2334
J . Phys. Chem. 1992, 96, 2334-2340
to characterize AOT-H20 interactions. From the values of 7a listed in Table I, we infer that excess electrons in isooctane are efficiently solvated by encapsulated water at values of R L 0.75 ( w 2 18.5), which is consistent with our earlier study! At values of R < 0.2 (w < 4.9) in the same system, electron solvation is too slow to compete with diffusion, and electron attachment is inefficient, which is also the case in T M S at all values of R that could be studied. This dependence of electron solvation in AOT/H20 reversed micelles on pc or De in the solvent that was studied was generally ignored in the earlier studies reviewed by Abdel-Kader and Krebs and could contribute to several of the discrepancies that they discuss.51 This dependence would be particularly significant in comparisons of electron solvation by water pools of AOT in low-p, solvents such as n-hexane or nheptane (p, < 0.1 cm2/(V s), ref 9) with solvation in the higher p, solvent isooctane (p, = 5.2 cm2/(V s), ref 9). The 50-fold longer 7,in the low-p, solvents would permit electron solvation to occur at much lower values of R or w than those required for efficient electron solvation in isooctane. In conclusion, it is noted that the capture of more conventional reactants such as excited ~ y r e n and e ~ ~pyrene e x ~ i p l e x e sby~ ~ (53) Wong, M.; Thomas, J. K. In Micellization, Solubilization and Microemulsionr; Mittal, K. L., Ed.; Plenum Press: New York, 1977; p 647.
inverted water pools depends on the surfactant head-group density at the surfactant-oil-water interface which was not considered to play a role in electron attachment. For the AOT-heptane systems the head-group density changes from 1.5 X 1014/cm2for 1 radius of 69 A to 9.4 X l O I 4 cmz a t a pool radius and the rate constant of exciplex capture varies from 9.4 X 1Oloto 3.2 X 1O’O M-I ssl over the same range. The observed difference in the exciplex capture rates was interpreted as the smaller and more dense micelle being less penetrable by the reactant. A similar effect was noted with inverted micelles constructed with the cationic surfactant benzyldimethylhexadecylammonium chloride. Such effects are considered to be unimportant for electron capture due to the size and nature of electrons relative to conventional reactants.
:;:::fr
Acknowledgment. All of us were supported in various phases of this work through NATO Collaborative Research Grants. G. Bakale also thanks Ms. D. Clancy for preparation of the manuscript and the US.Department of Energy for partial support of this research under Grant No. DE-FG02-88ER60617. J.K.T. thanks the N S F for support. Registry No. AOT, 577-1 1-7; TMS, 75-76-3; isooctane, 540-84-1. (54) Kikuchi, K.; Thomas, J. K. Chem. Phys. Lett. 1988, 148, 245.
Nanosecond Pulse Radiolysis Study of Metal Aggregation in Polymeric Membranes 0. Platzer, J. Amblard, J. L. Marignier, and J. BeUoni* Laboratoire de Physico- Chimie des Rayonnements, associP au CNRS, UniversitP Paris-Sud, B6t 350, 91 405 Orsay Cedex, France (Received: March 21, 1991; In Final Form: November 7 , 1991)
After an electron pulse, the transient optical absorption spectrum and the decay rate in a perfluorosulfonated ion-exchange membrane such as Nafion swollen by a 2-propanollwater mixture are found to be similar to those of the solvated electron in the same free solvent. The radiolytic yield is G(e;) = 2. In membranes swollen by a solution of silver ions, chosen as a model system for the study of metal aggregation in a microheterogeneous environment, the end-of-pulse spectrum is that of Ag2+ (C = 3.7). The following step is a fast dimerization of Ag2+giving Ag42+,occurring inside one single hydrophilic cavity, with a rate slightly higher than in the free solvent. On the contrary, the further coalescence rate drops suddenly to a value lo4 times lower than in a free solution, due to the slower diffusion of the transient species of silver aggregation through the hydrophilic channels. This process competes on the 10% range with a corrosion of the smallest aggregates by H,O+contained in the same cavity. Thus the polymeric medium allowed us to observe directly the corrosion of a noble metal aggregate by the acidic cations. In a repetitive pulse regime, it can be observed that the aggregates generated in the primary pulses act as growth centers and induce the coalescence of the new atoms further produced, thus protecting them from corrosion.
Introduction
It has been shown in the past decade’” that the radiolytic method is a powerful means of producing metal aggregates of controlled size, not only as subcolloidal solution^^-^^ or supported (1) Belloni, J.; Delcourt, M. 0.;Leclere, C. Nouu. J . Chim. 1982, 6, 507. (2) Belloni-Cofler, J.; Marignier, J. L.; Delcourt-Euverte, M. 0.; Miiiana-Lourseau, M. French CNRS Patent No. 84.09196 (June 13, 1984). (3) Belloni-Cofler, J.; Marignier, J. L.; Delcourt-Euverte, M. 0.; Mifiana-Lourseau, M. US Patent No. 4.629.709 (Dec. 16, 1986); US Patent No. 4.745.094 (May 17, 1988). (4) Henglein, A.; Tausch-Treml, R. J . Colloid Interface Sei. 1981,80,84. ( 5 ) Henglein, A. Top. Curr. Chem. 1988, 143, 113, and references therein. (6) Henglein, A. Chem. Rev. 1989, 89, 1861, and references therein. (7) Delcourt, M. 0.;Keghouche, N.; Belloni, J. Nouu. J . Chim. 1983,7, 131. (8) Delcourt, M. 0.;Belloni, J.; Marignier, J. L.; Mory, C.; Colliex, C. Radiat. Phys. Chem. 1984, 23, 485.
0022-365419212096-2334$03.00/0
clusters,”-13 but also as aggregates embedded in a polymer.I4 In the latter case, the principle is to insert metal ions by swelling a polymeric system with the adequate solvent mixture and then to reduce the ions by the radiation-induced species, produced in situ in their vicinity. In a solution, the radiolytic reducing species induce a population of atoms homogeneously distributed in the bulk. Under a pulse regime, the aggregation of these atoms may ~~~~~
~~
~
(9) Marignier, J. L.; Belloni, J.; Delcourt, M. 0.;Chevalier, J. P. Nature 1985, 31 7, 344. (10) Marignier, J. L.; Belloni, J. J. Chim. Phys. 1988, 85, 21. (11) Bruneaux, J.; Cachet, H.; Froment, M.; Amblard, J.; Belloni, J.; Mostafavi, M. Electrochim. Acta 1987, 32, 1533. (12) Bruneaux, J.; Cachet, H.; Froment, M.; Amblard, J.; Mostafavi, M. J. Electroanal. Chem. 1989, 269, 375. (13) Amblard, J.; Belloni, J.; Platzer, 0. J . Chim. Phys. 1991, 88, 835. (14) Platzer, 0.; Amblard, J.; Belloni, J.; Marignier, J. L. French Atwhem Patent No. FR 2 621 043 (March 31, 1989); European Patent Application No. EP 309 337 (March 29, 1989); Platzer, 0. Thesis, Orsay, 1989.
0 1992 American Chemical Society
Metal Aggregation in Polymeric Membranes then be observed by time-resolved ~pectroscopy.~ The polymeric systems we used were cation-exchange membranes of Nafion 117, constituted of copolymers of tetrafluoroethylene and perfluorovinyl ether grafted with ionic groups which stabilize hydrophilic cavities.'$ The cation-exchanger group is the sulfonate ion 40,which is compensated by positively charged ions according to the concentrations of the solution brought into contact.16J7 The impregnation of metal cations is obtained by immersing a membrane in a solution containing the metal salt, up to the proper exchange of H+ or alkaline cations by the metal ions to be reduced.18 The steady-state radiolysis study14 has concluded that the mean size of metal aggregates thus produced increases with the membrane swelliig factor and that this size is far from being sterically limited by the volume of the hydrophilic domains in the membrane, although that volume is also controlled by the extent of swelling. These results suggested that the final size of the metal aggregates is probably controlled by the size of the channels linking the hydrophilic cavities. In order to submit this hypothesis to direct experimental observation, the nucleation dynamics had to be studied by pulse techniques. The membrane is by itself rather transparent in the direction normal to the surface. However, the thickness of a single membrane (a few lo" mm, depending on the swelling factor) does not allow the generation of optical absorptions (due to the metal aggregates) intense enough to be detectable. A new device was therefore developed to solve the problem and to enable us to perform time-resolved spectrophotometric studies of the growth dynamics of metal aggregates included in a membrane. As in the previous study,14 monovalent silver ions were selected as a model system, because they undergo a monoelectronic reduction and have been extensively studied under homogeneous conditions.&6J9-22 The same preparation and the special device adapted to the pulse radiolysis study of membrane systems will be described in detail.
Experimental Section Source. The pulse radiolysis facility (706 Febetron delivering a 3-11s electron pulse), the splitting beam detection, and the computerized signal processing system have already been des ~ r i b e d . ~ The ~ . ~ experimental ~ dose, measured from the endof-pulse optical absorption of the hydrated electron in pure water, ranged from 300 Gy (Le., 1.9 X 10l8eV.mL-' or 30 krad) to 1.2 kGy (Le., 7.5 X 10ls eV.mL-' or 120 krad) per pulse. Irradiation Cell. The photosensitive properties of Ag salts require keeping the samples in the dark, even during the preliminary assays, until they are subjected to a pulse, in order to avoid any light-induced side reaction. The irradiation device of the membranes must at the same time preserve them from any contact with oxygen and keep the extent of swelling constant, which implies an envelope impermeable to gas or solvents, and nevertheless transparent to the analyzing light (Mylar foil). Since the optical path of a single membrane is too small to allow a detection, four or five pieces of membranes must be piled up, (15) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. 1981, 19, 1687. (16) Eisenberg, A.; King, M. Zon-Conruining Polymers; Stein, R. S., Ed.; Academic Press: New York, 1977; Vol. 2. (17) Eisenberg, A., Ed. Zons in Polymers; Adv. Chem. Ser. No. 187; American Chemical Society: Washington, DC, 1980. (18) Helfferich, F. fon Exchunge; McGraw-Hill: New York, 1962. (19) Von Pukies, J.; Roebke, W.; Henglein, A. Ber. Bunsenges. Phys. Chem. - .- . 1968. - . - -, 72. - , 842. - (20) Tausch-Treml, R.; Henglein, A.; Lilie, J. Ber. Bunsenges. Phys. Chem. 1978.82, 1335. (21) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. 2.Phys. D Aroms, Mol. Clusrers 1989, 12, 31. (22) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiur. Phys. Chem. 1989,34, 605. (23) Belloni, J.; Billiau, F.; Cordier, P.; Delaire, J. A,; Delcourt, M. 0.J . Phys. Chem. 1978.82, 532. (24) Marignier, J. L.; Belloni, J. Chem. Phys. Lett. 1980, 73, 461.
The Journal of Physical Chemistry, Vol. 96, No. 5, I992 2335
a
b
d
+
L e-
Figure l . Irradiation device for the pulse radiolysis study of membranes. (a) Detailed view of the sample holder H; (b) C = sample holder case; S = sheath; s = slit; SI,S2= screws for 9 variation; E = earthing; e- = accelerated electrons; L = analyzing light.
without any layer of solvent in between, so as to yield a sufficient signal (Figure la). The total sample thickness was limited to 2 mm in order to ensure a homogeneous dose throughout the whole depth. The sample to be irradiated must be renewed from pulse to pulse to avoid interference of the aggregates from a previous pulse with the dynamics of atoms newly generated in the next one. Contrarily to solutions, membranes do not allow free diffusion, so that, provided the irradiated zones are restricted by a slit opened in metal screens ( 5 mm thick duralumin), a new area of the same polymer sample may be observed, the behavior of which is independent of other pulses (Figure lb). The sample area must be accessible both to the electron beam and to the probe light. Due to the metal screen, the only possible configuration is to orientate the polymer multilayer at an angle 19 of about 45" relative to the electron beam and to the light beam which are perpendicular to each other. Practically, the membrane multilayer is pressed (Figure la) as a sandwich between two 25 pm thick Mylar foils which are held by a metal framework and impermeable joints (0.5 mm thick, made of natural rubber). Finally, 30 screws ensure the airtightness after joining the different pieces in a glovebox flushed with NZ. The total thickness of the support is 3 mm. The holder H (Figure 1b) may be orientated from 40 to 50° relative to the beams and may be shifted vertically, 5 mm by 5 mm, to offer new areas to irradiation through the slit s. Despite the different constraints and cautions imposed by such a system, the device operated quite well. Samples. The membranes of Nafion 117 are first thoroughly cleaned from impurities able to react with the radiolytic species. The cleaning process essentially consists in a careful elimination of photosensitive impurities possibly present in the membrane. The latter is first degreased, then immersed in a boiling mixture
2336 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Platzer et al.
TABLE I: Data Concerning the Samples Studied bv Pulse Radiolvsis" Na+/ Naf/50-50 XH'
XK' XAg'
XNa' vH*O v,.PrOH
P
7"
dc [H+I [i-PrOH]
0 0 0 1 50 50 0.19 255 13.8 lo-' 0 6.7
&+I
Naf/50-50 0.51 0.39 0.10
0 50 50 0.13 255 13.8 0.38 0.075 6.7
&+I
Naf/901 0.51 0.39 0.10 0 99 01 0.07 89 6.2 1.10 0.21 0.13
1.5
~~~
O x c + = ionic fraction of cations C+ inside the membrane; v = solvent volume fraction (W); p = optical path (cm); T , = volume increase ('3%); d, = average diameter of a hydrophilic cavity (nm), computed according to ref 15; [SI = concentration of a solute S in a cavity (mo1.L-').
(85-90 "C) 2-propanol-water (30 vol % 2-propanol) for 30-40 min, and finally rinsed several times with distilled water. After such a treatment, a sample of membrane no longer turns brownish during solar light irradiation. The membranes are then exchanged, totally with Na+ or partially with Ag+, by immersing them up to equilibrium in aqueous deaerated solutions of NaOH (1.5 mo1.L-I) or Ag2S04( mobL-'), respectively. However, the concentrations of ions inside the cavities may be much higher, particularly at low swelling factors. Only Na+ or Ag+ diffuse into the cavities of the membranes where they replace the countercations H+ of the sulfonate groups. Sulfate anions are electrostatically repelled by the negative charge of the sulfonate. The same has been observed for the anion hexa~hloroplatinate.'~ After they are rinsed with doubly distilled water, the membranes are swollen with mixed deaerated 2-propanol-water solvents and piled up in the holder under the protection of the glovebox against O2 and light. Another difficulty is to keep the swelling of the membrane constant so as to know the Ag+ concentration with accuracy. The exchange ratio is calculated from the pH change before and after exchange, considering the initial ion fractions xH+ = 0.58 and X K t = 0.42 in the membrane and the solution volume in equilibrium (Table I). From the swelling factor, the concentrations of Ag', H+, and (CH3)2CHOH (or i-PrOH) in the hydrophilic phase are derived. The equivalent optical path p corresponding to the liquid domain is estimated from the volume increase T, (derived from the measurement of the weight increase), assuming that the swelling expansion is identical in the three dimensions
p = ~ , ' / ~ e ( s i8) n where e is the thickness of the dry Nafion (185 pm) and 8 the angle between the sample and the analyzing beam plane. The parameters of the sample are summarized in Table I. The average diameter d, of a hydrophilic cavity is derived from the Bragg spacing parameter obtained for a given swelling factor by Gierke et al.I5 who modeled the membrane as a paracrystalline cubic lattice of cavities. Results and Discussion Nafion without Ag'. First, the primary radiolytic species produced from the solvent and contained in the hydrophilic phase of the membrane have been studied in the absence of Ag+ at early times by pulse radiolysis of Na+/Naf/50-50 samples, in the optical range 350-950 nm. The spectrum which shows an intense, broad band with a maximum at 710-740 nm is that of an electron solvated in a free 1:l (vol) 2-propanol/water mixture,25which constitutes the hydrophilic phase in the swollen polymer (Figure 2). Thus, the presence of a hydrophobic phase and of exchanging (25) Burrows, H. D. Radiat. Phys. Chem. 1982,19, 151. Leu, A. D.; Jha, K. N.; Freeman, G. R. Can. J . Chem. 1982, 60, 2342.
tw
1 -
as 500
700 800 900 X (nm) Figure 2. Spectrum of electron solvated in a 1:l (vol) 2-propanol/water 400
600
solvent: (+) this work; solid line, from ref 25. Inset: time evolution of the 600-nm signal from membranes containing no silver (sample Na+/ Naf/50-50). 1
A
I
cl C
280
300
320
340
360
380
X (nm) Figure 3. Transient optical spectra at various times in bs from a sample Ag+/Naf/50-50. Here the membranes were swollen with a 1:l (vol) 2-propanol/water mixture containing lo-' mo1.L-I Ag2S04.
groups surrounding the polar phase where e; was generated does not influence its spectrum. The time evolution of the 600-nm signal up to 700 ns, without Ag+ ions, is shown in the inset of Figure 2. Its decay rate is also comparable to that in the free solvent. This indicates that the membrane does not contain any impurity other than those of the swelling solution itself and that the Nafion structural groups are inert toward e;. The electron decay is pseudo first-order (tl12 1.5 X lo-' s). Assuming the same extinction coefficient as in the mixed free solvent, = 17 400 L.m~l-'.cm-'),~~ we derive from the end-of-pulse absorbance an initial concentration [es-l0 = 2.6 X mol.L-', and a dose of 120 krad per pulse if Gc;= 2e; per 100 eV26in such a medium on this time range. Nation with Ag. (a) Transient Spectra in Ag+/Nafion. The transient spectrum has been studied from 50 ns to a few minutes, in the optical range 280-900 nm. Two different types of Ag+ samples have been observed: Ag+/Naf/99-01 and Ag+/Naf/ 50-50. The end-of-pulse spectrum, which then decays over a few tens of microseconds, presents a maximum at 310 nm similar to that of Ag2+ in water.4 It is the same in both samples and at different doses per pulse (Figure 3). The second-order decay at 310 nm is correlated with the appearance of an absorption in the UV below 290 nm, corresponding to the dimer A&2+,which shows a maximum at 275 nm in pure water.27 Beyond 50 M, no absorption is detected any more in the visible range up to 500 ps (for 120 krad per pulse). Then, a band with a maximum at 380-400 nm slowly appears, during 0.5 s in the case of Ag+/Naf/99-01 irradiated by a single pulse of 120 krad (Figure 4 a-c). The slow buildup must be due to the extremely
-
(26) Hecquet, M . F.; Roux, J. C.; Simonoff, G. N.; Sutton, J. fnt. J . Radial. Phys. Chem. 1969, I , 529. (27) Mulvaney, P.; Henglein, A. J . Phys. Chem. 1990, 94, 4182.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2337
Metal Aggregation in Polymeric Membranes
a
t Ims)
t Im51
c
0
0
0,s
,d*,
0 0
1
t is1
10
20
30
40
t 15)
Figure 4. Time profiles of absorbances at 380 nm in a sample Ag+/
Naf/99-01 after one and two pulses of 120 krad for various ranges of time (a-d) (the reference absorbance is that before the pulse). slow diffusion of the large species resulting from coalescence. Finally, this absorbance slowly decays and stabilizes at a constant value after a 20%decrease (Figure 4d and Figure 5a). The stock of silver atoms involved in the nucleation is produced within the pulse duration, due to the high local concentration of Ag+ (Table I) in the hydrophilic volumes. After the early formation of Ag2+and then of Ag42+,the transient absence of any absorbance before the 400-nm increase is attributed to the sizedependent extinction coefficient found by Tausch-Treml et al. in the free solvent.20 Actually, with the same stock of reduced atoms, an increasing aggregation results in an increasing absorbance per atom at -400 nm. The silver growth in Nafion extends over times 4 orders of magnitude greater than in free liquids (10 s instead of millisecondsz2). (b) Kinetics in Ag+/Nafion. Through the splitting-beam detection, two different kinetics have been recorded for the same pulse, one at 320 nm in the nanosecond range in order to know the initial concentration of silver atoms complexed in the form Ag2+, and the second one a t 380 nm in the range of seconds so as to follow the time-resolved aggregation. Formation of Ag2+ and Ag42+. Local concentrations in the hydrophilic domains of H30+ (0.4-1.1 mol.L-'), and of Ag+ (0.08-0.2 mol-L-'), for Ag+/Naf/50-50 and Ag+/Naf/99-01 samples, respectively, are so high that the lifetime of e; under such conditions does not e x 4 -30 ps if calculated with the same rate constantsz8as in a free solvent. In fact, electron scavenging or Ag+ occurs within the radiolytic spurs and the scaby H30+ venged electron yield, equal to G(Ag2+)a t the end of the pulse, is about 3.7 per 100 eV @e., 4.6 X lo4 mo1.L-I a t 120 krad), on the basis of the same value t(Ag2+) in the membrane as in the free solvent.
+ H30+
e; e,
+ Ag+
-
H
+ H20
Ag,
(k', = k128= 3 x 1Olo L.mo1-l.s-I) (1)
(k'2 = k228= 3.6 X 1O'O L-mol-'d)
(2) Both reactions are assumed to occur with rate constants equivalent inside a cavity of the membrane (k') and in the free aqueous medium ( k ) . H atoms are readily scavenged by isopropyl alcohol (8.7 X lo7 L.mol-'.s-'), and by Ag+ ions (3 X lolo L.mol-W), which leads to Ag atoms produced within -50 ps. The complexation with a large excess Ag+ is also fast
Ag,
+ Ag+
-
Agz+
( k ; = k 2 = 5 X109 Lamol-Id) (3)
thus explaining that the end-of-pulse spectrum (Figure 3) corresponds to Ag2+.This suggests that the multistep process yielding Ag2+ has proceeded in the same hydrophilic volume of the (28) Anbar, M.; Bambenek, M.; Ross,A. B. N.B.S. Re$ Data Ser. 1973, 43.
< .-
. ' 4
Figure 5. Time evolution of the optical spectrum in Agt/Naf/99-01 after one (a) to four (d) repetitive 120 had pulses. The figures were obtained by an association of kinetics at various wavelengths, the sample being
renewed at each wavelength. The reference of absorbance is that before any new pulse. The normalization of absorbance at a given wavelength is made with respect to the absorbance of Ag,+ at the fixed wavelength (310 nm) on the nanosecond range. membrane, and justifies the assumption of rate constants equal to those in the free solvent. The kinetics of the decay of Ag2+at 320 nm (and the correlated increase below 290 nm) are close to a second order, as in a dimerization reaction yielding Ag,2+:
However, on the basis of a second order, the rate constant measured would be k ; = ( 4 f 2) X lo8 L-mol-'as-'. This value is slightly higher than that found by Henglein et ale4in a bulk aqueous solution: k4 = 2 X lo8 L-mol-W. The rate constant depends neither on the swelling factor nor on the dose absorbed. Actually, the dose increase does not increase the local concentration of radicals in a cavity but only the number of cavities receiving one spur. These facts indicate that dimerization is again only controlled by the diffusion restricted to a single cavity, and not by the diffusion through intercavity channels which should depend on the extent of swelling. The rate is also the same when the membrane already contains aggregates produced by previous pulses. Now, the chances that the new Ag2+is produced in a cavity containing already an aggregate are small: from their average concentration, there would be practically no chance to find more than one species per cavity. We therefore have to conclude that a cavity contains at least two Ag2+ and hence two reducing precursors from the same spur. However, they are not produced homogeneously but are concentrated in spurs of -2 nm diameter containing a few geminate pairs of eL-cation. In such a case, the probability to generate one spur inside a single cavity of size 6 nm is close to unity. Hence two Ag2+species issued from two precursors e[ of the same spur are produced in close vicinity, which explains that they dimerize faster than under homogeneous conditions, whatever the swelling factor. From the value of the absorbance at 320 nm extrapolated to the end of the pulse, and from the extinction coefficient of Ag2+,20 initial concentrations of silver atoms of 2.5 X lo4 and 4.3 X 10-4 mo1.L-l are found in Ag+/Naf/50-50 and Ag+/Naf/99-01 re-
Platzer et al.
2338 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
-
"t I
I
I
I
I
1
,
1
,
,
I
C /
.2t l ( S )
I
I
I
60
80
100
I
0
20
40
t (ms) figure 6. Time evolution of the mean extinction coefficient per atom at 380 nm in a free Agt solution (water 9975, 2-propanol 1%), and in differently-swollen membranes. Dose 120 krad. (a) Ag+/Naf/99-01; (b) Agt/Naf/50-50; (c) Agt/free solution/99-01.
spectively. These concentrations correspond in both cases to a radiolytic yield G(Ag2+) = 3.7. Aggregurion. The longer time evolution of the spectrum is characterized by the very slow increase of a band at 380 nm (Figure 4). Such a band is not specific for a particular size of the aggregates: the band arises from progressive coalescence of previous species, but according to the work of Henglein et its extinction coefficient per atom increases with the number of atoms per aggregate, at least in the 3-14 nuclearity range.
+ Agp
(k'~) (5) This explains why in the free solution4a constant number of atoms, as produced by one pulse, absorb the light with an intensity higher and higher as they are aggregating. In the membrane, the increase at 380 nm has the same cause, although it is very much slower (Figure 6). According to the conclusions arising from calculations in a free solution,22the coalescence rate constant is reduced from curve c to curve a by the same factor as the time is lengthened ( 10-3-10 s), Le., roughly 3-4 orders of magnitude. From a detailed model developed for membranes,29the coalescence rate constant is estimated to be PS= 5 x 104 L-mol-W, to bc compared with kS = 2 X lo* L*mol-'*s-' in the free l i q ~ i d . ~ The drastic change (4 X 103) in the coalescencerate constant arises from the necessary diffusion through the channels linking the cavities. Thus with a dose of 120 krad, the concentration of Ag42+in Ag+/Naf/99-01 is about 2 X lo4 mol.L-', Le., 6 X 10l6 Ag42+ species/cm3 of Nafion, to be compared with a concentration of 4 X 1OI8cavities/cm3: the average distribution is only 1.5 A h 2 + species per 100 cavities. Therefore, the coalescence requires diffusion from one cavity to another. The constant can be related to the diffusion constant (k'5)d and to the reaction constant at the encounter (k'Jr Agn
- 1= k'S
-+
&n+p
1 (k'S)d
1 +
the component (k'&r at the encounter should be independent of the medium and hence supposed to be the same as in the free liquid where it is slower than the diffusion and is nearly equal to ks: (k'Jr = (ks)r = ks = 2 X lo8 L.mol-'.s-' It follows that
(k'& = 5 x io4 L-mol-Id. a value which is about 104-105 times lower than a diffusion(29) Amblard, J.; Platzer, 0.;Ridard, J.; Belloni, J. J . Phys. Chem., following paper in this issue.
Figure 7. Kinetics of optical density evolution at 380 nm of silver aggregates after one to four repetitive 120 krad pulses in Agt/Naf/99-01. Reference is taken from the optical density just before the new pulse and normalization from pulse to pulse as in Figure 5 .
controlled rate constant in a free medium. Now, the diffusion process is much slower than the reaction itself, in contrast with the situation in the free liquid. This is confirmed by the dependence of the observed growth of the aggregates on the swelling factor. Figure 6 shows the evolution of the mean extinction coeficient per atom, derived from the optical density and the initial number of atoms, this coefficient being correlated with the size of the metal aggregate; contrarily to the Agz+ dimerization rate which was the same in both casea, the further aggregation process becomes slower when the hydrophilic phase is less strongly swollen; the rate is lower when the channels between the cavities, as well as the cavities themselves, are narrower. The slow diffusion through the channels is also supported by low diffusion coefficients foundMfor monomer molecules diffusing in various polymer matrices, cellulose acetate-butyrate (1.5 X c m 2 d ) , poly(viny1 butyrate) or poly(viny1alcohol) (8.5 X c m W ) , or poly(viny1 acetate) (6 X c m 2 d ) , instead of coefficients of a few 1O-5c m 2 dfor a free diffusion. The ratios are of the same magnitude as those found for silver coalescence in swollen Nafion compared to a free solvent. It is worth noting in Figure 6 that curves a and b (which both correspond to polymer samples) tend toward the same limit on the 100-ms range. This limit is about 70% of that observed for curve c (which corresponds to the free solution). Such an observation allows us to give an evaluation of the radiolytic yield G(Ag,,) close to 70% of 6.2, Le., 4.3. This value, hardly larger than the initial yield of 3.7 for Agz+, indicates that, contrarily to the case of a free aqueous solution with 2-propanol added, the (CH3)2COHradical does not seem to contribute significantly to the reduction of Ag+ for either samples a or b. If it did, a supplementary yield, changing from 3.2 for the first sample to 5 for the second, would be expected. Since the presence of the (CH3)2c0H radical does not seem to influence G(Ag,), it is possible that this radical (insofar as it is formed through the scavenging of OH' by i-PrOH) readily reacts with the polymeric structure instead of reducing Ag+. Actually, the comparison of samples a and b, which corrapond to markedly different situations of OH' scavenging (mainly by Ag+ in the former case, and by i-PrOH in the latter) and nevertheless lead to the same order of magnitude for G(Ag,), rather suggests that the species OH' react quite early with the polymeric structure of the membrane, instead of forming alcoholic radicals. Such an oxidative reaction could also explain the brittleness of membranes which have been y-irradiated with much higher doses (e.g., 50 Mrad).I4 (c) Corrosion. The steric hindrance of the coalescence is even more serious when, at lower dose per pulse (30 krad), the probability for the hydrophilic cavities of a Ag+/Naf/99-01 sample to contain one A g l + species drops to one Agd2+/250cavities (Figure 8, to be compared with Figure 7 at 120 krad). The first consequence is obviously to slow down even more strongly the ~~~
(30) Krongauz, V. V.; Yohannan. R.
M.Polymer
1990, 31, 1130.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2339
Metal Aggregation in Polymeric Membranes .121
I
I
I
I
I
5 1
P 0 c-i w
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8 a
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t (s) Figure 8. Kinetics of optical density evolution at 380 nm of silver aggregates after one to ten repetitive 30 krad pulses in Agt/Naf/99-01, Reference and normalization as in Figure 5.
coalescence process, thus shifted from the to the 10% range. A second consequence is that the final decrease, lowering by 20% the absorbance at long times (up to 100 s), is no longer observed at 30 krad. Now the maximum absorbance is only OD = 0.018, far more than 4 times less than the value at 120 krad, OD = 0.35. Thus, it is worth noting that the final concentration of silver atoms is not proportional to the dose per pulse. This suggests that both the 20%decay in Figure 8 and the very weak value at the maximum in Figure 7 are due to a reverse reaction of the smallest aggregates which undergo a slow corrosion by the acid ions H30+contained in the hydrophilic cavities:
This pseudo-first-order process would occur over the same time range t , independent of the aggregate content (dose), but fixed by the high local H30+concentration (fw-, = kk[H30+]). The dependence of kinetics on the H30+ concentration would be indeed another useful argument to support the corrosion invoked. However, the corrosion phenomenon is very slow, even over the range 0.4-1.1 mo1.L-l for [H30+](tl12 1 s). This constitutes an upper limit imposed by the exchanger structure of the membrane. Using less acidic conditions would lengthen t I l 2 beyond the time domain where the aggregates are still small and thus capable of being corroded. This is the situation encountered with silver growth in free solutions where generally no corrosion is detected by pulse r a d i o l y ~ i s . ~ ~ Instead of varying [H30+], we chose, other parameters being fixed, to lower the dose per pulse so as to lengthen the coalescence process and let the small aggregates undergo corrosion more efficiently. Actually, the absorbance increase at 380 nm following a 30-krad pulse, which would be caused by aggregation, is now so slow that it can be compensated to a large degree by the corrosion which occurs a t the same time. At 120 krad, the coalescence is much faster and precedes partly the corrosion, thus causing a maximum of OD (Figure 7). The absorbance which is finally observed therefore corresponds to those aggregates of size large enough to escape corrosion at least over the range 10-100 s. This is in agreement with the concept of a size-dependent redox potential for metal aggregates.4-6,2*22J1-33Such a concept, supported by observations of either the corrosion of nascent copper clusters3' or the oxidation of Ag atoms and oligomers by various oxidants,32accounts for a possible corrosion of silver by H30+, provided that the redox potential of the silver aggregate is lower than that of the hydrogen electrode. The same concept explains why no corrosion occurs (31) Delcourt, M. 0.; Belloni, J. Radiochem. Radioanal. Lett. 1973, 13, 329. ( 3 2 ) Henglein, A. Ber. Bunsenges. Phys. Chem. 1977, 81, 556. (33) Belloni, J.; Delcourt, M. 0.;Marignier, J. L.; Amblard, J. Radiation Chemistry; Hedwig, P., Nyikos, L., Schiller, R.,Eds.; Akadtmiai KiadB: Budapest, 1987; p 89.
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Figure 9. Final optical spectrum of silver aggregates produced after repetitive 120 krad pulses, referred to the unirradiated membrane: (a) Agt/Naf/99-01 after four pulses; (b) Agt/Naf/50-50 after 25 pulses; (c) Ag+/Naf/50-50 after 35 pulses.
when the potential turns to be positive, i.e., when particle size increases. The reoxidation by H30+of small silver aggregates has also been observed when these aggregates are produced as isolated oligomers on the surface of microcrystals of silver iodide." (a) Coalesceace Dynamics under a Repetitive Pulse Regime. The above described situation is still further accentuated in a situation of repetitive pulses (Figures 5, 7,and 8). (It is noteworthy that for each new pulse the reference signal is the final absorbance of the immediately preceding one, so that the kinetics only refer to the new pulse). In a repetitive pulse regime, the absorbance at any time is higher than for a single pulse. This means that the aggregates produced earlier and having escaped corrosion from a previous pulse, contribute to the protection of newly produced atoms from corrosion by coalescing with them. Another consequence of the sizedependent extinction coefficient per atom4 is to cause the average absorbance per atom to increase when new atoms coalesce with a previous aggregate, provided the final size is less than -13 atoms. This phenomenon is still perceptible in the 120 kradspulse-' case (Figure 8), except that the gain in absorbance per atom is more important for the second pulse. From the first to the second pulse, the absorbance is at all times higher (Figure 8, curves 1 and 2). Had the atoms generated in the second pulse coalesced independently of the preexisting aggregates, curve 2 would have been identical with curve 1. The increase observed from 1 to 2 is therefore due to the process of aggregation of the newly formed atoms with the preexisting germs, which enhances the optical density per atom and also, at long time, protects them more efficiently from corrosion. There is no further difference after three pulses at 120 krad (Figure 7), or after six pulses a t 30 krad (Figure 8), when the size-dependent extinction coefficient has reached its maximum value (15 500 Lmatom g-l-cm-l for 13-14 atom^).^ This means that the aggregation number of numerous aggregates after one single 120 krad pulse, or even after three or four 30 h a d pulses, is less than 13-14. In the 30 krad-pulse-' case (Figure 8) the initial concentration of atoms is lo4 mo1.L-I. From pulse to pulse, the induction time before the increase of the 380-nm band is reduced (300 ms for AOD, and 10-15 ms for AOD5). The maximum AOD occurs at the fifth pulse, then decays and is constant after ten pulses. The minimal size 13-14 is therefore reached after five pulses and apparently an aggregate of such a size is no longer oxidized by H30+. With further pulses, the new atoms contribute to the growth of preexisting aggregates with which their coalescence is more probable than the independent coalescence of newly formed atoms. The consequence is that, when both the number of pulses and the dose increase, the increasing number of atoms is distributed among a constant number of germs which are growing with the dose, in contrast to the situation in a free liquid with an added surfactant where aggregation leads to a constant final size for the aggregates, with a concentration depending on the dose absorbed. (34) Henglein. A.; Gutitrrez, M.; Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsenges. Phys. Chem. 1989, 93, 593.
2340 The Journal of Physical Chemistry, Vol. 96, No. 5, I992
*\-m,
Figure 10. Time evolution of the optical spectrum of newly-formed aggregates during a 25th 120-krad pulse in a sample Agt/Naf/50-50 (see Figure 5 for normalization).
The curves corresponding to two pulses of 120 krad or five pulses of 30 krad exhibit the highest amplitude because the growth induced by the last pulse concerns mostly the size with the highest derivative in the t = An)dependence. (e) High cumulated Doses. It was interesting to try and follow the spectral changes of the aggregates when their growth is provoked by a large number of repetitive pulses. At 120 krad pulse;' the final spectrum does not change after a few pulses, except in intensity (Figure 7) and in the slow corrosion decay. However, after a few tens of pulses, the overall final shape of the spectrum (reference: unirradiated membrane) is markedly different (Figure 9, b and c) and the same evolution of the spectrum is also observed regarding the kinetics following one of these pulses (Figure 10): we observe that, instead of a single maximum at 380 nm, a new one appears at 560 nm (after 25 pulses of 120 krad) while the membrane, instead of being yellow, turns orange. Later, i.e., after 35 pulses, an intense shoulder appears also at -720 nm and the membrane becomes brown red. The time evolution of any spectrum following one of these pulses starts with the classical 380-nm band (
