Water System

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The 1-Monooleoyl-rac-glycerol/n-Octyl-β-D-Glucoside/Water System. Phase Diagram and Phase Structures Determined by NMR and X-ray Diffraction Gerd Persson,*,† Håkan Edlund,† Heinz Amenitsch,‡ Peter Laggner,‡ and Go¨ran Lindblom§ Department of Natural and Environmental Sciences, Chemistry, Mid Sweden University, SE-851 70 Sundsvall, Sweden, and Austrian Academy of Science, Institute for Biophysics and X-ray Structure Research, Schmiedlstrasse 6, A-8042 Graz, Austria, and Department of Biophysical Chemistry, Umeå University, SE-901 87 Umeå, Sweden Received March 21, 2003. In Final Form: May 1, 2003 Using SAXD and NMR techniques as well as visual observations, a detailed study of the extension of the phase regions and their structures has been performed for the ternary system 1-monooleoyl-racglycerol (MO)/n-octyl-β-D-glucopyranoside (OG)/2H2O at 25 °C. OG in water forms a large region of a micellar solution phase, in which substantial amounts of MO can be dissolved. Increasing the MO concentration at constant water content results in the formation of two- and three-phase areas, consisting of liquid, lamellar, or cubic phases in equilibrium with a very dilute water/OG solution. Besides the different phases previously reported for the binary systems, an additional hexagonal phase occurs at high OG contents. Addition of minor amounts (≈1.5 wt %) of OG converts the cubic phases present in the MO/2H2O system to an LR phase, while the cubic phase in the OG/2H2O system is able to dissolve as much as 15 wt % MO. Since a major part of the phase diagram consists of planar bilayers, it is concluded that for most MO/OG ratios the spontaneous curvature is close to zero. The reason for this is discussed in terms of the molecular packing of MO and OG.

Introduction In recent years a great interest has been devoted to the study of amphiphiles with sugar headgroups, often in systems containing other surfactants or lipids and water. In this Introduction we will just briefly touch upon some of the most important achievements. Alkylglucosides (AGs) are nonionic surfactants that can be synthesized from renewable resources, and they are usually biodegradable.1 They are regarded as mild surfactants, suitable for a number of applications such as washing2 and cosmetic products3 as well as for solubilizing and reconstitution of membrane proteins.4-6 Though AGs have been known for more than a century,7,8 they have been less studied than the poly(ethylene oxide)-based surfactants. In many respects AGs do behave similar to the poly(ethylene oxide)based surfactants, but there are also significant differences.1,9 A lot of work has been published on commercial products, which often contain mixtures of AGs with different chain lengths and headgroups, having more than one glucoside unit. Moreover, binary systems with octyl-, * Corresponding author. Fax: +46 60 14 88 02. E-mail: [email protected]. † Mid Sweden University. ‡ Austrian Academy of Science. § Umeå University. (1) von Rybinski, W. Curr. Opin. Colloid Interface Sci. 1996, 1, 587. (2) Andree, H.; Middelhauve, B. Tenside, Surfactants, Deterg. 1991, 28, 413. (3) Busch, P.; Hensen, H.; Tesmann, H. Tenside, Surfactants, Deterg. 1993, 30, 116. (4) Helenius, A.; Fries, E.; Kartenbeck, J. J. Cell. Biol. 1977, 75 (3), 866. (5) Helenius, A.; McCaslin, D. R.; Fries, E.; Tanford, C. Methods Enzymol. 1979, 56, 734. (6) Rosenow, M. A.; Magee, C. L.; Williams, J. C.; Allen, J. P. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2002, 58, 2076. (7) Fischer, E. Chem. Ber. 1893, 26, 2400. (8) Fischer, E. Chem. Ber. 1895, 28, 1145. (9) Ryan, L. D.; Kaler, E. W. Colloids Surf., A 2001, 176, 69.

nonyl-, or decyl-β-glucoside and water,10-16 as well as ternary mixtures containing these surfactants, have been studied extensively.9,17-20 The binary n-octyl-β-D-glucopyranoside (OG)/2H2O phase diagram10,11 consists of a large micellar solution phase followed by a hexagonal phase, a bicontinuous cubic phase, and a lamellar liquid crystalline (LR) phase with increasing OG concentration. However, the hexagonal phase has a low melting point and disappears at temperatures above 23 °C. A miscibility gap was observed in the dilute regime of the binary system of decyl-β-glucoside/water, as well as for the longer homologues.9,14,15 No phase separation occurs in the dilute regime in the binary system of nonyl-β-glucoside/water.15 However, a miscibility gap exists in the ternary system with nonyl-β-glucoside/ decyl-β-glucoside/water,17 and this system shows an unusually large 1H/2H-isotope effect.18 Addition of nonanol to nonyl-β-glycoside and water also induces a separation into two liquids.19 Miscibility gaps are also found in the ternary OG/ether oil/water systems.9 The mechanism proposed for the phase separation occurring for poly(10) Sakya, P.; Seddon, J. M.; Templer, R. H. J. Phys. II 1994, 4, 1311. (11) Nilsson, F.; So¨derman, O.; Johansson, I. Langmuir 1996, 12, 902. (12) Zhang, R.; Marone, P. A.; Thiyagarajan, P.; Tiede, D. M. Langmuir 1999, 15, 7510. (13) Kocherbitov, V.; So¨derman, O.; Wadso¨, L. J. Phys. Chem. B 2002, 106, 2910. (14) Balzer, D. Langmuir 1993, 9, 3375. (15) Nilsson, F.; So¨derman, O.; Hansson, P.; Johansson, I. Langmuir 1998, 14, 4050. (16) Kameyama, K.; Takagi, T. J. Colloid Interface Sci. 1990, 137, 1. (17) Nilsson, F.; So¨derman, O.; Reimer, J. Langmuir 1998, 14, 6396. (18) Whiddon, C.; So¨derman, O. Langmuir 2001, 17, 1803. (19) Sottman, T.; Kluge, K.; Strey, R.; Reimer, J.; So¨derman, O. Langmuir 2002, 18, 3058. (20) Whiddon, C.; So¨derman, O.; Hansson, P. Langmuir 2002, 18, 4610.

10.1021/la034492f CCC: $25.00 © 2003 American Chemical Society Published on Web 06/03/2003

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(ethylene oxide)-based surfactants21 involves a conformational change in the headgroup, so that it becomes less hydrophilic. For the decyl-β-glucoside/water system a different mechanism based on the formation of a micellar network was proposed.15 1-Monooleoyl-rac-glycerol (MO) is an amphiphilic lipid that is almost insoluble in water, and a rich phase behavior, including lamellar, hexagonal, as well as cubic structures, is observed.22,23 The technical grade has been used as an emulsifying agent and a food additive since the 1950s, and it has also been utilized as a coamphiphile in pharmaceutical applications.24 The liquid crystalline structures formed by MO have been proposed to be utilized as alternatives to lipid vesicles for administration of drugs, providing slow release25-28 and stabilization of sensitive substances.27,29-31 Another potentially interesting application for these compounds is the use of them in electrochemical biosensors based on enzymes entrapped in a cubic phase.32 The cubic phase can be dispersed in excess water, producing a suspension of so-called cubosomes.33,34 A stabilization of the cubic phase can be obtained by a polymerization of the lipids building up the cubic structure.35 Another recent, important application of the MO-rich cubic phase is to use it as a medium for protein crystallization.36 To fully utilize these applications, several aspects must be thoroughly studied and a detailed knowledge of the phase behavior must be obtained. Several more or less complete ternary systems involving MO can be found in the literature, and they may contain from small- and medium-sized molecules and ions37-52 to polymers28,53,54 and proteins.29-32,55-59 Generally, inorganic (21) Andersson, M.; Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4957. (22) Hyde, S. T.; Andersson, S.; Ericsson, B.; Larsson, K. Z. Kristallogr. 1984, 168, 213. (23) Qiu H.; Caffrey, M. Biomaterials 2000, 21, 223. (24) Ganem-Quintanar, A.; Quintanar-Guerrero, D.; Buri, P. Drug Dev. Ind. Pharm. 2000, 26, 809. (25) Engstro¨m, S.; Larsson, K.; Lindman, B. European Patent 0 126 751, 1984. (26) Norling, T.; Lading, P.; Engstro¨m, S.; Larsson, K.; Krog, N.; Nissen, S. S. J. Clin. Periodontol. 1992, 19, 687. (27) Caboi, F.; Murgia, S.; Monduzzi, M.; Lazzari, P. Langmuir 2002, 18, 7916. (28) Johansson, A. K.; Linse, P.; Piculell, L.; Engstro¨m, S. J. Phys. Chem. B 2001, 105, 12157. (29) Nylander, T.; Mattisson, C.; Razumas, V.; Miezis, Y.; Håkansson, B. Colloids Surf., A 1996, 114, 311. (30) Sadhale, Y.; Shah, J. C. Int. J. Pharm. 1999, 191, 51. (31) Sadhale, Y.; Shah, J. C. Int. J. Pharm. 1999, 191, 65. (32) Razumas, V.; Kanapieniene´, J.; Nylander, T.; Engstro¨m, S.; Larsson, K. Anal. Chim. Acta 1994, 289, 155-162. (33) Ljusberg-Wahren, H.; Nyberg, L.; Larsson, K. Chim. Oggi 1996, 14, 40. (34) Larsson, K. J. Dispersion Sci. Technol. 1999, 20, 27. (35) Srisiri, W.; Benedicto, A.; O’Brien, D. F.; Trouard, T. P.; Ora¨dd, G.; Persson, S.; Lindblom, G. Langmuir 1998, 14, 1921. (36) Landau, E.; Rosenbusch, J. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 14535. (37) Takahashi, H.; Matsuo, A.; Hatta, I. Mol. Cryst. Liq. Cryst. 2000, 347, 231. (38) Saturni, L.; Rustichelli, F.; Di Gregorio, G. M.; Cordone, L.; Mariani, P. Phys. Rew. E 2001, 64, 040902(2). (39) Sva¨rd, M.; Schurtenberger, P.; Fontell, K.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1988, 92, 2261. (40) Eriksson, P.-O.; Lindblom, G. Biophys. J. 1993, 64, 129. (41) Gustafsson, J.; Ora¨dd, G.; Nyde´n, M.; Hansson, P.; Almgren, M. Langmuir 1998, 14, 4987. (42) Angelov, B.; Ollivon, M.; Angelova, A. Langmuir 1999, 15, 8225. (43) Aota-Nakano, Y.; Jie Li, S.; Yamazaki, M. Biochim. Biophys. Acta 1999, 1461, 96. (44) Barauskas, J.; Razumas, V.; Nylander, T. Chem. Phys. Lipids 1999, 97, 167. (45) Gustafsson, J.; Nylander, T.; Almgren, M.; Ljusberg-Wahren, H. J. Colloid Interface Sci. 1999, 211, 326. (46) Ai, X.; Caffrey, M. Biophys. J. 2000, 79, 394. (47) Borne´, J.; Nylander, T.; Khan, A. Langmuir 2000, 16, 10044. (48) Pitzalis, P.; Monduzzi, M.; Krog, N.; Larsson, H.; LjusbergWahren, H.; Nylander, T. Langmuir 2000, 16, 6358. (49) Borne´, J.; Nylander, T.; Khan, A. Langmuir 2001, 17, 7742.

Persson et al.

Figure 1. (a) n-Octyl-β-D-glucopyranoside (OG). (b) 1-Monooleoyl-rac-glycerol (MO).

ions37 and small molecules such as urea37 and sugars37,38,46 can be incorporated in substantial amounts (mol/L) into the cubic phases without causing major structural changes. The effect of larger molecules and proteins is highly dependent on the physicochemical properties of the added substance. The majority of these studies focus on the MOrich cubic phases. A number of interesting features are found in systems where the third component is another amphiphile. Thus, with oleic acid a cubic phase, built up of reversed micelles, of space group Fd3m49 with a very narrow stability range develops, while sodium oleate promotes the formation of a second, large LR phase area.49 In the MO/dioleoylphosphatidylcholine/2H2O system a very large region of a cubic phase is found.52 Extensive swelling of the cubic phase and a defect LR phase was found in the C16TAB/MO/water system, and sodium cholate in 0.9% brine yields an L3 phase.45 L3 phases are also found in MO/solvent/water systems for solvents such as PEG 400,53,54 ethanol, and DMSO.53 In multicomponent systems it has been found that MO forms microemulsions with AGs60 and improve fat solubilization with bile salts.61 Considering the possible applications of MO-containing systems, there is a need for further studies of such systems and, in particular, those enclosing a second amphiphile. Therefore, in this work we present a detailed investigation of the MO/OG/water system. Materials and Methods Materials. n-Octyl-β-D-glucopyranoside (OG) (>98% purity) (Figure 1a) and 1-monooleoyl-rac-glycerol (MO) (>99% purity) (Figure 1b) were purchased from Sigma Aldrich Chemie Gmbh, Germany, and the substances were used without further purification. 2H2O (99.9% in 2H) was obtained from Cambridge Isotope Laboratories, USA. Sample Preparation. The phase diagram was first roughly scanned, using nine samples of different MO/OG composition, which were diluted by successive additions of 2H2O. Each addition was followed by an equilibrium time of 1 week, during which the samples were stored in a water bath at 25 ( 0.2 °C, in darkness. These samples were prepared in 8-mm glass test tubes sealed (50) Caboi, F.; Amico, G. S.; Pitzalis, P.; Monduzzi, M.; Nylander, T.; Larsson, K. Chem. Phys. Lipids 2001, 109, 47. (51) Razumas, V.; Niaura, G.; Talaikyte, Z.; Vagonis, A.; Nylander, T. Biophys. Chem. 2001, 90, 75. (52) Gutman, H.; Arvidson, G.; Fontell, K.; Lindblom, G. In Surfactants in solutions; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 1, pp 143-152. (53) Engstro¨m, S.; Alfons, K.; Rasmusson, M.; Ljusberg-Wahren, H. Prog. Colloid Polym. Sci. 1998, 108, 93. (54) Evertsson, H.; Stilbs, P.; Lindblom, G.; Engstro¨m, S. Colloids Surf., B: Biointerfaces 2001, 26, 21. (55) Ericsson, B.; Larsson, K.; Fontell, K. Biochim. Biophys. Acta 1983, 729, 23. (56) Leslie, S. B.; Puvvada, S.; Ratna, B. R.; Rudolph, A. S. Biochim. Biophys. Acta 1996, 1285, 246. (57) Razumas, V.; Larsson, K.; Miezis, Y.; Nylander, T. J. Phys. Chem. 1996, 100, 11766. (58) Pebay-Peyroula, E.; Neutze, R.; Landau, E. Biochim. Biophys. Acta 2000, 1460, 119. (59) Tsapis, N.; Reiss-Husson, F.; Ober, O.; Genest, M.; Hodges, R. S.; Urbach, W. Biophys. J. 2001, 81, 1613. (60) von Rybinski, W.; Guckenbiehl, B.; Tesmann, H. Colloids Surf., A 1998, 142, 333. (61) Patton, J. S.; Carey, M. C. Science 1979, 204, 145.

Phase Diagram of Monoolein/Octylglucoside/Water

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with removable caps. A second set of samples were prepared by weighing the appropriate amounts of each substance into 8-mm glass tubes, which were flame-sealed and equilibrated in a dry bath at 25 ( 1 °C, in darkness. All samples were inspected both visually and between crossed polarizers to check homogeneity and birefringency, and the anisotropic samples were investigated by 2H NMR spectroscopy. The samples for investigations with 1H NMR spectroscopy were prepared in 8-mm test tubes, sealed with removable caps, and after an incubation time of 7 days at 25 ( 1 °C they were transferred into 5-mm NMR tubes. When more than one phase is present, the samples were separated into the constituent phases prior to measurement. The samples for the SAXD investigations were prepared in 8-mm test tubes sealed with removable caps, homogenized by repeated rotation in a table centrifuge, and stored at 25 ( 1 °C in darkness. All samples were inspected both visually and between crossed polarizers to check homogeneity and birefringency. This procedure was completed within 1 week. The samples were then cooled to 5 °C for 12 h, and then transferred to 1.5 mm capillaries, and finally stored at 25 °C in darkness for 4 days prior to measurements. Methods. Microscopy. Polarizing microscopy is a wellestablished technique to identify liquid crystalline phases.62 Anisotropic phases display distinct textures, such as the mosaic or Maltese cross patterns obtained for LR phases, and the fanlike one produced by hexagonal phases. Isotropic phases show only a dark background with no well-characterized texture. Microscopic studies were performed using an Olympus BH2-UMA microscope equipped with a Kappa CF 8/1 DX video camera and KappaImageBase software. NMR. All NMR experiments were performed on a Bruker Avance DPX 250 MHz NMR spectrometer equipped with a variable temperature control unit. At 25 °C the thermal stability was (1 °C measured over a period of 20 min. For the 2H experiments we used the standard Bruker 10-mm probe operating at 38.4 MHz. The recording of 1H NMR spectra was performed using a standard Bruker 5-mm probe operating at 250 MHz, and a standard Bruker diffusion probe equipped with a 10-mm coil was used to measure the translational diffusion coefficients. The 2H NMR method is a well-established, noninvasive method for investigating anisotropic, as well as multiphase, samples.63 The deuterium nucleus has a spin of 1 and, thus, possesses an electric quadrupole moment, which interacts with electric field gradients at the nucleus. Depending on the geometry of the liquid crystal, the resulting NMR signal will be a singlet or it is split into a doublet. When several different liquid crystalline phases are coexisting and the water exchange between these phases is slow on the NMR time scale, the resulting spectrum will be a superposition of spectra from the individual phases. If several deuterated sites are present within a single phase and the exchange of deuterium is slow on the NMR time scale, each site will produce a signal, either a singlet or a doublet, that may be shifted depending on the surrounding environment, as for the chemical shifts observed in proton NMR spectroscopy. The 2H NMR experiment was repeated at regular intervals, until no further change in the spectra could be detected.1H NMR was used to investigate the composition of the phases in the one- and two-phase areas in the aqueous corner of the phase diagram. The fractions of water, MO, and OG can be estimated from the integrals of the HDO, MO, and OG peaks. This will, however, only yield a qualitative estimate of the phase composition. In the self-diffusion experiment we used the stimulated spinecho pulse sequence.64,65 The self-diffusion coefficient, D, was obtained by fitting eq 1 to the measured NMR data.

I ) I0e{-(gGδ) D(∆-δ/3)} 2

(1)

(62) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628. (63) Lindblom, G. In Advances in Lipid methodology; Christie, W. W., Ed.; Oily Press: Dundee, Scotland, 1996; Vol. 3, pp 133-209. (64) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445. (65) Lindblom, G.; Ora¨dd, G. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 483.

where I denotes the observed echo intensity, I0 is the echo intensity in the absence of field gradient pulses, g is the magnetogyric ratio, G is the field gradient strength, δ is the duration of the gradient pulse, and ∆ is the time between the leading edges of the gradient pulses. In each experiment we used a fixed δ and a variable gradient G. ∆ was always set to 100 ms, G can be varied between 0.05 and 2.40 T/m, and depending on the expected diffusion coefficients, δ was set to a value between 1.0 and 3.5 ms. SAXD. The small-angle X-ray diffraction, SAXD, experiments were performed at the Austrian Academy SAXS station at the ELETTRA synchrotron, Trieste, Italy,66 using the 8 keV beam, corresponding to a wavelength of 0.154 nm. The temperature was controlled by an in-line DSC unit (Microcalix, CNRS, Paris, France; when used isothermally, a thermal stability < 0.1 °C is achieved). To ensure that a powder pattern is obtained, the capillary was oscillated around its axis during accumulation of one frame, with an angular velocity of 1.08 rad/s, using a purposebuilt device. For the isothermal samples the total exposure time was about 60 s per sample. The temperature-scanning rate used was 1 °C/min, and typically one frame per °C was accumulated. For a two-dimensional hexagonal lattice the Bragg reflections are related to the unit cell dimension, a, by the relation

dhk )

x3 2

a

x(h

2

(2)

+ k2 + hk)

where (h,k) are the Miller indices. The radius of the hydrocarbon core, rhc, of the rodlike micelles is obtained from15

x

x3φhc 2π

rhc ) a

(3)

where φhc is the volume fraction of the hydrocarbon core, and the area per headgroup, AH1, can be determined from

2Vchain rhc

AH1 )

(4)

where Vchain is the volume of one hydrocarbon chain or, in a mixture, the weighted mean value. The lamellar structure yields a diffraction pattern consisting of a number of equidistant peaks at 1:2:3, and the thickness of the hydrocarbon part (2rhc) and the area per headgroup, ALR, can be calculated from15

2rhc ) aφhc

(5)

Vchain rhc

(6)

and

ALR )

There are a variety of different cubic structures. Depending on the location in the phase diagram, they are built from either discrete aggregates or a bicontinuous network. For a cubic lattice with lattice parameter, a, the Bragg reflections are given by

dhkl )

a

xh

2

(7)

+ k2 + l2

where l is the third Miller index. The first three reflections of the Pn3m structure appear at (h,k,l) ) (1,1,0), (1,1,1), and (2,0,0), respectively. For the Ia3d structure the first four reflections appear at (2,1,1), (2,2,0), (3,2,1), and (4,0,0).67 TLC. The SAXD samples were checked for purity and radiation damage by TLC in a mixture of hexane, diethyl ether, and acetic acid in the ratio 80:20:1 (v/v/v), using a 20 cm long and 0.2 mm (66) (a) Preliminary SAXD measurements were performed with a standard laboratory small-angle X-ray camera (SWAX camera, HecusM.Braun Graz, Austria). (b) Amenitsch, H.; Bernstorff, S.; Kriechbaum, M.; Lombardo, D.; Mio, H.; Rappolt, M.; Laggner, P. J. Appl. Crystallogr. 1997, 30, 872. (67) Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989, 988, 221.

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Figure 3. Aqueous-rich corner of the phase diagram. Abbreviations: L1, isotropic solution phase; W, dilute OG solution or pure water; R, lamellar dispersion; cub, MO-rich cubic phases. Note that the two- and three-phase areas containing MO-rich cubic phases are not shown. Reproduced with permission from ref 79. Copyright 2003 Springer Verlag.

Figure 2. (a) Isothermal ternary phase diagram of the MO/ OG/2H2O system (25 °C): (+) locations of samples used in this study; (b) locations of the samples for which the self-diffusion coefficients are measured; (4) locations of the SAXD samples. Abbreviations: L1, isotropic solution phase; HI, OG-rich normal hexagonal phase; C1, OG-rich cubic phase; C2, MO-rich cubic phase of space group Pn3m; C3, MO-rich cubic phase of space group Ia3d; LR, lamellar liquid crystalline phase. (b) Tentative two- and three-phase areas. The dotted area indicates the part of the L1 phase where flow birefringency is observed. The equimolar line is also indicated. thick silica plate. As references we used MO, oleic acid, and OG dissolved in diethyl ether. Spots were developed by spraying the plate with PMOS (5 g of phosphomolybdic acid in 200 mL of 1 M sulfuric acid solution) followed by heating. Samples containing high concentrations of MO yielded, besides a large spot of MO, two unidentified spots and a third corresponding to oleic acid, showing that there is some small degradation of MO. However, a comparison with unexposed samples shows that this degradation is not a result of the exposure to X-rays but rather a result of the presence of water, leading to hydrolysis of MO. We have not observed any major changes in the phase behavior with time within the time frame of our study.50

Results General Appearance of the Phase Diagram. At low water content the phase diagram (Figure 2) is dominated by an LR phase, which extends from the MO- to the OG-

rich sides. At the OG-rich side of the equimolar line, a large solution phase appears, as well as two nonlamellar liquid crystalline phases, namely a normal hexagonal phase (H1) and a bicontinuous cubic phase (C1). The MOrich side shows, besides the LR phase, two bicontinuous cubic phases (C2 and C3). On the MO-rich side of the equimolar line toward high water content, the lamellar structure persists as one of the constituents in the multiphase areas. The sample properties gradually change as the water content increases from the opaque, birefringent, low-viscous appearance generally observed for LR phases to a white, nontransparent and rather viscoelastic material (R). Due to the difficulties to assign the 2 H NMR peaks, especially at low water contents, the twoand three-phase areas indicated in Figures 2b and 3 are tentative, mainly based on visual inspection. Isotropic Solution Phase (L1). In the binary OG/ 2 H2O system a large micellar solution phase is extending up to 70 wt % surfactant at 25 °C. The OG micelles can solubilize a relatively large fraction of MO. An isotropic solution is formed up to a molar ratio of MO/OG between 1:1.8 and 1:1.5, depending on the water content. At higher fractions of MO, a separation into two isotropic liquid phases occurs. Close to the liquid-liquid two-phase region, the samples appear bluish at high water content, while at low water concentration the samples are optically clear and show flow birefringency with a short relaxation time (indicated as the dotted area in Figure 2b). This area is not well-defined, since the observed flow birefringency increases gradually with increasing MO content and decreasing water content. Self-diffusion measurements were performed on two sets of samples, one at a constant MO/OG ratio of 80:20 (wt %) with varying water contents (Figure 4), and a second one at a constant OG/2H2O ratio of 40:60 (wt %) with varying MO contents (Figure 5). Some samples close to the liquid-liquid two-phase region (see next section) were also investigated with this method. Figure 4 shows the measured self-diffusion coefficients for OG, DOG. A simple two-site model was used to account for the presence of free surfactant

Dobs ) PDmic + (1 - P)Dfree

(8)

where Dobs is the observed diffusion coefficient, Dmic is the micellar diffusion coefficient, and Dfree is the diffusion coefficient measured below the cmc for OG (3.64 × 10-10


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Figure 4. DOG at constant MO/OG ratio equal to 1:4.4. The dashed line indicates the calculated result for spherical micelles using r ) 3.15 nm and k ) 1.7.

Figure 5. DOG (O) and DMO (2) at constant OG/2H2O ratio 1:21. Acceptable data were not obtained at MO contents below 10 wt %, due to the low intensity of the MO peak in the NMR spectrum.

m2/s). P is the fraction of micellized surfactant, given by

P)

Ctot - cmc Ctot

(9)

where Ctot is the total OG concentration and cmc ) 25 mM.68 We measured the cmc in the MO/OG solution, resulting in the same value as that for the pure surfactant. The translational self-diffusion coefficient of a sphere at infinite dilution, D0mic, can be obtained from the Stokes-Einstein equation. At finite aggregate concentrations, obstruction effects have to be accounted for. For spherical aggregates this can be accomplished by

Dmic ) D 0mic(1 - kφagg)

(10)

where k is a constant and φagg is the volume fraction of the micelles. For hard spheres, according to theoretical predictions, the value of k is between 1 and 2.5.69 However, if the micellar hydration is not included in the volume fraction, an apparent value of k must be used.70 We have evaluated eq 10 for the two k-values of 1.7 and 3.4. Further, we used two radii (2.91 and 3.15 nm) corresponding to the fully extended MO molecule with and without a hydration layer. The self-diffusion data were also evaluated for a prolate as well as an oblate geometry of the micelles according to procedures described in ref 71. However, it was not possible to conclusively determine the shape of the aggregates formed. (68) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J. Phys. Chem. 1959, 63, 648. (69) Evans, G. T.; James, C. P. J. Chem. Phys. 1983, 79, 5553. (70) Ora¨dd, G.; Lindblom, G.; Johansson, L.; Wikander, G. J. Phys. Chem. 1992, 96, 5170.

The obtained DOGs at constant MO/OG ratio are slightly larger than DOG for pure OG,11 while the self-diffusion coefficients of water, Dw, in the two systems are practically identical. A comparison between these two systems refers to the same water concentration. The MO self-diffusion coefficients, DMO (not shown in Figure 4), are smaller than the corresponding DOG, as expected for a substance mainly located in the micelles. At constant OG/2H2O ratio, DOG shows a shallow minimum, while the MO self-diffusion coefficient, DMO, increases monotonically with increasing MO concentration (Figure 5). Both diffusion coefficients are of the same order of magnitude, but DMO is slightly smaller. Dw scatters broadly about different values, but the overall trend is a decrease in the translational diffusion of water with increasing MO content, as one would expect (Table 1). Multiphase Areas in the Water-Rich Corner. As mentioned in the previous section, an increase in the molar ratio of MO/OG beyond 1:1.8-1:1.5 results in a separation into two liquid phases (Figure 6). The phase of higher density has a large water content and a small fraction of OG, while the other liquid phase contains mainly MO and OG, as determined by 1H NMR. The water-deficient phase is rather viscous and slightly bluish. Diffusion data of the water-rich phase reveal the presence of OG monomers together with slow-diffusing aggregates that are larger than a spherical OG micelle (Table 1). This will be further discussed below. Beyond this two-phase area, a number of different two- and three-phase regions are present (Figure 3). In all these multiphase areas, except at very high MO concentrations, a white, nontransparent, anisotropic, and viscous substance constitutes one of the phases. When dispersed in water, a white suspension of transparent particles is produced (Figure 7). In the polarizing microscope, the white substance shows a texture that is typical for an LR phase (Figure 8). Further, besides the large singlet peaks in the 2H NMR spectrum obtained from the aqueous solution phase, quadrupole splittings of rather constant size are observed upon dilution for these samples, and the SAXD diffractogram consists of two peaks indicating a lamellar structure (Figure 9). However, the obtained lattice constant is much larger than that in the LR phase, where also only one reflection is observed. Note also the sharp boundary between the region labeled R and the LR phase in Figure 6. We have also investigated the isotope effect in this part of the phase diagram by replacing 2H2O with 1H2O. However, such a replacement had no effect on the observed phase boundaries other than the shift caused by the different masses of 2H2O and 1H2O. The water-rich phase had a higher density than the water-poor one for 2H2O, while the opposite was observed with 1H2O.72 LR Phase. Figure 10 shows 2H NMR spectra for OG and MO in their LR phases along with typical 2H NMR spectra of the mixture of the amphiphiles. The extension of the LR phase region was determined from the occurrence of the central singlet NMR peak produced by either of the isotropic phases. As can be seen from Figure 10, pure OG produces only one set of peaks. These are originating from 2 H2O. When MO is present, several sets of peaks appear, originating from 2H2O and the two hydroxyl groups in the glycerol headgroup. The presence of these additional peaks indicates a slow exchange on the NMR time scale of 2H on the hydroxyl groups in MO. Because of the difficulties (71) Nilsson, F.; So¨derman, O.; Johansson, I. J. Colloid Interface Sci. 1998, 203, 13146. (72) Martin, A.; Leseman, M.; Belkoura, L.; Woermann, D. J. Phys. Chem. 1996, 100, 13760.

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Persson et al.

Table 1. Self-Diffusion Coefficients for MO, OG, and HDO in the L1 Phase and the Aqueous-Rich Phase in the Liquid-Liquid Two-Phase Area const MO:OG ratio

const OG:2H2O ratio

other two-phase

b

MO/OG/2H2O wt %

10-10Dw (m2/s)

10-12DMO (m2/s)

10-12DOG (m2/s)

10-12DOG (m2/s)

11.51/41.47/47.03 10.19/36.48/53.33 7.90/28.45/63.65 5.32/19.17/75.51 3.02/10.90/86.08 1.18/4.25/94.57 0.27/0.98/98.75 0.06/0.20/99.74 0.00/40.62/59.38 2.30/39.68/58.02 5.29/38.47/56.24 6.72/37.89/55.39 10.19/36.48/53.33 11.29/36.03/52.68 15.10/34.48/50.41 19.06/32.88/48.06 21.86/31.74/46.40 21.01/30.74/48.24 16.50/23.71/59.80 11.13/20.11/68.76 6.72/9.68/83.61 5.23/7.54/87.23 3.17/4.57/92.26 1.61/2.32/96.07

4.29 5.34 8.08 11.05 14.58 15.13 14.87 14.49 5.81 4.12 3.53 4.12 5.34 4.99 3.70 4.47 4.79 4.52 6.62 8.21 16.86 16.67 17.49 15.50

2.12 2.93 3.19 15.01 28.08 35.90 b b b b b 1.67 2.93 3.26 3.33 3.86 4.49 4.59 4.65 3.71 b b b b

3.78 3.80 8.02 31.21 69.81 137.34 391.82 304.74 5.12 5.17 4.89 4.57 3.80 4.08 4.44 5.13 5.90 5.55 6.57 6.88 373.05 341.10 381.88 367.86

7.45a 7.35a 12.97a 23.92a

a The occurrence of a biexponential decay for D OG in the latter phase area is due to an incomplete separation of the two phases. At low MO content DMO is difficult to obtain.

to assign the 2H NMR peaks, an unambiguous determination of two- and three-phase areas containing two anisotropic phases was not feasible. For the LR singlephase region, we assume that at sufficient water content the most intense pair of peaks originates from 2H2O (Figure 10). From the data obtained, it is concluded that there is an ideal swelling of the LR phase. The optical appearance of the LR phase at low water content indicated that there is a possibility of the existence of two separate LR phases. SAXD yielded only one reflection from these samples, but in one of them a ”hump” was present, supporting this view (Figure 11). Hexagonal Phase. In the binary system OG/2H2O, between 60 and 70 wt % OG, a normal hexagonal phase exists at 20 °C that vanishes with increasing temperature, and it is completely absent already at 25 °C. However, only small amounts of MO in the hexagonal phase increase the stability toward higher temperatures. The hexagonal phase can solubilize up to ∼15 wt % MO. At a molar ratio of OG/MO equal to 8.2 we obtained four reflections, from which the data in Table 2 were calculated. As can be inferred from Table 2, the area per amphiphile remains relatively constant upon addition of MO. This area is an average value, since the interfacial areas for MO and OG cannot be explicitly determined from these data alone. The radius of the hydrocarbon core increases slightly in the presence of MO. The 2H NMR data show ideal swelling behavior upon addition of water.63 Also, the size of the splittings decreases with increasing MO concentration. OG-Rich Cubic Phase (C1). The structure of the cubic phase in the binary OG/2H2O system has been determined to be bicontinuous of the normal type, belonging to the space group Ia3d.10,11 This structure can be described as a network of interconnected micellar rods immersed in water. Similar to the case of the hexagonal phase, ∼15 wt % MO can be incorporated into the cubic phase. The SAXD diffractogram of sample C1:1 is shown in Figure 12. If we allow for some missing peaks, the best fit is obtained with an Ia3d structure. Here, the first two peaks are very strong, the following two peaks are missing, and peak numbers 5 and 6 are very weak. For the second sample peak

numbers 5 and 6 are also missing, making the structural determination even more ambiguous. The obtained lattice parameters are shown in Table 2. MO-Rich Cubic Phases (C2 and C3). In the binary MO/water system two cubic phases occur.22,23 At high water contents the cubic structure formed belongs to the space group Pn3m, while at low water contents it belongs to the space group Ia3d. Both these cubic phases are built up of reversed bicontinuous structures, and they can be described by infinite periodic minimal surfaces.67 The SAXD data show that OG will change the Pn3m structure, first into an Ia3d structure, and eventually an LR phase will form. Furthermore, the two-phase area between the two cubic phases is narrow, while the two-phase area between the Ia3d and the LR phases is rather large. Addition of water to the cubic phases eventually results in a twophase system, where almost pure water is in equilibrium with a cubic phase. All three samples investigated in this area show that it is the Pn3m cubic structure that is in equilibrium with water. In a recent study, the thermal behavior of the MO-rich cubic phases was investigated. It was found that the Pn3m structure at 25 °C in sample H (see Table 2) converts to the Ia3d at a moderate increase in temperature.73 Discussion L1 Phase. From the phase diagram it can be concluded that the OG micellar aggregates can accommodate substantial amounts of MO. In the concentrated region of the L1 phase, 1 MO per 1.5 OG molecules can be solubilized. Upon an increase in the water content, this ratio decreases to 1:1.8. Increasing the MO/OG ratio further leads to a complex phase behavior, involving several multiphase areas, in which surfactant-rich phases are in equilibrium with almost pure water. The structure of the OG micelles has been investigated by several different methods, although the results obtained do not agree completely.11,12 Self-diffusion data indicate (73) Persson, G.; Edlund, H.; Lindblom, G. Eur. J. Biochem. 2003, 270, 56.


Langmuir, Vol. 19, No. 14, 2003 5819

Figure 7. Lamellar particles dispersed in water; upper frame in polarized light, and lower frame in normal light.

Figure 6. (a) Penetration scan using pure water; upper frame in polarized light, and lower frame in normal light. Abbreviations: Pn3m, MO-rich cubic phase of space group Pn3m or Ia3d; LR, lamellar liquid crystalline phase; Crystal, dry, pure MO. (b) Penetration scan using a 2.09 wt % OG solution; upper frame in polarized light, and lower frame in normal light. Abbreviations: L1, isotropic solution phase; 2φ, liquid-liquid twophase region; R, lamellar dispersion; Cubic, MO-rich cubic phase of undetermined space group; LR, lamellar liquid crystalline phase; Crystal, dry, pure MO. (c) Penetration scan using a 7.01 wt % OG solution; upper frame in polarized light, and lower frame in normal light. Abbreviations: L1, isotropic solution phase; 2φ, liquid-liquid two-phase region; R, lamellar dispersion; LR, lamellar liquid crystalline phase; Crystal, dry, pure MO.

cylindrical aggregates with an axial ratio of approximately 1:2, existing already at low OG concentrations,11 while SAXS and SANS measurements indicate spherical micellar structures at concentrations close to the cmc, and rodlike micelles at higher concentrations.12 Our self-diffusion data at a constant MO/OG ratio are to a close approximation the same as the ones obtained for the OG/2H2O system. This indicates that, in the dilute regime, adding up to 1 MO per 4.4 OG does not alter the micellar structure or size to an appreciable extent. When keeping the OG/2H2O ratio constant, we observe a minimum in DOG (Figure 5). A similar behavior has been observed in the aqueous corner of the didodecyldimethylammonium sulfate/hydrocarbon/water system. This was explained by a sphere-prolate-bicontinuous transformation.74 At MO concentrations higher than this minimum, we observe an increasing flow birefringency. From Figure 5 one can infer that in this region DOG is larger than DMO. Assuming that the contribution from free OG is negligible, this would imply a bicontinuous structure in this area (Figure 2b), since if the aggregates were discrete, DOG and (74) Nyde´n, M.; So¨derman, O.; Hansson, P. Langmuir 2001, 17, 6794.

Figure 8. Micrograph of the white substance, R, in polarized light.

Figure 9. Diffractograms of sample R at four different temperatures.

DMO would be exactly the same and equal to the aggregate diffusion.17,40 Thus, if we assume that OG aggregates are more or less spherical, addition of MO at low water content will cause a moderate micellar growth that results in these micelles connecting into a bicontinuous network, probably into an L3 phase. This transformation occurs gradually with increasing MO content.

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Figure 10. (a) 2H NMR spectrum of the OG/2H2O LR phase. (b) 2H NMR spectrum of the MO/2H2O LR phase. The most intense peaks (indicated by the arrows) are assumed to originate from 2H2O. (c) 2H NMR spectrum for the LR phase at low water content in the MO/OG/2H2O system. (d) 2H NMR spectrum for the LR phase at higher water content in the MO/OG/2H2O system. The most intense peaks (indicated by the arrows) are assumed to originate from 2H2O.

Figure 11. SAXS diffractograms for three samples at low water content in the LR phase. The arrow indicates the “hump” that may originate from a second LR phase. The black dots indicate the theoretical positions of the first and second peaks calculated for sample LR 3.

Liquid-Liquid Two-Phase Area. A high fraction of MO results in a separation of the micellar solution into two isotropic liquids. 1H NMR spectroscopy shows that a dilute OG solution is in equilibrium with a water-poor solution. Miscibility gaps have been observed for the decyl-βglucoside system15 as well as for mixtures of nonyl- and decyl-β-glucoside in water.14,17 A similar result was obtained when adding nonanol to the nonyl-β-glucoside/ water system17 and ether oils to an OG/water micellar solution.9 In the ternary system OG/octanol/2H2O an L3 phase is in equilibrium with a dilute water-rich L1 phase and an LR phase. The L3 phase is optically isotropic and birefringent under shear, and it scatters light strongly.19 As mentioned in the Introduction, for poly(ethylene oxide)-based surfactants the occurrence of a miscibility gap originates from a conformational change in the headgroup, leading to water becoming a less good solvent.21 Such an explanation is not plausible for the more rigid glucose headgroup. Instead, the observed phase separation has been suggested to occur because of formation of a bicontinuous liquid at sufficiently high concentrations of the amphiphile.15,17,19,20 On the basis of our findings for

Persson et al.

the L1 phase, this latter explanation is a possible one also for the present system. The phase separation can then be understood in terms of changes in the aggregate curvature. In the previous section it was concluded that a bicontinuous liquid of interconnected rods forms in the L1 area (dotted area in Figure 2). Upon dilution, this network swells, and as a consequence, the curvature of the interconnected rod structure becomes more positive as the distance between connection points increases. Stating that a mixing of OG and MO produces structures with a spontaneous curvature close to zero (see the section below on Liquid Crystalline Phases), we conclude that a more positive curvature leads to an increase in the free energy, which eventually becomes large enough to cause the phase separation. LR Dispersion. Addition of an ionic surfactant to a surfactant/MO/water system will develop a large area of an LR phase, extending into the dilute regime.41,45,49 The aggregate structure in this latter regime can vary; both normal LR,41,45,49 defect LR,41 and L3 phases45,59 as well as vesicles41,45 have been reported. Enhanced swelling of the cubic phases is observed when CTAB or sodium cholate is present.41,45 The structural determination of a highly swollen cubic phase is somewhat ambiguous, but the results presented indicate that the Ia3d structure is the dominating one in these systems.41,45 In the ternary system OG/octanol/2H2O an L3 phase is in equilibrium with a dilute water-rich L1 phase and an LR phase. To our knowledge, only one system consisting of a nonionic surfactant with MO and water has been investigated, namely dodecyl maltoside.46 This system was not fully investigated, but the results indicate the presence of a large LR phase that extends toward the aqueous corner.46 A large portion of the OG/MO/water system consists of multiphase areas, in which a dilute OG solution is found in equilibrium with a white, nontransparent substance of lower density (labeled R in Figures 3 and 6). A thin sample of this white substance observed between crossed polarizers in a microscope shows the typical textures of an LR phase (Figure 8), while a thicker sample (∼1 mm) is completely nontransparent, in contrast to typical LR phases, which are semitransparent. There is no evidence of vesicles. The sharp boundary observed in Figure 6 might at first sight lead to the conclusion that there are two separate phases, but this is in contrast to the 2H NMR data, showing the characteristics of a two-phase area. In the entire area, where this white substance is present, the 2H NMR spectra consist of a narrow singlet and a quadrupole splitting that is independent of the water content. When dispersed in water, a heterogeneous suspension containing particles of different sizes rather than an emulsion is obtained (Figure 7). We believe that these particles are fragments of an LR phase. The possibility that the particles are instead lamellar crystals (LC) could be excluded for the following reasons. First, even though the obtained lattice parameter is larger than that for the samples in the LR phase at low water content, it is comparable to the ones obtained in the cubic-LR twophase area (see Table 2). Second, in the binary MO/water system the LR phase transforms into lamellar crystals (LC) only at temperatures below 18 °C.23 Similarly, also in the ternary MO/diolein/water system the LC phase appears only at low temperatures and low water content.47 In the present system the crystallization has to be caused by dilution, which does not seem very plausible. In a recent study, it was shown that, for a sample of similar composition as ours, the bilayers are fluid from 0 to 52 °C in this region, and no transitions were detected in this temperature range.42 The lattice parameter obtained in ref 42 is


Langmuir, Vol. 19, No. 14, 2003 5821

Table 2. Lattice Parameters Obtained for the Different Liquid Crystalline Phasesa lattice parameter (Å) sample C1:1 C1:2 ref 10 ref 11 A B C D E F G H ref 23 ref 23 ref 23 Hex ref 10 ref 11 LR:1 LR:1 LR:2 LR:3 ref 10 ref 11 ref 11 ref 47 ref 47 ref 23 ref 23 R a

MO/OG/2H2O

wt %

0.00/74.75/25.25 8.92/67.46/23.62 0.00/74.9/25.1 0.00/79.0/21.0 57.81/0.34 /41.85 57.81/1.31/40.88 57.44/2.54/40.02 57.7/3.37/38.93 57.95/4.20/37.85 44.77/0.30/54.93 44.67/1.14/54.19 44.54/2.29/53.17 79.2/0/20.8 64.9/0/35.1 50.3/ 0/49.7 8.47/56.91/34.62 0.00/64.8/35.2 0.00/67.0/33.0 56.86/34.23/8.91 56.86/34.23/8.91 47.52/43.67/8.81 37.38/53.63/8.99 0/84.7/15.3 0/95/5 0/81/19 87.9/0/12.1 90.1/0/9.9 91.7/ 0/8.3 85.1/0/14.9 24.96/9.81/65.23

Pn3m

Ia3d

LR

HI

2rhc (Å)

rhc (Å)

LR

HI

area per headgroup (Å2) LR

HI

72.7 83.3 74.5 73.0 97.0 144.7 156.7 164.7 182.8 98.6 108.0 109.8

48.3 48.0 49.1

174.2 120 150

100 40.2 37.9 38.6 32.8 31.1 31.2 32.4 28.8 26.6 29.7 39.5 38.1 38 40 48.7

13.2 12.2

40.8 39.5

20.9 19.8 19.3 19.3 13.3

36.2 38.2 36.7 34.1 36.1

27.6 27.2

35.6 36.1

Data found in the literature are included for comparison.

Figure 12. Diffractogram of sample C1:1. The black dots indicate the positions of the first six peaks calculated for the Ia3d structure.

larger (54 Å) and increases more with temperature than in our study (data not shown), but this difference may be a result of the phosphate buffer used in ref 42. Thus, we believe that the particles in our system are indeed fragments of an LR phase. The questions are now how do they form and why do they not coalesce? At low water content, an LR phase is found at all MO/ OG ratios. There is a limit in the water content for the LR phase, and at ratios close to those for the respective 2H2O binary systems, dilution results in the formation of other liquid crystalline phases. The major part of the LR phase cannot, however, rearrange to another liquid crystalline structure upon dilution for reasons that will be discussed below. Instead, a large two-phase region forms, where the LR phase is in equilibrium with a dilute OG aqueous solution. The lamellae seem to be rather brittle and can easily be fractured to smaller particles, which probably are stabilized by an adsorption of OG on the edges. This

results in a stable dispersion of lamellar particles that strongly scatter light. However, the system appears isotropic when the lamellar particles are aligned between the cover glasses. Liquid Crystalline Phases. Both the OG-rich, nonlamellar liquid crystalline phases can accommodate quite a lot of MO (for the hexagonal phase MO/OG is 1:3.5, and for the cubic phase MO/OG is 1:5). The major effect is an increase in the lattice parameters with increasing MO content. Thus, replacing OG by MO molecules does not seem to affect the local curvature in a profound way. This is different from the behavior of the MO-rich cubic phases, which are able to accommodate only minor amounts of OG. The narrow two-phase region between the two cubic phases, along with the fact that no transition is traced by DSC,73 shows that these two structures are easily converted into one another. Furthermore, the Pn3m structure exists in a small region, indicating that this structure is on the edge of its stability range already in the binary system. Since structures consisting of planar bilayers (either as LR or as one of the phases in the multiphase areas) dominate the phase diagram, the conclusion is that the combination of MO/OG favors a flat structure. These effects can be understood from a simple geometrical consideration. If we define a hydrophobichydrophilic interface for a given aggregate structure and associate both headgroups with this surface, then the OG “tail” reaches down to the double bond in MO, leaving the other end of the MO “tail” free. It has been shown that in an aggregate the OG headgroups have a rather stringent demand for hydration11,75 and interact as hard spheres.76 Thus, the area of the headgroup of OG is more or less independent of the water content (40 Å2).11 The low (75) Pastor, O.; Junquera, E.; Aicart, E. Langmuir 1998, 14, 2950. (76) Waltermo, Å.; Manev, E.; Pugh, R.; Claesson, P. J. Dispersion Sci. Technol. 1994, 15, 273.

5822


hydration in aggregates seems to be a general feature of the glucoside moiety, since a similar result was obtained for monoglucosyldiacylglycerol.77 Since the headgroup area of the MO molecule is of the same magnitude (35 Å2),45,78 a replacement of OG with MO will not affect the area per molecule at the aggregate surface to a large extent. The longer MO molecule will increase the radius of the hydrocarbon core, thus producing an aggregate with lower curvature. This leads to a stabilization of the hexagonal phase located on the OG-rich side in the phase diagram (Figure 2), while, on the MO-rich side, the curvature is already close to zero and no hexagonal phase will form. Replacing MO with OG will probably have the same effect at the aggregate interface as described above, but the shorter hydrocarbon chain of OG allows for a larger spatial freedom for the MO acyl chain. Thus, the system can be pictured as a microemulsion in which the “oil” is a part of one of the amphiphiles. The hydrophilic-hydrophobic interfacial surface will be less flexible, and the preferred structure will consequently be flat. Conclusions We present the ternary phase diagram of MO/OG/2H2O at 25 °C as studied by SAXD, microscopy, and a variety of NMR methods. The OG-rich isotropic solution phase (77) Ora¨dd, G.; Rilfors, L.; Lindblom, G. Phys. Chem. Chem. Phys. 2001, 3, 5052. (78) Engblom, J.; Hyde, S. J. Phys. II 1995, 5, 171. (79) Persson, G.; Edlund, H.; Lindblom, G. Prog. Colloid. Polym. Sci., in press.

Persson et al.

can solubilize MO up to an MO/OG ratio between 1:1.8 and 1:1.5, while a separation into two liquids, one dilute and one concentrated, occurs with increasing MO/OG ratio. The aqueous-rich phase consists of a dilute OG solution, while the aqueous-poor liquid has a bicontinuous structure (possibly an L3 phase). Moreover, the MO-rich cubic phases are destabilized by the addition of OG, while addition of MO to the OG-rich liquid crystalline phases results in rather stable, nonlamellar phases. Generally, the lamellar structure seems to be the most favorable one in this system. This is obvious, since at low water content an LR phase (or possibly two LR phases) dominates the phase diagram, and in a large part of the dilute region the LR phase is dispersed as particles in equilibrium with either a dilute OG solution or a dilute OG solution and a third phase, consisting of another liquid or a cubic phase. The preference for the lamellar structure can be understood from geometrical considerations. Acknowledgment. We wish to thank M. Rappolt, M. Strobl, and S. Bernstorff at The Austrian Academy SAXS station at the ELETTRA synchrotron, Trieste, Italy, for their support during the experiments. EKA Chemicals Research Department, Sundsvall, Sweden, is gratefully acknowledged for giving us access to their microscope and video equipment. Mid Sweden University and The Swedish Research Council are acknowledged for financial support. LA034492F

Water System

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