4894
J. Phys. Chem.
m a , 87, 4894-4898
Singlet Molecular Oxygen In Micellar Systems. 1. Distribution Equlllbria between Hydrophobic and Hydrophilic Compartments Plato C. Lee and Mlchael A. J. Rodgers' Center for fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712 (Received: February 23, 1983)
A pulsed laser photolysis study of the effects of medium composition on the decay of singlet oxygen in microheterogeneous liquid systems is reported. In reverse micellar systems heptane and isooctane have been used as the dispersion medium with Aerosol OT as surfactant and added water (up to [H,O]/[AOT] = 68) as the interior phase. In aqueous dispersions, sodium dodecyl sulfate and cetyltrimethylammonium bromide were employed as surfactants up to concentrations of 1 M. Rose bengal was the sensitizer and excitation carried out at 532 nm. The decay of the near-IR luminescence from O2('Ag)was shown to be exponential under all conditions. The rate constant was found to vary with composition in a way that was nonlinear with concentration. Kinetic analysis of a model in which 02(lAg) was allowed to equilibrate between the interior and exterior compartments of the microheterogeneous medium resulted in an expression that fit the data. Equilibrium constants for the partition of O2(l4)between the compartments and natural lifetimes of 02(lAg) were extracted from the fitting procedures.
Introduction Photodynamic activity in biological systems is initiated by the absorption of visible light by a sensitizing molecule and results in varying degrees of damage to an organism.' This damage can be manifest as membrane lysis, enzyme dysfunction, gene modification, and/or cell death. In addition to light and photosensitizer, molecular oxygen is a necessary component. Because of this, the singlet excited molecular state of oxygen, Oz('Ag), has often been invoked as being the damage-initiating species.' The several physicochemical characteristics of O2(lAg) that are pertinent to its possible implication in photodynamic action are the following: (i) It has an excess electronic energy of ca. 23 kcal mor1 which appears to faciliate chemical reactions of 02(('%) not observed in the ground state of the rnole~ule(~Z;). Singlet oxygen reacts with alkenes, dienes, furans, indoles, thiols, amines, enamines, amino acids, and many molecules having biological significance.2 Many such reactions have little if any activation energy"+ and appear to be controlled by entropic c~nsideration.~+ (ii) O2(lAg)is a small, uncharged molecule and as such is all pervasive in condensed phases. It is not restricted in its diffusive characteristics by charged or neutral water-lipid interface~.~-'O Therefore, unlike many biologically important molecules, it is incapable of being compartmentalized. (1) For detailed reviews see (a) Foote, C. S. In "Free Radicals in Biology"; Pryor, W. A., Academic Press: New York, 1976; p 85. (b) Spikes, J. D. In "The Science of Photobiology";Smith, K. C. Ed.;Plenum Press: New York, 1977; Chapter 4. (2) Reviewed by Foote, C. S. In "Oxygen and Oxy-Radicals in Chemistry and Biology";Rodgers,M. A. J.; Powers, E. L., Ed.;Academic Press: New York, 1981; p 425. (3) Long, C. A.; Kelirns, D. R. J. Am. Chem. SOC. 1976, 97, 2018. (4) Gorman, A. A.; Lovering, G.; Rodgers, M. A. J. J. Am. Chem. SOC. 1979,101, 3050. (5) Hurst, J. R.; Schuster, G. B. J. Am. Chem. SOC.1982, 104, 6854. (6) Gorman, A. A.; Gould, I. R.; Hamblett, I. J. Am. Chem. SOC.1982, 104,7098. (7) Gorman, A. A.; Lovering, G.; Rodgers, M. A. J. Photochem. Photobiol. 1976, 23, 399. (8) Gorman, A. A.; Rodgers, M. A. J. Chem. Phys. Lett. 1978,55, 52. (9) Lindig, B. A.; Rodgers, M. A. J. J. Phys. Chem. 1979, 83, 1683. (10) Matheson, I. B. C.; Massoudi, R. J . Am. Chem. SOC. 1980, 102, 1942. 0022-3854/83/2087-4894~01.5010
(iii) Although a metastable entity, it has a significant natural lifetime (T*) in condensed phases." Even in water where T* is only 4 ps12 it has a mean radial displacement of 10 pm,13 a distance which is comparable with the dimensions of subcellular organelles. The combination of diffusability and natural lifetime means that photon energy absorbed in a relatively massive and perhaps phase-localized sensitizer molecule can be transferred to oxygen and thereby given a migratory capacity not possible if localized in the excited sensitizer molecule. A continuing effort in this laboratory has been the investigation of the nature, lifetime, and kinetic properties of 0 2 ( ' A ) in microheterogeneous media such as mi~ e l l e s ~ ~reverse ~ J ! mi~elles,'~ and vesicles.ls This stems from the concept that the forces responsible for the existence of compartmentalization in these relatively simple systems are the same as those that convey the same characteristic to cellular assemblies. In this respect, surfactant aggregates can be viewed as very primitive models of cellular membranes. In a recent study14of oil-in-water microemulsions (reverse micelles), we incorporated Ni2+ ions into the water pools to effect quenching of OZ(lAg). In this paper we present a kinetic model that supercedes that earlier and observe the effects of liquid-phase composition in both reverse and normal micelles. This serves as introductory material for later quenching studies.lB In the current work we have made use of the transient IR luminescence from singlet oxygen for monitoring its decay.17-" Recent improvements in time resolutiod2 have (11) Wilkinson, F.; Brummer, J. G. J.Phys. Chem. Ref. Data 1981,10, 809. (12) Rodgers, M. A. J.; Snowden, P. T. J . A m . Chem. SOC.1982,104, 5541. (13) Lindig, B. A.; Rodgers, M. A. J. Photochem. Photobiol. 1981,33, 627. (14) Matheson, I. B. C.; Rodgers, M. A. J. J.Phys. Chem. 1982,86,884. (15) Rodgers, M. A. J.; Bates, A. L. Photochem. Photobiol. 1983, 37, 551. (16) Lee, P. C.; Rodgers, M. A. J., to be submitted for publication. (17) Salokhiddinov, K. 1.; Byteva, I. M.; Dzagarov, B. M. O p t . Spectrosc. 1979, 47, 487 (translated from O p t . Spectrosk. 1979, 47, 881). (18) Hurst, J. R.; McDonald, J. D.; Schuster, G. B. J. Am. Chem. SOC. 1982, 104, 2065.
0 1983 American Chemical Society
The Journal of Physlcal Chemlstty, Vol. 87,No. 24, 1983 4895
Singlet Molecular Oxygen in Micellar Systems
made this the method of choice for 02(lA,) studies, even in microheterogeneous media.21
Experimental Section Materials. The following chemicals were used as received from the respective companies: rose bengal (MC/B), sodium dodecyl sulfate (SDS) (BDH), cetyltrimethylammonium bromide (CTAB) (BDH), sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (Fluka), deuterium oxide (D,O) (99.8% D, Aldrich), 2,2,4-trimethylpentane (isooctane) (99%, Aldrich), heptane (MC/B, 98%), dodecane (99%, Aldrich), hexadecane (99%, Aldrich), and acetonaphthone (99%, Aldrich). Instrumentation. A Quantel YG 481 Nd:YAG laser operating in a Q-switched mode was used as the excitation source. The frequency-doubledoutput at 532 nm was used to excite rose bengal (RB) and the third harmonic (355 nm) was used to excite acetonaphthone. A Judson germanium photodiode operating in reverse bias was used to detect the 1.27-pm luminescence of singlet oxygen from the solution in a 10 mm x 10 mm cuvette. The signal from the photodiode was fed via amplifier stages into a Biomation 8100 transient recorder and therefrom to an on-line DEC PDP 11/70 minicomputer. The details of the IR detection instrumentation12 and the analytical softwarez2have been published. Kinetic Considerations In our experiments the only components present prior to the 10-ns excitation pulse are the solvent, micellar constituents, water-soluble sensitizer, and oxygen. In less than 1ps postpulse the only difference is that some of the oxygen has been converted to 02('Ag). The kinetic model used here schematically represents the transfer of 02('Ag) between an exterior and an interior phase Aext
k'+ + interior phase e Aint + exterior phase
where k +' and k '-are bimolecular rate parameters. Taking the activities of the phases as unity we can thereby use k+ and k- as first-order constants governing the transfer. The model simplifies to the following: k+
Aext
e h-
exterior phase
Ikex,
Zext
j
Ai"t
1
t,,,
interiot phase
rint
where A and 2 represent excited- and ground-state oxygen species, respectively, and the subscripts "ext" and "int" refer to the exterior and interior, respectively. The individual rate equations for governing [ A] in the two phases are (1) d[Aextl/dt = k-[Aintl - {k+ + kextl[Aextl (2) d[Aintl/dt = k+[Aextl - {k- + kintl[Aintl It is recognized that this model is closely analogous to the equilibrium between transient monomer and excimer singlet states in fluid media and an analytical expression governing the concentrations of A in the two environments can be obtained by following the method of Birks et al.23 The resulting kinetic expressions are considerably simplified (see below) if the approximation can be made that (19) Parker, J. G.; Stanbro, W. D. J.Am. Chem. SOC.1982,104,2067. (20) Ogilby, P.; Foote, C. S. J.Am. Chem. SOC.1982, 104, 2069. (21) Rodgers, M. A. J. Photochem. Photobiol. 1983, 37, 99. (22) Foyt, D. C. Comput. Chem. 1981,5, 49. (23) Birks, J. B.;Dyson, D. J.; Munro, I. H. Proc. R.SOC.London, Ser. A 1963, 275, 575.
equilibration between the two states is rapid in comparison with the decay, i.e., if k+, k- >> kea, kbv These conditions obtain here since, even in aqueous phases the lifetime of A (7J is 4 ps,12 smaller by a factor of 5 or more than in typical hydrocarbon-based media." On the other hand, the rates by which 2 (and presumably A) enters into and exits from micellar aggregates have been reported24as not less than lo7 s-l. When the rapid equilibration conditions are applied, the Birks analysis23shows that the two states of the system decay with a common lifetime. Using this result for the microheterogeneous system we develop the model as follows: We put [AT] as the concentration of OZ(lAg)referred to the volume of the whole system then (3) [AT] = frn[Aintl + (1 - f m ) [ A e x t l where f, and (1- f,) are weighting coefficients equal to the volume fractions of interior (micellar) and exterior compartments, respectively. The time derivative of (3) is
-d[AT1 -
d[Aintl d[Aextl + (1- f m ) - dt (4) m f dt, , and substituting (1) and (2) into (4) we obtain d[ATl/dt = frn(k+[Aextl - (k- + kint.l[Aintl) + (1- fm)(k-[Aintl - (k+ + kextl[Aextl) (5) Using the fast equilibration condition we can see that [Aintl/[Aextl
= k + / k - = Keg
(6)
when (5) reduces to d[ATl/dt = - ( f ~ n ~ i n t [ ~ i n + t l (l - f n ~ ) ~ e x t [ ~ e x t ] ) (7) Since, under these conditions, Aint and Aext decay with a common lifetime defined by -d[A~l/dt = ( 1 / 7 d ) [ A ~ ] = kd[A~l Then, combining (31, (61, (7), and (8) we obtain
(8)
Ke is the equilibrium constant for the distribution of O2PAg)between the micellar and solvent compartments. This expression differs from that obtained in the earlier consideration of this model14 where the stationary state approximation was used. Whereas this is presumably valid under conditions of continuous irradiation in which 02('$) is a reactive state in low concentration,1° it cannot be valid in transient kinetics where a concentration pulse of 02(lAg) is rapidly injected and subsequent redistribution processes are in competition with its decay. Overall, the development here requires that the decay of singlet oxygen in these microheterogeneous phases be monoexponential and that its decay rate constant shall be a function of the medium composition according to (9). Experimental results follow that substantiate these requirements.
Results and Discussion As tests of the model and the equation derived therefrom, we employed micelles of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) in aqueous dispersions and reverse micelles (water-in-oil microemulsions) formed by Aerosol OT (AOT) in either n-heptane or isooctane. The xanthene dye rose bengal (24) Turro, N. J.; Aikawa, M.; Yekta, A. Chem. Phys. Lett. 1979, 64, 473.
4896 0.8
The Journal of Physical Chemistry. Voi 87. No 24. 1983
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i
40
P
I
'03 8
I
I 1 4
ai
00
ai
03
0.4
I t 1.1)
1
400
450
500
550
600
650
WAVELENGTH I nm I
Flgure 1. Absorption spectra of 10 p L M rose bengal in (A) H,O, (E) AOT-heptane solution, (C) SDS solution, and (D) CTAB solution.
(RB) was used as singlet oxygen sensitizer in all microheterogeneous dispersions. This anionic dye has been shown to associate with lipid-water interfacial regions in micellar dispersions of both anionic and cationic surfactani~~s This is supported by absorption spectral studies (Figure 1)which show that the spectral peak is significantly red shifted with respect to neat water in the microheterogeneous systems. Since the dye is insoluble in hydrocarbon liquids (e.g., heptane), this red shift presumably arises from association of the dye with the interface region. That sensitizer molecules are restricted to certain locations within the system is not critical in this study since O2 molecules are all pervasive and the natural lifetimes of 02(1$)are long enough to permit its distribution to randomize. AOT in n-Heptane and Isooctane. Micellar aggregates of Aerosol OT in alkane solvents will solubilize water into their inner region~.~'-~lThe size of the water droplet depends on the ratio [H,O]/[AOT] (= w ) and is independent of the nature of the dispersion medium.29 We have measured the lifetime of 02('A,) in both n-heptane and isooctane as a function of the volume fraction of water (f,) in the system (i) by increasing [H20]and [AOT] such that w remains constant (in this manner f m increases because more micelles of the same size are formed) and (ii) by increasing [H20]at a fixed [AOT] (now w increases, the size of the micelles increase, and their number decreases). In all cases the decay of the IR luminescence signal could be characterized by a single exponential time constant. Figure 2A is a plot of the observed decay rate constant, k d (= 1/7h) in an AOT/H20/n-heptane system of w = 22.2 giving water pools with diameters of near 40 A.31 These data were analyzed with eq 9 which can be simplified since, in the experiment, the highest value off, was 0.28 and by assuming that K,, < 1 (the solubility of 02 in alkanes is greater than in water) then fmKe,
