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Langmuir 1997, 13, 632-638
Phase Diagram of Copper(II) Bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2-Isooctane-Water J. Tanori,†,‡ T. Gulik-Krzywicki,§ and M. P. Pileni*,†,‡ Laboratoire S.R.S.I., U.R.A.C.N.R.S. 1662, Universite´ P. et M. Curie (Paris VI), B.P. 52, 4 Place Jussieu, F-75231 Paris Cedex 05, France; C.E.A-C.E.N. Saclay, DRECAM-S.C.M., F-91191 Gif-sur-Yvette, Cedex, France; and Centre de Ge´ ne´ tique Mole´ culaire-CNRS, F-91190 Gif-sur-Yvette, Cedex, France Received April 30, 1996. In Final Form: July 24, 1996X A phase diagram of copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2-isooctane-water, is presented. Keeping the concentration of the Cu(AOT)2-isooctane constant, an increase in the amount of water induces transitions. At low water content, spherical and cylindrical reverse micelles are formed. By increasing the water content, a phase transition takes place with formation of a bicontinuous system. Further addition of water leads to the formation of planar and bended (spherulite type) lamellae. As more water is added, only spherulites remain in the phase. Still further addition of water leads to a reappearance of an interconnected network and then reverse micelles.
1. Introduction In solution, surfactant molecules self assemble to form aggregates.1 At low concentration the aggregates are generally globular micelles,2 but these micelles can grow upon an increase of surfactant concentration and/ or upon addition of salt, alcohols, etc. In that case, micelles have been shown to grow to elongated more or less flexible rodlike micelles3-11 in agreement with theoretical prediction on micellization.12,13 The hydrophobic and hydrophilic media are not submitted to any local constraint except the density. They are separated by the surfactant film which has its own mechanical propertiessspontaneous curvature and elastic resistance to extra bending.14,15 Closely related to this quite loose set of constraints,16 the details of the morphology (curvature of the film) of the mixture at a local scale fluctuate strongly. The contribution of the entropy of the folded film is predominant in the free energy of the solution, while the morphology has little incidence. The § Centre de Ge ´ ne´tique Mole´culaire-CNRS. * All correspondence to this author. † Universite ´ P. et M. Curie. ‡ C.E.A.-C.E.N. Saclay. X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Larson, R. G. Rheol. Acta 1992, 31, 497. (2) Tanford, C. The hydrophobic effect; Wiley: New Yok, 1973. (3) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (4) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (5) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Deerian,P. J.; Drifford, M.; Zemb,T. N. J. Phys.Chem. 1988, 92, 2286. (6) Ninham, B. W.; Barnes, I. S.; Hyde, S. T.; Derian, P. J.; Zemb, T. N. Europhys. Lett. 1987, 4, 561. (7) Mazer, N.; Benedek, G.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075. (8) Blankschtein, D.;Thurston, G. M.; Benedek,G. Phys. Rev. Lett. 1986, 85, 7268. (9) Porte, G.; Appell, J.; Poggi, Y. J. Phys. Chem. 1980, 84, 3105. (10) Hoffmann, H.; Platz, G.; Ulbricht, W. J. Phys. Chem. 1981, 85, 3160. (11) Mishic, J. R.; Fisch, M. R. J. Chem. Phys. 1990, 92, 3222. (12) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (13) Safran, S. A.; Turkevich, L. A.; Pincus, P. A. J. Phys. Lett. 1984, 45, L69. (14) Talmon, Y.; Prager, S. J. Chem. Phys. 1978, 69, 2984. (15) Widom, B. J. Chem. Phys. 1984, 81, 1030. (16) Israelachvili, J. N.; Mitchell,D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
S0743-7463(96)00427-1 CCC: $14.00
interfacial curvature is toward the water and by convention is described as negative mean curvature. Reverse phases are characterized by a negative mean interfacial curvature and are commonly formed by double-chain amphiphiles. They are known as type II.17-20 Conversely, by increasing the amount of water the curvature increases to zero with formation of lamellar phases. Still further addition of water leads to a positive curvature with formation of normal phases (type I). In the present paper, we describe the progression of the phase diagram of the Cu(AOT)2-isooctane system with the increase of the water content, w ) [H2O]/[AOT]. At low w value, reverse micelles are formed. By increasing w, the hydration of the polar head groups changes the interfacial curvature. This induces the formation of interconnections. By increasing w, the interfacial curvature reaches zero and lamellar phases are observed. At higher water content, the hydration of the counter ions and polar head groups is reached. Further addition of water leads to a decrease in the interfacial curvature with formation of inverted phases (interconnected cylinders and then reverse micelles). 2. Experimental Section 2.1. Compounds. Synthesis of copper(II) bis(2-ethylhexyl)sulfosuccinate (Cu(AOT)2) has been described previously.21 Cu(AOT)2 in isooctane solutions for various water contents, w, are analyzed by Karl Fischer titration using a Mettler automatic titrator. The concentrations of Cu(AOT)2 are determined by adding a sample to a ≈0.03 M hydrochloric acid, ≈0.3 M ammonium acetate solution and subsequently titrating for copper(II) using a 0.01 M sodium EDTA solution with 4-(2pyridylazo)resorcinol as an indicator. Isooctane was supplied by Fluka (99.5% puriss), and ammonium acetate (98%), sodium EDTA, and 4-(2-pyridylazo)resorcinol (99%) by Prolabo. Singly distilled water was passed through a Millipore MilliQ system cartridge until its resistivity reached 18 MΩ cm. All chemicals were used without further purification. 2.2. Apparatus. Electrical conductivity measurements were made with a Tacussel CD 810 instrument using a TD 100 (17) Reactivity in reverse micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (18) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17207. (19) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17212. (20) Seddon, J. M.; Templer, R. H. Philos. Trans. R. Soc. London A 1993, 344, 377. (21) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598.
© 1997 American Chemical Society
Phase Diagram of Cu(AOT)2-Isooctane-Water (platinum) electrode from the same manufacturer. The measurements were made at 22 °C once a stable reading has been established. The conductivity measurements are made in each phase. Small-angle X-ray scattering (SAXS) experiments were performed on the high-resolution X-ray scattering apparatus from S.C.M-C.E.A. Saclay (France). The wave vector range was
Langmuir, Vol. 13, No. 4, 1997 633 write24
[J1(qR)/(qR)]2 ≈ exp(-q2R2/2) We find again that a plot of ln(qP(q)) vs q2 gives the radius of gyration of normal section of the cylinder:
Rg ) x(-2p)
0.02 Å-1 < q < 0.35 Å-1 where q ) 4π sin(θ)/λ, 2θ is the scattering angle, and λ is the radiation wavelength. Absolute scale is obtained knowing the intrumental constants of experimental setup. These constants are determined as described in ref 22 following the procedure reported in ref 23 by using a semitransparent beam stop. 2.3. Freeze-Fracture Electron Microscopy. A thin layer of the sample (20-30 µm thick) was placed on a thin copper holder and then rapidly quenched in liquid propane. The frozen sample was fractured at liquid nitrogen temperature, in a vacuum close to 10-7 Torr, with the liquid nitrogen cooled knife in a Balzers 301 freeze-etching unit. The replication was done using unidirectional shadowing, at an angle of 35°, with platinum-carbon, 1-1.5 nm of mean metal deposit. The replicas were washed with organic solvents and distilled water and were observed in a Philips 301 electron microscope. 2-4. Scattering Pattern Analysis. The scattered X-ray intensity I(q) is
The break in the linearity observed at very low q gives an estimation of the length of the independent cylinders or of the persistence length of the structure, 〈l〉, if the cylinders are connected:26
q ) π/〈l〉 An other estimation of the radius of the normal section is given by the minimum of the Porod plot: qminR ) 3.9. 2.4.3. Lamellar Phase. The characteristic distance, d, can be obtained from the structure factor peak using
d ) 2π[qpeak]-1 The intensity of the peak depends on the number of objects and on the bilayer rigidity. The concentration effect can be taken into account by an estimate of the invariante, Q. The Q value is obtained by28
I(q) ) P(q) S(q) P(q) and S(q) are the form and the structure factor, respectively. The latter describes the interactions between particles.24 2.4.1. Case of Spherical Structure.24 For spherical selfassemblies structures, at low volume fraction,25 the interactions are neglected and the scattered intensity is expressed as 2
I(q) ≈ P(q) ) ΦpolVF∆F [fs(qR)]
2
where Φpol, V, F, and ∆F are the polar volume fraction, the volume of one particle, the Thompson factor, and the variation of the electron density, respectively. In the Guinier regime, when q is close to 0, a good approximation of P(q) is given by24
P(q) ) ΦpolVF∆F2 exp(-q2R2/3) Hence, from the slope, p, of a plot of ln(P(q)) vs q2, the radius of gyration of the particle:
Rg )x(-3p) is deduced. From the Porod plot (I(q)q4 vs q) the characteristic radius Rc is deduced.24 In the case of homogeneous diffusing spheres, the characteristic radius is related to the first minimum or to the first maximum of this representation and equal to 4.5/Rc and 2.7/Rc, respectively. This is a convenient measurement of the droplet size. 2.4.2. Case of Cylinders. As for the previous cases we use I(q) ≈ P(q). For cylindrical self-assemblies we have24,26
qP(q) ) πΦpolVF∆F2[2J1(qR)/(qR)]2
qR < 3
where V is the volume per length unit of the cylinder and J1(x) the first Bessel function.27 ln(P(q)) varies as -ln(q). In this case, we are in the Guinier approximation (q f 0) and we can (22) Ne, F.; Gazeau, D.; Lambard, J.; Lesieur, P.; Zemb, T. J. Appl. Cryst. 1993, 26, 763. (23) Cotton, J. P. In Neutron X-Ray and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier Science Publishers, B.V.: Amsterdam, 1991; p 19. (24) Small Angle Scattering of X-rays; Guinier, A., Fournet, G., Eds.; Wiley and Sons: New York, 1955. (25) Small Angle X-ray Scatterings; Glatter, O., Kratky, O., Eds.; Academic Press: New York, 1982. (26) Cabane, B. In Surfactant in Solution, New Methods of Investigation; Zana, R., Ed.; 1985.
Q)
∫ q I(q) dq ∞ 2
0
2.4.4. Sponge Phase. The scatter pattern of a sponge phase is given by29
I(q) ≈ q-2 exp(-δ2q2/12) The representation ln(I(q)q2) versus q2 gives a straight line at low q value, and the slope indicates the finite thickness, δ, of the bilayer (slope ) -δ2/12)
3. Results The 5 × 10-2 M Cu(AOT)2 in isooctane solution is an isotropic phase. By water addition, the phase diagram evolves progressively. At w ) 5.5 a phase transition takes place. The lower phase is optically clear and keeps the blue color characteristic of Cu(AOT)2. The upper phase is pure isooctane. The increase of the water content induces an increase in the volume of the upper phase with a decrease of the lower phase. At w ) 11, a birefringent blue phase appears. From w ) 11 to 15, the volume of the isooctane and the isotropic phases decrease whereas the volume of the birefringent phase increases. At w ) 15, the isotropic phase totally disappears. The birefringent and isooctane phases remain in equilibrium. By increasing the water content, the volume of the upper phase (isooctane) decreases, whereas that of the lower phase increases. At w ) 20, a new isotropic phase appears in equilibrium with isooctane and the birefringent phase. From w ) 20 to w ) 30, three phases are in equilibrium (isooctane, isotropic and birefringent phases). By increasing the water content, from w ) 20 to 30, the volume of isooctane remains unchanged when that of the isotropic phase increases. At w ) 30, the birefringent phase totally disappears and the isotropic and isooctane phases remain in equilibrium. From w ) 30 to 35, the volume of isooctane progressively disappears. At w ) 35, one isotropic phase solution is obtained. 3.1. Below w ) 5.5. The solution is isotropic. The conductivity of Cu(AOT)2-H2O-isooctane solution is very (27) Handbook of Chemistry and Physics, 61st ed.; CRC Press: Boca Raton, FL, 1981. (28) Appell, J.; Marignan, J. J. Phys. France I 1991, 1447. (29) Filali, M.; Appell, J.; Porte, G. J. Phys. II, Fr. 1995, 5, 657.
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Figure 2. Variation of the viscosity with the water content, w. Figure 1. Variation of the conductivity, κ, in the various phases: (O) reverse micelles , (O) concentrated inverted phase, and (0) lamellar phase
low. It varies from 1 to 230 nS with increasing the water content to w ) 5.5 (Figure 1). At low water content, w ) 2, the behavior of the intensity, I(q), observed in the Cu(AOT)2-isooctane-water system is characteristic of a spherical structure. As has been previously shown,21 the radius of the microaggregate determined from the slope of the Guinier plot, ln(I(q)) versus q2 is found equal to 1 nm. By increasing the water content, above w ) 4, the scattering spectrum, at very small wave vectors, is no longer characteristic of a spherical structure. Cylindrical water in oil droplets are observed. The radius, determined from the slope of the linear relationship obtained by plotting ln(qI(q)) versus q2, is also in good agreement with that deduced from the Porod plot. The persistence length of the cylinder, 〈l〉, is found equal to 7 nm. From the conductivity and from SAXS measurements, it can be concluded that at low water content, water in oil spherical droplets are formed, whereas at higher water content cylindrical droplets are observed. These behaviors are similar to those previously observed for similar bimetallic surfactants.21 Such change in the droplet shape has been observed for many systems20 and is attributed to changes in the average curvature12,13,16 of the interfacial film. As matter of fact, the increase in the water content favors the hydration of the copper ions and the polar head groups which induces a decrease in the electrostatic interactions between the polar head group and copper ions and favors the increase in the curvature. 3.2. 5.5 < w < 11. Above w ) 5.5, the phase is destabilized and is separated into a more concentrated reverse micellar solution and an almost pure isooctane phase. By increasing the water content, the following are observed: (i) An increase in the isooctane volume. This induces an increase in the Cu(AOT)2 concentration in the lower phase. (ii) An increase in the conductivity of the lower phase (Figure 1) whereas the conductivity of the upper phase is similar to that obtained in pure isooctane. (iii) A progressive increase in the viscosity of the lower phase and then a decrease (Figure 2). The maximum in viscosity is reached for a w value close to 7. (iv) The slope of the plot of lnI(q)) versus ln(q), at low water q value, is equal to -1. This is characteristic of the scatter of cylinders. The gyration radius of the normal section is deduced from the slope obtained by plotting ln (qI(q)) versus q2 (Figure 3). The persistence length of the cylinders, 〈l〉, is deduced, at various water contents. Table
Figure 3. Variation of ln(I(q)q) versus q2. Table 1 w
gyration radiusa (nm)
persistence length (nm)
radiusb (nm)
6.5 7.5 8.5 9.5 10 11
1 0.9 1 1 1 1.1
3 3 3.2 3.1 3 3.1
1.3 1.3 1.5 1.5 1.5 1.5
a Estimation by plotting ln(qP(q)) vs q2. b Estimation given by the minima of the Porod plot: qminR ) 3.9.
1 shows no drastic change in either the gyration radius or in the persistence length with the increase in the water content. From this, the average size of the cylinders is deduced. It remains constant when the water content increases. The errors in the values given in Table 1 are evaluated to 10%. (v) Freeze fracture replicas show a homogeneous system made from only very small objects (Figure 4). The increase in conductivity and viscosity of the lower phase with an increase of the water content can be attributed to the increase in Cu(AOT)2 concentration: At low water content (below w ) 5.5), the water-in-oil phase is rather diluted. The increase in the water content induces a phase transition with an increase in the Cu(AOT)2 concentration in the lower phase. This favors the increase in the number of connections between cylinders to form a bicontinuous network. Similar behavior has been already observed with other self-assemblies surfactants and is attributed to formation of disordered open connected microemulsions.3-6 At high water content, w > 7, the decrease in the viscosity can be explained in term of branching of one cylinder onto another with locally a saddle structure. Similar behavior has been observed in oil-in-water micelles.30
Phase Diagram of Cu(AOT)2-Isooctane-Water
Figure 4. Freeze fracture electron micrograph of w ) 7.5 sample. Note the presence of only very small objects. The bar represents 200 nm.
3.3. 11 < w < 15. At w ) 11, a birefringent phase appears in equilibrium with the inverted phase and isooctane. By increasing the water content to w ) 15, the inverted phase progressively disappears. The SAXS pattern of the birefrigent phase is rather surprising. The Bragg peak is very broad and no second order is observed at w ) 12, 13, and 14. It does not follow a monotonic behavior with the increase of the water content, w. This could be attributed to disorders in the lamellar structure and (or) to the fact that at w ) 12, the volume of the birefrigent phase is very small (too close to the phase transition) and the scattering measurement is due to a birefrigent phase in the presence of the isotropic phase. From freeze fracture images is observed a coexistence of a rather well-ordered planar lamellar phase (Figure 5A) with poorly ordered spherulites (Figure 5B). Similar behavior is observed at w ) 12, 13, and 14. The characteristic distance is equal to 68, 55, and 57 Å for w ) 12, 13, and 14 respectively. The conductivity of the lamellar phase cannot be measured at w ) 12 (the volume of the lamellar phase is too small). From w ) 13 to 14, it decreases with increasing the water content. This is consistent with the fact that the freeze fracture images reveal the presence of two types of lamellar phase, namely, planar lamellae and spherulites. In the first case, the conductivity is expected to be higher than in the latter. As it is shown below, the planar lamellae disappear and spherulites remain. Hence the decrease in the conductivity and the nonmonotonic variation of the characteristic distance with increasing the water content can be attributed to an increase in the number of spherulites and a decrease in the planar lamellae. 3.4. 15 < w < 20. At w ) 15 the birefringent phase remains in equilibrium with pure isooctane. A drop in the conductivity is observed (Figure 1) with an average (30) Appell, J.; Porte, G.; Berret, J. F.; Roux, D. C. Prog. Colloid Polym. Sci. 1994, 97, 233.
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conductivity equal to 12.8 µS. This may be interpreted to indicate a decrease in the degree of interconnectedness. The birefringence and the decrease in conductivity strongly support the formation of a lamellar phase which consists of thin aqueous and thick isooctane lamellae. In all the range, the freeze fracture replicas show almost exclusively the presence of spherulites (Figure 6). They are formed here apparently without any external forces, in contrast to those usually observed after shearing of lamellar phases.31,32 These data are confirmed by SAXS measurements. As described above (paragraph 3.3), one Bragg peak without second order and a strong increase in the scattering at low angle are observed (Figure 7). From the Bragg peak, the characteristic distance, d, can be deduced. Figure 8 shows a linear increase of d with increasing w. This can be explained as the following: The equilibrium with almost pure isooctane forces the isooctane lamellae to be maximally swelled. Addition of water causes swelling of the aqueous lamellae and increases the characteristic distance, d. However, d is smaller than the value expected from the measured isooctane content. This may be due to onion-like lamellar structures containing the excess isooctane in between the spherulites and in their centers. 3.5. 20 < w < 40. At w ) 20, a new isotropic phase appears. By increasing the water content from w ) 20 to 30, the lamellar phase disappears with an increase in the volume of the isotropic phase. The conductivity of the isotropic phase is very high and is rather constant. The structure of the isotropic phase is not well defined. From SAXS measurements, it is difficult to differentiate between a sponge and an interconnected cylinder structure. As a matter of fact, Figure 9 shows at w ) 20 and w ) 26 a linear relationship obtained by plotting either ln(I(q)q2) or ln(I(q)q) versus q2. The correlation factors of these plots are similar (0.99). The freeze fracture images (Figure 10) show the presence of two types of objects: interconnected spherulites and densely packed, much smaller objects as observed in the region of interconnected cylinders (Figure 4). The interconnected spherulites may not represent the original three-dimensional structure. The spherulites may be an artifact that was induced by the quenching of the original sponge phase. This phase contains a large amount of isooctane which is extremely difficult to vitrify. At w ) 30, the lamellar phase totally disappears and the isotropic phase remains in equilibrium with isooctane. From w ) 30 to 35, volume of the upper phase decreases with an increase of the lower phase. This phenomenon is followed by a decrease in the conductivity (Figure 2). SAXS patterns show the scatter of cylinders. These data indicate a progressive dilution of the inverted phase with a decrease in the number of connections between the cylinders. At w ) 35 one isotropic phase is observed and the conductivity is very low. The increase in the water content to w ) 40 does not induce macroscopic changes and the conductivity of the solution remains very low (nS scale). SAXS experiments show the scatter of spheres characterized by radii given in Table 2. The low conductivity and the scatter of spheres led us to conclude that water-in-oil droplets are formed. (31) Diat, O.; Roux, D. J. Phys. II, Fr. 1993, 3, 9. (32) Berret, J. F.; Roux, D. C.; Porte, G.; Lindner, P. Europhys. Lett. 1994, 25, 521. (33) Khatory, A.; Kern, F.; Lequeux, F.; Appell, J.; Porte, G.; Morie, N.; Ott, A.; Urbach,W. Langmuir 1993, 9, 933.
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Figure 5. Freeze-fracture electron micrographs of w ) 14 (A and B) samples. Note the presence in both samples of planar lamellar phases and spherulites. The bars represents 500 nm.
Figure 6. Freeze-fracture electron micrographs of w ) 13 (A) and w ) 22 (B) samples. Note the presence of spherulites in all samples. The bar represents 1000 nm.
4. Discussion The phase progression of Cu(AOT)2 with increasing water content (w) is rather intriguing. It is thus more complex than that of NaAOT, which shows no restabilization of the inverted phase.17 The initial phase progression toward the lamellar phase can be rationalized in terms of a progression toward zero mean curvature of the surfactant monolayer (sphere, cylinder, network, lamel
lae). The interfacial curvature is oriented toward the water and is, by convention, described as negative mean curvature leading to inverse type II phases.20 The increase in the water content favors hydration of copper ions and the polar head groups and induces changes of the various electrostatic interactions at the interface. This indicates an increase in the mean curvature of the interface, so that the quasi-spherical micelles (observed at w ) 2) are less
Phase Diagram of Cu(AOT)2-Isooctane-Water
Langmuir, Vol. 13, No. 4, 1997 637 Table 2
w
structure
20 22 24 26 28 30 32 35
sponge phase and cylindrical micelles mixture
gyration radius (nm)
cylindrical micelles spherical micelles
2.4 2.4 2.4 2.5 2.6 3.9 4.3 4.8
radius (nm) Rqmax
5.9
Rqmin
persistence length (nm)
thickness of the bilayer (nm)
6.4 6.8 6.8 7.4 7.4 8 11
4.9 4.9 5 5.3 5.5
5.8
Figure 7. SAXS spectra obtained at various water content 15 < w < 19.
Figure 9. ln(I(q)q2) and ln(I(q)q) versus q2 for w ) 20 and w ) 26 samples.
Figure 8. Variation of the characteristic distance with the water content.
and less “favorable” and gradually grow to cylindrical micelles. At w ) 5, due to the increase in the interfacial curvature, the system releases isooctane which induces an increase in the Cu(AOT)2 concentration leading to the increase in the number of cylinders. Hence the fusion of two micelles to give a larger one corresponds to a gain in energy which is due to the transfer of the surfactant molecules in the two suppressed extremities from a spherical to a more favorable cylindrical surrounding. When the curvature increases (from w ) 5 to 11) the ends of the cylinders are eliminated by the branching of cylinders. The increase in the bicontinuous structure is well demonstrated by the increase in the conductivity (Figure 1). As the water content increases, the interconnected network growth induces an increase in the conductivity (Figure 1). This bicontinuous system forms an entangled network of long connections. Correspondingly, the viscosity increases steeply (Figure 2). Then connections become favorable, and the entanglements are progressively replaced by connections. An entanglement is a topological constraint to the relaxation stress while a connection is free to slide along the cylindrical micelles. Thus the viscosity decreases as entanglements are re-
placed by connections (Figure 2). Such effects have been already observed in the oil-in-water phase30,32 but are for the first time, to our knowledge, observed in the waterin-oil phase With the increase in the water content, the interfacial curvature reaches zero and lamellae are formed with a decrease in the interconnection (conductivity slows down Figure 1). Formation of spherulites could be explained by an excess of isooctane which swells the lamellae. A larger amount of water, 15 < w < 20, induces the appearance of a lamellar phase characterized by a zero curvature. The equilibrium with almost pure isooctane forces the isooctane lamellae to be maximally swelled. Addition of water molecules causes swelling of the aqueous lamellae and increases the characteristic distance, d. Hence the isooctane chains are tethered to the interface region with energetically preferred conformational states. This implies severe constraints with a thickness of the hydrocarbon region which cannot exceed the value set by the length of the fully extended chains (without oil). The small values of d are attributed to onion-like lamellar structures (spherulites) containing the excess isooctane between the spherulites as is observed on some freeze fracture images. It has to be noticed that such structure is usually observed after shearing.31,32 In the present case, it is ordered without any apparent external forces. However, noticeable is a large distribution in size of spherulites. The size varies from 100 to 8000 nm. The progression of the phase diagram when the curvature reaches zero is rather surprising. Above a water content corresponding to zero curvature, the system evolves to bicontinuous system which can be attributed to a sponge and interconnected cylindrical structures. At higher water content, the system is made of reverse
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progression toward zero mean curvature of the surfactant monolayer (sphere, cylinder, network, lamellae). Similar behavior has been observed by using dodecylammonium bromide surfactant, DDAB.3-6 This could be attributed to changes in the packing parameter due to the size of the counterion. The entire phase diagram behavior could be interpreted in terms of a competition between geometrically constrained interfacial curvature and entropic fluctuations in the bilayer.
Figure 10. Freeze-fracture electron micrograph of w ) 20 sample. The bars represent 500 nm.
micelles. This corresponds to a decrease in the curvature with the increase of the control parameter. In most of the system studied,3-6,9,12,13 when zero curvature is reached, the increase in the control parameter induces an increase in the curvature to a positive value with the formation of rods and then micelles in aqueous media. In our case, the system behaves differently, evolving toward a decrease in the curvature. This could be explained in terms of a
5. Conclusion The phase progression of Cu(AOT)2 with increasing water content (w) is described. At low water contents, reverse micelles are formed. By increasing the water content, destabilization takes place and the reverse micellar solution separates into a more concentrated inverted phase and an almost pure isooctane phase. Upon further addition of water, the coacervate becomes more and more concentrated and subsequently a turbid, birefringent, lamellar phase (LR) starts to coexist with the inverted phase and the isooctane phase. As more water is added, the proportion of lamellar phase increases and finally LR coexists alone with isooctane. It is rather surprising that further addition of water leads to a reappearance of concentrated interconnected cylinders that start to coexist with the LR and isooctane phases. The inverted phase progressively replaces LR and then becomes less and less concentrated. Subsequently, a homogeneous reverse micellar solution is recovered in a liquid-gas type transition, the reverse of that observed at low water content. Acknowledgment. The authors would like to thank Prof. B. Ninham for fruitful discussions. Thanks are due to Dr. G. Porte for critical comments on the manuscript. Thanks are due to J. C. Dedieu for his excellent technical assistance in freeze-fracture electron microscopy. J. T. thanks CONACYT for financial supports. LA960427C
