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Articles Micelle Shape Transformation in the Isotropic Phase of the Ammonium Perfluorooctanoate/Heavy Water Binary Mixture Gerson R. Ouriques,* Reneˆ B. Sander, and Vladimir Dmitriev† Departamento de Fı´sica, Universidade Federal de Santa CatarinasUFSC, Floriano´ polis, S.C., Brazil 88040-900 Received October 14, 1999. In Final Form: June 27, 2000 The size and shape of the aggregates in the homogeneous isotropic micellar phase of the binary system ammonium perfluorooctanoate/heavy water (APFO/D2O) have been extensively investigated by small- and wide-angle X-ray scattering, optical polarizing microscopy, self-diffusion NMR, and electrical conductivity techniques, as a function of temperature and surfactant concentration. The experimental results strongly suggest that there is a change in the structure of the micellar aggregates where the micelles undergo a shape transition, changing from spherocylinder (rod-shaped), at low surfactant concentration (WA < 26 wt % APFO), to disk-shaped micelles (oblate ellipsoids), at high surfactant concentration (WA > 28 wt % APFO).
1. Introduction One of the distinctive properties of lyotropic systems, contrasted to thermotropic ones, is that surfactant molecules in solution are dispersed units that can cluster together, forming nonrigid aggregates or micelles that remain thermodynamically stable, with properties distinct from those of monomeric solutions or molecular liquid crystals.1-6 Such volumetric and soft character of the building blocks of lyotropic systems provides the variety of the stable micellar forms (see, for example, ref 7) and presupposes the possibility for micelles of shape transformation, which either can induce a global symmetry change in a system or keep only local transformations when thermodynamic parameters vary. In the search for an example of such a possibility in real lyotropic systems, considerable efforts were focused on micellar nematic phases. Typically, a form transformation from spherocylinder to planar micelles was shown, by calculating the micellar elastic bending energy to be expected with an increase of the cosurfactant/surfactant molecular ratio in the nematic phase of the system potassium laurate/water/ decanol.8 The precise experimental study provides evidence for a structural rearrangement from disk-shaped micelles to elongated, flattened micelles in the nematic * To whom correspondence should be addressed. † On leave from the University of Rostov-on-Don, Russia. (1) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1982. (2) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley-Interscience: New York, 1980. (3) Myers, D. Y. Surfactants and Interfacial Phenomenon; VCH Publishers: New York, 1988. (4) Tadros, T. F. Surfactants; Academic Press: London, 1985. (5) Friberg, S. Lyotropic Liquid Crystals and The Structure of Biomembranes; Advances in Chemistry Series 152; American Chemical Society: Washington, DC, 1976. (6) Phase Transitions in Complex Fluids; Toledano, P., Figueiredo Neto, A. M., Eds.; World Scientific: Singapore, 1998. (7) Taddei, G. Colloid Polym. Sci. 1994, 272, 1300. (8) Amaral, L. Q.; Santin Filho, O.; Taddei, G.; Vila-Roman, N. Langmuir 1997, 13, 5016.
phase of the cesium pentadecafluorooctanoate/water system (see, for example, ref 9). In the isotropic phase, when the critical micelle concentration (cmc) is reached, the surfactant molecules aggregate to form spherical micelles. When the concentration of surfactant in water is increased, the micelles can grow, becoming anisometric and forming either rodor disk-shaped micelles. Generally, it is assumed that the structure of the building blocks in the isotropic phase is the same as that in the structured mesophases. However, measurements of geometrical shape are very difficult in the isotropic phase. It is clear that such shape transformations within the stability domain of the isotropic phase are of special interest. First, it is due to the clear great differences between intra- and interaggregate interactions. It is well-known that both shape transformation and orientational order parameters (OPs) transform as components of a traceless second-rank tensor; that is, they have identical symmetry properties. This must result in bilinear coupling between two OPs and, consequently, in OP-induced simultaneous symmetry changes. However, many systems provide examples where anisometric micelles are dispersed in a solvent without any long-range orientational order, forming the macroscopically isotropic state. One can conclude that the coupling between two identical symmetry mechanisms in such systems is negligible. Second, the transformational properties of the building blocks in the isotropic state clearly predetermine in an essential way the possible polymorphism of the corresponding complex fluids. In view of the importance of the shape behavior, in this work we have carried out experiments in the micellar isotropic phase and have shown a micelle shape transformation to take place in the stability region of this phase in the ammonium perfluorooctanoate (APFO)/D 2O binary (9) Holmes, M. C.; Leaver, M. S.; Smith, A. M. Langmuir 1995, 11, 356.
10.1021/la9913598 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000
Micelle Shape Transformation in the Isotropic Phase
Langmuir, Vol. 16, No. 21, 2000 7901 The electrical conductivity measurements in the frequency range 5 Hz to 13 MHz were made using a HP 4192 LF impedance analyzer and an immersion conductivity cell with 5 × 5 mm2 platinum electrodes coated with platinum black and an electrode separation of 5 mm, giving a cell constant of 0.800. This value was determined by measuring, in the cell, the resistance of a solution with potassium chloride at the concentration 102 mol/ m3. The temperature control was made by an electrical device which has high accuracy and stability and was previously calibrated by the manufacturer (Delristor, U.K.). The accuracy and stability errors in the temperature dependence experiment were (0.01 K and (0.003 K, respectively.
Figure 1. Partial phase diagram for the APFO/D2O system, taken from ref 10. Along the dashed curve, a two-phase region has so far not been detected.
system. The partial phase diagram of this system is shown in Figure 1. 2. Experimental Section The perfluorooctanoate acid ammonium salt (APFO) was purchased from Fluka, Buchs, Switzerland (purity better than 98%), and was purified by recrystallization twice from butanol. High purity (>99.9%) deuterium oxide (D2O) from Aldrich, Milwaukee, WI, was used. The test tubes with the mixtures were flame sealed to prevent evaporation and were left in a hot block at a controlled temperature in the isotropic solution phase to ensure homogenization (APFO surfactant and water mixtures are straightforward to homogenize in the isotropic phase). Samples with surfactant concentrations less than 40% (WA ) 0.40) by weight can be made isotropic at room temperature. In this case, the samples are easily homogenized by manual agitation and only a few minutes are needed for complete mixing. For samples with APFO contents greater than 40%, a combination of manual agitation, heating, and centrifuging was necessary to ensure good homogenization. Optical microscopy has been used to locate phase transition temperatures, identify, and characterize the mesophases through their textures and to determine the sign of the diamagnetic susceptibility anisotropy as well as the optical sign of the liquid crystalline phases. All the observations were carried out under a polarizing microscope in transmitted white light. X-ray diffraction experiments were carried out using a Philips generator with nickel-filtered Cu KR radiation of wavelength λ ) 1.54 Å and with flat photographic films used as detectors (Laue camera). The temperature was measured and regulated with an accuracy of (0.3 K. The alignment of the samples was provided by a small electromagnet (homemade) set at 100 V-2 A and with a pole diameter of 30 mm and a gap of 25 mm. The X-ray capillary (Lindeman, 0.5 mm i.d.) was vertically positioned into the heating block such that the magnetic field was perpendicular to both the long capillary axis and the X-ray beam. The peak to peak separations on the X-rays film were measured using a highprecision micro-photodensitometer L502-2 (U.K.) connected to a chart recorder set at 10 mV full scale deflection. All the samples in X-ray capillaries had their transitions checked with optical polarizing microscopy before and after each X-ray run. The pulsed field gradient spin-echo (PFGSE) experiment of 19F was carried out in the APFO/D O binary system using a 2 Bruker SXP spectrometer. Coupled to it, a Varian V3800, 115 in. electromagnet provides a magnetic field strength of 2.2 T when fully saturated. The system was previously calibrated with distilled water, at room temperature, by the method of variation of the field gradient amplitude, g. The experimental parameters used were δ ) 1 ms, γ ) 26.752, ∆ ) 10 ms, and T ) 10 ms, givem a gradient of -0.117 006. The amplitudes of the echos, with and without the pulse field gradient measured relative to the baseline intensity, were compared, and a plot of the natural logarithm of the ratio of these amplitudes against g2 yelds the gradient KDt, where K is a calibration constant determined from the calibration run with water. The temperature system gave a temperature stabilization of (0.05 K. The samples were sealed in flat-bottomed 10 and 5 mm (i.d.) sodaglass NMR tubes before insertion into the cell (samples in 0.5 mm X-ray capillaries were also used in NMR for consistency and comparison purposes with X-ray results).
3. Results and Discussions 3.1. X-ray Studies. The X-ray diffraction experiments were performed mainly in the micellar isotropic phase, I, on samples with surfactant concentrations ranging from 20 to 40 wt % APFO and temperatures from 290 to 340 K (on cooling) (Figure 1). Diffraction experiments were also performed in the micellar nematic and lamellar phases but only a few degrees below the transition line, from isotropic to nematic, N, or isotropic to lamellar, L, phases (for concentration checking). The diffraction patterns in the micellar isotropic phase were found to exhibit the same features as those previously reported in other micellar systems.11-13 The isotropic phase is easily identified by its characteristic pattern, generally constituted of concentric and diffuse rings, whose intensities are symmetrically distributed on the plane of the X-ray film, indicative of short-range positional and orientational disorder of the molecules. These diffuse rings mean that the aggregates undergo uncorrelated fluctuations of the orientational order and they are not due to a change in the micellar shape. By analogy with the results of refs 11-13, we could conclude that the basic structure units in the I, N, and L phases of the APFO/D2O binary system are composed of surfactant molecules that self-assemble, forming aggregates of not spherical but ellipsoidal shape. The micellar size was seen to change by changing the composition and/or temperature of the sample, and for the latter, there is no doubt of the micellar growth as T decreases. One of the reasons why micelles of spherical shape are ruled out from our consideration stemmed from the fact that the average micellar diameter, calculated with the position of the diffuse ring observed, for the hypothetical spherical aggregate is approximately two times higher than the calculated length of an APFO ion in its all trans conformation and, so, it is physically unrealistic. Measuring the diameter of the diffuse ring in the micellar isotropic phase allows the average aggregation number n to be defined as the number of surfactant molecules per micelle to be calculated. This can be done by dividing the average micellar volume Vm by the volume of the APFO ion. The geometrical parameters of the APFO molecule are collected in Table 1. The Vm values in the micellar isotropic phase were calculated using the equation
3 Vm ) x3ΦAd03 4
(1)
where ΦA is the volume fraction of amphiphile and d0 is the diameter of the diffuse ring.11 Equation 1 was deduced (10) Boden, N.; Clements, J.; Jolley, K. W.; Parker, D.; Smith, M. H. J. Chem. Phys. 1990, 93, 9096. (11) Boden, N.; Corne, S. A.; Holmes, M. C.; Jackson, P. H.; Parker, D.; Jolley, K. W. J. Phys. Paris 1986, 47, 2135. (12) Holmes, M. C.; Charvolin, J. J. Phys. Chem. 1984, 88, 810. (13) Holmes, M. C.; Charvolin, J.; Reynolds, D. J. Liquid Cryst. 1988, 3, 1147.
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Table 1. Geometric Parameters of the Molecule, Ion, and Fluorocarbon Chain of the APFO Used in the Calculations of the Average Aggregation Number, n, and the Micellar Diffusional Coefficient, Dt
APFO PFOfluorocarbon chain
length (Å)
Chain’s cross section (Å2)
volume (Å3)
13.70 12.30 11.00
28.70 28.70 28.70
376.00 352.40 315.20
Figure 3. Aggregation number versus composition in the homogeneous micellar isotropic phase for five different temperatures: 1, 293 K; 2, 303 K; 3, 313 K; 4, 323 K; 5, 338 K.
Figure 2. Temperature dependence of the aggregation number in the homogeneous micellar isotropic phase for five different concentrations (by weight fraction) of APFO in heavy water: 1, 0.21; 2, 0.25; 3, 0.29; 4, 0.35; 5, 0.39.
assuming that the reflections were scattered from the (111) plane of a fcc lattice with separation d0. This assumption must be treated carefully because the fundamental characteristic of an isotropic phase is the absence of any structure in the phase. However, the model, based on the consistency of the experimental results observed using different techniques in other micellar liquid crystals,2,14-16 seems to work satisfactorily. The average aggregation number, n, in the APFO/D2O system was found to be of the minimum of ∼80. This number is too large compared to the same n0 ) 18 calculated for a spherical micelle composed of APFO ions, considering its geometric packing constraints. Thus, again, it seems plausible to expect the micelles in the APFO/D2O mixture to have a nonspherical shape similar to that of the aggregates observed in an early work by Hendrikx and Charvolin17 in the sodium decyl sulfate/1-decanol/ water system. Figure 2 shows the variation of the average aggregation number, n, as a function of temperature for several samples at different APFO concentrations in the micellar isotropic phase. The general appearance of the curves is preserved over the range of compositions studied. In the micellar isotropic phase the X-ray measurements show that the separation distance, d0, increases little as the temperature decreases for samples at high surfactant content but, at low APFO concentration, d0 increases more rapidly. The behavior of the micellar size with surfactant concentration is unexpected: at low concentrations, in the region between WA ) 0.20 and WA ) 0.28, the aggregation number increases relatively rapidly with increasing concentration, but at WA ) 0.29 APFO, approximately, the continuity is interrupted by an abrupt change in n. At this concentration (WA ) 0.29) and at the temperature T ) 338 K, n changes from ∼98 to ∼85 monomers per micelle. The changing in n is much more pronounced in the low-temperature region where, for T (14) Israelachvili, J. N.; Marcelja, S.; Horn, R. G. Q. Q. Rev. Biophys. 1980, 13, 121. (15) Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1977, 60, 221. (16) Bem-Shaul, A.; Szleifer, I.; Gelbar, W. M. J. Chem. Phys. 1985, 83, 3612. (17) Hendrikx, Y.; Charvolin, J. J. Phys. Paris 1981, 42, 1427.
) 293 K, n drops from ∼184 to ∼151 monomers per micelle. For samples with concentrations higher than WA ) 0.29 APFO, n increases monotonically, reaches a maximum, and then decreases with increasing concentration. At lower temperatures, n is a maximum for WA ) 0.35, but as T increases further, the maximum value of n moves along the curves n versus WA, reaching a maximum for WA ) ∼0.42 at T ) 338 K. A better visualization of the sharp discontinuity observed in the isotropic phase can be seen in Figure 3, which clearly shows the variation of the size of the aggregates plotted as a function of concentration. For each temperature there is a corresponding increase in n with surfactant concentration. However, on crossing from WA ) 0.29 to WA ) 0.31, n drops drastically and becomes even smaller than the sample with the smallest concentration (WA ) 0.21). 3.2. NMR Self-diffusion. The behavior of n with concentration can be interpreted as a change in the structure of the aggregates. This is a plausible explanation once the same equations were used in the calculations of n and, importantly, in the same phase region (isotropic). On one hand, Burkitt with coauthors proposed, on the basis of SANS investigation, that the aggregates in APFO/ D2O dilute solution are cylindrical micelles 48 Å in length and 20 Å in diameter.18 We use the data of ref 18 as a starting point in the following calculations. On the other hand, different authors found that in the more concentrated part of phase diagram of APFO/D2O and similar CsPFO/D2O systems disk-shaped micellar aggregates are stable (see, for example, refs 9, 10, and 19). In view of the preceding discussion, it is logical to assume self-assembled aggregates undergo the shape transformation from rods to disks as the concentration is varied. Such an assumption is supported initially by a rough estimate of the aggregation numbers ratio, nSC/nD, for a spherocylinder (SC) and a disk (D) of equal volume compared to the experimentally found values of n before and after the aggregation number drop just mentioned above. The corresponding calculations give nSC/nD ) 0.82 while the experimental ratio varies between 0.8 and 0.92, which can be considered as a fairly good correlation. The measured and calculated values of the micellar translational self-diffusion coefficient Dt as a function of APFO concentration are shown in Figures 4 and 5. The plot in Figure 4 represents the measured values while the plot in Figure 5 represents the calculated values. A simple comparison shows a slight discrepancy of the measured and calculated Dt values, but the behaviors of the curves (18) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (19) Boden, N.; Corne, A.; Jolley, K. W. J. Phys. Chem. 1987, 91, 4092.
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γ(p) ) 0.312 + 0.565/p + 0.100/p2
Figure 4. Experimental data of the diffusional coefficient Dt as a function of surfactant concentration WA at several temperatures: 1, 333 K; 2, 318 K; 3, 313 K; 4, 303 K.
Figure 5. Calculated values of the diffusional coefficient Dt as a function of surfactant concentration WA at several temperatures: 1, 333 K; 2, 318 K; 3, 313 K; 4, 303 K.
are similar for all temperatures and concentrations studied. In the calculations made here, the micelles were modeled as oblate ellipsoids for concentrations WA > 0.3 of APFO and as rods for concentrations WA < 0.3 of APFO. For an oblate ellipsoid the Stokes-Einstein equation
Dt ) KT/f
(2)
was used. K is the Boltzmann constant, and f is the translational frictional coefficient. The frictional coefficient f is given by
f ) 6πηrSf-1/p2/3
(3)
where η is the solvent viscosity and r is the radius of a sphere with the same volume, Vm, as that of the ellipsoidal micelle (Vm ) 4πab2/3). The axial ratio p is defined as the equatorial axis divided by the revolution axis (major axis). The shape factor Sf is defined by
Sf ) (p2- 1)-1/2 tan-1(p2 - 1)1/2
(4)
for p > 1 and
Sf ) (1 - p2)-1/2 ln{[1 + (1 - p2)1/2]/p}
(5)
for p < 1. For rod-shaped micelles of length L and diameter d, the translational self-diffusion coefficient was calculated using the equation
3πηLDt/KT ) ln(p) + γ(p)
(6)
where p, the axial ratio, is defined as the ratio of the micellar length L to its diameter d and γ, a correction term dependent on the micellar shape, can be calculated from
(7)
which is particularly accurate if p < 20.20 It is worthwhile to note that, in opposition to the micellar self-diffusion measurements, the water self-diffusion experiment, in the isotropic phase, may not distinguish between two forms of micelle due to the weak shape dependence of the obstruction factor (see, for example, ref 21 and references there). Both Figures 4 and 5 show a decrease in Dt as the concentration increases, passing through a minimum at approximately WA ) 0.28 APFO, and then Dt begins to increase with increasing amount of surfactant. The temperature dependence of Dt shows an increase in Dt as T increases. There are two main reasons for the observed dependence of Dt on temperature. The first reason is attributed to the solvent viscosity, which decreases as T increases, lowering the frictional coefficient f. The other cause is attributed to the variation in size of the micellar aggregates, which became smaller and smaller with increasing temperature; thus, the micelles interact less with each other, and so they can flow freely in the solution (the dependence of Dt on concentration is a complex matter, and a deep analysis on this is outside the scope of this article but calculations have been done and will be the subject of a forthcoming report). However, some information can be extracted and will help us to understand better the phase behavior of the APFO/D2O micellar system. Let us focus on the variations attributed to a change in the micellar shape. The region of the curve in which Dt is minimal corresponds to a region of instability where the micelles undergo a shape transformation from rods (for WA < 0.28) to disk aggregates (for WA > 0.28). Again, globular micelles must be ruled out because the aggregation number is too large to be consistent with spherical aggregates and also because the theoretical values calculated for Dt, assuming spherical aggregates, did not show the same qualitative behavior when the data were compared to the measured Dt values. Diffusion by APFO ions in the solution can also be ruled out; their calculated average Dt value was approximately 10 to 11 times higher than the measured Dt value, and this is unreasonable. To explain the deviation of the measured and calculated Dt values observed in several surfactant systems, many theoretical studies based on the energy barrier have been published (see, for example, refs 11 and 22). However, these theories fail to explain adequately why the calculated Dt values are higher than the measured Dt values. Damodaram and Song23 proposed that, in addition to the energy barrier, the potential energy of the molecule may play an important role in the adsorption phenomenon, such that the diffusion coefficient Dt and the potential energy Up are related by the equation (Dt)1/2 ) (D0)1/2 exp[(-p∆A + Up)/KT]. In their proposal, if the free energy of the molecule is positive and greater than the energy barrier (b ) p∆A, where p is the surface pressure and ∆A the area occupied by the molecule at the interface), a higher value for Dt would be expected. If the potential energy is positive and equal to p∆A, then the Dt calculated will be equal to the Dt measured. If it is negative, then the Dt calculated would be smaller than the Dt measured. The potential energy is related to the hydrophobic and hydrophilic free energies of the molecule in solution (Up ) (20) Skripov, V. P.; Firsov, V. V. Zh. Fiz. Khim. 1968, 42, 1253. (21) Jo´hanesson, H.; Halle, B. J. Chem. Phys. 1996, 104, 6807. (22) Chandrasekhar, S. Liquid Crystals, 2nd ed.; Cambridge University Press: Cambridge, 1992. (23) Donoradan, S.; Song, K. B. In Surfactants in Solution; Mittal, L. L., Ed.; Plenum Press: New York, 1989.
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Figure 6. Electrical conductivity versus surfactant concentration measured at different temperatures: 1, 305 K; 2, 320 K; 3, 325 K.
UHDO + UHDI). In water, the hydrophobic energy is positive and the hydrophilic energy is negative; thus, the magnitude and sign of Up depend on the number of polar and nonpolar groups of the molecule. For an APFO ion, Up is positive and so it seems that the proposal of Demoradam and Song would explain satisfactorily the higher values (relative to the Dt values measured) obtained in the calculations of the diffusion coefficient in the APFO/D2O system. 3.3. Electrical Conductivity. APFO is a salt of a weak base and a strong acid that, when in solution, becomes slightly acidic due to hydrolysis (pH ) 6). In solution, APFO molecules are practically neutral, so that the concentration of H+ and OH- is small compared to the concentration of the ammonium ions. Due to the fact that the APFO molecules, above the cmc, are self-aggregated into several shapes and their mobility is low, they do not contribute significantly to the conductivity. This means that the mobile ammonium ions are mainly responsible for the charge transport in the electrical conductivity experiments, and this would be sensitive to structural changes in the micellar aggregates: micelles with different shapes will have different counterion densities bound to the micellar surface, contributing differently to the mechanism of conduction. Thus, it seems reasonable to expect a change in the conductivity data on crossing the boundary region from rods to disks. Figure 6 shows the conductivity (Kzz) measurements plotted versus amphiphile concentration WA for different temperatures within the stability domain of the isotropic phase. First, one can see that the values of Kzz increase as T increases, independent of surfactant concentration. This behavior is related to a change in the viscosity of the solvent (which is low at high temperature) and to the micellar size (which, at higher temperature, is smaller than that of the micelles at lower temperature, leading
Ouriques et al.
to an increase in the population of the counterions free in the solution). Second, the conductivity increases monotonically as WA increases, but for WA between 0.26 and 0.28 there is a sharp discontinuity in the Kzz value which can be interpreted, in view of the preceding results, as a change in the shape of the aggregates; that is, the micelles undergo a shape transition from rods, at low WA, to disks, at high WA. This happens because of a decrease in the population of the free transport carriers (counterions) in solution when the shape transition from disks to rods takes place. The sign and magnitude of ∆Kzz allow us to revise the micelle parameters proposed in ref 18. If the dimensions of the rod-shaped micelles proposed by Burkitt et al.18 are used in the calculations of the micellar superficial area and then compared with the average value calculated assuming disks, the former gives a smaller area (Arod ∼ 3709 Å2, Adisk ∼ 5139 Å2). This means that there are less counterions bound to the rod-shaped micellar surface, and consequently, an increase in Kzz would be expected when rods are transformed to disks, instead of a decrease as already observed. The cause of this behavior might be attributed to both the overestimated oblate ellipsoid model parameters and the underestimated rod-shaped micellar size in the proposal of ref 18. Assuming that the micellar ellipsoidal model is valid and only the speculative micellar dimensions of the rods are the cause of the discrepancy observed in the Kzz measurements, then larger micelles would be needed to explain such a discrepancy. One can find that a micellar length of ∼66 Å is necessary to match the superficial area of an assumed rodlike structure. The average aggregation number is also unusually small for such a rod-shaped micelle (n ∼ 42). However, if a length of 66 Å is used in the calculation, n will be approximately 89 monomers per micelle, matching the micellar size calculated from X-ray experiments. Thus, the interpretation of rod-shaped micelles by Burkitt et al.18 seems to be consistent though the lengths of the rods are most probably a little greater than those of the micellar model. In summary X-ray, diffusion, and electrical conductivity studies strongly suggest that there is a change in the shape of the micellar aggregates in the homogeneous isotropic phase of the APFO/D2O binary mixture where the micelles change from spherocylinders (for WA < 0.26) to disks (for WA > 0.28). Acknowledgment. We would like to thank the Federal University of Santa CatarinasUFSC for providing financial support. Thanks also to professors Neville Boden and Pierre Toledano for useful discussions we had. LA9913598
