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Excess of Solubilization of Lindane in Nonionic Surfactant Micelles and Microemulsions F. Testard* and Th. Zemb C.E.A./D.R.E.C.A.M./Service de Chimie mole´ culaire, C.E.N. Saclay, Laboratoire de Diffusion de Rayons X aux petits angles, Bat.125, 91191 Gif-sur-Yvette Cedex, France Received September 8, 1997. In Final Form: March 6, 1998 The solubilization of lindane by nonionic polyoxyethylene glycol surfactants has been studied in micellar solution, as well as in Winsor I and Winsor III microemulsions. The maximum of solubilization was determined when a saturated lindane solution was in equilibrium with solid lindane. At this saturation point, the interfacial molar ratio λmax of the solute to the surfactant was determined, taking into account the amount of lindane solubilized in oil. We first studied the evolution of the maximum excess solubilization λmax as a function of surfactant volume fraction in water-C8E6 binary solutions. The excess solubilization was found to be constant between 0 and 50% in surfactant volume fraction. For higher volume fractions, λmax increased with the surfactant concentration. This behavior is the result of a competition between water and lindane to be solubilized by the surfactant film. We also observed a decrease of the clouding temperature for the water-C8E6 system when lindane is solubilized. We have evidenced a linear decreasing of λmax with the reduced temperature (TLC - T)/TLC, where TLC is the cloud temperature for the saturated system. By comparison with results obtained with ternary microemulsions for two different curvatures, we have observed a strong dependence of λmax on the average curvature of the surfactant film. The excess solubilization is maximum for the bicontinuous balanced structure obtained in the Winsor III case, where fluctuations produce the most stable structure versus the amount of added solute located in the surfactant film. When only excess oil is present (Winsor I), the o/w droplets at the emulsification failure boundary show an intermediate value of the maximum solubilization power.
Introduction Our aim in this paper is to demonstrate the link between solubilization and interfacial curvature. Ninham and coworkers showed 20 years ago that curvature is the main variable in free energy of surfactant systems.1 Therefore, curvature could also drive the solubilization of a solute (and the reverse) in flexible self-associated systems such as nonionic micelles and microemulsions. As a hydrophobic model solute, we chose lindane, which is of great importance given its ubiquity in pesticides and subsequent environmental problems.2,3 Lindane is the γ-isomer of hexachlorocyclohexane (1R,2R,3β,4R,5R,6β) (often denoted γ-HCH) and was, until recently, one of the most widely used organochlorine pesticides for agricultural purposes. This pesticide has a very long residence time after dispersion.2,3 Our aim in this study is to understand the basic mechanisms of lindane solubilization in the simplest self-assembling system for which the phase behavior is known as a function of composition and temperature, that is the ternary system water-oil-linear nonionic surfactant with polyethylene head groups.4 We have used the polyoxyethylene glycol surfactants CiEj series as a model system, and we chose one oil which solubilizes lindane: cyclohexane. We chose the lindane molecule as the apolar solute because it is slightly soluble in oil with a well-defined solubility limit. It has a very low solubility in water. Our aim is to investigate its behavior at the liquid/liquid interface, in order to make a later comparison with that at the liquid/solid interface (1) Hyde, S.; Anderson, S.; Larsson, K.; Blum, Z.; Landh, T.; Lidin, S.; Ninham, B.W. The language of shape; Elsevier Science B.V.: Amsterdam, 1997. (2) Metcalf, R. L. In Pesticides In The Environment; White-Stevens, R., Ed.; Marcel Dekker: New York, 1997; Vol. I, Part I, pp 67-144. (3) Ware, G.W. The Pesticide Book; W. H. Freeman: San Francisco, CA, 1978; Part II, pp 27-52. (4) Strey, R. Colloid Polym. Sci. 1994, 272, 1005.
on clay. Moreover, lindane may be a useful model, since some studies have shown that the accumulation of these slowly degrading molecules is a latent threat to the biotope in agriculture.2 Binary water-CiEj solutions have a lower consolute temperature. They separate into two phases when the temperature is increased above the cloud point temperature. This temperature is a characteristic of the surfactant and is modified by the addition of a solute. In the case of added salt, the variation of cloud point temperature has been extensively documented.5-7 It has been demonstrated that the addition of salt decreases or increases the lower consolute boundary of the water-CiEj system, depending on the nature of the salt. For example, NaCl and NaBr depress the lower consolute boundary of the water-C8E5 system whereas the addition of NaI elevates it.6 In the same way, the solubilization of organic compounds has been studied in terms of cloud point variation.8,9 The direction of the shift of the cloud point temperature has been interpreted as variations in the overall attractive interaction between the nonionic micelles.10-12 Ternary water-oil-CiEj systems have also been well studied in the last 20 years,13-16 mainly because there is (5) Doscher, T.; Myers, G.; Atkins, D., Jr. J. Colloid Sci, 1951, 6, 223. (6) Weckstro¨m, K.; Zulauf, M. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2947. (7) Khenhare, P.; Hall, C.; Kilpatrick, P. K. J. Colloid Interface Sci. 1996, 184, 456. (8) Briganti, G.; Puvada, S.; Blankschtein, D., J. Phys. Chem. 1991, 95, 8989. (9) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Ogino, K., J. Colloid. Interface Sci. 1992, 152, (2), 402. (10) Blankschtein, D.; Thurston, G.; Benedek, G. Phys. Rev. Lett. 1985, 54, 995. (11) Thurston, G.; Blankstein, D.; Fish, M.; Benedek, G. J. Chem. Phys. 1986, 84, 4558. (12) Blankschtein, D.; Thurston, G.; Benedek, G., J. Chem. Phys. 1986, 85, 7268.
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no need of addition of a cosurfactant to have a large singlephase microemulsion domain at a given temperature. Kahlweit and co-workers give a general unified description of the phase behaviors, including that of the balanced microemulsion, in equilibrium with excess water and oil (Winsor III), as well as the oil or water emulsification failure boundaries, which are commonly referred to as Winsor I and II, respectively. Standard experimental procedure allows precise determination of the composition and temperature of the microemulsion domains with the lowest possible surfactant content, the so-called “fish-tail” domains.4,15 In this balanced Winsor III case, the average curvature of the interface is zero; that is, the surfactant parameter p ) V/al is 1 to a first-order approximation.17-19 Winsor I and Winsor II domains are easier to locate experimentally, since determination of the radius of the droplets formed by the dispersed phase allows determination of the average curvature of the surfactant interface. Moreover in the binary micellar domain, the radius of curvature of the interface is equal to the chain length by definition. In microemulsions, oil and water microdomains are separated by the surfactant interface. We attribute to this interface the volume of the hydrophobic chains. Thus, when a hydrophobic solute is solubilized in a microemulsion, it may be solubilized either in the oil volume fraction or in the volume fraction corresponding to the interfacial layer. Following the definition of Hatton,20-22 we will define the excess of solubilization as the amount of solute solubilized in the interfacial volume. This excess is always deduced by subtraction of the quantity solubilized in the oil volume present. A natural way to quantify such solubilization is to use the interfacial composition parameter λ introduced by Hatton for reverse micelles of AOT20-22 to study the solubilization of amino acids. The parameter λ is the molar ratio of solute to surfactant at the interface. When the solution is saturated with solute (i.e. in equilibrium with an excess of solute), the parameter is called λmax. It represents the maximum solute content of the interfacial film. Our aim is to compare the excess of solubilization of lindane in binary water-C8E6 systems and in ternary water-oil-C8E6 systems up to the saturation point. In the binary system, we have determined simultaneously the maximum excess solubilization in the film (λmax), the variation of the cloud point with degree of solubilization, and the possible microstructural modification induced by the solubilization of lindane. Furthermore, we determined the λmax parameter in a water-(cyclohexane + lindane)C8E6 bicontinuous microemulsion with a zero spontaneous curvature. An identical procedure was also used in a Winsor I microemulsion (oil in water microemulsion with excess oil). These measurements show the evolution of (13) Shinoda, K.; Kunieda, H. In Microemulsion, Theory and Practice; Prince, L., Ed.; Academic: New York, 1997. (14) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3 (3), 233-234. (15) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654-668. (16) Kahlweit, M.; Strey, R.; Busse, G., J. Phys. Chem. 1990, 94, 3881-3894. (17) Chan, D. Y. C.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1975, 71, 119. (18) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans 2 1976, 72, 1525. (19) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (20) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6400; J. Phys. Chem. 1990, 94, 6411. (21) Leodidis, E. B.; Bommarius, A. S.; Hatton, T. A. J. Phys. Chem. 1991, 95, 5943. (22) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1991, 95, 5957.
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the saturation value of interfacial composition with the average curvature of the interface for different microstructures. Experimental Section Materials. Hexaoxyethylene glycol mono-n-octyl ether (C8E6) was purchased from NIKKO Chemicals Tokyo and was shown to be >99% pure by gas chromatography. Cyclohexane was obtained from SDS (Peypin) with a nominal purity > 99%. We used Millipore-filtered water. Lindane was supplied by Aldrich with a nominal purity > 97% and was recrystallized in a mixture of CHCl3-EtOH. The molecular densities (in g/cm3) used to calculate the volume fractions are 1 for H2O, 1.01 for C8E6, O.779 for cyclohexane, and 1.73 for lindane at 25 °C. The melting point of lindane is 112.5 °C. Method of Phase Separation Observation. The samples were placed in a temperature-controlled bath with a temperature stability of (0.1 K. The phase separation temperature was induced by cycling the bath temperature above and below the transition. We started with a sample in the one-phase region and increased the temperature until turbidity was observed. Then, we determined the reclarification temperature upon decreasing the temperature. The exact cloud temperature was taken as the average of these two temperatures. Starting with a concentrated sample, the cloud point as a function of composition was determined by making several dilutions and repeating the temperature cycles each time. The same procedure was applied to the upper as well the lower boundary of the single-phase microemulsion region, located between the biphasic regions with excess oil or excess water. Analysis of Lindane and Surfactant in the Sample. We used a gas chromatograph equipped with an OV-17 column (T ) 140-225 °C) to measure the amount of lindane and surfactant in the solution. Water was titrated by a Karl-Fisher method with the KF684 coulometer (from Metrhom). For the saturated solutions, we prepared a sample with excess lindane at a higher temperature and we titrated the lindane and the surfactant after cooling. For the Winsor I microemulsion (oil in water microemulsion with excess oil), we used the phase separation technique described in ref 23 to determine the composition of each phase. Knowing the cmc of C8E6 in water (9.9 × 10-3 M at 20 °C24,25) and the solubility of lindane in water (3.45 × 10-5 M26), we obtained the composition of each phase and the lindane content. Subtracting the lindane solubilized in the oil droplets, assuming this is the same composition as that in the excess oil, we deduced the molar ratio λ of lindane to surfactant in the film. X-ray Small Angle Scattering. Scattering experiments on the microemulsions were performed in borosilicate capillaries of 0.2-mm thickness for good temperature control. Flat cells of 0.2-mm thickness with Kapton walls were used for the micellar solutions at room temperature. We used a home-built HuxleyHolmes type, high-flux camera using a pinhole geometry. The X-ray source was a copper rotating anode operating at 15 kW. The K R1 radiation was selected by the combination of a nickelcovered mirror and a bent, asymmetrically cut germanium monochromator. Data correction, radial averaging, and absolute scaling were performed using routine procedures. The precise description of this camera is given in refs 27 and 28. The spectra are recorded with a two-dimensional gas detector which had an effective q-range from 0.02 to 0.4 Å-1 (q ) (4π/λ) sin θ).
Binary System Phase Diagram. Figure 1 represents the binary water-C8E6 phase diagram, schematically redrawn from (23) Testard, F.; Zemb, Th.; Strey, R. Prog. Colloid Polym. Sci. 1997, 105, 332. (24) Corkil, J. M.; Goodman, J. F.; Ottewill, R. H. Trans. Faraday Soc. 1961, 57, 1627. (25) Degiorgio V. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Academic Press: New York, 1985; pp 303-335. (26) Kan, A. T.; Tomson, M. B. Environ. Sci. Technol. 1996, 30, 1369. (27) Leflanchec, V.; Gazeau, D.; Taboury, J.; Zemb, Th. J. Appl. Crystallogr. 1996, 29, 110-117. (28) Ne´, F.; Gazeau, D.; Lambard, J.; Koksis, M.; Zemb, Th. J. of Appl. Crystallogr. 1993, 26, 763.
Solubilization of Lindane
Figure 1. Water-C8E6 phase diagram schematically redrawn from ref 29: (1) two isotropic liquid phases; (2) one isotropic liquid phase; (3) hexagonal phase.
ref 29. A large homogeneous isotropic liquid phase exists at all surfactant concentrations between T ) 20 °C and T ) 75 °C. Therefore, there are constant-temperature paths through this isotropic phase from pure water to pure surfactant without phase separation. For this system, the microstructure is made of discrete micelles at low surfactant concentration that change continuously into a connected network as the surfactant concentration increases (along path A in Figure 1).30,31 This connected network is made of random hydrocarbon tubes, with a radius given by the apolar chain length, as evidenced by a strong broad scattering peak located at 2 nm, even for the pure liquid surfactant without added water.30 The X-ray scattering spectrum of the pure nonionic surfactant shows a correlation peak at q = 2 nm; otherwise, the neutron scattering spectrum is flat because there is no neutron contrast between the head and the tails of the surfactant. Dividing X-ray by neutron scattering has allowed determination of the radius of the connected cylinders making this microstructure. Due to the high connectivity,32 it is equivalent to consider the structure as connected polar cylinders or connected hydrocarbon domains, because the volume fractions are of the same order of magnitude. Dividing X-ray by neutron scattering to get rid of the interaction peak and discuss microstructure versus the prediction of the different models has been described in ref 30. For a given surfactant volume fraction, upon raising the temperature, the micellar phase separates into a dilute phase and a concentrated phase (along path B in Figure 1, for example). The cloud curve is characteristic of such binary water-CiEj solutions33 and is attributed to a dehydration of the polar head of the surfactant as the temperature increases.34 We studied the solubilization of lindane in water-C8E6 solutions along path A and path (29) Clunie, J. S.; Corkill, J. M.; Goodman, J. F.; Symons P. C.; Tate, J. R. Trans. Faraday Soc. 1967, 63, 2839. Marland, J. S.; Mulley, B. A. J. Pharm. Pharmacol. 1971, 23, 561. (30) Barnes, I. Microstructure of bicontinuous phases in surfactant systems. Thesis from Australian National University, 1990. Zemb, T. N.; Barnes, I. S.; Derian, P.-j.; Ninham, B. W. Prog. Colloid Polym. Sci. 1990, 81, 20. (31) Zulauf, M. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions, Academic Press: New York, 1985; pp 663-673. (32) Zemb, Th. N. Colloid Surf., A, in press. (33) Mitchell, D. J.; Tiddy, G.; Waring, L.; Bostock, T.; Mc Donald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975-1000. (34) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 1982, 78, 20252042.
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Figure 2. Interfacial molar ratio λmax ) nlindane/nsurfactant versus surfactant volume fraction in water, at 20 °C, for a saturated solution in equilibrium with excess solid lindane. The line is a guide for the eye. The right abscissa scale is the molar ratio nwater/nOCH2CH2, the available water per oxyethylene unit of the surfactant headgroup. The statistical error of dosage is 5 wt % for lindane and 6 wt % for the surfactant. Therefore we have an instrumental error of 11% for λmax.
B in Figure 1, for different lindane contents, up to the saturation when solution is in equilibrium with solid lindane. Determination of the Saturation Point. The first study was along path A in Figure 1. We determined the maximum of the solubilization of lindane in a waterC8E6 binary solution at 20 °C by titration. Thus we obtained the variation of the molar ratio λmax (lindane/ surfactant), as a function of the surfactant volume fraction in the aqueous solution. The results are plotted in Figure 2. We see in this figure two different domains. In the first one, for a surfactant volume fraction between 0 and 50%, λmax is relatively constant. In the second one, for a higher surfactant volume fraction, λmax is increasing as the surfactant volume fraction in the solution increases. In the first region, the molar ratio λmax value is around 0.025 (which is equal to 0.011 in volume ratio). It corresponds to the maximum of saturation of the micelle with lindane, where each individual micelle contains, on average, 0.025 lindane molecule per surfactant molecule, that is, of the order of two lindane molecules per micelle. This ratio is independent of the micellar concentration. Introducing more lindane molecules per micelle would represent a large bending energy cost, so it is expected that the micellar aggregation number does not change with added lindane. Structural Study. At 20 °C, we conclude that the molar ratio λmax does not depend on the surfactant concentration when the volume fraction is less than 50%. To determine if there are any structural modifications of surfactant aggregate induced by solubilized lindane we studied, by SAXS, aqueous solutions of C8E6 saturated with lindane. Shown in Figure 3 are the spectra of a water-C8E6 binary solution at 5 wt %, without lindane and another saturated with lindane. These spectra are very similar, and this was also observed for other concentrations up to 30 wt % of surfactant. Therefore, the solubilization of lindane in micellar solution does not induce any structural variation of the aggregates in aggregation number or in the size of the polar layer stabilizing the micelle. Since no shift was observed in the oscillation seen in the spectra, we conclude that the addition of lindane in this regime does not decrease the hydration number of the polar head to values lower than the usual four molecules per oxyethylene group evidenced
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Figure 4. Cloud temperature of the binary system waterC8E6 alone (squares) or saturated with lindane (triangles) (λmax ) 0.096 at T ) 58 °C). The abscissa is the weight percentage of surfactant in the water-surfactant mixture.
Figure 3. (a, top) SAXS data of a 5 wt % water-C8E6 micellar solution, at 20 °C (line), compared to the same micellar solution in equilibrium with excess solid lindane. (b, bottom) SAXS data of pure C8E6 (line) and C8E6 saturated with solid lindane.
by NMR or vapor osmometry.35-37 Therefore, there is no average curvature variation induced by solubilization of lindane in the water-C8E6 solution. This was also observed for the pure surfactant, where no variation of the X-ray scattering peak between spectra of pure C8E6 and C8E6 saturated with lindane was evident, as shown in Figure 3b. Phase Diagram Variation. A second investigation of the effect of lindane solubilization was made along path B shown in Figure 1. The cloud curve was measured in the pure water-C8E6 system and again in the same system saturated with lindane (solution in equilibrium with excess solid). These cloud curves are represented in Figure 4. Over the range of surfactant concentration studied, the temperature of each cloud curve is constant within (0.2 K. Therefore, we can approximate this temperature to a cloud point temperature Tc, by comparison with the literature values.38,39 We observe a decreasing of Tc by 17 (35) Lyle, I. G.; Tiddy, G. Chem. Phys. Lett. 1986, 124,(5), 432. (36) Carlstro¨m, G.; Halle, B. J. Chem. Soc., Faraday Trans. 1 1989, 85 (5), 1049. (37) Zulauf, M.; Weckstro¨m, K.; Hayter, J.B.; Degorgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411. (38) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley Interscience Publications: New York, 1988. (39) Schubert, K. V.; Strey, R.; Kalhlweit, M. J. Colloid Interface Sci. 1991, 141, (1), 21-29.
Figure 5. Variation of the cloud temperature of the system water-C8E6 versus the molar ratio λ ) nlindane/nsurfactant of the host molecule solubilized in the micellar solution. The dashed line corresponds to the saturation limit of the solution with lindane.
°C when the water-C8E6 system is saturated with lindane. The cloud point temperature was also determined for water-C8E6 solutions with different intermediary lindane contents. We consider the ternary system water-C8E6lindane as a pseudobinary system where the molar ratio λ (lindane-surfactant) is fixed. The plot in Figure 5 shows the evolution of Tc with the molar ratio λ. We see that the Tc versus λ curve is linear until saturation of the solution with lindane. The depression of Tc is due to the increasing attraction of the effective interaction between aggregates. A similar decrease of the cloud point temperature has already been observed for hydrophilic solutes and particularly for solutes which are located at the interface.40,41 Lindane is hydrophobic, and because of its strong effect on the cloud curve, the lindane molecules are probably located, on average, in the polar layer of the micelles and induce an attraction potential between micelles roughly proportional to λ. Variation of Molar Ratio versus Temperature. In Figure 5, at 58 °C, we notice the λmax value corresponding to the saturation of the water-C8E6 solution. By comparison with the value found at 20 °C in Figure 2, we see (40) Schick, M. J. Nonionic Surfactant: Physical Chemistry; Surfactant Science Series, Vol. 23, Marcel Dekker: New York, 1987. (41) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Ogino, K. J. Colloid Interface Sci. 1992, Vol 152, (2), 402-409.
Solubilization of Lindane
Figure 6. Variation of the molar fraction λmax at saturation versus reduced temperature: TLC is the cloud point temperature for the system saturated in lindane (58 °C); (squares) λmax in molar ratio; (triangles) λmax in volume ratio. The instrumental error is 11 wt % for λmax.
that the maximum solubilization λmax increases from 0.025 to 0.91 upon increasing the temperature by 38 °C. To determine the evolution of λmax versus temperature between these two points, we determined the amount of each component in the saturated solutions of water-C8E6 at 15 wt % in the temperature range 20-58 °C. We plot the results in Figure 6. We observe a linear decrease of λmax with the reduced parameter (TLC - T)/TLC, where TLC is the temperature of the cloud curve for the waterC8E6 system saturated with lindane. This dependence of λmax on the reduced temperature shows that λmax must be an important themodynamic parameter of the system. Comparison of the Solubility Value of Lindane in a Solvent. Coming back to Figure 2, we observe a large increase of λmax for surfactant volume fractions larger than 50%. Figure 2 shows also in parallel the molar ratio of water per “OCH2CH2 group”, which is the maximum hydration number. The λmax value decreases with an increase in the hydration number of the polar head of C8E6. Thus, it seems that the lindane is expelled by the water of hydration. The situation here is similar to the water-lindane competition in clays already observed in studies of pesticides.42,43 We find a similar result if we examine the solubilization of lindane in pure polyglycol E6, that is the isolated headgroups of the surfactant. In Figure 7, we compare the solubility of lindane in different molecular systems as a function of temperature. First, we see that the solubilization is always enthalpy driven; that is, the slope of the curve is always positive. Adding water to pure E6 decreases the maximum amount of lindane solubilized in the mixture E6 + water. When we add 1% or 3% water to E6 (saturated with lindane), we observe a lower value for the molar ratio at saturation. This is again due to the competition between water and lindane, up to the point where all the headgroups are fully hydrated. Moreover, it can be seen in Figure 7 that the saturation value λmax in pure C8E6 is different from the sum of saturation values obtained in octane and in E6, the two isolated parts of the surfactant molecule. Thus, the excess of solubilization observed when the polar heads and the tails are associated to form the surfactant molecules is clearly due to molecular organization. (42) Barlow, F.; Hadaway, A. B. Nature, 1956, 8, 1299-1300. (43) Lee, J. F.; Mortland, M. M.; Boyd, S. A.; Chiou, C. T. J. Chem. Soc., Faraday. Trans. 1, 1989, 85 (9), 2953.
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Figure 7. Temperature dependence of the maximum solubility expressed as the molar ratio λmax in several binary systems. (squares) λmax ) nlindane/noctane; (circles) λmax ) nlindane/nE6 (E6 headgroups only); (crosses) the same with the headgroups E6 including 1, 2, 4, and 8 wt % of added water; (diamonds) λmax ) nlindane/nsurfactant for a micellar solution at 15 wt %; (triangle) case of pure surfactant C8E6. The instrumental error is about 11 wt % for λmax.
Binary mixtures of surfactant and water do not show solubilization powers larger than 1 solute for 40 surfactants. We now compare these values to the solubilization of lindane in water-oil-CiEj ternary microemulsions. Ternary System Zero Spontaneous Curvature Bicontinuous Microemulsion. We have previously demonstrated, in ref 23, the existence of excess solubilization of lindane in a bicontinuous microemulsion with a zero average curvature. This excess has been quantified by several techniques for water-(cyclohexane + lindane)-C6E5 microemulsions. If we consider Figure 8b, we see again that the maximum amount of lindane solubilized in a microemulsion exceeds the amount solubilized in the same amount of pure oil. Therefore, there is an excess of solubilization in a bicontinuous water-(cyclohexane + lindane)-C8E6 microemulsion with a zero spontaneous curvature. In Figure 8a, we report for a water-(cyclohexane + lindane)-C8E6 ternary system a part of the pseudobinary phase diagram at equal water to oil volume fraction for a different initial weight fraction β of lindane in the oil (cyclohexane + lindane). This figure is focused on the end of the “fish tail”, the region close to the point (γ˜ , T ˜ ), with γ˜ representing the minimum amount of surfactant needed to solubilize a mixture of equal volume of water and oil and T ˜ representing the temperature of zero spontaneous curvature. We verified that the three-phase region still exists for each composition in lindane. In Figure 8b, we have then plotted the temperature of zero spontaneous curvature of the bicontinuous microemulsion water-(cyclohexane + lindane)-C8E6 for different initial weight fractions β of lindane in the oil (cyclohexane + lindane) superposed to the solubility curve of lindane in cyclohexane. The experimental procedure used is described in ref 23 using the knowledge of the phase diagram of the water-oil-CiEj system as determined by Kalweiht studies.15 Stable microemulsions exist with a larger amount of lindane relative to the oil (β) than the saturation value (βS) in the same oil alone and at the same temperature. At saturation, the composition of oil is initially β ) 11.5% at T ) 37.5 °C. At this temperature, the maximum solubility of lindane in bulk cyclohexane is
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Figure 9. SAXS of the microemulsion water-(cyclohexane + lindane)-C8E6 at zero spontaneous curvature for three different initial weight compositions β of lindane in oil. The spectra are on a logarithmic scale. The straight line is an ideal Porod behavior, varying as q-4. Table 1. Composition of the Two Phases in the Winsor I Equilibrium, That Is, in the Microemulsion (µE) and in the Upper Oil Phase sample I µE I oil
Figure 8. (a, top) part of the pseudobinary phase diagram of the water-(cyclohexane + lindane)-C8E6 system. The figure is focused on the fish tail, the region close to the point. Around this point we have four domains: one monophasic domain (denoted by 1), two two-phase domains: (Winsor I, denoted by 2, and Winsor II, denoted by 2), and a Winsor III region just schematically drawn for one system (denoted by 3). Samples are prepared at equal volume to oil ratios. (b, bottom) Variation of the temperature of zero spontaneous curvature in water(cyclohexane + lindane)-C8E6 (dark squares) versus the initial weight composition β of lindane in the oil. The dashed line represents the maximum solubility of lindane in bulk cyclohexane. The plot for water-(cyclohexane + lindane)-C10E8 is drawn too (circles).
βS ) 6.8% (the solubility of lindane in water is negligible compared to these values). Therefore, all of the excess lindane is in the surfactant film. By subtraction, we obtain the saturation value λmax ) 0.28. Investigating the microstructure of these two samples by SAXS, we can see in Figure 9 that when more lindane is solubilized in a ternary microemulsion at the temperature at which the spontaneous curvature is zero, the diffusion of the film increases and does not follow the Porod behavior. The diffusion of the film is becoming more and more important.44,45 This demonstrates either a penetration of lindane in the film or a dehydration of the headgroups. We did similar experiments,23 to investigate the dependence of the temperature of zero spontaneous curvature T ˜ on the lindane content of the oil, using toluene as (44) Auvray, L.; Cotton, J. P.; Ober, R.; Taupin, C. J. Phys. 1984, 45, 913. (45) Teubner, M. J. Chem. Phys. 1990, 92, 4501.
water wt %
cyclohexane wt %
surfactant wt %
lindane wt %
54
20.8
21.2 2.75
4 6
oil instead of cyclohexane, for the C6E5 surfactant. The result is opposite in two cases, demonstrating that with cyclohexane T ˜ decreases with β, the amount of lindane in the oil, while T ˜ increases in the case of toluene. The T ˜ variation is linked to the penetration power of the oil; therefore, we demonstrated that the penetration power of lindane was between those of cyclohexane and toluene. Winsor I Microemulsion. We now turn to the determination of the maximum excess solubilization when the microstructure is a Winsor I microemulsion, that is, an oil in water microemulsion where an excess of oil exists, due to emulsification failure. We prepared the solutions by mixing a volume V of water + surfactant with 30 wt % surfactant and a volume V′ of cyclohexane saturated with lindane. After mixing and cycling the temperature above 20 °C, we let the solid lindane reprecipitate. Then, by determining the compositions of the excess oil and of the microemulsion phase, we obtained, by difference, the molar ratio λmax of the excess solubilization for the saturated Winsor I system. The compositions of the demixed sample are given in Table 1. We make the usual assumption that the oil microdomain in the microemulsion and the excess bulk oil contain the same concentration of lindane. As in the bicontinuous microemulsion at zero spontaneous curvature, we observe an excess of solubilization for such a microemulsion. The radius of the oil droplets is derived by the composition of the sample using the equation R ) 3Φ/Σ, where Φ is the apolar volume fraction and Σ is the total interface per unit volume, obtained from the product of the surfactant at the interface and the known headgroup area per C8E6 surfactant.46 The radius (R ) 51 Å) obtained by this procedure is quite coherent with the scattering peak obtained by SAXS (Table 2). To demonstrate this, we have fitted the experimental curve using a sphere form factor and a hard sphere potential for the structural factor, (46) Sottmann, T.; Strey, R.; Chen, S.-H. J. Chem. Phys. 1997, 106 (15), 6483.
Solubilization of Lindane
Langmuir, Vol. 14, No. 12, 1998 3181
Table 2. Interfacial Composition λmax and Average Droplet Radius Obtained from Dosage and from Fitting the Observed Scattering by a Monodisperse o/w Hard Sphere Dispersiona sample
λmax
R ) 3Φ/Σ
Rfit
σSD
I µE
0.17
51 Å
56 Å
118 Å
a φ is the apolar volume fraction, Σ is the specific area, deduced from surfactant content and area per molecule, R is the scattering radius of the microemulsion droplet, mainly the apolar part of high electronic density, and σSD is the hard sphere diameter including part of the interfacial film.
Figure 11. Molar ratio λmax ) nlindane/nsurfactant at saturation versus reduced curvature l/R, where l is the C8 hydrophobic chain length (Tanford’s formula) and R is the radius of the droplet, equal to the average curvature of the surfactant film.
Figure 10. SAXS experiment and best fitting to hard spheres for sample 1: an oil in water microemulsion of water(cyclohexane + lindane)-C8E6.
using the classical Hayter-Penfold technique with one shell, where the radius of the doplet is the only free parameter. In Figure 10, the experimental spectra and the best fit are plotted together: the best fit gives a microemulsion droplet radius of 56 Å. Conclusion The first indirect link between curvature and solubilization has been given by Fletcher47 using AOT reverse micelles with salts, or p-nitroaniline as solute and by (47) Fletcher, P.D. I. J. Chem. Soc., Faraday Trans 1 1986, 82, 2651.
Hatton on AOT reverse micelles with amino acids as a solute.20-22 They demonstrated a curvature-dependent interfacial partition coefficient in a AOT w/o microemulsion system. The experiments presented here allow a comparison of the solubility limit of a guest molecule in three surfactant phases at different curvatures. Plotting the maximum molar ratio of solubilized lindane versus the radius of curvature shows that the maximum solubilization is a strongly decreasing function of the surfactant aggregate curvature, as shown in Figure 11. This crucial relation, demonstrated first with amino acids in AOT reverse micelles, is here demonstrated on a larger scale in a nonionic system. This behavior is most likely universal and follows the work of Ninham and coworkers, who showed some 20 years ago that the chemical potential of any surfactant aggregate can be developed as a function of either the interfacial curvature 〈H〉, or the surfactant parameter p.1,19 In the same way, the maximum number of solubilized molecules, obtained when a surfactant system is in equilibrium with excess solute, is also a nearly linear function of the curvature. LA971006D
