Langmuir 1995,11, 2405-2409
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Percolation of Water-in-OilMicroemulsions Studied by the Iodine Laser Temperature Jump C. Petit,+*$ J. F. Holzwarth,§ and M. P. Pileni*ltt$ Laboratoire de Structure et Rdactivite des Systbmes Interfaciaux, URA CNRS 1662, Universitd P. et M. Curie, 75231 Paris Cedex 05, France, CEA, DRECAM Service de Chimie Moldculaire, CE Saclay 91 191 Gif sur Yvette, Cedex, France, and Fritz Haber Institut der Max Planck Gesellschaft, Faradayweg 4-6, 0-14195 Berlin-Dahlem, Germany Received September 7, 1994. In Final Form: March 21, 1995@ The iodine laser temperature jump (ILTJ) is used to perturb an AOT water-in-oil microemulsion. From the dynamic response to such a perturbation, it is possible to calculate the bending elastic modulus of the AOT interfacial monolayer. At large water content, this value is close to 0.4 kT, which is consistent with the value obtained by other techniques. It is shown that increasing the droplet concentration at constant micellar radius induces a slight increase of the interfacial fluidity. From the amplitude of the response to the thermal perturbation, the percolation process of a reverse micellar solution can be studied with great accuracy.
Introduction Water-in-oil droplets are formed by dissolving surfactant in organic solvents. A widely used surfactant is AOT, because it can solubilize large amounts of water in alkanes, like isooctane, without adding cosurfactant. Water is readily solubilized in the polar core forming the so-called "water-pool".' These reversed micellar systems have attracted considerable attention recently owing to their ability to host various hydrophilic components in organic solvent.2 AOT reverse micelles are spherical droplets whose radius varies linearly with the water content, W = [H20l/[AOTl. Furthermore, due to Brownian motion, there are numerous collisions between droplets, which in some cases allow the exchange of materials solubilized in the water pools. Thus, AOT reverse micelles can be considered as a liquid chemical m i c r ~ r e a c t o r .However, ~ most of the physical work performed on this system provides a static view of the systems. Only few experiments deal with the dynamical properties of AOT microe m u l ~ i o nin ; ~particular, the interfacial dynamics in the percolation region are not well understood. In this paper, we study how the interfacial dynamics of AOT reverse micelles depend on the water content and the polar volume fraction (i.e., concentration of droplets). As an experimental tool, the iodine laser temperature jump (ILTJ) method5 is used. This is a convenient thermal relaxation technique to study the interfacial behavior of AOT reversed micelles after a rapid and small increase of their temperature.6 The detection system allows the + Universite
P. et M. Curie.
1: DRECAM Service de Chimie Moleculaire. 9 Fritz Haber Institut der Max Planck Gesellschaft.
Abstract published in Advance A C S Abstracts, June 15,1995. (1)Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdan, 1989. (2)Luisi, P. L.;Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochem. Biophys. Acta 1988,947,209. (3) Pileni, M.P. J. Phys. Chem. 1993,97,6961. (4)(a) For review see: Kellay, H.; Binks, B. P.; Hendrickx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994,49,85. (b) Binks, Langevin, D. Lanmuir 1989,5,415. (c) B. P.; Meunier, J.;Abillon, 0.; Guering, P.; Cazabat, A. M. J . Phys. (Fr.)LETTRES 1983,44,L-601. (d) De Gennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86, 2294. ( 5 ) (a) Holzwarth, J. F.; Meyer, F.; Pickard, M.; Dunford, H. B. Biochemistry 1988,27,6628.(b) Fletcher,P. D. I.; Holzwarth, J. F. J . Phys. Chem. 1991,95,2550.(c) Holzwarth,J.F. In The Enzyme Catalysis Process; Cooper, A,, Houben, J. L., Chien, L. C., Eds.; Plenum: New York, 1989;p 383. (6)Alexandridis,P.; Holzwarth, J. F.; Hatton, A. T. Langmuir 1993, 9,2045. @
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observation of the whole relaxation process, between lop6 and 1 s in a single measurement. In the case of AOT reverse micelles, this permits a study of the intra- and intermicellar behavior approaching the percolation threshold.
Experimental Procedure Materials. AOT (sodium di(ethylhexy1)sulfosuccinate) and isooctane were purchased, at the highest purity available, from Fluka and Sigma and were used without further purification. The composition of reversed micellar solutions is expressed in the term of W, the ratio of water-to-surfactant molar concentrations. All the samples were prepared in isooctane with triplydistilled water. Conductivity Experiments. The conductivity measurements were made with a Tacussel CDSlO conductivity meter using a GK2401C cell placed in the sample being studied. The temperature is controlled by a thermostatic bath with an accuracy of f0.2". Iodine Laser TemperatureJump Experiments. (Seerefs 5 and 6 for a detailed description.) The ILTJ technique uses the photon emission of an iodine laser in the near-IR (1315 nm) to create avery fast temperature rise in the water pool. This occurs by photon absorption of overtone vibrations of the OH bonds of water molecules. A very homogeneous temperature jump in layers up to 3 mm can be achieved. The typical temperature rise time in our experiments was 1 p s . The longest time observable is limited by the back-coolingofthe sample to its thermostatically controlled originaltemperature (5-10 s). AKrypton Laser tuned at 406.7 nm was used to detected the effects of the temperature perturbation on the sample scattering intensity. The signal was registered with a Tektronix 7904 oscilloscope and followed by two Tektronix 390 AD transient digitizers and stored on a computer. Further data processing involved sampling and averagingof the signal as well as calculating the relaxation time by appropriate fitting of the signals to relaxation equations. In a typical experiment, an iodine laser pulse, carrying an energy of approximately 1 J in 1p s , was focused into a 10-mm (width) x 3.5" (height) area in the microemulsion placed in a 5- x 10-mm quartz cuvette. The cuvette was located in a temperature-controlled holder of the ILTJ detection chamber and was allowed to equilibrate to the desired temperature. Recording the sample scattering intensity at 90" started 5 p s before the Tjump and covered the time range up t o 100ms. Five relaxation signals were averaged for each experiment. To avoid artifacts due to the very low intensity of the signal, each experiment was repeated at least twice.
Results and Discussion The amplitude and sign of the scatter, A(OD), and the relaxation time, z, of the microemulsion system in response
0 1995 American Chemical Society
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2406 Langmuir, Vol. 11, No. 7, 1995 W = 40, [AOTI = 0.1 M,T = 22 C
: v 600
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Figure 1. Typical relaxation trace obtained after the iodine laser temperature jump ofAOT reversed micelles. The change in the scatterings intensities is given in differential optical density. The bold line indicates the exponential curve fit resulting in the relaxation time. In this experiment, t = 2.5
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to the fast increase of its temperature (Figure 1) are obtained. The optical density variation observed a t 407 nm, due to the change in scattering intensity after the temperature jump, can be analyzed in term of relaxation amplitude and relaxation time:
It = (I, - I , ) P
+ I,
where It,IO,I,, and z are the scattered light intensities a t time t, 0, and m and the single exponential relaxation time, respectively. The variation of the optical densities is deduced from the light intensities ratio. Figure 1shows that the observed signal can be quite well fitted to this expression. No further evolution is observed for longer times up to 0.2 s. In AOT reverse micelles, the observed relaxation time has been interpreted as the surfactant monolayer rigidity of the AOT microemulsion:6 After the fast heating of the water pool (during the ILTJ pulse), the volume of the water pool increases (thermal expansion) and the surfactant interface is "pushed out", resulting in deformation of its equilibrium shape followed by relaxation of the spheroidal shape of the droplets toward their equilibrium spherical form. Hence, the observed changes are related to interfacial relaxation. This phenomenon is similar to the well-known Kerr effect, which gives directly the deviation of the droplets from the spherical shape.8 The classical theory assumes that the relaxation time is inversely proportional to the bending rigidity of the surfactant layer (i.e., bending modulus), K , (412) and proportional to the third power of the micellar radius, R. A complete expression adapted to the reverse micelles, which are a deformable polydisperse system, was proposed by Safran et al.1°and successfully used in the case of the Ken- effect experimentll or T-jump experiment on AOT reverse micelled
where z is the characteristic relaxation time, ,5 = l/kbT, E = qdqi, vi and qo are the viscosities of the fluid inside (water) and outside (solvent) the droplets, respectively, and (T = (R2)/(R)2 - 1, related to the polydispersity of AOT reverse micelles. The case of 1 = 0 corresponds to the (7)Towey, T.F.; Khan-Lodl, A.; Robinson, B. H. J. Chem. SOC., Faraday Trans 2 1990,86,3757. (8)Borkovec, M.; Eicke, H. F. Chem. Phys. Lett. 1988,147,195. (9)Hilficker, R.;Eicke, H. F.; Sager, W.; Steeb, C.; Hofmeier, U.; Gehrke, R. Ber. Bunsenges. Phys. Chem. 1990,94,677. (10)Milner, S.T.;Safran, S. A. Phys. Rev. A 1987,36, 4371.
Figure 2. Variation of the relaxation time at T = 22 "C with the water content of reverse micelles in isooctane. [AOT]= 0.1 M. polydispersity, the case o f i = 1to the displacement of the micelles. Hence, only the case of1 = 2, which corresponds to ellipsoidal shape fluctuation of the droplets, is taken into account to estimate the bending modulus (the others terms of the development, i> 2, have small amplitude). As mentioned in previous experiments on AOT reverse micelles, this gives a good estimation of the relaxation time obtained by ILTJ experiments on the microsecond scale.'j Figure 2 shows the evolution of the relaxation time with the water content (i.e., the micellar radius). No drastic change is observed by changing the water content. The relaxation time is small, about 2ps. There is probably a small increase oft at low water contents, but the accuracy of the measurements is too small to obtain quantitative information (2'-jump rise time of about 1ps). Considering the relaxation time at the plateau (W > 15))z = 2 ps, the Safran expressionlo as expressed above yields a value of the bending elastic modulus, K , close to 0.35 kT a t W = 15. This is in good agreement with the values reported from other techniques for AOT reverse micelles. By analysis of the size polydispersity,12 Kerr effect,l' timeresolved fluorescence,13 and neutron spin-echo,14 the bending modulus, K , is found equal to 0.4, 0.5, 1, and between 3 and 5 kT, respectively. The bending elastic modulus, K , slightly increases by increasing the water content (Table 1). From Kerr effect measurements, a similar increase in the bending elastic modulus, K , with the micellar radius has been observed.ll It must be noticed that in the determination of K from eq 1,the polydispersity of the microemulsion, (T, is an important parameter. A small change in its value could drastically change the estimated value of K . This parameters change with the size of the droplets; the greater are the reverse micelles, the higher the polydi~persity.~ Furthermore, a t low water contents, the water structure differs largely from bulk water and the relaxation process a t the molecular level is probably very different. These could explain the lower (11)Linden, E. v. d.; Bedeaux, D.; Hilfiker, R.; Eicke, H. F. Ber. Bunsenges. Phys. Chem. 1991,95,876. (12)Kotlarchik, M.; Stephens, R. B.; Huang, S. J. J.Phys. Chem. 1988,92,1533. (13)Almgreen, M.; Johannsson, R.; Eriksson, J. C. J.Phys. Chem. 1993,97,8590. (14)(a)Huang, J.S.;Milner, S. T.; Farago, B.; Richter, D. Phys. Rev. Lett. 1987,59,2600.(b) Farago, B.; Huang, J. S.; Richter, D.; Milner, S. T.; Safran, S. A. Prog. Colloid Polym. Sci. 1990,81,60. (c) Farago, B.; Richter, D.; Huang, J. S.; Milner, S. T.; Safran, S. A. Phys. Rev. Lett. 1990,65,3348.
Langmuir, Vol. 11, No. 7, 1995 2407
Percolation of Water-in-OilMicroemulsions Table 1. Change in the Micellar Radius, Relaxation Time, and Bending Elastic Modulus with the Water Content of AOT Reverse Micelles in Isooctane ([AOT] = 0.1 M, T = 22 "C) 5 10 15 20
a
3 f 0.5 2 f 0.5 2.5 f 0.5 2 f 0.5 2 f 0.5 2 f 0.5
18
25 33
40
40 70
50
85
0.35 (f0.02) 0.35 (f0.02) 0.35 (f0.02) 0.36 (f0.02) 0.45 (f0.05) 0.50 (f0.05)
Deduced from the linear relation R = 1.5W
+ 10 (see ref 21). 14 0
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Polar Volume Fraction, PHI (76)
Figure 4. Evolution of the relaxation time with the polar volume fraction of reversed micelles of AOT in isooctane. Table 2. Variation of K , the Bending Elastic Modulus, with the Polar Volume Fraction, (W= 40, T = 22°C. Solvent Isooctane) @, % 5, P K , kT 1.5 3
.,, 0
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Figure 3. Variation of the maximal signal amplitude signal amplitude with the water content. [AOT]= 0.1 M, T = 22 "C.
variation oft with the micellar radius, lower than expected from the classical model where, for a monodisperse hard sphere, tvaries with the third power of the micellar radius. The amplitude of the signal strongly increases with the water content and reaches a plateau a t W = 15 (Figure 3). As mentioned previously in similar systems6 because of the visual detection of the experiments, this strong increase of the signal occurring between W = 5 and W = 15 could reflect a change in the refractive index of the microemulsion with the water content a n d o r reflect the large changes in the water structure, since water, a t low water content, exists only a s hydration water.15 Contrary to other results published for this system, there was no change ofthe sign of the amplitude by increasing the water content a t 22 "C. This is probably due to a lower droplet concentration in our case (AOT = 0.1 M) which excluded important collective phenomena (aggregation of micelles) during the experiment. At W = 40 and [AOT] = 0.1 M, increasing the temperature from T = 22 "C to T = 30 "C induced a n increase of the relaxation time from 2 to 3 . 5 , ~ In ~ .SAXS, no change was observed in the size or in the polydispersity of the droplets by slightly increasing the temperature.16 So according to the model described above, the change in the relaxation time with temperature could only be due to a decrease of the bending elastic modulus, because all the other parameters are equal and the micellar concentration is always too low to authorize interdroplet phenomena a t the used scale time. The calculated values were 0.45 at 22" and 0.40 kT at 30 "C. This indicates a small decrease in the rigidity of the interfacial layer. This (15) (a)Motte, L. Thesis, Universite P. et M. Curie, France, 1994. (b) Onori, G.; Santucci, A. J. Phys. Chem. 1993,97,5430. (16) Huruguen, J. P.; Authier, M.; Greffe,J . L.; Pileni, M. P. J.Phys.: Condens. Matter 1991,3, 865.
7 11 15 26 35
2 2 2.5 3 3 4 4.5
0.45 0.45 0.43 0.41
0.41 0.39 0.37
is consistent with an increase in the interfacial fluidityg with temperature. Hence, the ILTJ experiment is a powerful technique to get information on the interface dynamics. ILTJ experiments on AOT-isooctane-water micellar solutions a t W = 40 and 22 "C showed a linear increase of the relaxation time with the polar volume fraction, (IJ (Figure 4). In conductivity and SAXS experiments, no percolation phenomena were observed16 below a polar volume fraction of 30%and no change in the droplet size and polydispersity could be detected.17 Therefore, a s previously, the increase in the relaxation time with the polar volume fraction can be attributed to a decrease in the bending elastic modulus, K. This means the higher the droplet concentration, the lower the bending elastic modulus (Table 2). In a n AOT/water/decaline microemulsion, using Kerr effect measurements, similar data have been observed and attributed to the aggregation process.'l Under our experimental conditions, this explanation cannot be taken into account. As a matter of fact, the increase in the polar volume fraction induces a decrease in the intermicellar potential17 and in the intermicellar exchange rate constant.18 So the decrease in the bending elastic modulus, K (the increase in the interfacial fluidity), with increasing polar volume fraction induces a decrease in the collision efficiency. Hence, the number of collisions increases because of the increase in the number of droplets but the efficiency in the intramicellar exchange process decreases. The collisions are more elastic. To reinforce this picture, it was also recently shown that increasing the polar volume fraction at constant Wvalue leads to a decrease of enzymatic activity of AOT-solubilized enzyme.l9 At T = 22 "C and W = 40, above a polar volume fraction of 3O%, a percolation phenomena appears. This is clearly (17) Pitr6, F.; Regnaut, C.; Pileni, M. P. Langmuir 1993,9,2855. (18) Jain, T. K.; Pileni, M. P. Unpublished results. (19) Michel, F.; Pileni, M. P. Langmuir 1994,10, 390.
2408 Langmuir, Vol. 11, No. 7, 1995
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Figure 5. (A, top) Variation ofthe conductivity with the volume fraction of water in AOT-isooctane-water solutions for different temperatures. (B, bottom) Variation of the maximum relaxation amplitude with the volume fraction ofwater in AOTisooctane-water solutions for different temperatures. The W value is constant and equal to 40.
shown by the divergence of the conductivity (Figure 5A) by increasing the polar volume fraction. The conductivity onset gives the percolation threshold. It is attributed to the formation of an infinite cluster of droplets allowing the Na+ ions to percolate through the system.16 It should be noted that in AOT reverse micelles, percolation is a dynamic process, where the reverse micelles are still spherical and keep the same radius before and after the percolation process.20 Percolation can also be induced by increasing the temperature. As shown on Figure 5A, raising the temperature decreases the percolation threshold. This is due to a n increase of the aggregation process resulting from an increase of the micellar interaction. Figure 6 shows that the ILTJ signal is drastically modified below and above the percolation threshold. The deviation of the optical density is negative below and positive above the percolation onset. Such changes can be explained a s follows: (i) Below the percolation threshold, the case is the same as exposed above; the relaxation is observed in a very short time, indicating intramicellar perturbation. The decrease in the optical density is due to a n increase in the interfacial fluidity, which induces changes in the solvent penetration. This induces a decrease in the refractive index.6 The relaxation time is related to the elasticity of the membrane of the reverse micelle, (20) Huruguen, J. P.; Zemb, T.; Pileni, M. P. Prog. Colloid Polym. Sci. 1992,89, 39.
(ii) Near and above the percolation threshold, the previous model is no more valuable. Here a small increase of temperature could induce the formation of larger aggregates of droplets which form in the course of the sticky collision between droplets.22 This induces an increase of the scattering intensity a t 407 nm. Hence, the optical deviation becomes positive, showing the presence of large aggregates of micelles. The relaxation process is now a n intermicellar process, related to the kinetics of aggregation of the reverse micelle near and above the percolation threshold; this phenomena is drastically different from the previous one, and eq 1is no more valuable: no direct information could be obtained on the fluidity of the interface. However, the change in the sign of the optical density deviation gives a clear indication of the percolation process (Figure 5B). Table 3 shows the difference in percolation threshold as determined by ILTJ and conductivity a t various temperatures. Due to the high sensitivity of the ILTJ technique for the formation of aggregates, the percolation threshold determined is smaller than that determined by conductivity experiments. This illustrates the large potential of this technique to get precise information on the percolation process in microemulsion systems. (21) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985,118, 414. (22) (a) Eicke, H. F.; Stepherd, J. C. W.; Steineman, A. J. Colloid Interface Sci. 1976,56,168. (b) Robinson, B.; Steytler, D. C.; Tack, R. D. J.Chem. SOC.,Faraday Trans. 1979,75,781. (c) Ober, R.; Taupin, C. J. Phys. Chem. 1980, 84, 2418.
Percolation of Water-in-OilMicroemulsions
Conclusion The iodine laser temperature jump experiment is a powerful technique to study the physical properties of water-in-oil microemulsions. It is possible to measure the bending elastic modulus of the monolayer of AOT forming the interface from a simple and fast experiment. The calculated value is close to 0.4 kT and slightly dependent on the water content. Increasing the droplet concentration induces a small but significant decrease of the bending elastic modulus. This can be explained in terms of a decrease in the collision efficiency due to an increase in the fluidity.
Langmuir, Vol. 11, No. 7, 1995 2409 It is also shown that ILTJ experiments allow us to study the percolation and the aggregation process in micellar solutions and to obtain a very precise value of the percolation threshold.
Acknowledgment. Financial assistance from the CNRS-MPG exchange program in the form of Travel Grants is gratefully acknowledged. J.F.H. thanks P. Alexandridis for helpful discussions about the bending modulus. LA940733V
