Langmuir 1990,6, 471-478
471
Photophysical Properties of Pyrene in Zeolites. 2. Effects of Coadsorbed Water K.-K. Iu and J. K. Thomas* Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 Received May 12,1989. In Final Form: August 15, 1989 T h e effects of coadsorbed water, following pyrene adsorption, on oxygen and T1+ and perylene quenching of pyrene fluorescence in zeolite A and zeolite X have been examined. For zeolite X, coadsorbed water drastically decreases the oxygen-quenching efficiency but enhances the T1+ quenching of pyrene fluorescence. On the other hand, coadsorbed water does not affect the energy-transfer quenching between 'pyrene* and perylene. In both hydrated and dehydrated zeolite X samples, T1+ quenches 'pyrene* via an electron-tunneling mechanism. We have determined the active radius for the hydrated and dehydrated zeolite X samples to be 16.0 f 1.6 and 11.1 f 1.0 A, respectively. T h e perylene quenching of 'pyrene* in zeolite X follows a Forster dipole-dipole interaction mechanism with a critical transfer distance of 38.9 f 3.0 8, for hydrated samples and of 37.6 f 1.4 A for dehydrated samples. For zeolite A, coadsorbed water does not block oxygen quenching of 'pyrene*, which requires close contact of reactants. In addition, T1+ ions do not quench 'pyrene* in zeolite A under any conditions stated above. Pyrene is adsorbed at the surface of zeolite A, giving ready access to oxygen, while T1+ ions are adsorbed in the zeolite pores at same distance from pyrene.
Introduction Zeolites have applications in many technological such as catalysis, waste water treatment, etc. Unlike other adsorbents, zeolites have unique uniform pores and tunnel sizes (3-13 A), which provide selective exclusion of molecules or ions. A n u m b e r of reports on the remarkable effects of constrained zeolites on various photochemical a n d photophysical processes have been ~ o m p i l e d . ~ Although t h e cage effects of coadsorbed water on the photolysis of ketones in pentasil zeolites5 and the interaction of Pd3+and Pd+ with coadsorbed water in Pd-Ca-X zeolites have been documented, studies of coadsorbed water in photophysical quenching a r e still lacking. W e have previously reported the photophysical properties of pyrene in zeolites, which include surface polarities, dimer formation, a n d Cu2+ a n d oxygen quenching of excited pyrene.' The present work is a n extension of t h e previous s t u d y and investigates the effects of coadsorbed water o n t h e reactions of 'pyrene* in both zeolite A a n d zeolite X. Our studies show that photophysical quenching is affected or unaffected b y the coadsorbed water molecules, depending o n the t y p e of reaction and t h e location of the reactants. Because activated d r y zeolites are extremely hygroscopic, these studies are i m p o r t a n t for an understanding of the behavior of coadsorbed water.
Experimental Section Zeolites and Chemicals. Sodium-exchanged zeolite X (Si/ A1 = 1.18) and zeolite A (Si/Al= 1)were obtained from Exxon and Aldrich, respectively. Thallium(1) chloride was obtained
* Author to whom correspondence should be addressed. (1) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (2) Molecular Sieves ZZ; Katzer, J . R., Ed.;ACS Symposium Series 40; American Chemical Society: Washington, DC, 1977. (3) Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monographs 171; American Chemical Society: Washington, DC, 1976. (4) Turro, N. J. Pure Appl. Chem. 1986,59, 1219-1228 and refer-
ences cited therein. (5) (a) Turro, N. J.; Cheng, C.-C.; Lei, X . 4 . J. Am. Chem. SOC. 1985,107,3739-3741. (b)Turro, N. J.; Cheng, C.4.; Abrams, L.; Corbin, D. R. J . Am. Chem. SOC.1987, 109,2449-2456. ( 6 ) Michalik.. J.:. Naravana. _ . M.:. Kevan. L. J.Phvs. Chem. 1985.89.
4553-4560.
(7) Liu, X.; Iu, K.-K.; Thomas, J. K. J.Phys. Chem. 1989,93,41204128.
0743-7463/90/2406-0471$02.50/0
from Mackay Inc., perylene was obtained from K & K Laboratories Inc., and pentane (anhydrous, 99+%) was obtained from Aldrich. All these chemicals and zeolites were used as received. Pyrene (99%) from Aldrich was purified by liquid chromatograph, by adding a pyrene cyclohexane (Aldrich, HPLC grade) solution to a silica gel (60-8, pore size, from Merck) packed column. The purified pyrene was obtained by evaporation of the cyclohexane of the eluded solution. Preparation of Zeolite Samples. Thallium(1)-exchanged zeolites were prepared via ion exchange with a stock aqueous TlCl solution. The amount of exchanged T1+ was controlled by using different amounts of the stock solution. The mixture, zeolite and TlCl(aq), was incubated for 1 h after an initial violent mixing. The supernatant of this mixture was then tested with N%S(aq) to produce T1,S (K, = lo-") precipitation. This was done to ensure that all the rfl+ ions had exchanged into the zeolite. The precipitation test was done by adding the supernatant into a test tube which contained a concentrated Na,S aqueous solution. After being washed twice with distilled water, the T1+-exchanged zeolites were then under vacuum and ready for further study. Both sodium- and thallium(1)-exchanged zeolites were dehydrated for at least 8 h a t 550 "C. After the hot dehydrated zeolites had cooled inside a desiccator, a stock pyrene pentane solution was added into the dehydrated zeolites to prepare the pyrene-impregnated zeolites. A fixed amount of perylene pentane stock solution was also added to prepare the perylenedoped samples. The zeolite-pentane mixtures were kept in the dark overnight. The supernatant pentane was checked with a UV-vis spectrometer to ensure that no pyrene or perylene was left behind in the solvent. These zeolite and pentane mixtures were then vacuum-dried Torr) and kept a t 130 OC for 30 min while evacuating before the dehydrated experiments were performed. After the dehydrated experiments, the dry zeolite samples were transferred to a wide mouth glass bottle and exposed for 1 day to the water vapor inside a Parafilm-sealed, watercontaining beaker. These samples were then taken out and exposed to atmosphere for another day. The final fully hydrated zeolite samples contained 32% water by weight for zeolite X and 28% for zeolite A. For effects of coadsorbed water on oxygen-quenching experiments, a fixed amount of water was introduced to the dehydrated zeolites via rapid evaporation from a microcapillary side arm tube. The side arm tube was connected to the sample tube through a three-way stopcock. The connected side arm tube, which contained a fixed amount of water, was degassed while frozen at liquid nitrogen temperature. After the capillary tube 0 1990 American Chemical Society
472 Langmuir, Vol. 6, No. 2, 1990
Iu and Thomas
Table I. Parameters Obtained from the Fitting of Perylene-Quenching Pyrene Fluorescence Decays with Fbrster Type Energy-Transfer Model (Eq 1) for Dehydrated and Hydrated Zeolite X perylene concn per UC,*lo3 M
10"kd,
Y
s-l
103
c, M"
Ro,
h9 dehy' hyd dehy' hvd dehv' hvd 8.02 1.335 7.76 0.177 0.222 7.54 6.01 39.0 42.1 2.225 7.31 0.281 0.348 7.92 7.22 38.4 39.6 3.115 6.72 0.370 0.418 8.42 7.45 37.6 39.2 4.005 6.80 0.440 0.475 9.10 8.43 36.6 37.6 5.780 6.66 0.629 0.592 9.19 9.76 36.5 35.8 7.21 8.43 7.77 37.6 38.9 (&0.61)e (k0.89)' (k1.74)e (4~1.4)~ (k3.0)' a Calculated values from either eq 2 or 3 in text based on the y value from the fitting. uc denotes unit cell. These samples were prepared in the routines described in the Experimental Section, presumably in a very dry condition. The experiments were performing under vacuum Torr). These hydrated samples were prepared in the routines described in the Experimental Section; they adsorbed about 32% water by weight. e Within 95% confidence limit. 0 0.012 0.020 0.028 0.036 0.052
dehy" 10.00 10.80 10.20 9.87 10.50 9.10 10.10 (+0.60)'
0
'
Table 11. Parameters Obtained by Fitting Tl+-Quenching Pyrene Fluorescence Decays with Electron-Tunneling Model (Eq 4) for Dehydrated and Hydrated Zeolite X T1+ concn 109A 10"kd, s-l lo-%, s-1 Der U C , ~ M dehv' hv' dehv' hv' dehv' hv' 0 0 10.0 8.2 0.673 0.0750 13.8 2.61 10.0 8.4 3.0 367 0.859 0.1246 19.0 4.55 9.7 8.2 3.7 252 1.122 0.1744 26.4 6.42 9.6 7.4 5.4 238 1.571 0.2243 40.8 8.13 7.4 7.3 14.0 291 9.3 7.9 6.5 287 (f2.0)d (k0.9)d (f8.1)d (k92)d
'
uc denotes unit cell. These samples were prepared in the routines described in the Experimental Section, presumably in a very dry Torr). These hydrated samples were prepared in the routines condition. The experiments were performing under vacuum (described in the Experimental Section; they adsorbed about 32% water by weight. Within 95% confidence limit. faced to a Zenith 2-200 computer (an IBM PC-AT compatible), was also used for steady-state fluorescence studies. The pyrene concentrations8 were 2 x lo* and 1.2 X mol/g for zeolite X and zeolite A, respectively. All samples were contained either in 6-mm-diameter Pyrex tubes or in 1cm2 quartz tubes, and the experiments were performed at room temperature (20 "C). For the fully hydrated zeolite X experiments, samples were exposed to atmosphere while performing the experiments; the dehydrated zeolite samples, however, were under vacuum (IOv3Torr) during the course of experiments.
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F i g u r e 1. Effect of coadsorbed water in pyrene-impregnated zeolite X on the time-resolved pyrene fluorescence monitored a t 400 nm. Dotted line represents the fully hydrated sample (also see Experimental Section). Short dashed line and long dashed line represent the dehldrated sample at 15.2 Torr of oxygen and under vacuum (IO- Torr), respectively. Four solid lines starting from the fastest decay are 1.7%, 2.7%, 4%, and 5.3% coadsorbed water by weight. The oxygen pressure was 15.5 k 0.3 Torr for the partially hydrated samples. was pumped to vacuum, the stopcock was closed and the liquid N, removed; the frozen water was heated until all the water vaporized. The water vapor was then introduced into the dehydrated zeolite, which had been maintained under vacuum. To avoid inhomogeneous adsorption, the hydrated zeolite was kept at 140 "C for about 30 min with the system closed. After the samples cooled off, a fixed amount of oxygen was introduced to the tube for the oxygen-quenching experiments. Photophysical Properties Measurement. The instrumentations and the data handlings had already been described in detail in our previous paper.7 A SLM Instruments Inc. Model SPF500C equipped with a 250-W Xe lamp, which was inter-
Results and Discussions Zeolite X in Dehydrated and Hydrated Form. Effect of Oxygen-Quenching Pyrene Fluorescence. We reported earlier' that a small amount of residual adsorbate (e.g., water) inside zeolite X markedly affects the oxygen quenching of 'pyrene*. Figure 1shows the timeresolved pyrene fluorescence (AEM = 400 n m ) i n zeolite X under the influence of different amounts of adsorbed water. The fluorescence decay of a fully h y d r a t e d sample (exposed to atmosphere, dotted line in Figure 1) is slower than the d e h y d r a t e d sample under vacuum Torr; long dashed line in Figure 1). The samples were k e p t at 130 OC for 30 m i n while evacuating the tubes, before the experiments were performed. It is unlikely' that a small amount of oxygen remains i n the sample to quench 'pyrene* and cause a faster decay. The lifetime of 'pyrene* i n both hydrated and dehydrated form is 130
-
(8) The concentration unit is alternately mol/L (M) or mol/g. The conversion factor is 1 X 10" mol/g = 1.502 X M. It is calculated based on the unit cell formula weight of zeolite X (dehydrated) (Na [(A10 ),(Si0 = 13504.6 au) and the unit cell volume (1.5 X 10' %), and it can%e simply expressed as [adsorbate] (M)= (n/N X 6.7 x loz2M, where n is the number of quencher per unit cell and N is Avogadro's constant. The molarity unit (M) is only making sense in the zeolites if the adsorbate can be adsorbed inside the zeolites. (9) The oxygen pressure needed to decrease the 'pyrene* lifetime from 125 to 100 ns is 0.1 Torr. It is calculated from our reported oxygen-quenching rate (k, = (2.20 f 0.02) X 10' Torr-' s-*) in ref 7.
Langmuir, Vol. 6, No. 2, 1990 413
Photophysical Properties of Pyrene in Zeolites 1 00
A
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Figure 2. Effect of coadsorbed water in zeolite X on the steadystate spectra. Solid line represents the fully hydrated sample (32% water by weight), while the dotted line represents the dehydrated sample. (A) Fluorescence spectra. The excitation was 337 nm (band-pass = 10 nm), and the emission band-pass was 0.5 nm. (B) Excitation spectra. The monitoring emission centered at 400 nm (band-pass = 20 nm), and the excitation band-pass was 0.5 nm. The subtle differences on the fine structure are apparent.
and -110 ns, respectively (see Tables I and 11). The lifetime of 'pyrene* in a polar environment is generally shorter than that in a nonpolar environment (e.g., T, 200 ns in water vs 400 ns in cyclohexane). The experimental results suggest that the environment inside the zeolite X is relatively polar, and the dehydrated zeolite X is slightly polar than the hydrated one. Both pyrene fluorescence and excitation spectra of the hydrated sample provide better spectral resolutions than the spectra of the dehydrated sample. Parts A and B of Figure 2 show this observation. It was shown earlier that, under our experimental conditions, pyrene is located inside the cage of zeolite X.7 The water molecules, which are adsorbed after the pyrene loading, serve to fill up the internal space rather than to flush the pyrene molecules out of the cages. This argument is supported by no observable pyrene dimer emission even in fully hydrated samples. In addition, the highly exothermic hydration of zeolite suggests that the bonding between water and zeolite framework is strong. The coadsorbed water might become part of the highly organized and rigid zeolite framework. The net result causes not only sequential blocking of diffusion processes (like oxygen quenching) but also provides a space-limited environment for pyrene molecules. This environment limits the pyrene motion and results in structured spectra for both fluorescence and excitation.
-
,
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M O
(
,
.
-. i.
,
-. 500
nm 1
Figure 3. (A) Steady-statefluorescence spectrum for perylenedoped, pyrene ([pyrene] = 2 x lo4 mol/g) impregnated zeolite X. Dashed line represents [perylene] = 3 X lo-' mol/g; solid line is the sample without perylene. The excitation was 337 nm (band-pass = 10 nm), and the emission band-pass was 0.5 nm. (B) Excitation (solid line) and fluorescence (dotted line) spectrum for perylene-impregnated (9 x mol/g) zeolite X.
The experimental conditions for the fluorescence spectrum were the same as in A, except the excitation was 370 nm. The monitoring emission centered at 525 nm (band-pass = 10 nm), and the excitation band-pass was 0.5 nm for excitation spectrum. All samples were fully hydrated (also see Experimental Section).
Pyrene-Perylene Singlet-Singlet Energy Transfer. A well-known example of Fiirster type dipoledipole interaction in singlet-singlet energy transfer is 'pyrene* as an energy donor and perylene as an energy acceptor.'O The fluorescence spectra of the pyrene-impregnated zeolite X with and without perylene are shown in Figure 3A. The fluorescence and excitation spectra of perylene-impregnated zeolite X are also shown in Figure 3B. The decrease of the III/I ratio of pyrene fluorescence (391-nm peak/372-nm peak) in perylene-doped samples is due to the increasing absorption of perylene (the energy acceptor) at 391 nm. In a Forster type interaction, the time dependence of the pyrene fluorescence can be expressed as'' I = I, exp[-kdt - 2y(kdt)'/']
(1)
with
Y = C*/C*O (2) In eq 1, Io denotes the initial pyrene fluorescence inten(IO) (a) Forster, Th.Discuss. Faraday SOC.1959, 27,7. (b) Mataga, N.; Obashi, H.; Okada, T. Chem. Phys. Lett. 1967,1, 133-134. (11) (a) Bennett, R. G. J. Phys. Chem. 1964, 41, 3037-3040. (b) Birks, J. B. J. Phys. B 1968, I , 946-957.
Iu and Thomas
474 Langmuir, Vol. 6, No. 2, 1990
A
A
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TIME
Inanosecondsl
TIME
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-
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-1
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100
200
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300
100
500
line on the raw data) with the FBrster model (eq 1 in text). The signal at 0 M perylene concentration is fitted with a single-exponential function at times long after the laser pulse; the dashed line shows that the signal deviated from first-order decay. (B) Unweighted residuals for all the fits in A. Starting from 3.115 x 4.005 x the top: 0,1.335 x loT3,2.225 x and 5.780 X M perylene. sity (at time zero), and k, denotes the decay rate constant in the absence of quencher. By application of the nonlinear least-squares fitting routine to the timeresolved pyrene fluorescence (400 nm) with the form of eq 1, the y values were extracted at different perylene concentrations. The critical acceptor concentration (CAo)was calculated by using eq 2 above, which further yields the critical transfer distance (R,)via eq 3 below:
R, = [1500/(~~/~NC:)]~/~ X 108A
(3) where N denotes Avogadro's constant and the unit of CAois mol/L. Figures 4A and 5A show the time-resolved pyrene fluorescence (400 nm) together with the computer fits for both dehydrated and hydrated pyrene-impregnated zeolite X samples with added perylene. The unweighted residuals of the fits in Figures 4A and 5A are also shown in Figures 4B and 5B. From the random distribution of the unwei hted residuals, the goodness of the fits can be justified." The parameters obtained from the fits are listed in Table I; in addition, the calculated critical parameters, based on the y values from the fits, are also included (12) Indeed, the judgement of the fits is based on (1) random distribution of the unweighted residuals and (2) x 2 magnitude.
101
500
TI ME ( n a n o s ec 0 n d s I
(nanoseconds)
Figure 4. (A) Time-resolved pyrene fluorescence monitored at 400 nm for dehydrated perylene-doped, pyrene-impregnated zeolite X. Six traces starting from the slowest are 0, 1.335 X lod3,2.225 X 3.115 X 4.005 X and 5.780 X M perylene. All traces, except 0 M perylene, are fitted (smooth
300
200
100
Figure 5. Same description as Figure 4, except the samples were fully hydrated (see Experimental Section).
for comparison. From the values reported in Table I, hydration of the samples does not affect this type of quenching. The value for the critical transfer distance is slightly less than the reported value in a PMMA polymer matrix (44 A);13 this difference might be due to (1) shortening in the 'pyrene* lifetime (e.g., -130 ns in zeolite X compared to -400 ns in PMMA) or to (2) increase in the refractive index of the medium.13 T1+ Quenching of 'Pyrene*. We have previously reported that Cu2+ quenches 'pyrene* instantaneously in zeolite X7 if the distance between these two species is less than the active radius 13.6 f 0.2 A,14 and the quenching mechanism is electron-tunneling in nature. Exchanged T1+, like Cu2+, can readily quench the 'pyrene* in the supercage of zeolite X. Basically, the electron-tunneling model is an electron-transfer process as follows:
+ T1'
pyrene' + T1 The kinetics follow an electron-tunneling model via eq 1pyrene*
+
4416-18
(13) Johnson, P.C.; Offen, H. W. J. Phys. Chem. 1972,57, 14731475. (14) We tried to increase the calcination temperature for further dehydration of the Cu2+-exchangedzeolite X. Unfortunately, the Cuz+ can destroy the zeolite framework and form CuO if the calcined temperature is higher than 300 'C. This phenomenon is well documented for the dehydroxylation of divalent cation in zeolite Y.lS For this reason, we decided not to use the Cu2+ as a quencher in this coadsorbed water studies. (15) Reference 1, pp 462-468. (16) De Mayo, P.;Natarajan, L. V.; Ware, W. R. In Organic Phototransformations in Nonhomogeneous Media; Fox, M. A., Ed.;ACS Symposium Series 278; American Chemical Society: Washington, DC, 1985. (17) Bitting, W.; Milosavljevic, B. H.; Thomas, J. K. Int. J.Rodiat. Chem. Phys. 1988,32, 181-184.
Langmuir, Vol. 6, No. 2, 1990 475
Photophysical Properties of Pyrene in Zeolites
TIME
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In -I
-1.7
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TIME
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(nanoseconds)
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0.4
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too
200
300
400
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0
TIME
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Figure 6. (A) Dehydrated T1+-exchanged pyrene-impregnated zeolite X. Time-resolved pyrene fluorescence monitored at 400 nm. Five traces starting from the slowest are 0, 0.0750, 0.1246, 0.1744, and 0.2243 M T1+. All traces, except 0 M T1+, are fitted (smooth line on the raw data) with the electron-tunneling model (eq 4 in text). The signal at 0 M perylene concentration is fitted with a single-exponentialfunction at times long after the laser pulse; the dashed line shows that the signal deviates from first-order decay. (B) Unweighted residuals for all the fits in A. Starting from the top are 0, 0.0750, 0.1246, 0.1744, and 0.2243 M T1+.
I = I, exp(-kdt - A[ln3 ( u t ) + h, ln2 ( u t ) + h2In ( u t ) + h31) (4)
where A is a factor which depends linearly on the quencher concentration
A = [Q1/(Ro/a)3Co
(5)
k , is the rate constant of the fluorescence decay in the absence of quencher. h,, h,, and h, are coefficients related to derivatives of the r function derived by Tachiya and Mo~umber’~ h, = -3I”(l) = 1.731 646 99 h, = 3I’”(1) = 5.934 335 97 h, = -I””(,) = 5.444 874 45
(6)
t is the time, and v is the vibrational frequency in a square (or rectangular) potential well. For a first-order approximation, the rate of electron transfer in an electron tunneling model is similar to a free particle in an onedimensional box, as described in quantum mechanics.20 (18) Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1985, 89, 1830-1835. (19) Tachiya, M.; Mozumder, A. Chem. P t y s . Lett. 1974,28,87-89.
zoo
IO0
TIME
100
(nanoseconds)
Figure 7. Same description as Figure 6, except the samples were fully hydrated (see Experimental Section).
The square potential well is essentially the potential curve of ‘pyrene* in our case. Fitting the time-resolved pyrene fluorescence (400 nm) of the T1+-exchangedzeolite X yields the parameters A, kdl and v. Figures 6A and 7A are the fits of the tunneling model to both dehydrated and hydrated zeolite X samples. The provided unweighted residuals in Figures 6 B and 7B show the goodness of the fits. The results of the fits are summarized in Table 11. The electron-transfer rate in terms of the distance between the two species may be expressed in the form
k ( r ) = v exp(-r/a) s-l (7) This equation requires a, which is the attenuation length of the wave function, and may be obtained from eq 5. The nature of electron-tunneling kinetics, a very fast fall off in decay rate with distance, is well approximated by the Perrin static model. In other words, the initial rapid quenching at small r may be considered to be static, while the slow quenching at large r may be considered to be approximately that observed in the absence of quencher. Following this assumption, we applied the Perrin model to the T1+ quenching data and sought C, and R, in eq 5. Rotwhich has the same meaning as the radius of “active sphere” in the Perrin model, is the critical transfer distance for the electron-tunneling model. The C,, as in the Perrin model, is the critical quencher concentration. In the Perrin static model, a plot of In ( I o / I ) vs [Q] should be linear and yield a slope of l/C,,. R, can be (20) (a) Brocklehurst, B. Chem. Phys. 1973, 2, 6-18. (b) Marcus, 1985,811, 265-322.
R. A.; Sutin, N. Biochim. Biophys. Acta
476 Langmuir, Vol. 6, No. 2, 1990
Iu and Thomas 1.00
3.00
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> I-
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0.n
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I-
4
0.m
W
0: 0.00
TIME (nanoseconds)
I
I
/C
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M
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Figure 8. (A) Perrin plots of steady-state quenching of pyrene fluorescence monitored a t 400 nm in both fully hydrated ( X ) and dehydrated (0) and T1+-exchanged zeolite X samples. (B) Parameter A of the electron-tunneling model (eq 4 in text) as a function of T1+ concentration of both hydrated (X) and dehyT1+-exchanged zeolite X samples. The A values are drated (0) reported in Table 11. A linear fit is applied to each set of data.
further determined by using the equation
Ro = [3 x 102'/(4nNCo)]"3 A (8) where N is Avogadro's constant. Figure 8A shows the Perrin plots of In (&,/I) vs [Tl'] for both hydrated and dehydrated zeolite X samples containing pyrene and T1+. From the slopes of the hydrated samples, 10.42 f 3.58 M-', and the dehydrated samples, 3.45 f 0.96 M-l, the calculated critical T1+ concentrations are (9.6 f 3.3) X and (2.9 f 0.8) X lo-' M for hydrated and dehydrated samples, respectively. The critical transfer distances (R,)are then determined for the hydrated samples (16.0 f 1.6 A) and for the dehydrated samples (11.1 f 1.0 A). Figure 8B shows the plots of A vs [Tl+]for t h e hydrated and dehydrated samples. The slope is (3.7 f 0.2) X for hydrated samples and (1.7 f 0.5) X for dehydrated samples. From the slope and the known R, and C,, the a value is determined for both systems (1.13 f 0.28 8,for hydrated; 1.89 f 0.53 A for dehydrated). Finally, a distance (rbdependent rate constant can be obtained via the expression in eq 7. Equation 9 (hydrated) and 10 (dehydrated), given below, are the expressions for the distance ( r ) dependence of the rate constant for both systems: k ( r ) = (2.87 f 0.92) X 10" exp[-0.88 f 0.22 A-']r s-l (9) k ( r ) = (6.5 f 8.1) X 10' exp[-0.53 f 0.15 A-']r s-l
(10)
(nanoseconds)
Figure 9. Comparison of time-resolved pyrene fluorescence signal monitored at 400 nm before (dashed line) and after hydration (solid line) in zeolite X. The difference, a t times long after the laser pulse, between hydrated and dehydrated samples is due to the slower 'pyrene* decay in the hydrated samples (k (dehydrated) > k (hydrated), also see Tables I and 11). (A! Pyrene-contained%l+ ([Tl+] = 0.2243 M) exchanged zeolite X samples. A faster fall off a t the early times on hydrated sample (solid line) is apparent. (B) Perylene ([perylene] = 5.78 X M) doped, pyrene-impregnated zeolite X samples.
From the experimental results, T1+ quenches 'pyrene* about 2 orders of magnitude faster in the hydrated samples comparing to the dehydrated samples. The results suggest that the water molecules in the hydrated samples serve to enhance the rate of electron transfer. I t is interesting to note that we observed the same phenomenon, which is a faster 'pyrene* decay in wet sample, in a cellophane matrix.21 On the other hand, coadsorbed water in the perylene-doped, pyrene-impregnated zeolite X has no effect on the perylene quenching of 'pyrene* but increases the 'pyrene* lifetime as we mentioned in the last section. Figure 9 shows the timeresolved pyrene fluorescence (400nm) for the zeolite X samples before and after hydration in both quenching systems. The faster initial decay of the fluorescence in the hydrated samples (Figure 9A, solid line) compared to those in dehydrated samples (Figure 9A, dashed line) illustrates the enhanced quenching in the hydrated T1+exchanged zeolite X. This is further supported by (1)a larger slope of the plot of In ( I o / I )vs [TP], (2) a smaller a value, and (3) a larger v value. The enhancement of (21) Iu, K.-K.; Thomas, J. K.,un ublished data. However, a faster 'pyrene* decay is observed in a Cu&/pyrene cellophane system (ref 22). A further investigation is in process for this matter. (22) Milosavljevic, B. H.; Thomas, J. K. Chem. Phys. Lett. 1985, 114, 133-137.
Langmuir, Vol. 6, No. 2, 1990 477
Photophysical Properties of Pyrene in Zeolites A
C
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Figure 10. Comparison of time-resolved pyrene fluorescence signal monitored at 400 nm in zeolite A. (A) Pyrene-loaded Na+-exchanged zeolite A. Solid line represents the dehydration sample under vacuum Torr). Dashed line represents the fully hydrated sample under 15.3 Torr of oxygen. (B) Dehydrated pyrene-loaded T1+-exchangedzeolite A under vacuum Torr). Solid line represents [T1+]= 0 M; dashed line represents [T1+]= 0.2243 M.
the rate of the electron-transfer reaction in hydrated zeolite X may be explained by the exothermic Frank-Condon restriction as Beitz and Miller described.23 Although the a value in eq 7 can change the magnitude of the electron-transfer rate, the frequency factor ( u ) is the dominant element in our case.24 The frequency factor ( u ) in eq 7 may show a Gaussian distribution, which is the result of a simple theoretical treatment, assuming the equilibrium nuclear configuration and the frequencies of the quantum modes are identical in the initial and final states,25 in the mild exothermic region as Miller described,26and can be expressed as the following: u
where AGO,,
= uo exp[-(AGoET+ A)2/(4AkT)]
(11)
is the free energy change of the electron-
(23) Beitz, J. V.; Miller, J. R. J. Chem. Phys. 1979, 71, 4579-4595. (24) A simple calculation described in ref 26 shows that changing the a value from 0.75 to 2.45 A will decrease by 1 order of magnitude the electron-transfer rate. In our case, the difference in the a value for hydrated and dehydrated zeolite X is not a lot; however, the frequency factor ( u ) does make a big difference between both cases (see eq 9 and 10 and Table 11). (25) (a) Marcus, R. A. J. Chem. Phys. 1965,43,1978-1989. (b) Levich, V. G. Adu. Electrochem. Eng. 1966, 4, 249. (c) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975,63, 4368-4368. (26) Miller, J. R.; Peeples, J. A,; Schmitt, M. J.; Closs, G. L. J.Am. Chem. SOC.1982, 104,6488-6493 and references cited therein.
transfer reaction, A is the total rearrangement energy, and k is the Boltzmann constant. We have calculated that the AGO,, for 'pyrene*/Tl+ in CH3CN is -(-1.8 eV). Although coadsorbed water might affect both AGOET and A, the highly rigid environment in hydrated zeolite X suggests that the influence on the total rearrangement energy (A) might be bigger. From the larger value of u (see Tables I and 11)in hydrated samples, it is suggested that the absolute value of AGOET + A, which controls the rate of an electron-transfer reaction, is smaller in the hydrated zeolite X sample than the dehydrated sample. Zeolite A in Dehydrated and Hydrated Form?' We have reported that the location of adsorbed pyrene in zeolite A is solely on the external surface.' Because the aperture of the entry of zeolite A is about 4 A,it is impossible for the pyrene molecule (kinetic diameter about 7 A) to diffuse into the zeolite A framework without serious lattice distortion. For this reason, we choose zeolite A as a comparisonto zeolite X, where pyrene locates inside the cage of zeolite X under our experimental conditions (also see Experimental Section). Oxygen and T1+ Quenching of 'Pyrene*. Figure 10A shows a comparison of the time-resolved pyrene fluorescence (400 nm) of a dehydrated sample under vacuum vs a hydrated sample under 15.3 Torr of oxygen pressure. Unlike the zeolite X samples in Figure 1 (dotted line), where coadsorbed water can totally block the oxygen quenching (even the samples exposed to air; partial oxygen pressure is about 200 Torr), coadsorbed water in zeolite A does not block oxygen quenching. Figure 10B shows a comparison of the pyrene fluorescence under vacuum on dehydrated zeolite A with and without T1+. I t is important to note that the [T1+]in these samples was about the same as in zeolite X (also see Figure 6A), in which an apparent quenching was observed. Again, the subtle differences in quenching behavior between zeolite X and A are due to the location of pyrene. As mentioned earlier, pyrene is adsorbed on the external surface while T1+ is trapped inside the zeolite A cage. The situation is different in zeolite X, where both pyrene and T1+ are trapped inside the zeolite. Because the electron-transfer process in an electrontunneling mechanism becomes inefficient at large distance (vide supra), the large distances between internal and external surface of zeolite A result in no observable quenching of 'pyrene*. Furthermore, T1+is trapped inside the zeolite cage and within the lifetime of 'pyrene* cannot move to quench 'pyrene*.
Conclusion This study reports the effects of coadsorbed water on different types of quenching in both zeolite X and zeolite A. In zeolite X, coadsorbed water blocks quenching that requires reactant encounters (e.g., oxygen quenching). For the quenching involving an interaction through space (e.g., Forster model, electron-tunneling model), coadsorbed water does not stop the quenching action. In the case of electron-tunneling quenching, coadsorbed water increases the critical transfer distance and enhances reaction. For zeolite A, where pyrene is solely located on the (27) We decreased the pyrene concentrationin the zeolite A, in order to avoid the self-quenching caused by high pyrene concentration under vacuum condition. This self-quenchingbecomes insignificant, if the oxygen pressure increases. Therefore, our reported oxygen-quenching rate for 'pyrene* on zeolite A is still valid.
Langmuir 1990,6, 478-481
478
external surface, oxy en can still effectively quench 'pyrene*. However, T1 does not quench 'pyrene* under these experimental conditions.
also wants to thank Dr. M. Wolszczak and Dr. K. Koike for a helpful discussion on the electron-tunneling mechanism.
Acknowledgment. We thank the National Science Foundation for the support of this work. Kai-Kong Iu
Registry No. 0,, 7782-44-7; T1, 7440-28-0; H,O, 7732-18-5; pyrene, 129-00-0; perylene, 198-55-0.
Q
Partitioning and Stability of Aqueous Dispersions. Particle Size of Dye Dispersionst Erik Kissa Research and Development Division,$ Jackson Laboratory, Chemicals and Pigments Department, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898 Received April 25, 1989. I n Final Form: August 17, 1989 The particle-water interface in aqueous dye dispersions stabilized with a polymeric dispersant has been probed with a water-immiscible solvent in which the dye but not the dispersant is soluble. Unprotected dye particles dissolve in the solvent on contact and are extracted into the solvent phase. Dye particles protected by sodium lignosulfonate, a polymeric dispersant, are hydrophilic and favor the aqueous phase when extracted with the solvent. The extraction rate of the dye from the aqueous phase into the solvent phase depends on the resistance of the protective barrier to penetration of the solvent and indicates the effectiveness of particle shielding. When the composition of the dispersion and the extraction conditions are constant, the extraction rate decreases exponentially with the increasing fineness of the dispersion and the stability to sedimentation and coagulation. The extraction technique promises to be a useful tool for studying dispersion stability and probing the particle-water interface.
Introduction Water-insoluble dyes, pigments, and pharmaceutical and agricultural chemicalsare dispersed in water by reducing their particles to a size which can be stabilized by a di~persant.'-~The dispersant aids wetting of dye particles with water, facilitates breaking of agglomerate^,^,^'^ and stabilizes the dispersion by forming a steric or an electrostatic barrier by adsorption on the surface of the Ionic polymers stabilize aqueous disper-
'
Presented at the 62nd Colloid and Surface Science Symposium, June 22, 1988, University Park, PA. Publication No. 648. (1) Dispersion of Powders in Liquids, 2nd ed.; Parfitt, G. C., Ed.; Wiley: New York, 1973. (2) Callahan, W. B.; Manz, W. J.Oil Colour Chem. Assoc. 1964,47,
*
1964. (3) Parfitt, G. D. J. Oil Colour Chem. Assoc. 1967, 50, 822. (4) Carr, W. J. Oil Colour Chem. Assoc. 1971,54, 155. (5) Carr, W. Powder Technology 1977,17, 183. (6) Rehbinder, P. A. Colloid J. USSR 1968,20,493. (7) Kissa, E. Text. Res. J. 1989,59, 66. (8) Napper, D. H. J. Colloid Interface Sci. 1977,58, 390. (9) Ottewill, R. H. J . Colloid Interface Sci. 1977,58, 357. (10) Parfitt, G. D.; Peacock, J. In Surface and Colloid Science; Matievic, E., Ed.; Plenum Press: New York, 1978; Vol. 10, p 163. (11) Ottewill, R. H. Colloid Polym. Sci. 1980, 67, 71. (12) Tadros, Th. F. Adv. Colloid Interface Sci. 1980,12, 141. (13) Laible, R.; Hamann, K. Adv. Colloid Interface Sci. 1980, 13, 65. (14) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymeric Adsorption; Marcel Dekker: New York, 1980. (15) Vincent, B.; Whittington, S. In Surface and Colloid Science; Plenum Press: New York, 1981; Vol. 12, p 1. (16) Tadros, Th. F. In The Effects of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic Press: London, 1982; p 1. (17) Napper, D. H. Polymeric Stabilization of Colloidal Dzspersions; Academic Press: London, 1983.
0743-7463/90 f 2406-0478$02.50/0
sions by the electrosteric mechanism, which combines electrostatic and steric mechanism^.'^^'^*^*^ One of the mechanisms may be dominant under certain conditions. Dilute dispersions are stabilized mainly by electrostatic repulsion, but in concentrated dispersions the steric stabilization mechanism is dominant. The objective of this paper is to show that the stability of dispersions can be examined and the particle-water interphase probed by extraction with a waterimmiscible solvent in which the dispersed solid but not the dispersant is soluble. We studied aqueous dye dispersions stabilized with sodium lignosulfonate, an ionic amphiphilic polymer which is derived from lignin and features sulfonic acid and phenol The sta(18) Vincent, B. In Polymer Adsorption and Dispersion Stability; Symposium Series 240; Goddard, E. D., Incent, B., Eds.; ACS American Chemical Society: Washington, DC, 1984; p 1. (19) Hirtzel. C. S: Raiaeoualan. R. Colloidal Phenomena: Noves: Park Ridge, NJ,.1985.' ' (20) Lyklema, J. In Flocculation, Sedimentation, and Consolidation; Moudgil,B. M., Somasundaran, P., Eds.; United Engineering Trustees. 1985. (21) Silberberg, A. J. Colloid Interface Sci. 1986, 111,486. (22) Fleer, G. J.; Scheutjens, J. M. H. M.; Cohen Stuart, M. A. Colloids Surf. 1988, 31, 1. (23) Herb, C. A.; Ross,S. Colloids Surf. 1980, 1, 57. (24) Buscall, R. J. Chem. SOC.,Faraday Trans. 1 1981, 77,909. (25) Glennie, D. W. In Lignins; Sarkanen, K. V., Ludwig, C. H., Eds.; Wiley: New York, 1971. (26) Le Bell, J. C.; Hurskainen, V. T.; Stenius, P. J. J.ColloidInterface Sci. 1976, 55, 60. (27) Tadros, Th. F. Colloid Polym. Sci. 1980,258 439. (28) Heimann, S. Melliand Textilber 1982,53,885. (29) Heath, D.; Tadros, Th. F. Colloid Polym. Sei. 1983,261,49. (30) Hatfield, G. R.; Maciel, G. E.; Erbatur, 0.;Erbatur, G. Anal. I
Chem. 1987,59, 172.
0 1990 American Chemical Society
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