J. Phys. Chem. 1983, 87,5166-5172
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possible to choose another cation as a reference point, especially when it is necessary to operate a t p H values where complete extraction of the ion of interest would occur. This reference ion should be a t a much higher concentration than the ion of interest and it must have a smaller value of p, than does the competing ion. For example, calcium ion is a common constituent of biological and environmental samples, so a calcium chelate in an organic solution may be equilibrated with an aqueous
sample containing a trace transition-metal ion and free calcium ion. The exchange will not substantially change the activity of the reference ion and equations similar to those derived above can be applied to relate the distribution coefficient to activities. Registry No. CuCl,, 7447-39-4;HC1, 7647-01-0;KCl, 7447-40-7; CU(NO~)~, 3251-23-8; “OB, 7697-37-2;KNOB,7757-79-1;HC104, 7601-90-3; NaC104, 7601-89-0; acetylacetone, 123-54-6;copper, 7440-50-8.
Quenching of Aromatic Hydrocarbon Fluorescence by Counterions in Aqueous Micellar Solution. Relationship to Ion Exchange Elsa Abuin, Eduardo Lissl, Departamento de Qdmica, Facuitad de Ciencia, Universidad de Santiago de Chile, Avda. Matucana 28-0 Int., Casiiia 5659-Correo Santiago, Chile
2,
Natal Bianchl, Laerle Mlola, and Frank H. Quina* Instituto de Qdmica, Universidade de S5o Pauio, Cx. Postal 20.780, Cidade Universit6aria, 0 1000-S50 (Received: January 7, 1983)
Paulo,
SP, Brazii
The quenching of the fluorescence of ionic micelle-solubilizedaromatic hydrocarbons by counterions is examined from the point of view of ion exchange at the micelle surface. Counterion quenching of the fluorescence of a series of aromatic hydrocarbons is studied over a wide range of detergent (hexadecyltrimethylammonium bromide, chloride, sulfate, or thiosulfate) and added common or foreign salt concentrations by using two different experimental approaches. The data are analyzed by using a simple pseudophase ion-exchange model which assumes that the observed quenching is a direct function of the local quencher counterion concentration(s) at the micellar surface. For Cl-/Br- and N03-/Br- exchange at the hexadecyltrimethylammonium micellar surface, the apparent selectivity coefficients thus derived agree well with values determined by independent (ground-state) methods. Selectivity coefficients are also reported for iodoacetate/Br-, I-/Br-, and S042-/S2032exchange. Quenching rate parameters are consistent with little or no intrinsic effect of the micellar environment on the counterion quenching efficiency relative to aqueous medium.
Introduction ’ The quenching of the fluorescence of micelle-solubilized arofiatic hydrocarbons by counterions’ has played a central role in the investigation of photophysical phenomena and fast reactions in ionic micellar solutions.2 In this regard, two aspects of this quenching have been of par(1) (a) Thomas, J. K. Acc. Chem. Res. 1977,10, 133-8 and references cited therein. (b) Patterson, L. K.; Vieil, E. J . Phys. Chem. 1973, 77, 1191-2. (c) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. Ibid. 1981, 85, 1198-202. (d) Geiger, M. W.; Turro, N. J. Pkotochem. Photobiol. 1975,22,273-6. (e) Rodgers, M. A. J.; da Silva e Wheeler, M. F. Chem. Azys. Lett. 1976,43, 587-91. (f) Rodgers, M. A. J.; da Silva e Wheeler, M. F. Ibid. 1978,53, 165-9. ( 9 ) Van Bockstaele, M.; Gelan, J.; Martens, H.; Put, J.; Dederen, J. C.; Boens, N.; De Schrijver, F. C. Ibid. 1978,58, 211-5. (h) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. Ibid. 1979,68,451-4. (i) Ziemiecki, H. W.; Holland, R.; Cherry, W. R. Ibid. 1980, 73,145-8. 0’) Grieser, F. Ibid. 1981,83,59-64. (k) Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC. 1979,101,279-91. (1) Grieser, F.; Tausch-Treml, R. Zbid. 1980,102,7258-64. (m) Burrows, H. D.; Formosinho, S. J.; Paiva, M. F. J. R. J.Photochem. 1980,12,285-92. (n) Burrows, H. D.; Formosinho, S. J.; Paiva, M. F. J. R. J. Chem. SOC., Faraday Trans. 2 1980, 76,685-92. (0)Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2551-8. (p) Wolff, T.; Von Buenau, G. Ber. Bunsenges. Phys. Chem. 1982, 86, 225-8. (4) Pownall, H. J.; Smith, L. C. Biochemistry 1974, 13, 2594-7. (2) For recent reviews of micellar effects on photophysical processes and photochemical reactivity, see: (a) Fendler, J. H. “Membrane Minetic Chemistry”; Wiley-Interscience: New York, 1982. (b) Kalyanasundaram, K. Chem. SOC.Rev. 1978, 7, 453-72. (c) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19,675-96. (d) Whitten, D. G.; Russell, J. C.; Schmehl, R. H. Tetrahedron 1982, 38, 2455-87. 0022-365418312087-5166$0 1.50lO
ticular interest: the overall kinetic description and the details of the intramicellar quenching process. In general, the kinetics of fluorescence quenching in micellar solution should be a rather complex function of the unquenched probe fluorescence lifetime and the absolute and relative rates of micellar entry and exit of both the probe and the quencher. This problem has been rigorously s o l ~ e d l for * ~ the ~ case in which the probe is completely micelle incorporated, the rate constants for micellar entry and exit of the quencher are independent of the concentration of detergent and added salt, and the quencher incorporation obeys a Poisson distribution; the resulting expressions for the probe fluorescence decay function and quantum yield have been recently r e ~ i e w e d . ~ Extension of these expressions to the case of fluorescence quenching by counterions in ionic micellar solution has been criticized5 on the grounds that the micellar entry and exit rate constants for an ionic quencher are necessarily a function of the micellar surface potential and hence dependent on the concentration of added salt and micelles (3) (a) Infelta, P. P.; Gratzel, M.; Thomas, J. K. J.Phys. Chem. 1974, 78, 190-5. (b) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289-92. (c) Infelta, P. P. Ibid. 1979, 61, 88-91. (4) Yekta, A,; Aikawa, M.; Turro, N. J. Chem. Phys. Lett. 1979, 63, 543-8. (5) Almgren, M.; Gunnarsson, G.; Linse, P. Chem. Phys. Lett. 1982, 85, 451-5. See also ref lj.
0 1983 American Chemical Society
Quenching of Aromatic Hydrocarbon Fluorescence
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5167
present. At higher mean occupation numbers (bound quenching counterions per micelle), the binomial distribution of the bound counterions6 should also be taken into account. Two models have been proposed for the intramicellar quenching process. In several early studies,laIb the quenching of the excited aromatic hydrocarbon by the counterion was presumed to occur a t the (largely unperturbed) micelle-water interface. For a probe solubilized predominantly within the micellar core, this model would require radial diffusion of the excited probe to the micellar surface (Stern region) prior to the quenching act. The apparent lack of a marked dependence of the quenching probability on probe lifetime is consistent with preferential solubilization of most aromatic hydrocarbons a t or near the micelle-water interface.lkJ An alternative interpretation, proposed by Rodgers and da Silva e Wheeler,le is that the incorporated aromatic hydrocarbon creates a local perturbation of the micellar structure, forming a waterfilled channel along which the (highly moblile) counterion can penetrate to quench the excited probe. In the present work, we approach the problem of the quenching of the fluorescence of aromatic hydrocarbons by counterions from a somewhat different viewpoint, viz., in terms of the relationship between the observed quenching and counterion exchange a t the ionic micellar surface. The concept of counterion exchange, which has proved to be quite useful in the quantitative analysis of ground-state reactivity in ionic micellar so1ution,2a,6has been suggested in several previous studies of counterion quen~hing.~~,~J',' We show here that, in the limit of fast counterion redistribution or exchange,6 the observed quenching should be directly relatable to the selectivity for counterion exchange. This relationship is confirmed experimentally over a wide range of added salt and detergent concentration by using a series of aromatic hydrocarbon fluorescence probes, quenching counterions, and detergents.
water (surface tension, 30 "C) of 8.8 X and 1.3 X M, respectively. Hexadecyltrimethylammonium thiosulfatelo ((CTA),Sz03or CTAT) was kiqdly provided by Dr. Hernan Chaimovich (Instituto de Quimica, USP) and had a cmc in water (surface tension, 30 "C) of 1.3 X M. Hexadecyltrimethylammonium sulfate ((CTA),SO, or CTAS) was prepared from CTAB by exchange with solid Ag2S04in ethanol (30-min sonication) followed by two recrystallizations from acetone-methanol; the conductimetric cmc in water was found to be 2.2 X M. Iodoacetic acid (BDH Ltd., 98+%) was used as received. All inorganic salts were analytical grade or superior and all solutions were prepared in deionized water doubly distilled in glass. Methods. Fluorescence spectra were recorded on either a Perkin-Elmer 204s or a Hitachi-Perkin-Elme MPF-4 spectrometer operated in the ratio mode by using airequilibrated solutions (3.00 mL) contained in 1 cm path length quartz fluorescence cells with Teflon stoppers (Hellma). Pyrene fluorescence lifetimes were determined a t 30 "C by using a TRW Systems Model 75A decay time fluorimeter with Model 32A decay time computer, both coupled to a Tektronix type 7704 oscilliscope system; the excitation (N, flash lamp) and the emission were filtered with appropriate Corning colored glass filters (CS 7-54 and CS 3-75, respectively). Fluoresence lifetimes of benzo[ghilperylene were determined from oscilliscope (Tektronix 7736) traces of the output from a fast rise time photomultiplier, using a PRA Nitronite laser as excitation source. The experiments with pyrene as probe in CTAB and either the iodoacetate ion (IAc-) or the iodide ion (I-) as quenchers were performed a t 30 "C. The final solutions, containing CTAB (0,010, 0.020, or 0.040 M) and salt (0-0.110 M NaBr, NaC1, or NaN03), were prepared from a stock solution of pyrene (5 X M) in 0.10 M CTAB, pyrene being incorporated by stirring overnight in the dark followed by Millipore (2200-A) fiItration. After registering the initial fluorescence spectrum (337-nm excitation), we Experimental Section added successive aliquots of a concentrated quencher stock Materials. Pyrene (Aldrich 99+%) was purified by the solution (total addition 1 4 8 pL), registering the emission method of Geiger and TUrro.ld Naphthalene (Hopkins and spectrum following each addition. The IAc- stock solution Williams, purified) was sublimed and biphenyl (Fluka) was prepared by careful neutralization of aqueous iodorecrystallized from ethanol. Perylene (Aldrich Gold Label), acetic acid and the final concentration determined by benzo[ghi]perylene (Aldrich), and 4-(1-pyreny1)butyricacid absorption spectroscopy using t = 425 M-l cm-l at 260 nm. (Eastman Kodak) were used as received. In all cases, the The procedure for the fluorescence measurements with absorption and emission spectra of the probes were in these quenchers was entirely analogous. In all cases, the accord with literature data. Hexadecyltrimethyldecay was a single exponential (within the precision of the ammonium bromide (CTAB, Merck p.a.) and chloride apparatus) with no indications of a fast nonexponential (CTAC1, Herga Inddstrias Quimicas, Rio de Janeiro) were decay component a t short times. In the remaining expurified as previously describedg and exhibited cmc's in periments, performed a t room temperature (20 f 2 "C), 10 p L of a stock solution of the fluorescence probe (6) (a) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979,83,1844-50. (10-3-10-4 M) in ethanol was added to 3.00 mL of detergent (b) Chaimovich, H.; Aleixo, R. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, a t the desired final concentration (5.0 X 10-3-2.0 x F. H. In "Solution Behavior of Surfactants-Theoretical and Applied M, with or without added common salt). After registering hpecta"; Mittal, K., Fendler, E. J., Eds.; Plenum Press: New York, 1982. (7) (a) Schmehl, R. H.; Whitten, D. G. J. Am. Chem. SOC.1980,102, the initial emission spectrum, we added successive aliquots 1938-41. (b) Schmehl, R. H.; Whitesell, L. G.; Whitten, D. G. Ibid. 1981, of a concentrated stock solution of foreign salt or quencher 103,3761-4. (total addition 5100 pL, correcting for dilution when (8)The available evidence for the rapidity of monovalent-monovalent counterion exchange has been considered elsewhere.@The basic experinecessary), recording the emission spectrum after each mental criterion for fast counterion redistribution is single exponential addition. Excitation wavelengths in these experiments decay of the probe emission in the presence of the quencher s p e c i e ~ . ' ~ J ~ ~ were 337 or 340 nm for pyrene, 340 nm for benzo[ghi]The basic model should also describe reasonably well data for systems in which the probe fluorescence decay is predominantly single exponential perylene, 400 nm for perylene, 260 nm for biphenyl, and in nature, especially when the net fractional coverage by quenching 290 nm for naphthalene. The fluorescence intensity data counterions is maintained high throughout. Equation 6 should apply, in for biphenyl and naphthalene were corrected for residual principle, under any kinetic circumstances (e.g., completely static emission from non-micelle-incorporated probe." quenching) as long as the variation of the relative fluorescence quantum yield ratios is a reasonably linear function of the fractional coverage. (9) (a) Chaimovich, H.; Bonilha, J. B. S.; Politi, M. J.; Quina, F. H. J. Phys. Chem. 1979,83,1851-4. (10) Bonilha, J. B. S.; Chiericato, G., Jr.; Martins-Franchetti, S. M.; Ribaldo, E. J.; Quina, F. H. Ibid. 1982,86, 4941-7.
~ _ (10) Cuccovia, I. M.; Aleixo, R. M. V.; Erismann, N. E.; Van der Zee, N. T. E.; Schreier, S.; Chaimovich, H. J. Am. Chem. SOC.1982,104, 4544-6.
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1983
Quenching of sodium 4-( 1-pyreny1)butyrate (Na+PBA-) fluorescence by IAc- in the absence of micelles was performed in 0.1 M NaCl as described previously12 for quenching by I-.
Photophysical Model In the limit of fast counterion redistribution on the time scale of the excited-probe lifetime, the entire population of excited micelle-incorporated probes should sense the same average local quencher (counterion) concentration, leading to first-or pseudo-first-order (single exponential) decay of the probe emission.s Accordingly, if the probe is totally incorporated into the micellar phase, if interprobe interactions are unimportant, and if the quenching is entirely dynamic (negligible static quenching), the following Stern-Volmer relationships should hold:
Kx/Y = ~XYf/(~YXf)
(7)
For a micellar solution of detergent DY (or D,Y) containing Yad and Xad, the appropriate expression^^*^^ for Yf and xf, the analytical concentrations of non-micelle-bound (free) counterions, and for By and 0x (when KXiY # 1) are
Y f = aCD + cmc
+ OxCD + Yad
xf= Xad - OXCD OY = OX
= {-AI
Yb/CD =
Boy - 0 X
= Xb/CD =
+ [(AI)'
=
eoY/[1
ooY(OX/OY)/[l
(8) (9)
+ (0X/&)]
(10)
+ ('%/OY)I
4(1 - Kx/y)XadKx/y(1~ ) C D I ~ ' ~ ) / [ -~ Kx/Y)I C D ( ~ (11)
where
Ai = ~ C i DCmC +
In these equations, Cpo and T O are the probe emission quantum yield and lifetime in micellar solutions of the nonquenching detergent DZ or of the (potential) quencher detergents DY or DX (with or without added common salt, Yad or Xad, respectively); GX,: and T X , are ~ corresponding parameters in micellar solutions of DY (with or without Yad) containing added foreign salt Xador, equivalently, of DX containing Yad. 0OY and Box are the average fractional counterion coverages of the micelle surface in solutions containing only counterions Y or X, respectively, and OY and BX are the corresponding coverages in solutions containing both Y and X. Finally, the pseudo-first-order rate constants for quenching in the micellar pseudophase (k,) are defined in terms of the quenching probability a t unit coverage (0 = 1) of the micelle surface by the quenching ~0unterion.l~ Assuming equilibrium mixing of the counterions a t the micelle surface and an average degree of counterion dissociation ( a )insensitive to counterionic composition,gbwe may write that 00,
= BOX = ox
+ 0y = 1 - a
(4)
Combination of eq 1-4 leads directly to the desired working relationships:
-0oy =-
Toy
@X,Y
TX,Y
= 1 + (k,(X, - Rq(Y))T0Y0X
The selectivity coefficient for exchange of counterions X and Y of the same valence is given by6sgb (11) Abuin, E.; Lissi, E. J . Phys. Chem. 1980, 84, 2605-7. (12) Quina, F. Hi Toscano, V. G. J. Phys. Chem. 1977, 81, 1750-4. (13) As defined, k, = Nk,/lZI, where N is the mean micelle aggregation number (monovalent detergent monomers per micelle), 121 is the counterion valence, and k, is the (more con~entional)'~~~~~~'~~~~ pseudofirst-order rate constant for quenching of the probe in a micelle containing a single quencher species.
Yad
4- KX/yXad
Kx,y(l - a ) c ~ (12)
The critical micelle concentration (cmc) and the total (C,) and micellized (CD = CT - cmc) detergent concentrations are expressed throughout in molarity with respect to the counterion (as opposed to normality of the surfactant ion in the case of a detergent of the type DzY). Interchange of X and Y in eq 7-12 provides the corresponding expressions for a micellar solution of DX (or D2X) containing Xad and Yad. At least two approaches, neither of which requires prior knowledge of the quenching rate constants, can be employed to derive KXIy values from fluorescence quenching data in the limit of rapid counterion redistribution. Thus, if the ratio is experimentally accessible, determination of Ooy/@x,y or soy/ permits calculation of BX/Oy (eq 6) and thus Yf, Xf, Oy, and Ox (eq 8-11). A plot of log (Ox/Oy) vs. log (Xf/Yf)should be linear with unit slope and intercept log KxIy, in accord with (compare eq 7) log (0x/0y) = log Kxiy
+ log (X,/Yf)
(13)
Alternatively, if the quenching by X is studied over a sufficiently large concentration range of detergent and Yad, the value of Kx can be obtained by simultaneous resolution of eq 11 lor diverse sets of experimental conditions (CT, Xad, and Yad) which provide identical values of Ooy/@x,y or T ~ ~ / (implying,l' T ~ , ~ via eq 5 , that BX = Ox). Once KXIYhas been determined, 0, values can be calculated from eq 11and the quenching constants derived from plots of the quenching data according to eq 5.
Results and Discussion Four detergents, hexadecyltrimethylammonium bromide (CTAB),chloride (CTACl),sulfate (CTAS), and thiosulfate (CTAT), were employed in this work. The relative probe fluorescence quantum yields in 0.010 M solutions of these detergents are collected in Table I along with fluorescence lifetimes in CTACl and in aqueous ethanol. Particularly noteworthy is the equivalence of the fluorescence quantum yields in CTACl and CTAS, implying that chloride and sulfate can be considered to be nonquenching counterions for the probes employed. The ion-selective-electrode data of Larsen and MagidI4 provide values of BBr for 0.010 M CTAB solutions at a series of added NaN03 concentrations. In Figure 1, the corresponding pyrene fluorescence quantum yield ratios in the (14) Larsen, J. W.; Magid, L. J. J. Am. Chem. SOC.1974, 96, 5775-82.
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
Quenching of Aromatic Hydrocarbon Fluorescence
5169
TABLE I: Relative Probe Fluorescence Q u a n t u m Yields in Micellar Solutions of Pure CTAC1, CTAB, CTAS, and CTAT (0.01 M Detergent, 20 " C ) and Fluorescence Lifetimes in CTACl (0.020 M Detergent) and in Aqueous Ethanol (1:l by V o l u m e ) 0 L? C l P Ox
probe
X = Br
biphenylb naphthalene pyrene perylene benzo [ghilperylene
6.3 7 (7.85)e
x = so,
1.5f (1.3)g 1.0
1.0
1.15
1.0
1.0
x =s,o,
T°C1,
ns
T ' , ~ , ~ ns
14d
13' 55e 20 0 ,( 157 )g 5' 83
60 4.1 19
60d 244,d 1 6 5 h
83h
a In ethanol-water (1: 1 b y volume). Corrected f o r non-micelle-incorporated probe emission. ' Estimated from relative fluorescence q u a n t u m yield data. Reference 21; degassed. e Reference l g . f 30 "C. g Reference I d . This work; air equilibrated. Reference l b .
1.0---
0
0
7
t
/
L
05
_
_
A
10
OB,
Flgure 1. Effect of added NaN03 (0-0.040 M) on the relative fluorescence quantum yield of pyrene in 0.010 M CTAB at 20 OC, plotted as a function of BBr values calculated from the ion-selectiveelectrode data of Larsen and Magid.14
absence (@OB,) and presence (@NO3,Br) of NaN03 are plotted vs. these OBr values. The most noteworthy feature of the data is that of the observed quenching efficiency is a linear function of OBr. Furthermore, the similarity between the value of @oNo3/@oB, = 1.32 calculated from the intercept (8Br = 0) and @'Cl/@'Br a t 20 "C in Table I suggests that NO,- is also a nonquenching counterion in this case. Indeed, assuming kq(N031= 0-and roB,= 120 ns a t 20 "C,the slope of Figure 1 provides kq(Br) = 2.4 X lo6 s-l, in excellent agreement with the value of 2.3 X lo6 s-l derived from lifetime data in CTAB and CTACl.ld Finally, the extrapolated value of BoB, = 0.85 compares favorably with a recent literature value15 of 0.84 and the normal range of a = 0.2 f 0.05.6,9 Two probes, naphthalene and biphenyl, were employed to investigate chloride/bromide ion exchange. The fluorescence experiments were performed by adding NaJ3r to micellar CTACl and by adding NaCl to micellar CTAB. The ratios 8Br/8CI (eq 6) and Brf/Clf (eq 10 and 11 with a = 0.20) derived from the experiments with biphenyl are plotted in log-log fashion in Figure 2. As required by eq 13, all of the data fall on a single line of unit slope; the antilog of the intercept provides K B , / c l = 4.2 f 0.8. A similar plot of data with naphthalene as probe provided a value of KBrjCl= 5 f 1. Divalent-divalent counterion exchange was examined by adding NazS2O3to micellar CTAS and Na2S04to micellar CTAT. When plotted according to eq 13, the data for a series of probes fall on a single line of unit slope (Figure 31, corresponding to K s ~ o ~ ~=s2.5 o , f 0.6. Exchange of two quenching counterions was examined by using pyrene as probe and sodium iodoacetate (IAc-) as quencher in micellar solutions of CTAB containing NaBr. The ratios of pyrene fluorescence quantum yields and lifetimes in the absence and presence of IAc- are identical within experimental error, in accord with eq 5. When plotted as a function of [IAcad],the total analytical concentration of added IAc- (Figure 4),it is apparent that the overall quenching behavior, and thus by implication (15) Zana,R. J . Colloid Interface Sci. 1980, 78, 330-7.
Flgure 2. Determination of KBriC, via eq 13 from Br- quenching of biphenyl fluorescence in micellar CTAB and CTACI. The ratios 8,,/8,, (eq 6) and Br,/CI, (eq 8-1 1 with a = 0.2) were calculated from the variations in the biphenyl fluorescence quantum yield upon addition of NaCi to 0.020 M CTAB (0)or to 0.020 M CTAB containing 0.0156 M NaBr ( 0 )and upon addition of NaBr to 0.020 M CTACl containing 0.0613 M NaCl (0). "-1
I
-
-"/ t
1
I
I
I
1
1
I
-10 0 10 lOg([%OJf/[~lf 1 Figure 3. Determination of KS20,,S0,via eq 13 from S2OS2-quenching
-20
of aromatic hydrocarbon fluorescence in micellar CTAS and CTAT. The ratios 8sp,/8so, (eq 6) and [S203],/[S04],(eq 8-1 1 with a = 0.1) were calculated from the variations in the relative probe fluorescence quantum yields. The data refer to addition of Na2S,0, to 0 0050 M CTAS with pyrene as probe (O),to 0.010 M CTAS with pyrene (0)or benzo[ghi]perylene (W) as probes, or to 0.0096 M CTAS containing 0.0384 M Na2S04with perylene as probe (A)and to addition of Na,SO, to 0.010 M CTAT with pyrene (0)or perylene (A)as probes.
BIAO is determined solely by the total concentration of Brpresent (=c, + [Brad]). This corresponds to a special situation encountered only when the ion-exchange selectivity coefficient is unitary. Indeed, all of the data are correlated when these ratios are replotted (Figure 4,insert) against values of 8 1 calculated ~ ~ (with a = 0.20) from the expression appropriate6afor the case of K I A c / B r = 1.0:
hc=
(1 - a)[IAc,dl/(C,
-I- [Brad]
[IAC,d]) (14)
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1983
Figure 4. Quenching of pyrene fluorescence by the iodoacetate ion (IAc-) at 30 OC in micellar CTAB. Fluorescence quantum yield (0,O; independent determinations)and lifetime (B) ratios at the concentrations of CTAB and added NaBr indicated ([CTAB]/[NaBr] in lo3 M) are plotted as a function of [ IAc,,], the analytical concentration of added IAc-. The curves are calculated for KIAc/Br = 1.0 by using the quenchlng constant of (kq(Brl)~OBr = 91 derived (eq 5) from the plot shown in the insert of the quantum yield ratios vs. oIAc (eq 14 with cy
= 0.2).
From the slope of 91 f 5 and the experimental pyrene fluorescence lifetime in aerated CTAB (TOB, = 110 f 5 ns a t 30 "C, independent of we find (eq 5) a quenching rate constant of kq(IAc)= (8.3 f 0.8) x lo8 s-l. The overall correspondence between the experimental data for the pyrene/IAc-/CTAB/NaBr system and the photophysical model suggested that replacement of NaBr by a monovalent foreign salt (NaZ) should provide a sensitive method for estimating selectivity coefficients of the type K B r , n For the mutual exchange of three monovalent counterions, the selectivity coefficients should be related as6aKBrIZ= KBr/IA&IAc/Z. In this regard, the lack of selectivity in the IAc-/Br- exchange is particularly convenient since KBrIZ= KIA+. This possibility was examined by studying the quenching of pyrene fluorescence by IAcin micellar CTAB containing either NaNO, (Figure 5A,B) or NaCl (Figure 5C,D). The ratio of pyrene fluorescence quantum yields or lifetimes in the absence and presence of NaZ (Z = NO3 or C1) is given by (compare eq 5 )
The corresponding ratios before and after addition of IAc (16) In contrast to the report of Gratzel and Thomas,17 we find that both the lifetime (.OB,) and the fluorescence quantum yield (@OB,) ?f pyrene in micellar CTAB (0.01-0.04 M) are independent within experimental error of the concentration of added NaBr (0-0.10 M). The presence of a small amount of R quenching contaminant (e.g., traces of 1.) in their NaBr would rationalize this discrepancy. (17) Gratzel, M.; Thomas, J. K. J . Am. Chem. SOC.1973,95,6885-9.
Figure 5. Quenching of pyrene fluorescence by IAc- at 30 OC in micellar CTAB containing added NaZ (A, B: Z = NO3; C, D: Z = CI). Experimental and calculated (eq 15) fluorescence quantum yield (0) and lifetime ( 0 )ratios in the absence of IAc- are compared in A and C. I n B and D, the corresponding ratios in the absence and presence of IAc- at the indicated concentrations of CTAB and NaZ ([CTAB]/ [NaZ] in 1O3 M) are plotted vs. [ IAc,,], the analytical concentration of added IAc-. The curves are calc_ulated(e_q16, see text) with KRIw3 = 0.9 (A, B), KBrICI= 6 (C, D), (kq(z)- kq(Br))~OBr = -0.4, and the parameters of Figure 4.
to micellar CTAB containing NaZ can be readily shown to be
--@Z,Br
- - =TZ,Br
@IAc,Z,Br
1+
TIAc,Z,Br
(Kq(1AC)
- kq(Br))70Br01Ac -k
- kq(Br))7°Br0Z
(16) -k (&q(Z) - Rq(Br))TOBro'Z
Using the value of (h - K q ( B r ) ) ~ O B r= 9l_from the data of Figure 4 (insert) anf'aAChommon value of (hq(z,- &q(Br))~@Br = -0.40 for both NOs- and C1-, we fitted these equations to the experimental data of Figure 5 by varying the selectivity coefficient K B r I Z = KNc/D The fractional coverage of the micelle surface by Z in the absence of IAc- (WZ) was calculated from eq 11 (replacing X by Z throughout and taking a = 0.20); the values of oIAcand of BZ in the presence of IAc- were calculated by using, respectively, eq 17 and 16 of ref 6a (with Kxiy = = 1.0). The best-fit value of K B r / N O 3 = 0.90 f 0.10 (Figure 5A,B) is the same as that determined by Sepdlveda e t a1.18 from competitive (equilibrium ground state) counterion binding experiments and ultrafiltration and falls within the range of values estimated by Larsen and Magidl* on the basis of ion-selective-electrode measurements of intermicellar (free) counterion activities. Likewise, the best-fit value of KBrja = 6 f 1 compares favorably with the value of 5 reported by Sepdlveda et al.18 and the values of 4.2 f 0.8 (biphenyl, Figure 2) and 5 f 1 (naphthalene) estimated in this work from Br- quenching data in NaCl/CTAB and NaBr/ CTACl mixtures. The last system investigated was the quenching of the fluorescence of pyrene by NaI in micellar CTAB, a system which has previously received some attention in the lit(18) (a) Bartet, D.; Gamboa, C.; SepGlveda, L. J. Phys. Chen. 1980, 84,272-5. (b) Gamboa, C.; Sepiilveda, L.; Soto, R. Ibid. 1981,85,1429-34.
Quenching of Aromatic Hydrocarbon Fluorescence
*.
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983 5171 TABLE 11: - Pseudo-Firsborder Quenching R a t e Constants ( h , ) at Unit Counterion Coverage in t h e Micellar Pseudophase a n d Bimolecular Quenching R a t e Constants in Homogeneous Solution (h,')
probe
20
counterion quencher
biphenyl naphthalene pyrene
BrBrBr-
pyrene Na'PBApyrene Na'PBA-
I1IAc-
s-'
5
t
1
1 0 - S k a o , bM-' s-' 1.6'
1 . 3 (1.5)d 0.025 (0.023)f 0.036g 2 . 8 t 0.5g
0.48' ( 1 . 6 ) e 0.026' (0.031)e 1.36' ( 2 . 8 ) e
5.Zh
8.3 i 0.8g 38
IAc-
I
4
a 20 t 2 "C, e x c e p t as noted. Ethanol-water 1:l ( b y volume) €or t h e neutral aromatic hydrocarbons or aqueous Reference 21. solution ( f i = 0.1, 30 "C) for Na'PBA-. Reference l g . e Reference 20; ethanol-water 20:80 (naphthalene) o r 30:70 ( p y r e n e ) . Reference I d . 30 "C. Reference 12.
1.o Flgure 6. Quenching of pyrene fluorescence by the iodide ion (I-) at 30 OC in micellar CTAB. The dynamic (0,lifetime ratios) and static (0, quantum yield ratio/lifetime ratio) contributions to the quenching are plotted vs. calculated 8, values (eq 11 with a = 0.2 and KIIBr= 13, see text) in accord with eq 17. The data refer to experiments at the following concentrations of [CTAB]/[NaBr] (in IO3 M): 10/0, 10/30, 10/100, 20/0,20/20,20/90,40/0, and 40/70. Quantum yield (0)and lifetime ( 0 )ratios for two of these conditions are plotted in the insert vs. [Iad], ?e analytical concentration of added I-; the curves are calculated for (kqo,- kq(Br))70B, = 31 and K,, = 14.5.
e r a t ~ r e . ' ~ ~As ~ Jreported ~ by Rodgers and da Silva e Wheeler,le we find that the pyrene fluorescence decay is effectively single exponential in nature over the entire range of conditions investigated. However, our data, which cover a much wider range of concentration of detergent (0.01-0.04 M) and added NaBr (04.10 M, maintaining CT + [Brad] constant a t 0.02, 0.04, or 0.11 M) and include parallel determinations of fluorescence lifetime and quantum yield ratios, provide a somewhat different picture of the quenching behavior in this system. Thus, we find marked deviations between the fluorescence quantum yield and lifetime ratios (Figure 6, insert), indicative of mixed static (or pseudostatic) and dynamic contributions to the overall quenching. Moreover, the apparent quenching efficiencies (lifetime data) are not a simple function of either [Brad]or CT + [Brad]as would be anticipated on the basis of the Gouy-Chapman type model (which ignores selectivity) suggested by Rodgers and da Silva e Wheeler.Ie That ion-exchange selectivity indeed plays a role in determining the quenching behavior was confirmed by equating points corresponding to identical lifetime ratios under diverse conditions (eliminating Ox = OI in eq ll),a procedure which provided the quite reasonable value of KIIBr= 13 (f3) for the I-/Br- ion-exchanger selectivity coefficient. Significantly, the same value of KI Br was obtained upon equating points corresponding to identical fluorescence quantum yield ratios, implying that, whatever (19) Pownall and Smith'q reported quenching of anthracene fluorescence by 1- in micellar CTAB. Grieser'j recently reported a detailed study of the quenching of pyrene fluorescence by 1- in micellar dodecyltrimethylammonium chloride; in this detergent, the decay of the pyrene fluorescence is multiexponential in the presence of I-, permitting estimation of the quencher entry and exit rates.
the precise nature of the quenching kinetics, both the quantum yield and the lifetime ratios are directly related to OI. In Figure 6, the fluorescence quenching data are plotted vs. BI values (eq 11with K I , B r = 13 and a = 0.20). The lifetime ratios correlate linearly with 8, over the entire range (eq 5) with = (2.8 f 0.5) X lo8 s-l, whereas the quantum yield ratios obey the empirical relationship
-a 0 -B r @I,Br
ToBr
- --(I
71,Br
+ K,,BI)
with a phenomenological static quenching constant of K,, = 14.5 f 4. Our data are insufficient to determine the origin of this apparent static contribution to the quenching in CTAB. Ground-state complexation is unattractive since there is no evidence for static quenching in homogeneous solution.12~20~21 Likewise, simple proximity in the micellar phase at the moment of excitation is unattractive since the quenching by IAc- (a more effficient quencher) is entirely dynamic in CTAB (Figure 4). The data presented here demonstrate that, in the limit of fast counterion redistribution,8 the quenching of the fluorescence of micelle-solubilized aromatic hydrocarbons is a direct function of the average fractional coverage of the micelle surface by the quencher counterion(s) and hence of the local quencher concentration in the vicinity of the micellar surface ( x b = xb/(cDn = 0x/v, where is the effective reaction volume per mole of micellized detergent).6,9bUnder these conditions, the pseudo-firstorder rate constant kq for counterion quenching of the probe fluorescence in the micellar pseudophase should be related to the bimolecular rate constant k,O for quenching in homogeneous aqueous medium via
v
k,O = k q m / F m =
(kqQ/Fac)/Fm
(18)
The factor F , incorporates all intrinsic effects of the micellar microenvironment on the relative bimolecular quenching efficiencies in the micellar (kqm)and aqueous phases, while the factor Factakes into account any kinetic effect due to a difference in probe-quencher accessibility upon going from a homogeneous medium to the_micellar pseudophase.1c,22Table I1 collects the values of k , derived (20) Shizuka, H.; Saito, T.; Morita, T. Chem. Phys. Lett. 1978, 56, 519-22. (21) Shizuka, H.; Nakamura, M.; Morita, T. J . Phys. Chem. 1980,84, 989-94. (22) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Baungarten, E.; Martins-Franchetti, S. M.; Chaimovich, H. J . Phys. Chem. 1980, 84, 361-5.
J. Phys. Chem. 1983,.87, 5172-5176
5172
from the data analysis, along with values of kqo for quenching of the various probes in ethanol-water mixtures2OS2land for quenching of the fluorescence of sodium 4-(l-pyrenyl)butyrate (Na+PBA-) a t high ionic strength. In view of typical estimates of P (0.37 M-1)639band Fa, (0.33lcfor a probe solubilized predominantly in the region of the micelle-water interface) the similarity between the magnitudes of Eq and k,O for quenching by Br-, I-, and (when the more efficient quenching of Na+PBA- is taken into account) IAc- points to little or no intrinsic effect (F, ca. 1)of the micellar environment on the quenching efficiency.
Conclusions The data presented here demonstrate that, in the limit of fast counterion redistribution, the quenching of the fluorescence of micelle-solubilized aromatic hydrocarbons is a direct function of the local concentration of counterionic quencher(s) a t the micelle surface. For all the systems investigated here, the observed quenching behavior can be reproduced over a wide range of detergent and added common and foreign salt concentrations by using a simple pseudophase ion-exchange formalism which assumes constant ion-exchange selectivity_coefficients and constant intrinsic quenching efficiencies (kqvalues). From the agreement between model and experiment, one can conclude that, in these systems, the average solubilization site(s) of the probe and the average relative quencherprobe geometry in the micellar pseudophase are insensitive to variations in the overall counterionic content, both a t the micellar surface and in the intermicellar aqueous phase.
On the other hand, the fact that the observed quenching can be rationalzed by using a model which treats the micelles (mathematically) as a separate pseudophase (as opposed to an ensemble of individual micellar entities) necessarily implies that the quenching data in these systems are inherently uninformative as regards the details of the intramicellar quenching process (i.e., for distinguishing between quenching a t an unperturbed micellewater interface vs. probe-induced water channels, vide Introduction). Acknowledgment. F.H.Q. is a senior research fellow of the Conselho Nacional de Desenvolvimento Cientifico e Tecnolbgico (CNPq), Brasilia, supported in part by the Departmento de Bioquimica with funds from the Financiadora de Estudos e Projetos (FINEP). N.B. and L.M. acknowledge partial leaves of absence from the Instituto de Biociencias, Letras e Ciencias Exatas, UNESP, Ssio Jose do Rio Preto, SP, during the course of graduate studies. We thank Herga Indiistrias Quimicas, Rio de Janeiro, for generous donations of CTAC1. This collaboration was stimulated by travel funds from PNUD-UNESCO (RLA 024); the research in SBo Paulo was supported by the Funda@io de Amparo ii Pesquisa do Estado de Ssio Paulo (FAPESP 79/0272 to F.H.Q.). Registry No. NaNO,, 7631-99-4; NaBr, 7647-15-6; NaC1, 7647-14-5;S202-, 14383-50-7;Na2S203,7772-98-7;Br-, 24959-67-9; I-, 20461-54-5; IAc-, 152-34-1; Na+PBA-, 63442-80-8; CTAC1, 112-02-7; CTAB, 57-09-0;CTAS, 67355-36-6;CTAT, 82209-37-8; ethanol, 64-17-5;biphenyl, 92-52-4; naphthalene, 91-20-3; pyrene, 129-00-0; perylene, 198-55-0;benzo[ghi]perylene, 191-24-2.
X-ray Absorption Fine Structure Investigation of V,O,-TiO, Support
Catalysts. 1. The Titania
R. Kozlowskl,+ R. F. Pettifer," Department of Physics, University of Warwick, Coventry CV4 7AL, England
and J.
M. Thomas
Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, England (Received: January 26, 1983)
X-ray absorption fine structure (EXAFS) measurements have been performed on crystalline TiO, in the anatase form and on highly dispersed TiOz prepared by hydrolysis of titanium butoxide. The dispersed material had a mean particle radius R, = 35 A in which 30% of the titania octahedra lie on the surface. EXAFS analysis shows that the dispersed material is in the form of anatase with coordination distances identical with those of the crystalline material, aR < 0.005 A, for all coordinations involving the coupling of three octahedra. The variance of shell radii in the dispersed material shows a progressive increase with respect to the crystal as a function of the mean shell radius. Correlation to within 0.1 A of atomic position is lost beyond distances involving greater than three-octahedra coupling. The results are explained on thehasis of increased compliance of the surface over that of the bulk. No major surface reconstruction is observed.
I. Introduction Vanadium oxide based catalysts have long been emplayed in the selective oxidation of hydrocarbons. particular, the studies have concentratedon their use in
the industrially important oxidation of o-xylene to phthalic anhydride, the precursor of anthraquinone, a n d several useful esters. Wainwright and Foster1 reviewed the recent literature concerned with this reaction, placing emphasis on the influence of supports and promoters in catalyst
On leave from the Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland.
(1) M. S. Wainwright and N. R. Foster, Catal. Reu., 19, 211 (1979).
0022-3654/83/2087-5 172$01.50/0 0 1983 American Chemical Society
