Langmuir 199411, 4347-4354
4347
Droplet Clustering in Ionic and Nonionic Water in Oil Microemulsions: Rate of Exchange between Clusters Studied by Phosphorescence Quenching Holger Mays, Jorg Pochert, and Georg Ilgenfritz" Institut fur Physikalische Chemie der Universitat zu Koln, Luxemburger Strasse 116, 0-50939 Koln, Germany Received June 12, 1995. I n Final Form: August 17, 1995@ Time-resolved phosphorescence quenching measurements with a terbium complex as lumophore and methyl viologen and bromphenol blue quenchers allow one to monitor processes in ionic and nonionic water in oil microemulsions in the long (millisecond)time range. We interpret the decay in this time range as exchange of quencher between clusters of microemulsion droplets. We show that material exchange due to this clusteringstarts below the percolation threshold,the rate increasingalong the percolation transition. The activation energies, which are formally negative in the nonionic system, are discussed with respect to the processes involved in the fusion of droplets and clusters. Comparison of ionic AOT- and nonionic Igepal-stabilized microemulsions gives evidence that the droplets in the ionic system maintain their compartmental structure even at temperatures above the percolation threshold.
Introduction Microemulsions are thermodynamically stable and transparent one-phase mixtures of water, oil, and amphiphile. Their phase behavior has been extensively ~ t u d i e d l due - ~ to the theoretical interest in understanding the balance between interfacial forces6and the importance of microemulsions for applications, e.g., in tertiary oil recovery' and as a medium for chemica1819and biochemical reactions.1°-12 However, important aspects of structure and dynamics are presently not fully understood. This especially concerns questions of droplets clustering in the percolation transition. The microscopic structuring of the water and oil domains separated by a surfactant monolayer occurs in the nanometer range.3r4J3 Detailed information has been obtained from various methods, like small angle neutron scattering (SANS),14J5light scattering (QELS),16 and freeze fracture electron microscopy (FFEM)." Direct information on the dynamics and material exchange between water in oil (w/o) microemulsion droplets is available from kinetic experiments like metal complex
* To w h o m correspondence should be addressed. Abstract publishedinAduance ACSAbstracts, October 15,1995. (1)Kahlweit, M.; Strey, R. Angew. Chem. 1985,97,655. (2)Kahlweit, M.; Strey, R.; Busse, G. J . Phys. Chem. 1990,94,3881. (3)Howe, A. M.; McDonald, J. A.; Robinson, B. H. J . Chem. Soc., Faraday Trans. 1 1987,83,1007. (4)Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989,5, 1210. (5)Clark, S.;Fletcher, P. D. I.; Ye, X. Langmuir 1990,6, 1301. (6)Gompper, G.;Schick, M. Self-Assembling Amphiphilic Systems; Academic Press: London, 1994. (7)Rosen, M. J.; Zhi-Ping, L. J . Colloid Interface Sci. 1984,97,456. (8) Kahlweit, M.; Strey, R.; Schomacker, R. In Reactions in Compartimentalazed Liquids; Knoche, W., Schomacker, R., Eds.; Springer: -~ Berlin, 1989;p 1. (9) Schomacker,R.; Stickdom, K.; Knoche, W. J.Chem. SOC., Faraday Trans. 1991,87,847. (10)Luisi, P. L. Angew. Chem. 1985,97,449. (11)Pileni, M. P., Ed. Structure and Reactivity inReuersed Micelles; Elsevier: Amsterdam, 1989. (12)Luisi, P. L. Kineticsand Catalysis in MicroheterogeneousSystems; Marcel Dekker: New York, 1991;p 115. (13)Almgren, M.; Johannsson, R. J. Phys. Chem. 1992,96,9512. (14)Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges.Phys. Chem. 1981,83,863. (15)Kotlarchyk, M.; Chen, S. J. J. Phys. Chem. 1982,86, 3273. (16)Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979,92,480. Strey, R. J. Phys. Chem. 1988,92,2294. (17)Jahn, W.; @
formation,18where a microemulsion containing the metal ion in the aqueous phase is mixed with a microemulsion containing the ligand. Another method is time-resolved electric bi~-efringence.'~-~~ Such measurements confirm the highly dynamic nature of the microemulsion structure. Previous work14J8,23 and our own studies on electric field induced percolation by monitoring electric conductivity and electricbi~-efringence'~-~~ have revealed that processes of droplet aggregation exist in the time range of milliseconds. A particularly useful method for obtaining both structural and dynamic information has been proven to be time-resolved fluorescence and phosphorescence q u e n ~ h i n g . ~ r ~The J ~ ~time ~ ~ - range ~ ~ accessible with fluorescent probes is restricted by their relatively short lifetime in the nanosecond to microsecond time range. The widely used Ru(bpy)s2+has a lifetime of about 570 ns and is able to monitor processes in that time scale. The present paper investigates the dynamics of droplet exchange with the phosphorescence quenching method in the long, i.e., millisecond, time range. We have applied a terbium complex as phosphorescent probe which exhibits one of the longest lifetimes accessible in aqueous solution (1.9 ms). We will argue that different types of exchange processes are observed with a short living probe and a long living one, and we follow the reasoning of Almgren's group that in the long time range the properties of droplet clusters become a p ~ a r e n t . Different ~ ~ , ~ ~ w/o microemul(18)Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. SOC., Faraday Trans. 1 1987,83,985. (19)Runge, F.;Rohl, W.; Ilgenfritz, G .Ber. Bunsen-Ges.Phys. Chem. 1991,95,485. (20)Schlicht, L.; Spilgies, J . H.; Runge, F.; Lipgens, S.; Boye, S.; Schiibel, D.; Ilgenfritz, G. Biophys. Chem., in press. (21)Runge, F.; Schlicht, L.; Spilgies,J . H.; Ilgenfritz, G.Ber. BunsenGes. Phys. Chem. 1994,98,506. (22)Hilfiker, R.; Eicke, H. F. J . Chem. SOC.,Faraday Trans. 1 1987, 83,1621. (23)Johannsson, R.;Almgren, M.; Alsins, 3. J. Phys. Chem. 1991, 95,3819. (24)Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989,93,10. (25)Lang, J.;Jada, A.; Malliaris, A. J . Phys. Chem. 1988,92,1946. (26)Atik, S.S.;Thomas, J. K. J.Am. Chem. SOC.1981,103,3543. (27)Veerbeck, A,; De Schryver, F. C. Langmuir 1987,3,494. (28)Almgren, M.; Johannsson, R. Langmuir 1993,9,2879. (29)Fletcher, P. D. I. J . Chem. SOC.,Faraday Trans. 1 1987,83, 1493. (30)Modes, S.;Lianos, P.;Xenakis, A. J.Phys. Chem. 1990,94,3363. (31)Fletcher, P. D. I.; Horsup, D. I. J. Chem. Soc., Faraday Trans. 1992,88,855.
0743-746319512411-4347$09.00/00 1995 American Chemical Society
4348 Langmuir, Vol. 11, No. 11, 1995
Mays et al.
sions stabilized by ionic and nonionic surfactants using AOT and Igepal CO-520 are compared. The quenching behavior is studied when the aqueous phase in the microemulsions undergoes a temperature-induced percolation transition. We will also show that the temperature dependence of the rate constants of slow exchange processes correlates with the percolation transition ofboth the ionic and nonionic microemulsions. This leads to formal negative activation energies for the intercluster exchange in a nonionic microemulsion. Phase Behavior and Percolation. A characteristic feature of the phase behavior is the existence of a continuous one-phase channel in the temperature1 composition diagram from the water rich side (L1 phase, oil in water (olw) microemulsion) to the oil rich side (Lz phase, wlo microemulsion). Going from one side to the other one may achieve a continuous change from normal micelles to reversed micelles over an intermediate bicontinuous structure with an inversion of the surfactant monolayer curvature from positive to negative values. The change in structure from water droplets in oil (Lz) to oil droplets in water (L1)is accompanied by a drastic increase in electric conductivity for ionic surfactanuwater or nonionic surfactantfbrine systems.32 Percolation of the aqueous phase can also be induced in wlo microemulsions with constant composition by changing the temperature. This change in state occurs within a narrow temperature range of few kelvin. Even systems which only contain 5 vol % aqueous phase show the temperature-induced percolation transition. Percolation at such low volume contents of the conducting phase requires attractive interactions leading to droplet clustering and formation of an infinite network of clusters or conducting water channels. Experimental indications for the occurrence of droplet clustering in microemulsions are obtained from several methods.5,23,31,33 Percolation in wlo microemulsions can also be brought about by variation of pressure34and application of high electric f i e l d ~ . l ~ -Also, ~ l addition of solutes20can induce the percolated high-conductivity state. Measurement of electric conductivity as a function of temperature is the most simple method to monitor percolation. But other methods such as viscosity35or electric birefringen~el~-~l can also probe the structural changes. The strong increase in conductivity is due to a transport of charge capiers through the microemulsion. For the low-conductivity state it has been suggested that ion fluctuations resulting in charged droplets and their migration in the electric field are responsible for the large conductivity compared to apolar solvent^.^^^^' With the percolation transition another mechanism of ion transport must become predominant. Transfer of ions through water channels between droplets in an infinite cluster during sticky collisionsor transient merging of droplets have been discussed as possible transport mechanism^.^^ Lang et al. performed a detailed fluorescence quenching study on the rate of exchange between droplets in various AOT microemulsions with different alkane and correlated their results with the percolation phenomena.24 The ~~~~
-3 2: IgepaVc,n-Hcxanc(l:I), Wo=24.5, C,=O.173 M 3: Igcpal/c,n-Hexane(l:I).Wo=49.C,-0.082 M 4: IgepsVc,n-Hexane(I:l), Wo=12.2. Cr=0.166M 5: AOT/n-Dodecane,Wo=21, Ct=O.l M 6: AOTIn-Heptane, Wo=55, C,- 0.089 M
-4
-5 log K Scm-1
-6 -1
"
15
20
25
30
2
35
40
45
50
"C Figure 1. Electric conductivity curves of different microemulsione as a function of temperature. W O= [waterl/[surfactantl, and CT= total surfactant concentration. The aqueous phases in all microemulsions contain 0.01 M KCl. Arrows in the diagram indicate phase separation. The percolation temperatures were not influenced by the addition of luminescent probe and quencher.
authors found that the second-order rate constant of exchange between droplets must be sufficiently high (larger than lo9 M-l s-l) to reach the highly conducting state, irrespective of the surfactants nature, oil, and volume fraction of the dispersed phase. There is an inverse temperature dependence of percolating wlo microemulsion systems stabilized by ionic and nonionic surfactants. While with ionics (like AOT) the highly conducting state is found at higher temperatures, nonionics (like alkyl glycolethers)have the percolated state on the low temperature side (see Figure 1). This opposite behavior can be explained by the opposite temperature dependence of surfactant hydration and solubility in water.'
Fluorescence and Phosphorescence Quenching The fluorescence quenching method has' been extensively applied since the work of Turro and Yekta,39where micellar aggregation numbers have been deduced from static fluorescence measurements. However, comprehensive analysis requires time-resolved measurements. An analysis of time-resolved fluorescence in micelles with exchange of quenchers over the continuous phase was given by Infelta et aL40 and T a ~ h i y aAfter . ~ ~ a briefexciting pulse the fluorescence intensity I(t) decreases with the time t as follows:
I ( t ) = Io exp[-A2t
+ A,(exp(-A4t)
-
1)l
(1)
IO denotes the intensity at t = 0, and the parametersA2are related to the kinetic constants of the quenching reaction steps. It has been shown that expression 1can also be applied to w/o microemulsions under certain conditions (especially the validity of the Poisson distribution of solubilized material), with the parameters given in eq 2:26327
A4
~
(32)Lagues, M.;Ober, R.; Taupin, C. J.Phys. Lett. 1978,39,487. (33)Smeets, J.;Koper, G. J. M.; van der Ploeg, J. P. M.; Bedeawr, D. Langmuir 1994,10,1387. (34)Zhang, J.;Felton, J. L.; Smith, R. D. J.Phys. Chem. 1993,97, 12331. (35)Chen, S.J.;Evans, D. F.; Ninham, B. W. J.Phys. Chem. 1984, 88,1631. (36)Eicke, H.F.;Borkovec, M.; Das-Gupta, B. J.Phys. Chem. 1989, 9.1 - - , 314 - - -. (37)Hall, D. G. J.Phys. Chem. 1990,94,429. (38) Mathew, C.;Patanjali, P. K.; Nabi, A.; Maitra, A. Colloids Surf 1988,30,253.
A4 = Kq KO
+ K,CM
(2)
is the rate constant for the natural decay of the probe
(39)Turro, N.J.;Yekta, A. J.Am. Chem. SOC.1978,100, 5951. (40)Infelta, P. P.; Gratzel, M.; Thomas, J. K. J.Phys. Chem. 1974, 78,190. (41)Tachiya, M. Chem. Phys. Lett. 1976,33,289.
Langmuir, Vol. 11, No. 11, 1995 4349
Droplet Clustering in Microemulsions
(ko = l h o ) , k , the first-order rate constant for intradroplet quenching by one quencher molecule, and k , the rate constant for exchange of quencher between droplets. CQ and CM are the total concentrations of quencher and droplets, respectively. The average occupation number n of droplets with quencher is given by the ratio n = CQJCM. The rate constant k , of intramicellar quenchingdepends on the collision probability between probe and quencher and reflects the mobility of these molecules in a droplet. The constant k , was found to scale with the inverse droplet volume.27 Expressions 1and 2 have also been applied to percolating micro emulsion^,^^,^^ although the droplet model is not strictly applicable in this case. If the decay function (1)with the above expressions for the parameters A2-A4 is considered for slow processes after a time when rapid events are finished (i.e., k , >> k,CM, and after an expansion of the exponential A4 term), the intensity as a function of time reads
it is possible to study both processes. When using a phosphorescent probe with increased lifetime (e.g. Cr( b ~ y ) 3 ~the + ) , long time behavior (It) with k,t >> 1is given bY
= -kot - n,
+ (n, - n ) exp(-k,,t)
(6)
In this case the fraction (1- exp(-n)) ofdroplets containing an excited lumophore and at least one quencher molecule is rapidly quenched on the long time scale investigated yielding an initial drop in intensity. The subsequent exchange of molecules within a cluster leads to the exponential term with k,,. , The limiting slope of the logarithmic decay curve is determined by the natural lifetime of the unquenched lumophore:
(6a) Thus, after an initial drop caused by the rapid intramicellar quenching the subsequent decay behavior can be described by a single exponential function. The initial drop at a given quencher concentration is determined by the droplet concentration. New aspects in the theory of fluorescence quenching were discussed by Almgren et al.23 They postulated clusters of droplets in wlo microemulsions and developed an equation to describe the time-resolved quenching experiments for small clusters:
Z(t) In= -kDt IO
+ n[exp(-k,t)
-
The systems studied by Almgren et al.13,23 do indeed show this behavior. Our experiments give evidence that in the very long time range additional processes can be observed, which we interpret as exchange of quencher between clusters. It can be assumed that the rate of exchange depends on a second-orderrate constant as the responsible step should be the collision of clusters. We propose to extend eq 6a with an additional term describing the slow exchange between clusters. The decay function then becomes
+
11
(7)
(n, - n)[exp(-k,,t) - 11 (4) where k , and n have the same meaning as above and the index c refers to the properties of clusters, i.e., k,, the first-order rate constant for quenching within a cluster and n, the average number of quencher molecules in a cluster. In the decay law (4) an additional exponential term appears compared to eq 1. The luminescence of the droplet fraction (1 - exp(-n)) containing an excited lumophore and at least one quencher molecule is quenched with the rate constant k,. The excited fluorophores in droplets containing no quencher molecule at the beginning are deactivated in clusters with the rate constant kcT The whole deactivation is superimposed by the natural deactivation. It has been pointed out by the authors that the observed process which yields k , or k,, can be selected by the choice of a suitable probe molecule according to its natural lifetime. A short time approximation (index st) for short living probes (like F'TSA with a lifetime of about 11ns) with linearization of the last term in eq 4 results in = -(k0
+ (n, - n)k,,)t - n + n exp(-k,t)
(5)
Again, this expression is of the Infelta type. For the case of fast intramicellar quenching compared to exchange processes (Le., k , * kcq,ke&) the parameters A3 and A4 are the same as in eq 2 while the first-order term K,CQ in A2 is replaced by (n, - n)k,,. The second-order exchange between droplets is interpreted here as a first-order quenching process within a cluster. With short living fluorescent probes only the intramicellar quenching can be observed. By using the widely used probe Ru(bpy)s2+
The index vlt denotes the approximation for very long time. k,, is the second-order rate constant for the quenching reaction between different clusters and between clusters and remaining droplets. Comparison of this expression with the previously treated formalism shows that (7) is evidently analogous to eq 3, but eq 7 results from the qualitative interpretation that on the large time scale the observable process is due to clustering. Equation 7 will be used to describe our experiments for conditions far away from the percolation temperature, where clusters start to develop. We also use eq 7 in the percolating regime. For the description of quenching reactions in ramified and large compact clusters other expressions have been A general decay law developed in the for low occupation of clusters with quencher reads
with S(k,t) the average number of distinct sites (i.e., micelles in a cluster) visited by a walking molecule with the walking frequency k,. Here lo refers to the initial intensity after the intramicellar quenching events. In fractal clusters, S(k,t) correlates with the spectral dimension d, of the clusters
whereas in large compact clusters the number of visited (42) Lianos, P.;Modes, S. J.Phys. Chen. 1987,91, 6088.
Mays et al.
4350 Langmuir, Vol. 11, No. 11, 1995 micelles is given by
1000
S(k,t) = a,(k,t)
(10)
a n d a2 are scaling constants, on the order of unity. Finally, if the quencher occupation of micelles is high, the quenching event will occur either immediately in a micelle or after the first walk step. Then the decay can be described by
,I
I
I
I
1
I
B
B
800
a1
I(t) In -= -hot IO
- k,[1
- exp(-n)lt
(11)
Experimental Section The surfactants Aerosol-OT(AOT, Sigma) and Igepal CO-520 ( C Q H ~ Q - C ~ H ~ - ( O - C ~ H ~ ) , a- Omixture H, of n-nonylphenyl oligoethyleneglycolethers with the average numberj = 5, Aldrich) were used as received. Deionized water was destilled twice before use. Buffer solution pH 8 was prepared by dissolving tris(hydroxymethy1)aminomethane(0.02 M, Merck p.a.1 and potassium chloride (0.01 M, Merck p.a.1 and adjusting the pH value with concentrated HCl (Merck p.a.1. The oils cyclohexane, n-hexane, n-heptane, n-dodecane, and Decalin were obtained from Merck (analytical grade) and used without further purification. Tris(2,2'-bipyridyl)mthenium(II)chloridewas obtained from Aldrich. The luminescence probe Tb(pdah3- (pda = pyridine-2,6-dicarboxylicacid)was synthesized in buffer solution accordingto the method of Barela and Sherry43to yield a complex concentration of 100 pM. The quenchers potassium hexacyanoferrate(II1)(Merck),methyl viologen chloride ( M V ,Sigma),ferroin (Merck),and 3',3",5',5"-tetrabromophenolsulfonephthdein (Bromphenol Blue, BPB, Merck) were used as supplied. The microemulsions were prepared by weighing in the calculated amounts of oil, surfactant, and water phases and mixing by intense shaking for 1 min followed by brief sonication. The compositions are characterized by W O(number ofwater molecules per surfactant molecule) and CT(surfactant concentration with respect to the total microemulsion volume) which are specified in Figure I. The electric conductivitv was measured with a WTW-LTA electrode and a Wayne-Kek Autobalance Precision Bridge B331 at the circular frequency w = 104rad/s. Optical absorption spectra were carried out with a Perkin-Elmer A-19 spectrometer. Phosphorescence spectra and time-resolved decays were determined with a Perkin-Elmer spectrometer LS 50 B. In this instrument a xenon flash tube produces polychromaticlight with 10 ps pulse width and 20 kW power. A Monk-Gillieson type monochromator with an accuracy of fl nm selects the desired exciting wavelength. The cell geometry was a standard 90" arrangement, and the cell holder for 1cm2quartz cuvettes was modified for better temperature control. The emitted light passes another monochromator and was detected by a EM1 9781B photomultiplier. No filters were used since scattered light plays no role in transient phosphorescence measurements. The first data point can be taken after l o p s , so measurements at shorter times are not possible. Unless otherwise stated the emitted photons were collected over an integration time of 100 ps and were slightly corrected in amplitude by a term with respect to the integration time. This procedure does not affect the time constant of decay. Linear fittings were performed aRer 100 ps to avoid influences of faster processes and were calculated with a least square fit method by a microcomputer. The temperature was controlled with a Lauda RC 20 Cryostat and kept within ~k0.05"C.
Results and Discussion Figure 1 shows the electric conductivity of the microemulsions studied by the phosphorescence quenching method as a function of the temperature. In the microemulsions with the ionic surfactant AOT the conductivity increases with rising temperature, but in the microemulsions with the nonionic Igepal increased conductivity (43)Barela, T.D.;Sherry, A. D. Anal. Biochem. 1976, 71, 351.
600 Ir
400
200 0
300
200
400
nm
500
600
Figure 2. Phosphorescence spectrum ofthe Tb(pda)s3-complex in aqueous solution with light intensity in arbitrary units after a 30 p s delay time. 2 1.5
I 0.5 InI(t) 0 -0.5 -1 -1.5
L
I
1
I
0
0.1
0.2
0.3
I
10.4
I
0.5
y
\&
0.6
0.7
ms
Figure 3. Time-resolvedintensities of Tb(~da)3~with different MV concentrations in aqueous solution at T = 25 "C. The top curve is for zero, the lowest for maximal quencher concentration. The integration time was 10 ps. occurs with falling temperature. This demonstrates the inverse temperature behavior of ionic a n d nonionic systems. Since the probelquencher system Tb(pdah3-/MV2+ has not been applied for quenching experiments in microemulsions before, it will be briefly introduced here. The terbium central atom of the electrostatic complex can be excited directly by f-f transitions, b u t the forbidden nature of this exciting process leads to very weak absorption a n d small emission light i n t e n ~ i t i e s .The ~~ emission intensity can be enhanced by using a suitable ligand which transmits energy to the central atom, resulting in direct excitation of the f-f transitions. The phosphorescence spectra show s h a r p emission bands at the wavelengths 492 a n d 545 n m that are not broadened by vibrational coupling, as seen in Figure 2. The complex has the long lifetime of TO = 1.9 m s in aqueous solution, because the location of the emission transition center is well shielded from the surrounding solution. This shielding is t h e reason for oxygen having no effect on the lifetime too. Thus, this complex is well suited to perform measurements at very long times. The emission can be quenched by electron transfer reactions from the excited metal atom or by Forster energy transfer. M V is a suitable electron acceptor and reduces the lifetime of the probe molecule (Figure 3). The rate constant for the quenching reaction in water was determined from a Stern-Volmer plot yielding k, = 2.1 x lo7 M-l s-l at 25 "C. The activation energy is E, = 14.5 kJ (44)Stryer, L.;Thomas, D. D.; Meares, C. F. Annu. Rev. Biophys. Bioeng. 1982,11, 203.
Langmuir, Vol. 11, No. 11, 1995 4351
Droplet Clustering in Microemulsions I 6 lnl(t) 5
CQ= o CQ= 20 pM
4
-0.2
-
C; =40w
3
A
CQ= 60 pM
v C; IC'"
-0.25 0
0.05
0.1
L ms
0.15
0.2
0.25
Figure 4. Time-resolved decay of Tb(pda)s3- in the AOT/ heptane system, WO= 55 and CT= 0.089 M. The microemulsion with quencher contains CQ = 88 pM MV (1 mM relative to water phase). Data points are taken from 10 measurements.
=Sow =loOuM
non-percolated
2
0
-I
0.2
0.4
fms
0.6
0.8
s
mol-l. Although the strongest absorption of Tb(~da13~occurs at 240 and 285 nm it is not advisable to use these wavelengths, since the quencher MV begins to adsorb strongly at about 300 nm. The problem of inner filter effects can be avoided if the wavelength 320 nm is chosen to excite the complex. Figure 3 reveals that under this condition no intrinsic absorption through the quencher percolated appears, which would result in an initial drop in intensity. I C', = lOOuM 2 L 1 I 1 I BPB as a quencher works by radiationless energy transfer 0 0.2 0.4 0.6 0.8 with kq = 6.7 x lo7 M-l s-l (at 25 "C)and has a lower ms activation energy than MV. But because this molecule Figure 5. Decay curves of Tb(pda)s3-in the presence of BPB absorbs strongly in the range of the possible exciting in the AOT/dodecane microemulsion, WO= 2 1and CT= 0.1 M, wavelengths, the initial drop in intensity cannot be at 20 "C (upper diagram)and 34.5 "C (lower). CQ*marks the quencher concentration with respect to water. Note, that the analyzed according to eq 7. inner filter effect of the quencher contributes to the apparent Ionic Microemulsions with AOT. In Figure 4 the time-dependent emission intensity of the T b ( ~ d a ) ~ ~ -initial drop. complex dissolved in a AOTheptane microemulsion is droplets is presumably determined by the desorption of shown. The decay behavior of the methyl viologen the bound MV molecules. However, it is interesting to quenched system in both the nonpercolated and percolated note that the initial drop increases with the percolation state is quite similar: after an initial drop in intensity (by transition. Summarizing,methyl viologen is not a suitable extrapolation to t = 0) the decay is given by a single quencher to study microemulsions with anionic surfacexponential function. The lifetime of the lumophore tants. Unfortunately we have found no negatively charged complex in the microemulsion is not affected by the molecule which quenches the Tb(pda)s3-luminescence via surfactant. electron transfer which would allow simultaneous deterAnalyzing the initial drop according to eq 3 yields an mination of k,, and C,. unreasonably high concentration of micelles (1.6-2.2 mM) We performed further experiments with energy transfer compared to the value calculated from surfactant conquenchers. BPB is negatively charged and can be assumed centration and surfactant area (-0.04 mM). The change to be localized within the water droplets. Figure 5 shows in slopewhen quencher is added, in general a characteristic the decays for this quenching system in the dodecane of exchange, is minimal. The same observation was made microemulsion. In general, there is an initial drop in light with the dodecane microemulsion. intensity followed by a single exponential decay. Although The unexpected small initial drop and the absence of the initial drop is hard to evaluate quantitatively because exchange processes may be explained by a very strong of the quencher's intrinsic absorption, the initial drop adsorption of the positively charged MV molecules at the becomes larger with increasing temperature while the negatively charged surfactant interface, as found for intrinsic absorption is constant for a given quencher instance with the dimidium cation.45 The distance concentration. The percolation transition in this system between the nitrogen atoms in MV carrying the highest is accompanied by a clear increase in the rate of exchange, charge density is approximately 0.6 nm, calculated from apparent from the change in slope in the logarithmic plot. covalent radii. The surface area occupied per AOT At intermediate temperatures the images show a is about 0.5 nm2 yielding a mean distance systematic evolution from one extreme (nonpercolated) between two surfactant molecules in the interfacial layer to the other (percolated). The same type of decay was of 0.8 nm. Thus the MV molecules geometrically fit well obtained for the AOT-heptane microemulsion. The on the surfactant layer. Due to the strong adsorption, the observed decay curves were single exponential in all cases concentration of free quencher within the aqueous phase (far below the percolation threshold, at the threshold, and is reduced to an effectivequencher concentration available above the threshold). The second-order rate constant of for the probe. The latter is preferentially localized in the deactivation k,, from eq 7 in the nonpercolated state of bulky water core ofthe droplets because ofits high negative the dodecane system at 20 "C was 1.85 x lo8 M-l s-l charge. Therefore, the initial drop is smaller than for an (Table 1). unbound quencher, and the very slow exchange between We tested the exchange mechanism by applying the positively charged ferroin cation, which yields k,, = 1.86 (45) Fletcher, P. D.I. J . Chem. SOC.,Faraday Trans. 1 1986,82, 2651. x lo8 M-I s-l, in excellent agreement with BPB. Ferroin
I
4352 Langmuir, Vol. 11, No. 11, 1995
Mays et al.
Table 1. Values of the Rate of Exchange k,, in AOT/ n-Dodecane: W O= 21, CT = 0.1 M, Quencher = BPB state T ("C) k,, (s-l M-l) nonpercolated percolated
1.85 x lo8 4.75 x 108 7.0 x los
20 29 34
0.5 0
~~~~
~~~
(46)Vollmer, D.; Vollmer, J.;Eicke, H. F.Europhys. Lett. 1994,26, 389.
I
I
1
I
-0.5
InU 10
is a bulky probe, and its charge is preferentially localized around the central atom and not concentrated at the outer sphere. Thus the quencher exchange is not hindered by binding of these molecules to the surfactant interface, despite the positive charge. The luminescent probe as well as all quenchers used show no noticeable solubility in a solution of AOT in oil, which was checked by UV absorption measurements. Thus it is highly likely that also on the long time scale fusion and fission of droplets andor clusters is the dominant exchange mechanism.13Js To classify the observed exchange process we repeated measurements of the rate of exchange between individual droplets with the shorter living probe R~(bpy)3~+ and analyzed the decays according to eqs 1and 2. In the timeresolved quenching experiment of this probe witli hexacyanoferrate(II1)we obtained for the nonpercolated dodecane system at 20 "C k, = 2.8 x lo9 M-' s-l, in reasonable agreement with the value obtained by Lang et al.25for the same system under similar conditions. (We thank Mr. M. Gutmann, Koln, for allowing us to use his fast laser pulse equipment.) From comparison ofthis with the rate constant of quenching on the long time scale, which is 1 order of magnitude slower, we conclude that the observed process is not due to exchange between individual droplets but appears with the existence of clusters. This is supported by the observed increase in initial drop with increasing temperature, both for MV and BPB. Regarding the quenching behavior in the percolated state (cf. Figure 5) the increased initial drop gives evidence that the droplet structure is maintained. An increased quenching on shorter time scales can be explained by extended cluster formation at the expense of droplets or smaller clusters, allowing additional quenching steps within the cluster. The finding that droplet structure is maintained with AOT reversed micelles gives strong support to the conclusions drawn from other The decay function of the phosphorescent probe Cr(bpy)g3+in a percolating microemulsion has been studied by Almgren et al. r e ~ e n t l y .It~ was ~ ~ ~found ~ that the tendency of the decay functions changes with the percolation transition. The experimental result was that at low temperatures after an initial drop the slope of the quenched decay curve was the same as in an unquenched microemulsion. With increasing temperature the decay function changed from single exponential to one with a stretched exponent. The authors interpreted their results as a change in structure with temperature, from isolated droplets near the lower phase boundary (solubilization point) to small clusters of droplets (described by eq 6) then leading to a fractal cluster of aggregated droplets in the percolative regime (eqs 8 and 9). From our experiments with Tb(~daI3~we have no indications that such a change occurs. As mentioned before, the decay is always single exponential. An exchange between clusters is observed in the unpercolated state too (cf. Figure 5 ) . Equations 8 and 11also do not account for the increased initial drop we find apart from the intrinsic quencher absorption. Our results suggest that relativejy small clusters of droplets are existent while the microemulsion undergoes
,
I
-1
-1.5
-2 -2.5
I
C$=O C$ = 50 pM
a
= 100 pM Co* = 150 pM
v a
CQ* = 200 pM CQ*= 250 pM
Cp*
cQ*=300pM
0
0.05
non-percolated 0.15
0.10
0.20
0.25
iiis 0.5
0
-2.5
I
0
percolated 1
0.05
I
I
0.10
0.15
I
0.20
i I
0.25
iFiS
Figure 6. Decay of Tb(pdah3-in a nonpercolated (2' = 28 "C, upper diagram)and a percolated (2' = 21.5 "C, lower diagram) Igepal microemulsion with different concentrations CQ*of MV in the aqueous phase. Oil = c,n-hexane, W O= 24.5, and CT = 0.173 M. The linear fit starts at 0.1 ms and is averaged from 5 measurements. Table 2. Values of the Rate of Exchange k,, in AOT/ n-Heptane: W O= 65, CT = 0.089 M, Quencher = BPB state T ("C) k,, (s-l M-l)
nonpercolated percolated
30 40 45.5
4.62 107 2.99 x lo8 9.47 x 108
percolation, but the dynamics of exchange between these clusters become faster and contributes to an enlarged ion migration. The values of the intercluster exchange k,, in Tables 1and 2 increase with the percolation transition by 1 order of magnitude, reaching lo9 M-' s-'. With the percolation transition, the rate of intercluster exchange becomes almost as fast as the intracluster e x ~ h a n g e . ~ ~ ~ ~ ~ In summary, the dynamics of percolating clusters become apparent from measurements in the millisecond time range. Nonionic Microemulsions with Igepal CO-520. The nonionic amphiphile Igepal CO-520 is of the nalkylphenyl oligoethyleneglycolether type, similar to the widely used C12Ec. The main difference of this technical product is a distribution of the polar tail lengths. This surfactant also does not disturb phosphorescence measurements, since the lifetime of the probe is not affected by this amphiphile. The decay functions of the phosphorescer with MV in a microemulsion based on Igepal in the nonpercolated state at higher temperatures (Figure 6, upper diagram) show a behavior like that in the AOT systems and can be described with eq 7. After the decay of faster processes the kinetic curve is clearly exponential. At short times (
