Oxygen Transfer Rates across Water-Mlcelle Interfaces Derived from Measurements of N12+ Quenchlng of Slnglet Molecular Oxygen In Aerosol-OT-Heptane Reverse Micelles I. B. C. Matheson' and M. A. J. Rodgers Center for Fasf Kinethx Research, University of Texas at Awtln, Austln, Texas 78712 (Received: Octobw 2, lS81; I n Flnai Form: November 10, 1981)
Ni2+quenching of singlet molecular oxygen has been used as a probe of reverse micelles of water-aerosol-0T in n-heptane. Nonreverse micelle measurements of the quenching of singlet molecular oxygen by Ni2+in D20 yielded a quenching rate constant of 3.3 X lo7M-'s-'. Experiments on reverse micelles yielded after data analysis a singlet molecular oxygen entry rate into the water pools of 9 X los s-' and an exit rate of 2.8 X lo7s-'. The partition coefficient for oxygen between the water pools and heptane is 3.2. Introduction Micellar chemical kinetics are of interest since the micellar system constitutes an simple model of a biological membrane in that an interface between an aqueous phase and a lipid phase is present. Micelles composed of ionic surfactants additionally have a peripheral charge layer. A particularly interesting type of membrane reaction is photodynamic inactivation of cells, i.e., killing in the presence of light and dyes.' A case of photodynamic inactivation is the photohemolysis of red blood cell2which is thought3 to involve the singlet molecular oxygen attack on the cell membrane followed in turn by cell lysis. In this context the dynamic and reactive properties of singlet oxygen in micellar media are highly relevant. Since the initial report by Gorman et al.4 on the reaction of singlet molecular oxygen with the singlet oxygen monitor 1,3-&phenylisobemfuran (DPBF) associated with sodium dodecyl sulfate (SDS) micelles, there have been several studies of singlet oxygen reactions in micelles.611 The majority of these reports have concerned reactions in "normal micelles" which are in effect globules of lipid with a polar coat dispersed in an aqueous phase. There have only been a few reports of singlet oxygen reactions in reversed micelles, i.e., dispersions of surfactant in the hydrocarbon phase with water molecules entrained in the polar region. These rep~rts'~-'~ have concerned reversed micelles formed by using dodecylammonium propioniate in cyclohexane solution and used fluorescein and the thiazine dyes as sensitizers and DPBF as the singlet oxygen monitor. Evidence was obtained for the participation of both type I (radical) and type I1 (singlet oxygen) photosensitized oxidation, as demonstrated by amine enhance(1) Blum, H. F. "Photodynamic Action and Diseases C a d by Light" Van Nostrand-Reinhold New York, 1941. (2) Harber, L. C.; Fleisher; and Baer, R. L. J.Am. Med. Assoc. 1964, 189, 191. (3) Kaplan, M. M.; Trozzolo, A. M. in "Organic Chemistry"; Waeserman, H. H., Murray, R. W., Eds.;Academic Rese: New York, pp 5-75-95; Vol. 40. (4) Gorman,A. A.; Lovering, G.; Rodgers, M. A. J. Photochem. Photobiol. 1976,23, 299. (5) Matheson, I. B. C.; King,A. D.;Lee, J. Chem.Phys. Lett. 1978,55, 52. (6) Gorman, A. A.; Rodgem, M. A. J. Chem. Phys. Lett. 1978,55,55. (7) Barboy, N.; Krajlik, I. J. Photochem. 1978,9, 322. (8) Usui, Y.; Tsukuda, M.; Nakamma, H. Bull. Chem.Soe. Jpn. 1978, 51, 379. (9) Lindig, B. A,; Rodgem, M. A. J. J. Phys. Chem. 1979,83, 1683. (10) Matheson, I. B. C.; Maaaoudi, R. J. Am. Chem. SOC. 1980, 106, 1942. (11) Lindig, B. A.; Rodgers, M. A. J. Photochem. Photobiol. 1981,33, 627. (12) Miyoahi, N.; Tomita, G. 2.Naturforsh. E 1979,34,1552. (13) Miyoshi, N.; Tomita, G. 2.Naturforsch. E 1980,35, 107. (14) Miyoshi, N.; Tomita, G. 2.Naturforsch. B 1980,35, 1144.
ment of DPBF o x i d a t i ~ n ~and, ~ * ' ~for the thiazines, a parallelism between DPBF oxidation and dye photoreduction by al1ylthio~rea.l~ We wish to report singlet molecular oxygen quenching in the water pools of sodium bis(2-ethylhexy1)sulfosuccinate, aerosol-0T (AOT), micelles in n-heptane solution which recently have been physically characterized by Robinson and ~ o - w o r k e r s . ~ The ~ J ~ decay of singlet molecular oxygen was found to be a function of both Ni2+ concentration and volume fraction of water. The results were analyzed and yielded estimates of the entry and exit rates of molecular oxygen to and from the water pool reverse micelles. Such rates for the normal micelles formed from SDS have been previously reported by Matheson and Massoudi.lo Materials and Methods 2-Acetonaphthone and DPBF were' supplied by the Aldrich Chemical Co., Milwaukee, WI, and were used as supplied. AOT was supplied by the Fluka Chemical Co., Buchs, Switzerland, and was purified" by charcoal chro1 5 vol/vol, solution matography of methanol-clohexane, followed by removal on a rotary evaporator and oven drying at 50 "C in vacuo. Bilirubin was supplied by the Sigma Chemical Co., Saint Louis, MO, and was used as supplied. Bilirubin solutions were freshly prepared as M bilirubin in M base and kept over ice to minimize decomposition. Bilirubin solutions more than 2 h old were discarded. Working solutions were prepared by dilution of the M master solution into DzO and the pD measured. No attempt was made to control the pH in the water pools. tram-8-Carotene was supplied by HoffmanLaRoche, Nutley, NJ, and was used as supplied. Cetyltrimethylammonium chloride (CTAC1) was supplied by the Eastman Kodak Chemical Co., Rochester, NY, and was used as supplied. DzO, 99.8% D, was supplied by Stohler botope Chemicals, Waltham, MA. n-Heptane (Omnisolv, suitable for spectroscopy) was supplied by MCB, Cincinnati, OH. NiClz was Fisher Scientific Co. certified grade, Fairlawn, NJ. The N2 laser (337 nm) photolysis system employed for kinetic measurements has been previously described.g A rectangular section cuvette with four optical faces was employed for the kinetic measurements, and the path length along the monitoring beam was 5 mm. Experiments were carried out at ambient temperature (ca. 20 "C) with (15) Day, R. A.; Robinson, B. H.; Clark, J. H. R.; Doherty, J. V. J. Chem. Soc., Faraday Trans. I 1979, 75, 119. (16) Robinson, B. H.; Steytler, D. C.; Tack, R. D. J. Chem. SOC., Faraday Trans. 1 1979, 75,481. (17) Fendler, J. H., private communication.
0022-3654/82/2086-0884~01.25/0 0 1982 American Chemical Society
Oxygen Transfer Rates across Water-Mlcelle Interfaces
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982 885
12-1
2
0
10
NI'I~MI
20
3
1
Flgure 1. Decay rate of singlet molecular oxygen as a function of Nf+ concentratlonat 22 O C : upper Ilne, DPBF monitor; lower line, bilirubln monitor, pD 10.6.
oxygen-saturated solutions. Experimental control, data acquisition, and analysis were carried out with the DEC PDP 11/34 array at the Center for Fast Kinetics Research (CFKFt).lB The quencher used was the Ni2+cation, whose quenching of singlet oxygen in 2-butoxyethanol has been reported by Carlsson et al.19 The quenching of singlet molecular oxygen by Ni2+ in D20 was derived from the increase of the decay rate of singlet molecular oxygen monitored by both DPBF in CTACl micelles and the nonmicellar water-soluble acceptor bilirubin. The quenching of singlet molecular oxygen by Ni2+in the water pools of AOT-heptane reverse micelles was observed by Ni2+enhancement of the decay rate of @-carotene triplet in the heptane phase. The formation of this triplet in aerated solutions has been shown to be rate limiting at high &carotene concentrations.*22
Results Ni2+ Quenching of Singlet Molecular Oxygen. The effect of added Ni2+on the rate of decay of singlet molecular oxygen in D20 is shown in Figure l. The lower line is for the singlet oxygen monitor b i l i r ~ b i n ~ atbpD ~~ 10.6 in D20 and upper line for DPBF in CTACl micelles. Bilirubin decay was monitored by following the bleaching at 435 nm and, similarly, DPBF at 415 nm. The sensitizer was 2-acetonaphthone, 1.3 mM in CTACl micelles in both cases (CTAC1 = 0.10 M). The sensitizer was associated with the cationic micelles which were chosen to obviate Ni2+quenching of triplet sensitizer. This had been found to be troublesome in nonmicellar experiments. The lines in Figure 1 may be analyzed by using the equation
k = ko +
(1)
where k is the observed decay rate measured for a quencher concentration [Q], ko is the decay rate for [Q] = 0, and kQ is the bimolecular rate constant for quenching. ko represents the s u m of the singlet molecular oxygen decay rate due to the solvent kD and that due to the monitor kA[A]. All experiments were carried out at an approximately constant [A], f 5 % , so that ko was approximately (18) Foyt, D. C. Comput. Chem. 1980,5,49. (19) Carbon, D. J.; Mendenhall, G. D.; Suprunchuk, T.; Wiles, D. M. J. Am. Chem. SOC.1972,94,860. (20) Farmilio, A.; Wilkinson, F. Photochem. Photobiol. 1973,18,447. (21) Wilkimon, F.; Ho, W. T. Spectrosc. Lett. 1978, 21, 455. (22) Rodgem, M. A. J.; B a d , A. Photochem. Photobiol. 1980,31,533. (23) Matheson, I. B. C.; Curry, N. U.; Lee,J. J. Am. Chem. SOC.1974, 96,3348. (24) Matheeon, I. B.C. Photochem. Photobiol. 1979,29, 875.
20
0
1
60
40
Nlz'mh41
F@we 2. Decay rate of slnglet molecular oxygen in AOT-HpO reverse micelles at 22 O C . The monitor was &carotene at -23 pM, whose triplet absorption decay was measured at 515 nm.
I
0
.05
I
I
I
.10
d5
.20
Figure 3. Data of Flgure 2 replotted as a double reciprocal plot, Le., llk, - k o as a function of 1/[N12+].
constant. CTACl (0.1 M) comprises -4% of the bulk volume given a CTACl partial molar volume of 365 mL.% The lines shown in Figure 1have similar slopes and their mean slope yields a Ni2+quenching rate constant for singlet molecular oxygen of (3.3 f 0.3) X lo7 M-ls-l. Ni2+Quenching of Singlet Molecular Oxygen in AOTHeptane Reverse Micelles at a Constant Water Volume Fraction. The effect of Ni2+in water pools at a constant volume fraction H20:heptane of 0.091 on the decay rate of singlet molecular oxygen in 1.0 M AOT in n-heptane is shown in Figure 2. The singlet molecular oxygen decay was monitored as triplet carotene decay at 520 nm. In contrast to quenching by Ni2+ in the aqueous systems (Figure l),Ni2+ quenching in reverse micelles is clearly nonlinear. The rate of decay of singlet molecular oxygen in the absence of Ni2+represents the sum of quenching contributions of the solvent mixture and -23 pM @-carotene used for this data set. The same data have been replotted as a double reciprocal plot in Figure 3. This exhibits a linear fit and the derivation of a kinetic relation reproducing this behavior is given in the Discussion section. Attempts to study Ni2+ quenching for solutions with lower AOT concentrations,0.1 M, were unsuccessful; under these conditions water containing Ni2+separated out over a period of a few minutes. Ni2+ Quenching of Singlet Molecular Oxygen at a Constant Ni2+ Concentration as a Function of Water Volume Fraction. The effect of water volume fraction, f , a t a constant Ni2+ concentration on the decay rate of (25) Corkhill, J. M.; Goodman, T. F.; Walker, T. J . Chem. SOC.,Furuduy Tram. l 1967,63,768. Corkhill reported a partial molar volume
for CTAB of 368 mL; CTACl is assumed to be similar.
888
Matheson and Rodgers
The Journal of physlcal Chemistry, Vol. 86, No. 6, 1982
sheath of 0.9 nm thickness and a water pool core of 2.2 nm diameter. This smaller diameter leads to a micelle molar volume of 3.4 L and, since water has a molar volume of 0.055 L, the aggregation number of HzO molecules is N = 61. Thus,for HzO = 5 M the micelle concentration C given by C = 5 / N = 82 mM (2)
201
-
0
0
10
20
I/{
Quro 4. l l k ,
-
ko as a functlon of l / f f o r two dlfferent N12+ concentrations: &carotene monitor, 22 O c .
HEPTANE Flguro 5. Schematlc diagram of a reverse mlcelle.
singlet molecular oxygen in AOT-heptane reverse micelles is shown in Figure 4. As in the case of Figure 3, this is shown as a double reciprocal plot and the derivation of equations describing such behavior is given below. It is notable that in the double reciprocal plota the two different Ni2+concentrations used yield similar elopes and that the intercepts differ by approximately a factor of 5.
Discussion The rate constant for Ni2+quenching of singlet molecular oxygen in D20solution derived from Figure 1,Le., 3.3 X lo7 M-l s-l, is significantly lower than the 3 X 108 M-' s-l value in 2-butoxyethanol found by Carlason et We suggest that the difference may arise from the fact that the solvent is different and/or that the earlier eatimate was based on relative B measurements rather than time-resolved measurements as in the present study. That two different monitoring solutes give the same value convinces us that our value is to be preferred for aqueous media. The mechanism of Ni2+quenching of singlet molecular oxygen has been postulated to be energy transfer on the basis of near-IR The relatively efficient quenching found in this study tends to support such a mechanism. In order to explain the results of Figures 2-4, it is necessary to create a kinetic model of the reverse micellar system. In Figure 5 a schematic diagram of a reverse micelle is shown. Singlet molecular oxygen is generated by collisions with 2-acetonaphthone triplet states which are predominantly situated in the heptane phase. It is assumed that the singlet molecular oxygen monitor, 8carotene, is situated predominantly in the heptane phase and the quencher Ni2+in the aqueous phase. The ratio [HzO]/[AOT] = R was 5.0 for the experiments of Figure 2. The physical measurements of R o b i n s ~ n ' ~and J ~ coworkers suggest that for R = 5 the overall diameter of the reverse micelles is 4.0 nm comprising a surfactant tail (26) Evans, D. F. J. Chern. SOC.,Chern. Cornrnun. 1980,1134.
The maximum concentration of Ni2+used in the experiments of Figure 2 was 56 mM relative to the bulk volume. The question that now arises is: how does the Ni2 distribute itself in the water pools or, in other words, what is the effective Nis+ concentration in the water pool? This has been answered by recent studies of Fletcher et al.n who have found for R = 5 a bimolecular transfer rate for Zn2+and Ni2+between colliding water pools of 110' M-ls-l. Since C approaches 0.1 M for these experiments, the unimolecular Niz+transfer rate will approach or exceed lo6 5-l. It should be noted that the singlet molecular oxygen decay rate ko, due to heptane and P-carotene quenching, is 4 X lo6 s-l as shown by the intercept of Figure 2. Thus, the Ni2+distributes itself through the water pools at a greater rate than singlet molecular oxygen decays and so the Ni2+concentration in the water pools may be taken to be, to a first approximation, [Ni2+]m/f, where f is the water volume fraction. In any case, the maximum [Ni2+]is 60 mM, i.e., (r) = [Ni]/C > 1and the probability of multiple occupancy at (r) > 1is very small. After formation, singlet molecular oxygen (A hereafter) can migrate from the heptane medium into the water pooh at a rate k- s-l. It is also assumed that the only significant loss processes for A in the water pools are outmigration at a rate k- 8-l and reaction with a quencher Q at a micellar concentration [&I, at a rate of k[Q]M 8-l. In other words, quenching of A by water in the water pools is not competitive with outmigration rate, k+. Experiments with AOT-heptane mixtures shows similar decay rates whether or not H20was present, confiiing the above supposition. The following analyeis assumes implicitly that a singlet oxygen stationary state is established across the micelle interface. This assumption has been used previously to analyze the reaction of A generated in the aqueous phase with DPBF associated with SDS rnicelles.l0 It follows that ~-[A]Be (k+ i~Q[&IM)[A]M zz 0
(3)
i.e. (4)
In eq 4, [A], represents the A concentration in the reverse micelles and [A]Bthe concentration referred to the bulk volume. What is measured experimentally is quenching referred to the bulk volume, i.e. -d[Ah]~/dt= kl[AlB (5) Equation 5 defines the experimental quenching rate kl measured relative to the bulk volume. The bulk concentration, [A],, is controlled by quenching by heptane and P-carotene, which together contribute a decay rate k,. kl here refers to the Ni2+contribution to the A decay rate. All the Ni2+cations are localized in the water pools which constitute a volume fraction f, so we may write k i [ A l ~= ~ ~ Q [ & ~ M [ A ] M (6) (27) Fletcher, P. D. I.; Robinson, B. H.; Bermejo-Banvera, F.; Oakenfull, D. G. In "Microemulsions";Robb, I. D., Ed.; Plenum Press: New York, in press.
Oxygen Transfer Rates across Water-Mlcelle Interfaces
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982 007
Substituting for [AIM, using eq 4, and eliminating [AIB, we obtain (7)
Equation 7 has the quencher concentrations referred to the micelle and, since the quencher cations may be reasonably supposed to reside exclusively in the water pools, it follows that [ Q ~ B= [ & l ~ / f (8) Combining eq 7 and 8, we obtain
[QIB is the quencher concentration referred to the bulk volume and eq 9 may be used to analyze all the singlet molecular oxygen quenching in AOT-heptane reverse micelles of this study. Equation 9 may be recast into more convenient forms. Where f is held constant and [QIBis varied
in this equation K = k + / k - and represents the partition coefficient for molecular oxygen (of either ground or lowest excited state) between the bulk heptane phase and the water pools. When [&le is held constant and f is varied
Equation 9a may be used to analyze the data of Figure 3 yielding from the intercept fk- = (8.1 f 0.9) X lo6 s-l and from the slope kQ/K = (1.03 f 0.09) X lo7 s-l whence, for f = 0.091 k- = (8.9 f 1.0) X lo6 s-l and for kQ = (3.3 f 0.3) x lo7 M-l s-l K = 3.2 f 1.0 Also, since k+ = Kkk+ = 2.8 X lo7 s-l The smooth curve drawn through the points of Figure 2 was generated by using the above values of the coefficients f k - and kQ/K and eq 9. Equation 9b may also be used to analyze the date of Figure 4. Equation 9b suggests that the slope should be independent of quencher concentration and the intercept should be inversely proportional to the quencher concentration. This appears to be the case, the lines are approximately parallel, and a five times increase in quencher concentration produces an approximately five times reduction in the intercept. The data of Figure 4 are more scattered than those of Figure 2 and Figure 3, possibly because of difficulty in achieving exactly the same Ni2+ concentration for each point in a data set. As a result, the agreement with the results derived from Figure 2 is only fair, yielding as mean values from both lines of Figure 4 k- = (6 f 1) X lo6 s-l kQ = (1.4 f 0.3) X lo7 M-' s-l
TABLE I: Comparison of Oxygen Transfer Rates and Partition Coefficients for AOT Reverse Micelles and SDS Micelles AOT-heptane reverse micelles SDS micelles
K 'k k-
3.2 2.8 x 10's-l 8.9 x lo6 s-l
2.8
1.0 x 106 s-1 3.7 x 10's-1
Thus, it appears that the proposed kinetic analysis will account for the experimental results. The success of this relatively "simple" kinetic analysis is probably due to the fact that both singlet molecular oxygen and Ni2+distribute themselves through the water pools at a faster rate than singlet molecular oxygen decays. Thus, there is no need for the complication of P o i s s ~ nor~ other30 ~ * ~ ~statistics. These are necessary when the characteristic rate of the analytic process, e.g., fluorescence, is much faster than fluorophore transfer between micelles. It is of interest to compare the transfer rates and partition coefficients of this study to values obtained in a previous kinetic study for normal SDS micelles. These are collected in Table I. It should be noted that k+ represents the transfer rate from water to the hydrocarbon phase for both types of micelle. It should be further noted that the oxygen partition coefficient for 2.8 for SDS has been supported by a nonkinetic direct m e a ~ u r e m e n t . ~ ~ It appears that oxygen partitions between the aqueous and hydrocarbon phases to about the same extent in both types of micelles. This is somewhat surprising; since this system is largely hydrocarbon, it was anticipated that the partition coefficient should approach the 9:l solubility ratio for oxygen in heptane relative to water.31 The relatively low value of K for reverse micelles suggests that either oxygen is less soluble in the hydrocarbon than expected or more soluble in the water. This implies that the hydrocarbon phase may have some water content and vice versa. It also appears that, since k+ and k- are about a factor of 3 lower for the reverse micelles, reverse micelles are less easily penetrated &e., have a higher microviscosity)than normal micelles. These transfer rates are still relatively fast compared to say H20 quenching of A, kH& = 3 X lo6 s-l,OJ0so that this process is not sufficiently competitive to significantly quench A in reverse micelles. In conclusion, it appears that reverse micelles are almost as accessible to singlet molecular oxygen as normal micelles and that by extension more complex structures, i.e., biological membranes, would be expected to be similarly accessible. Thus singlet molecular oxygen could be generakd at a site remote from that of its reaction.
-
Acknowledgment. Work at the Center for Fast Kinetics Research is supported by NIH grant RR00886 from the Biotechnology Branch of the Division of Research Resources and by the University of Texas at Austin. Partial support for this work was provided by NIH grant GM24235. In addition, we thank Dr. B. H. Robinson for discussions of his and this work. (28) Khuanga, U.; Selinger, B. K.; McDonald, R. A u t . J.Chem. 1976, 29, 1. (29) Rodgers, M. A. J.; da Silva e Wheeler, M. F.; Chem. Phye. Lett. 1978, 53, 1.
(30) Dorrance, R. C.; Hunter, T. F. J. Chem. SOC.,Faraday Tram. 1 1977, 73, 891. (31) Matheson, 1. B. C.; King, A. I).J. Colloid Interface Sci. 1978,66,
464.
