432
Langmuir 1986,2,432-438
line with no significant intercept or curvature and a standard deviation of 0.013 dyn/cm. The apparatus was relatively insensitive to vibrations. Pounding of the bench on which it was set lowered the pressure at which the bubble detached by less than 0.05 dyn/cm. This shows that the bubble is rather stable against mechanical disturbances, presumably due to its smallness. The general procedure was therefore to thermostat the transducer with the capillary removed and establish the zero pressure reading. Then the capillary was attached and immersed to the desired depth into the solution, generally 5 mm, and the latter thermostated. The pressure was then raised slowly to the approximate ST to be measured and a t least half an hour later the measurements were performed, first at longer BI's using the manostat and then at shorter BI's using flow rate control mainly by the precision pressure reducer. The immersion zero was checked either after the measurements or before measurements involving rapid bubbling if the solution was strongly foaming. Finally the capillary was removed and the pressure zero verified after about an hour.
For most precise measurements the suppression of the CD12 was set so that the pressure corresponding to a convenient BI was set to the middle of the recorder chart without the bucker. Pressure could then be changed by *0.5 dyn/cm by the bucker alone with immediate return to the original pressure when desired by switching off the bucker. At high bubbling rates, where the manostat could not produce a constant enough pressure to be measured by the DVM, the chart of the recorder was used as the first basis of measurement, and when the pressure changes extended beyond the chart, the suppression of the Validyne output was relied upon with some loss of precision. As the high bubbling rates are intrinsically less reproducible because of irregular convection, this did not represent a real problem.
Acknowledgment. This work was made possible by the hospitality of Prof. Bruno Zimm's laboratory and a grant from the Research Corporation Foundation. I am also grateful to Dr. Mark Troll for the design of the amplifier and many stimulating discussions.
Structure and Transport in the Microemulsion Phase of the System Triton X-100-Toluene-Water Mats Almgren,*t Jan van Stam,+Shanti Swarup,t*tand J.-E. Lofrothe The Institute of Physical Chemistry, University of Uppsala, S-751 21 Uppsala, Sweden, and Department of Physical Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden Received November 20, 1985. I n Final Form: February 27, 1986 The microemulsion phase of the system Triton X-l Wtoluene-water has been studied by time-resolved fluorescence quenching, conductivity measurements (of added NaCl), and determination of self-diffusion coefficients. The fluorescence quenching results are compatible with the existence of reversed micelles over all compositions studied, which means down to 17% toluene and 15% water, but with a rapid exchange of the probes and quenchers between the droplets, in particular at high water and low toluene concentration. The exchange is assumed to be mediated by a fusion-fission process in which two droplets merge into a labile large drop, which rapidly splits up in two drops again. Qualitatively, the conductivity of added NaCl changes in the same way as the exchange rate of the solutes from the fluorescence quenching. The self-diffusion coefficient of water changes comparatively little and is larger than what can be explained by the fusion-fission process alone. In addition to the exchange information, the fluorescence quenching study yields the concentration of droplets. With an assumption about the composition of the intermicellar solution, the size of the droplets can be estimated. The phase diagram shows that extensive water solubilization occurs only above a critical concentration of the surfactant in toluene. Assuming that the intermicellar solution has this critical composition throughout, the droplet sizes were calculated. The hydrophilic radius was found to vary between 30 and 46 A, increasing with increming water content, whereas the area available per surfactant at the surface of the hydrophilic drop was constant at 77.2 f 1.6 A2. Introduction The term microemulsion, introduced by Hoar and Schulmanl already in 1943, was given a generous definition by Danielsson and Lindman2as "a system of water, oil, and amphiphile which is a single optically isotropic and thermodynamically stable liquid solution". Most interesting, technically and scientifically, are those microemulsions in which oil and water are both major constituents. As 'University of Uppsala. Present address: Rutgers University, Department of Chemistry, Piscataway, NJ 08854. Chalmers University of Technology and University of Goteborg.
* *
0743-7463/S6/2402-0432$01.50/0
witnessed by several recent conference volume^,^ the microstructure and transport mechanisms in microemulsions are far from well-known, although the main features of some systems appear well established. Ternary systems composed of Aerosol OT (AOT),water, and a hydrocarbon are notable examples where well-defined and comparatively well-characterized water droplets exist over a wide (1)Hoar, T. P.; Schulman, J. H. Nature (London) 1943, 152, 102. (2) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391.
(3) (a) Robb, I. D., Ed. Microemukrions; Plenum Press: New York and London, 1982. (b) Mittal, K. L., Lindman, B., Eds. Surfactants in Solution; Plenum Press: New York and London, 1984;Vol. 3. (c) Lindman, B., Olofson, G., Stenius, P., Eds. Prog. Colloid Polym. Sci. 1985, 70.
0 1986 American Chemical Society
Structure and Transport in Triton X-100-Toluene-Water
Langmuir, Vol. 2, No. 4, 1986 433
composition range. In other systems the microemulsion phase extends from a normal micellar solution in the water corner to a reversed micellar solution in the oil-rich part, with a less well-defined transition region in between. The microstructure and the transport mechanisms in this region are intriguing problems. Time-resolved fluorescence quenching methods appear promising in this field. These methods can provide direct evidence for the confinement of probes and quenchers in small compartments. In favorable cases the number of compartments, and hence their size, can be determined. It is also possible to get information on the size polydispersit9 or on the rate of exchange of the probe or quencher between the compartments. Although it has been demonstrated that size and exchange information may be obfluorescence quenching methods have tained in this not been applied systematically to water-in-oil microemulsions or reversed micellar systems. One problem is the selection of a good probe-quencher pair for a particular system. We have chosen Ru(bpy)32+-methylviologen (MV2+),where the long lifetime of the excited state and the very efficient quenching by MV2+are beneficial. The couple is suitable for nonionic or cationic systems but hardly for anionic ones. We chose the nonionic surfactant Triton X-100, which is readily available in a photophysically very pure quality but comprises a wide distribution of homologues with varying length of the poly(oxyethy1ene) chain. With toluene as the oil an interesting phase diagram was obtained, with a large L2 or microemulsion area. Several features of this diagram are unusual but were fortunately confirmed by unpublished work at the Institute of Surface Chemistry in Stockholm.' A second problem was that the fluorescence quenching kinetics had not been treated with sufficiently generality for an application of the methods to microemulsions. The Infelta modeFg for quenching in micellar systems allows a migration of the quencher between the micelles. In typical microemulsion systems, however, the exchange of both probe and quencher may be important. An exchange of the droplet contents by the fusion of two droplets into an unstable dimer, which rapidly undergoes fission, has been suggested to occur in the AOT-oil-water system.lOJ1 The Infelta model was generalized to include this and other exchange mechanisms in a previous article which will be refered to as 1.l2 A brief account of the pertinent features of this generalized model will be given below. The experimental fluorescence decay results fit well to this generalized model with an assumed exchange by the fusion-fission process. This does not imply, however, that the system necessarily is composed of rather monodisperse aqueous droplets undergoing fusion-fission processes at a rate comparable to the fluorescence decay rate of the excited probe. We will discuss, therefore, the information contained in the fluorescence decay curves also in more general terms and compare this with the results from measurements of the conductivity of added salt and NMR
determinations of self-diffusion coefficients. Fluorescence Decay Kinetics. In the model of Infelta et al.8 the decay of the excited species is given by
(4)Almeren. M.: Lofroth. J.-E. J . Chem. Phvs. 1982. 76. 2734. ( 5 ) Atiky S. S.; Thomas, J. K. J . Am. Chem. koc. 1981, i03,3543; J. Phys. Chem. 1981,85, 3924. (6) Gelade', E.; De Schryver, F. C. J. Photochem. 1982, 18, 223. (7) We are indebted to Dr. Irenea Burachewska and Prof. Per Stenius for making these data available to us. (8) Infelta, P. P.; GrHtzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. Infelta, P. P.; Gratzel, M. J . Chem. Phys. 1983, 78, 5280. (9) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (10)Fletcher, P. D. I.; Robinson, B. H. Ber. Bumenges. Phys. Chem. 1981, 85, 863. (11) Eicke, H. F.; Shephard, J. C. W.; Steinemann, A. J . Colloid Inteface Sci. 1982, 90,92. (12) Almgren, M.; Lofroth, J.-E.;van Stam, J., submitted for publi-
cation in J . Phys. Chem. (Submitted).
F ( t ) = Al exp(-A2t + A3(exp(-A4t) - 1))
(1)
where A, = F(0) is the fluorescence intensity immediately after 6-pulse excitation, and A2 = ko + k,k-n(k,
+ k-)-l
A, = kq2n(k,+ k J 2 A4
=
k,
+ It-
Here n is the mean number of quenchers per micelle, ko is the fluorescence decay rate constant in a micelle without quenchers, and x k , and xk- are the rate constants for quenching and escape, respectively, for micelles with x quenchers. The general appearance of the decay according to eq 1 is that of a two-stage process, where an initial rapid, nonexponential phase is followed by a slow exponential one. These gross features are expected whenever probes in different environments are accessible for quenching to very different degrees. Consider the case when there is a poissonian distribution of quenchers over confinements which may also contain probes. At the moment of excitation the distribution of quenchers over cells with excited probes is the same as that over all cells. Rapid quenching occurs in those cells where x is large so that the surviving excited states are preferentially found in cells without quenchers or with few quenchers. At long times a steady-state situation evolves in which a certain distribution of quenchers is maintained in the cells with surviving excited states. If there is no exchange between the cells only cells without quenchers are represented at the steady state; for other given mechanisms the stated-state distribution can be determined by numerical calculations as shown in I. The rate of deactivation of excited states follows -dB/dt = (kot k , ( x ) ) B (2)
+
where B = E,"=,$, is the concentration of micelles with excited probe and B, is the concentration of micelles with excited probe and x quenchers; since only very few probes are excited, B is also the total concentration of excited probes. ( x ) is the average number of quenchers in micelles with excited probes and evolves in time from ( x )o = n at the moment of excitation to ( x ) in ~ the steady-state phase of the decay. For most exchange mechanisms a solution of eq 2 can only be obtained by numerical solution of the equation system for the rate of change of all B, species. It was shown in I that a good approximate solution with the same form as eq 1can be obtained. The parameters A2-A4 were determined by the following identifications. From eq 1 and 2 at the steady state A2 = ko
+ k,(x),
(3)
Comparison of the initial values for the first and second derivatives from eq 1 and 2 yields A, = n ( l
- (x)Jn)'
A4 = k,n(n -
(x),)-l
(4)
(5)
ko is obtained from measurements without added quencher, and n , ( x ) ~ and , k , are estimated via the parameters A2-A4 by fitting eq 2 to the experimental decay. From n and the total quencher concentration the concentration of droplets and thus their size are obtained. k, is the first-order rate constant for quenching in a cell with
434 Langmuir, Vol. 2, No. 4, 1986
one quencher. By multiplication with the molar volume of the droplet a second-order quenching constant k z is obtained, which may be compared to quenching constants in homogeneous solutions. For diffusion-controlled quenching, information about the mobility or viscosity in the droplet is obtained. Information about the exchange processes is contained in ( x ) ~ .It was shown in I that different exchange models predict different dependencies of ( x ) , / non n, or experimentally on the quencher concentration, an effect that in principle can be used for discrimination between different mechanisms. When the exchange mechanism is known the estimates of ( x ) , and n can be translated to values of the pertinent rate parameters by using numerical results presented in I or calculated as described there. Here we will consider only exchange by fusion-fiasion, characterized by a fist-order rate constant kt,which is the reciprocal of the average lifetime of a droplet between two fusion-fission processes. The distribution of quenchers over all micelles, M,, is assumed Poissonian a t all times. The fusion-fission process can be designated By +M m Bx + M m + y - x
-
Almgren et al. Table I. Estimated Parameter Values and Goodness-of-Fit Measures from a Global Analysis of Several Decay Curves for Different Quencher Concentrations for Each of Two Compositions quencher concn x io3, mol-' kg-' n W ) 8 x,20 Zb T/TX = 60140;water = 7.5% w/wc 0.661 0.368 f 0.011 0.013 f 0.003 1.14 0.56 1.257 0.676 f 0.013 0.019 f 0.004 1.09 1.61 1.895 0.982 f 0.013 0.026 f 0.004 1.12 1.14 2.458 1.255 f 0.015 0.024 f 0.005 0.87 0.29 3.086 1.546 f 0.017 0.025 f 0.006 1.08 0.53 3.680 1.720 f 0.019 0.022 f 0.007 1.15 0.17
T/TX = 40160;water = 17.9% w/wd 0.693 1.991 2.743 3.332 4.004
0.227 f 0.018 0.730 f 0.022 1.082 f 0.022 1.381 f 0.025 1.682 f 0.026
0.092 f 0.015 0.337 f 0.022 0.457 & 0.026 0.566 f 0.035 0.660 f 0.040
1.27 1.17 0.93 1.04 1.00
0.60 0.15 0.53 0.56 0.03
Reduced X-square test. *Runs test. Probe concentration 5 X M. T~ = (580f 9) ns (x: = 1.20;z = 0.25);k, = (5.98f 0.25) X lo6 s-l. dProbe concentration 5 X M. T,, = (578 f 9) ns. (x? = 1.00;z = 1.69);k, = (1.756k 0.044)X lo6 s-l. a
The conditional probability w ( x ; y )of a transformation giving B, from By is given by
Toluene
m)=O
where P(m;n)is the Poissonian distribution with average
n. The transition matrix w(x;y)can be evaluated numerically for a given n. As discussed in I, the steady-state distribution can then be calculated by an iterative method and thus also ( x ) , , for different values of the ratio of the rate parameters k,/k,, allowing the determination of k t / k , from estimated ( x ) , / n .
Materials and Methods Triton X-100 (Merck, 99.5%) is a polydisperse preparation of [p-(1,1,3,3-tetramethylbutyl)phenyl]poly(oxyethylene) with an average of 9.5 oxyethylene units per molecule. R ~ ( b p y ) , ~ in + , the form of ruthenium 2,2'-dipyridyl perchlorate from G. Fredric Smith, and MV2+, methylviologen (Serva Chemicals), were used as supplied. Other chemicals were of highest quality. For the conductiometric measurements in the Lzphase the solutions were made up using 0.200 M NaCl instead of water. A Philips conductivity meter Model PW 9505 was used at lo00 Hz with platinized electrodes a t 24.0 "C. Self-diffusion coefficients were determined at 25.0 OC by the Fourier transform 'H NMR pulsed-gradient spin echo method of Stilbs.', The fluorescence decay measurements were performed in a time-correlated photon counting instrument which has been described e1~ewhere.l~For each composition in the L2 phase, as marked out in Figure 4,measurements were made both without quencher and at five quencher concentrations, at a constant probe concentration (about 5 X 10" M, yielding an absorbance of about 0.7 cm-l at 452 nm). The data from the six runs were analyzed together in a global analysis15 (no deconvolution was necessary due (13)Stilbs, P.; Moeeley, M. E. Chem. Scr. 1980,15,176;1980,15,215. Stilbs, P. J. Colloid Interface Sci. 1982.87.385. (14) Lbfroth, J.-E. Ph.D. Thesis, The University of Gbteborg, Gateborg, Sweden, 1982. (15)(a) Lbfroth, J.-E. Eur. Biophys. J. 1986,13,45; J. Phys. Chem. 1986,90,1160; Anal. Instrum. (N.Y.),in press. (b) Knutson, J. R.; Beechem, J.; Brand, L. Chem. Phys. Lett. 1983,102,501.
Water
E
Triton X-100
Figure 1. Phase boundaries in the system Triton X-100toluene-water at 24 O C . L1and L2are connected isotropic phases and D is a lamellar and E a hexagonal liquid crystalline phase. Conductivity measurements (with NaCl added) and NMR selfdiffusion determinations were performed at compositions indicated. to the long lifetime of the excited probe) using a model defined by eq 1 and 3-5. The value of ko was fixed as obtained in the measurement without quencher, and k, was required to have the same value at all concentrations of quencher. Values of ( x ) , and n were obtained for each quencher concentration. The fit to the model was excellent in all cases, as exemplified by the data given in Table I, which refer to the same compositions as the decay curves shown in Figure 3.
Results and Discussion Phase Diagram. The boundaries of the various onephase areas are shown in Figure 1. An unusual feature is the narrow channel connecting L1 and L2 between the lamellar and hexagonal liquid-crystalline phases (the assignments were made by inference from similar systemd6). Samples with composition in this region were isotropic and very viscous liquids. Added sodium chloride gives a high (16)Friberg, S.;Mandell, L.; Fontell, K. Acta Chem. Scand. 1969,23, 1055.
Langmuir, Vol. 2, No. 4, 1986 435
Structure and Transport in Triton X-100-Toluene- Water Table 11. Self-Diffusion Coefficients in the L2Phase
at 24 OC
samde n0.O 7 81 9 10 101 102 103 12 121 13 14 15 16
neat
self-diffusion coefficients x~O'O, m2 s-l water toluene TX 1.23 2.00 1.37 0.93 1.04 1.15 1.02 1.15 1.11 1.10 1.35 22.7
0.96 1.39 2.50 5.42 5.19 5.10 4.93 7.38 8.76 11.6 11.2 11.6 13.1 24.0
0.060 0.20 0.20 0.34 0.36 0.38 0.61 0.50 0.66 0.83 0.96 1.29 1.73
IO-^ -
0 5
6
0'9
(17)Shinoda, K.; Ogawa, T. J. Colloid Interface Sci. 1967, 24, 56. (18) Lindman, B.; Stilbs, P. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York and London, 1984; Vol. 3, p 1650.
::
8
IO-^ -
(2
0
13
0
14
0
aThe compositions are marked out in Figure 1. conductivity indicating a water-continuous system. The L2 phase is split into two regions by a two-phase area; a similar behavior was found by Shinoda et al." for other nonionic surfactant-oil-water systems. At low water content there is probably a molecular solution of hydrated Triton X-100 (TX) and toluene (T). At more than 1.8 H20/TX a new phase appears, in which the additional water is encapsulated in reversed micelles; this phase is formed only if the Triton concentration exceeds 16% w/w in toluene. A further suggestive feature is that several border lines meet in an intersection close to the point T X / T = 16/84 on the Triton-toluene axis. This fact suggests that the reversed micellar phase comprises TXwater aggregates with between 6.6 and 23.1 water molecules per surfactant, embedded in an intermicellar solution consisting of essentially toluene and Triton X-100 in an 84/16 w/w ratio. The intermicellar solution contains also some hydration water. Although these statements about the segregation in the reversed micellar phase are rather speculative, the calculations of micellar sizes from the fluorescence quenching data were based on an intermicellar solution having this composition. Transport Properties: NMR Self-Diffusion Data and Conductivity of Added Salt. The measured selfdiffusion coefficients for toluene, water, and Triton X-100 in the L2phase are collected in Table I1 for compositions shown in Figure 1. The self-diffusion coefficients of both toluene and Triton X-100 decrease smoothly with decreasing toluene content, whereas the water self-diffusion coefficient varies comparatively little and is indeed larger than that of toluene at the lowest toluene concentration. This is certainly not what one would expect for a system with closed water droplets. The values in the toluene-rich part of the L2 area are in better accord with such a supposition: water and Triton X-100 diffuse about equally slow, and toluene retains a self-diffusion coefficient at about half of the value in neat toluene. The water self-diffusion coefficient is still rather large as compared to the values reported for the AOT-oil-water system, where the existence of closed water droplets is well established. Lindman and Stilbs18obtained values for the water self-diffusion coefficient in this system which were always below 10-lo m2 s-l. Another comparison can be made with the nonionic surfactant microemulsions studied
10
OOO 0
L
,
L".
,
,
.
I
,
-
YOyy To1
15 16
6b
4~
Figure 2. Equivalent conductanceof NaCl in solutionswith NaCl added to a molar concentration c. Compositions indicated in Figure 1. by Nilsson and Lindman.lg The surfactant was tetraethylene glycol dodecyl ether and the oil either hexadecane or decane. The phase diagrams are very different from that of the present system and contain both surfactant and L2phases. The authors argue against the presence of closed water droplets in most of these systems-they expect them to be present only close to the oil corner in the decane system where the diffusion of water is slowed down by a factor of 10-100 compared to neat water. By this criterion the present system appears as a borderline case, with a self-diffusion coefficient of water that remains about 20 times smaller than that of neat water over most of the L2 area, although both the surfactant and the oil diffusion are strongly retarded toward the surfactant corner. Also the conductivity, shown in Figure 2 as a logarithmic plot of the equivalent conductance of NaC1, behaves similar to what is often reported for microemulsions. The values decrease monotonically from the water-rich L1 phase via the channel and the surfactant-rich part of the L2 phase to the very low values in the toluene-rich part. The conductivity changes by more than a factor 100 in the L2 phase. The change is smooth and does not suggest the existence of a percolation threshold at any particular composition,unless it occurs already between composition 15 and 14; it is also in this region that the self-diffusion of water becomes faster than that of the surfactant. In this region, however, the fluorescence quenching results give clear evidence for the presence of closed droplets with relatively slow exchange. This will be discussed next. Fluorescence Quenching Results. The decay curves in Figure 3a are typical for a case where a fraction of the excited probes is rapidly quenched by quenchers that were present in the vicinity of the probes at the moment of excitation, whereas the rest of the excited probes are almost inaccesible for quenchers. This provides a clear indication that the ionic probes and quenchers are confined in closed water droplets at this composition, which is rich in toluene. The decay rate increases slightly with the quencher concentration in the exponential tail, showing that some exchange of probes and quenchers occurs. If (19) Nilsson, P.-G.;Lindman, B.
J.Phys. Chem. 1982, 86,271.
436 Langmuir, Vol. 2, No. 4, 1986
Almgren et al.
Table 111. Results from Fluorescenoe Quenching Studies at Several Compositions" composition
droplet concn, mmol/ kg 7.20 3.92 2.58 3.52 4.37 1.68 2.00 2.44 2.64 3.17 1.16 1.48 1.93 0.73
HzO,
T/TX
% w/w
20180 3oj70 40160 40160 40160 50150 50150 50150 50150 50150 60140 60140 60140 70130
15.00 17.70 17.90 14.44 11.40 18.40 15.90 13.20 12.60 9.50 13.90 12.00 7.53 8.50
NH~O
NTX
1155 2504 3854 2276 1448 6084 4412 3000 2648 1660 6670 4490 2165 6500
144 216 267 203 170 315 272 230 214 184 340 271 218 336
(x),/n atn=1 0.39 0.47 0.42 0.195 0.125 0.34 0.28 0.123 0.080 0.048 0.190 0.125 0.026 0.18
k,
X
lo4,
5-l
R,8,
3.26 2.01 1.76 3.48 5.09 1.93 2.14 3.34 3.90 5.55 2.05 2.70 5.98 2.06
30.4 36.2 40.1 35.4 32.3 44.3 41.0 37.6 36.5 33.4 45.6 41.1 35.7 45.3
4?rR2/N~x,bAz 80.8 76.6 75.7 77.4 77.4 78.4 77.8 77.3 78.0 76.1 76.8 78.4 73.4 76.3
"The droplet concentration, @),In, and k , were obtained primarily; other quantities calculated as described in the text. *Mean value: (77.2 f 1.6) A2. 0
1000
2000
I
I
1
nano conds a
:ounts
channel no 0
1 counts
256
1000
2000
1
I
nanoseconds b
Figure 3. Fluorescence decay curves for ruthenium bipyridyl ions with methylviologen as quencher in solutions with compositions belonging to the L, phase. (a) Weight ratio Triton X-100 to toluene is 40160; the weight fraction water is 0.075. Quencher concentrations: 0, 1.895 X 3.680 X mol kg-'. (b) Weight ratio T X t o toluene is 60140; water wei h t fraction is 0.179. Quencher concentrations: 0, 4.004 X 10- mol kg-'.
P
one moves further down in the L2area, and in particular toward higher water content, the initial phase of the decay curves is gradually suppressed. The decay has still a well-defined final exponential part but with a strongly increased decay rate, as exemplified in Figure 3b. This is in accord with the idea that the droplets are preserved and the rate of exchange between them increases. We can not rule out, however, that the droplets simultaneously become more polydisperse, both in size and shape. It is then reasonable to consider the following question: Is it possible that the droplets have grown so much that we are in effect dealing with a bicontinuous system? The exponential tail of the decay curves should then be due to normal second-order quenching in these large aqueous pools. The volume of the aqueous part of the system may be estimated as the sum of the volumes of water and the poly(oxyethy1ene) tails of the surfactant. The concentration of quencher in the aqueous part is then obtained, and from the measured decay constants we may estimate the second-order quenching constant. For the compositions with most water and weight ratios of Triton X-100 to toluene of T X / T = 40/60 or more (see Figure 4) the values obtained were between 0.8 X lo8 and 1.4 X 10 M-' s-l. This is only about a factor of 5 less than the value in water with 0.10 M salt, which indicates that the probe and quencher are very mobile in this environment. Since the self-diffusion coefficient of water is reduced by a factor of about 20, and the conductivity of NaCl even more, these systems may not be truly bicontinuous. Let us now turn to the results obtained by an analysis based on the generalized Infelta model, assuming a narrow size distribution and exchange by the fusion-fission process. The results presented in Table IV refer to the rather sparse grid of compositions indicated by crosses in Figure 4. The number of water molecules per droplet varies over the Lz area roughly as shown by the lines in Figure 4. The number of TX molecules per droplet, the hydrophilic radius, and the area available per surfactant molecule at the surface of the assumed spherical drop of water and poly(oxyethylene) groups are presented in Table 111, all calculated under the assumption that the intermicellar solution contains TX and toluene in a 16/84 w/w ratio. The remarkable constancy of the surface area for the surfactant molecules, 77.2 f 1.6 A2, without any noticeable trend, supports this assumption, although the value is much larger than what is usually expected for a surfactant in a reversed micelle. We have no explanation for this; Triton X-100 is an unusual surfactant, however, with a short,
Structure and Transport in Triton X-100-Toluene- Water Toluene
E
Water
Triton X-100
Figure 4. Number of water molecules per droplet is constant at the value indicated along the lines in the L2area. The compositions used in the fluorescence quenching study are also in-
dicated.
5-
L321-
10
20
30
LO
50
60
Toluene % w/w
Figure 5. The second-orderfluorescence quenching rate constant k2 (0) and the fusion-fission frequency kt (X) in the L2area at compositionsshown in Figure 4. The dashed lines connect values for compositions with constant T/TX ratio.
branched hydrocarbon chain connected via the phenyl group to a rather long poly(oxyethy1ene) chain, and the solvent, toluene, could act as a cosurfactant at the interface. Figure 5 shows the variation of the micelle fusion frequency k, and the quenching rate inside the droplets, as given by the second-order rate constant k2 = k V-, where Vdm? is the molar volume of the hydrophilic droplet. The lifetime of the reversed micelle is rather short, decreasing from about 1.5 X lo* s a t the highest to 0.15 X lo4 s at the lowest toluene concentration. There is also a clear decrease in lifetime with increasing water content at a fixed T X / T ratio. Also the quenching rate increasing regularly with the toluene concentration and decreases with increasing water content at fixed TX/T. The total variation is smaller for k2 than for k,-it is within a factor of 3-but difficult to rationalize. The variation correlates neither with the composition nor with the size of the droplet; it may be an artifact. The value of the second-order rate constant is otherwise as expected for this couple enclosed in small aqueous confinements. The values are slightly smaller than what is typically found in water-the strong ionic strength dependence allows only rough comparisons. A value of 5 X 108 M-' s-l for a 0.1 M salt solution was found.
Langmuir, Vol. 2, No. 4 , 1986 437
The possibility of confinement size polydispersity requires further study. When there is no exchange, polydispersity would result in an apparent decrease of the fluorescence quenching aggregation number with the quencher c~ncentration.~ In the present study the noexchange situation was best approached at low water and high toluene concentration. Results presented in I show the variation with the quencher concentration, Q, of the estimated value of n, divided by the quencher concentration (this parameter is proportional to the micelle aggregation number for a given composition). These results give no evidence for a polydispersity; in general an increase in n / Q was observed, which in itself is a disturbing irregularity. Self-Diffusionand Fusion-Fission. The fluorescence quenching study indicates that the probes and quenchers are confined together, with a rather rapid exchange between the confinements. It turns out, however, that the observed exchange of the probes and quenchers between the reversed micelles, interpreted as due to a fusion-fission process, cannot quantitatively explain the rapid self-diffusion of water in the TX- and water-rich part of the Lz area. Let us consider the composition T/TX = 30/70 and 17.7% water. The average distance between the center of two droplets with R = 36.2 A, assumed packed in a fcc lattice, would be about d = 86.4 A, leaving a hydrophobic layer of a t least 14 A between the drops. At such close spacing there would be some packing problems that make the assumption of spherical shape questionable, in particular as the drops are mobile; let us disregard these problems. A given water molecule would transfer from one drop to another with the frequency k,/2, in this case 2 X lo6 8. A random walk contribution to the diffusion coefficientz0is given by ktd2/12 = 1.2 X mz s-', which is an order of magnitude smaller than the measured coefficient. The water molecules can dissolve in the hydrophobic portion to some extent and then pass over by diffusion. The rate of this process can be overestimated by regarding the aqueous sphere as surrounded by a uniform, 14-A-thick layer in which the diffusion coefficient is assumed to be 2X m2 s-l, and the solubility is the same as in the intermicellar solution of composition TX/T = 16/84, Le., about 0.37 M. The permeabilityP," would then be roughly 1X m s-', and the escape frequency of a water molecule 3Pf/R = 8 X 106 s-', which added to the fusion-fEsion process would give a random walk contribution to the diffusion coefficient of about 6 X 10-l' m2 s-l, still more than a factor of 2 less than the observed value. The diffusion of free water molecules and of micelles could hardly contribute more than 2 X lo-" m2 s-l. To remove this discrepancy a mechanism is required which gives rise to compartmentalization with exchange of probe and quencher and still allows the rather rapid water diffusion. One possibility would be that the different compartments are transiently connected by narrow holes or channels, which rarely are large enough to permit quencher or probe to transfer but allow a rapid exchange of the small water molecules. Other measurements are required to test this proposition. A clear result from this study, evident already from the fluorescence decay curves, is that the hydrophilic probes and quenchers are forced together in common confinements, as would be provided by reversed micelles. This (20) Almgren, M.; Stilbs, P.; Alshs,J.; L m ,P.; Kamenka, N. J. Phys. Chem. 1985,89,2666. (21) T h e permeability was estimated as the product of the distribution constant and the diffusion coefficient divided by the layer thickness.
438
Langmuir 1986,2,438-442
seems to occur in most of the Lzarea, down to a 20/80 T / T X ratio. It is also evident that a rapid exchange between the confinements occurs, more rapidly at high water and low toluene content. This is in qualitative accord with the self-diffusion and conductivity data, although the water molecules move faster than the bulkier solutes. The numerical values estimated for various parameters have to be regarded with caution for several reasons: the exchange mechanism is unknown, there may be an important po-
lydispersity effect, and some irregularities were observed in the n / Q and k2 values. Acknowledgment. We are indepted to Professor Peter Stilbs and Krzysztof Rapacki, FK, for the NMR experiments. This work was supported by the Swedish Natural Science Research Council. Registry No. Triton X-100,9002-93-1;toluene, 1088&3;water, 7732-18-5.
Pyrene Excimer Formation in Micelles of Nonionic Detergents and of Water-Soluble Polymers Nicholas J. Turro* and Ping-Lin Kuo Department of Chemistry, Columbia University, New York, New York 10027 Received December 3, 1985. In Final Form: February 26, 1986
Pyrene excimer formation has been investigated in two nonionic micellar systems, one composed of water-soluble copolymers of poly(ethy1ene oxidepropylene oxide) (E/P 0.8) and one composed of surfactants of the Triton type (alkylphenolethoxylate). The ratio of excimer emission intensity to monomer emission intensity (Ie/Im)is employed to investigate the variation of certain micellar properties as a function of surfactant structure, temperature, pressure, and added electrolyte. It is proposed that variations of Ie/Zm serve as a qualitative monitor of variation in micellar propertes with experimental variables. Analysis of quenching of pyrene fluorescence by nitrite ion in Triton micelles shows it to be consistent with occupancy of pyrene in the hydrophilic ethylene oxide outer core and penetration of the nitrite ion into the core. Introduction Pyrene excimer formation is a well-known concentration-dependent phenomenon in organic solutions.' However, because of the low solubility of pyrene in water (ca. M), pyrene excimer formation is not observed in pure water solution. Solubilization of pyrene by micelles formed from ionic surfactants allows observation of pyrene excimer formation in aqueous solutions under proper c0nditions.l This measurement of pyrene excimer emission as a function of surfactant concentration allows the determination of aggregation numbers of micelle formed from nonionic and ionic surfactants.2 We report the results of investigations of pyrene excimer formation in aqueous solutions of nonionic surfactants of the Triton type and of watersoluble polymers of the ethylene oxide/propylene oxide types. Experimental Section Materials. Pyrene (P, Aldrich Chemical Co.) was purified by recrystallization (3X) from ethanol. The synthesis of 1,3-di-anaphthylpropane (DNP) is described in the literat~re.~ Sodium dodecylsulfate (SDS, Biorad. Lab.), the polyethylene glycol nnonylphenyl ethers (CahEO,, n = 5,10,15,18,20,respectively, Tokyo Kasei Co.), the poly(ethy1ene oxide-propylene oxide) block copolymer with an ethylene oxide/propylene oxide ratio of 0.8 (E/P 0.8, MW 2917, Polysciences), and sodium chloride (Alfa) were used as received. Sodium nitrite (Mallinckrodt, Chem. Works) was purified by recrystallization (2X) from water. (1) (a) Dorrance, R. C.; Hunter, T. F. J. Chem. SOC., Faraday Trans. 1974,70,1572. (b) Dorrance, R. C.; Hunter, T. F. J. Chem. SOC., Faraday Tram. 1974,68,1312. (c) Craig, B. B.; Kirk, K.; Rodgers, M. A. Chem. Phys. Lett. 1977, 49, 437. (d) Infelta, P. P.; GrBtzel, M. J. Chem. Phys. 1979, 70, 179. (2) (a) Atik, S.; Nam,M.; Singer, L. Chem. Phys. Lett. 1979,6775. (b) Lianos, P.; Lang, J.; Strazielle,C.; Zana, R. J. Phys. Chem. 1982,86,1019. (c) Levitz, P.; Van Damme, H.; Keravis, D. J.Phys. Chem. 1984,88,2228. (3) Chandross, E. A.; Dempster, C. J. J. Am. Chem. SOC.1970,92,3586.
Fluorescence Measurements. Fluorescence spectra were acquired on either a Perkin-Elmer MPF-3L or a SLM 8000 spectrometer. Fluorescence lifetimes were acquired on a PRA single photon counting apparatus! Quenching parameters were determined by Stern-Volmer analysis of fluorescence lifetimes as a function of quencher concentration. All samples were deaerated by nitrogen purging. The ratio of pyrene excimer emission (Ze, ca. 470 nm) to pyrene monomer emission (Zm, ca. 375 nm) is defied as &/Im. The emissions were obtained by excitation at 332 nm. The ratio of naphthyl excimer emission (reN,ca. 397 nm) to naphthyl monomer emission (I", ca. 337 nm) is defined as IeN/ImN.The emissions were excited at 290 nm. The conversion of ZeN/ImN into microviscosities,q, of colloidal aggregates follows a literature method.& The ratio of pyrene monomer fluorescence intensity at 373 nm (Il) to that at 383 nm (ZJ is defined as Il/Z3 (intensities of the first and third vibrational bands of pyrene monomer fluorescence) and follows a literature method, ref 5b, to estimate micropolarity CM of colloidal aggregates. A stainless steel high pressure cell (Union Giken Engineering Co.) was employed for the high pressure studies? The microviscosity, 9, of collidal aggregates under pressure was estimated by fluorescence polarization methods.'~~The polarization of pyrene monomer was acquired on a SLM 8OOO spectrophotometeremploying a two polarizer system. Results Although pyrene excimer is not observed for saturated aqueous solutions of pyrene (ca. M) in water, aqueous solutions containing pyrene (loT4M) and micelles of sodium dodecyl sulfate (SDS), C9PhElo,or E / P (0.8) show (4) (a) Turro, N. J.; Liu, K. C.; Chow, M.-F.; Lee, P.; Photochem. Photobiol. 1978,27,52. (b) Turro, N. J.; Aikawa, M. J. Am. Chem. SOC. 1980,102 4866. ( 5 ) (a) Avouris, P.; Kordas, J.; El-Bayoumi, M. A. Chem. Phys. Lett. 1974, 26, 373. (b) Glushko, V.; Thaler, M. S. K.; Karp, C. D. Arch. Biochem. Biophys. 1981,210, 33. (6) Turro, N. J.; Okubo, T. J. Am. Chem. SOC.1981,103,7224. (7) (a) Thomas, J. K.; GrBtzel, M.; J. Am. Chem. SOC.1973,95,21. (b) Aoudia, M.; Rodgers, M. A. J. J. Am. Chem. SOC.1979, 101, 6777. (8) Perrin, F. J. Phys. Radium 1936, 7, 1 .
0 1986 American Chemical Society
