The solution was filtered, and the solvent rotary evaporated (below 40 "C). In the limits of our experimentation no differences were found between these two methods of AOT preparations. Perdeuterated AOT, AOT-d, was obtained by incubating purified AOT in C 2 H 5 0 D (Sigma), rotary evaporating the solvent, and repeating the process three times. Sodium dodecyl sulfate, SDS, was purified by soxhlet extraction by petroleum ether for 24 h, followed by dissolution in acet0ne:methanol:water = 90:5:5 (v/v) and recrystallization. No minima were observed in the surface tension plots. 2,2,4-Trimethylpentane, isooctane, absolute methanol, and D,O were used as received. Deionized water was doubly distilled in an all-glass apparatus. The final stage of distillation included a superheated oxygenated quartz column. Additionally, double-distilled water was filtered through a 0.2-pm Millistak filter system (Millipore Corp.). Usually, it gave a pH of 5.5. HC1 and NaOH, or D2S04and NaOD, were used to adjust the pH or pD to the required value. A Radiometer pHM8 meter was used, in conjunction with a combination microelectrode for pH determinations. pD was taken as the meter reading +0.4. Absorption measurements were obtained on a Cary 118C. Fluorescence spectra were recorded on a Spex Fluorolog spectrofluorometer. Nanosecond laser flash photolysis was carried out with a Quanta-Ray DCR Nd:YAG laser using the third harmonic (353 nm) line, delivering 8-ns pulses at 1-25 mJ per pulse. Transients were accumulated on a Tektronix 7912 digitizer, and the data were analyzed with a PDPll-35 minicomputer. All spectra were taken at 30 "C. Fluorescence lifetimes and time-resolved fluorescence anisotropies were determined on a single-photon-counting system using tunable laser pulses as the excitation source. A Spectra-Physics cavity dumped rhodamine 6G dye laser synchronously pumped by a mode-locked argon ion laser (No. 171) was used to provide tunable 15-ps pulses at 4 MHz. The second harmonic (296 nm, vertically polarized) was generated by means of a temperaturetuned ADA crystal. The residual 592-nm radiation was removed by a 7-54 Corning glass cutoff filter. The ORTEC 457 TAC was used in the normal mode. The "start" signal was obtained from a portion of the 592-nm pulses via a Texas Instruments TIED 56 photodiode and an ORTEC 436 discriminator. The emission signal, viewed at 90" and passed through an ultraviolet Polacoat polarizer (OM type 105, uv WRMR), set at 54.7" for lifetime and 0' ( I , , ( t ) )or 90" (I,(?)) for anisotropy determinations, was used to activate the TAC. Photon counting and data treatment by the Marquardt algorithm have been previously d e ~ c r i b e d . ~ Precision of the fit was estimated by the x2 parameter and by inspecting the residuals and matrix covariances. (42) Eicke, H . F., private communication, 1983.
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985 2347
Proton Transfers in Reversed Micelles
Theory
112112 = Yf
The F ~ r s t e cycle r ~ ~ serves as the model on which the analysis of the data is based. The kinetic equations for the excited-state reactions are given by dx*/dt = -(koff* + k,,
+ kf)x* + k,,[H+]y* + r
- Ys
(17)
These quantities may now be used in eq 5-7 to solve for the rate constants, in terms of measuredf, Yf, ys,and a known value for kf:
(1)
kOff* = f Y f + (1
-nrs - kf
(18)
and
+ k,,: + k{)y* + r’
dy*/dt = koff*x*- (k,,*[H+]
(2)
where x* = [(POH)*], y* = [(PO-)*] in Scheme I, and r and r’are the rate of generation of (POH)* and (PO-)*, respectively, by steady-state photon absorption. Fluorescence Lifetimes. The initial conditions are that x* = xo* and y* = yo* at t = 0 for the pulsed laser experiments and that r = r’= 0. In the limit pKa* < pH < pKa, [H+] is unchanged by the small fraction of protons emitted by (POH)* upon excitation. The pseudo-first-order rate constants for the reprotonation, k,*’, is defined by k,*’ = k,,*[H+], and the linearized equations of (1) and (2) are given by dx*/dt = -a+* + ko,*‘y* (3) dy*/dt = kOff*x*- a f l *
(4)
where
al = koff*+ k,, a2 = ko,*’
+ kf + k{
+ k,;
(5)
(6)
Equations 18-20 were used for our data analysis; however, they can be considerably simplified by assuming kf = k{:
kOff* = f(Yf - 7s)
(21)
kd = Ys
(22)
ken* = (1 -n(Yf
(23)
- 7s)
Steady-State Speptrofluorometry. In steady-state fluorescence dx*/dt = 0 and dy*/dt = 0 in eq 1 and 2. The constants r and r’(Scheme I) represent tbe excitation rate of POH and PO-. The fluorescence intensity of (POH)*, ZU5, is proportional to krx*, and that of (PO-)*, Z510, is related to kp*. If one defines IMSmaX as the fluorescence intensity of (POH)* at high acidities (no (PO-) present) and 9, = (Z445/Z445max), eq 1 and 2 lead to
and m defined for later use as
+
m = (al - a2)2 4k0ff*k0,*
(7)
Solving eq 3 and 4, with the initial conditions that x*(O) = xo* and y*(O) = 0 (i.e. prior to the laser pulse only POH is present), leads to the well-known sum of two exponentials for the time development of the concentrations of each43 x*(t) =
xo*lferff+ (1 - n e T J )
(8)
where f is the fraction of the fast-decaying component of the total concentration of (POH)* and yf and ys are the reciprocals of the ) slow ( T ~ decay ) constants: fast ( T ~ and
f=
m1f2+ al - a2 2m1/2
(11)
-1 -- Ys = al + a2 - m1f2
(12)
2
2
71
The fluorescence intensities of (POH)*, determined at 445 nm (ZU5(t)), and that of (PO-)*, determined at 510 nm (ZSlo(t)),are given by Z44S(t) =
kfxo*(ferff+ (1 -j)eTsf]
koff ZSlo(t) = k(xo*-(-eTfr m1/2
+
=frr + (1 -nrs
(15)
a2 = (1 - n Y f + f Y ,
(16)
(43) Ireland, J. F.;Wyatt, P.A. H.Adv. Phys. Org. Chem. 1976, 12, 13 1-219.
= koff[POH]- ko,[PO-] [H”]
(25)
We define x ( t ) = [PO-] - [PO-],, the change in [PO-] from its equilibrium value [PO-],. It follows that
+x [H+] = [H+], + x
[PO-] = [PO-],
(26)
[POH] = [POHI, - x at equilibrium
(13)
A computer analysis deconvolutes the known impulse response of the detection system from the measured fluorescence a t each wavelength, to obtain the constantsf, Tf, and T ~ .What follows is a procedure for calculating koff*, ko,*, and k( f r o p these constants taking known values for kf and neglecting k,, and k,:. Solving eq 10-12 for a l , a2, and m1I2yields a1
fast under our experimental conditions) bleaching of the ground-state POH absorbance at 400 nm and the parallel development of the PO- absorbance at 450 nm. Subsequent decay of the PO- absorbance, mqnitored at 450 nm, or the recovery of the POH absorbance, monitored at 400 nm, occurred on the microsecond time scale. This process corresponds to the ground-state reprotonation of PO- (see Scheme I). Following a recent we obtain the expression used for treating our flash photolysis data. Equation 2, written for the ground state rather than the excited state, becomes d -[PO-] dt
_1 -- Yf = al + a2 + m1f2 Tf
Laser Pulse Initiated pH Jump. Excitation of POH by a 2-5-mJ, 353-nm laser pulse results in the transient formation of PO- and H+. This could be seen in the prompt (unmeasurably
kodPQH10 = kon[PO-l~[H+l~ Introducing (26) and (27) into (25) gives --dx dt - kon([H+Io + [PO-], + koff/kon)x
(27)
+ konX2 (28)
-
For pKa* < pH C pK,, [Ho+]>> [PO-l0 and koff/kon
