© Copyright 2002 American Chemical Society
JUNE 25, 2002 VOLUME 18, NUMBER 13
Letters Lyotropic Mesophases Next to Highly Efficient Microemulsions: A 2H NMR Study Cosima Stubenrauch,* Christian Frank, and Reinhard Strey Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany
Daniel Burgemeister and Claudia Schmidt Institut fu¨ r Makromolekulare Chemie, Albert-Ludwigs-Universita¨ t, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany Received February 19, 2002. In Final Form: April 16, 2002 Phase diagram investigations of symmetric microemulsions of water-n-decane-C10E4-PEP5-PEO5 in the semidilute regime suggested the existence of anisotropic mesophases (C10E4, n-decyl tetraoxyethylene glycol ether; PEP5-PEO5, poly(ethylenepropylene)-co-poly(ethylene oxide), MPEP ) MPEO ) 5 kg mol-1). Performing a temperature scan on a sample at fixed composition, we observed an alternating sequence of eight two-phase regions and six one-phase regions. The composition of the sample was chosen to contain equal volumes of water and oil and an overall weight fraction of surfactant and polymer of 0.20. We examined the nature of the lyotropic mesophases and the coexisting phases by measuring the line splitting of 2H NMR spectra. The experiments unequivocally reveal the existence not only of a lamellar (LR) but also of a normal hexagonal (H1) and a reverse hexagonal (H2) phase.
Ternary mixtures of water, oil, and a nonionic surfactant form thermodynamically stable microemulsions. A typical example is the system water-n-decane-C10E4 (C10E4, n-decyl tetraoxyethylene glycol ether). Variation of the surfactant mass fraction γ at equal volume fractions of water and oil (φ ) 0.5) leads to the phase diagram shown in Figure 1, i.e., the well-known “fish”.1-3 At low temperatures an oil-in-water microemulsion coexists with an excess oil phase (denoted 2), whereas at high temperatures a water-in-oil microemulsion coexists with an excess water phase (denoted 2). At lower surfactant mass fractions, γ, the three-phase region (3) occurs
in the intermediate temperature range, whereas at higher γ the isotropic one-phase region appears (1). The point X ˜, where the three-phase and the one-phase regions meet, has the coordinates γ˜ ) 0.132 and T ˜ ) 30.15 °C, where γ˜ is the minimum surfactant concentration that is needed for complete solubilization of water and oil. γ˜ is a measure for the efficiency of the surfactant.2 For C12E4 one has γ˜ ) 0.042 at T ˜ ) 18.20 °C (see Figure 1). The gain in efficiency for C12E4 is accompanied by the formation of a lamellar phase (LR) at low surfactant concentrations.1 A way to control the efficiency and the formation of lyotropic mesophases separately is to use amphiphilic block copolymers as “efficiency boosters”.4,5 Specifically, the poly(ethylenepropylene)-co-poly(ethylene oxide) (abbreviated
(1) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. (2) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993, 47, 4197. (3) Strey, R. Colloid Polym. Sci. 1994, 272, 1005.
(4) Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. German Patent Application No. 19839054.8, 1998. (5) Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. Langmuir 1999, 15, 6707.
I. Introduction
10.1021/la0201725 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/31/2002
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Figure 1. Sections through the phase prism at equal volumes of water and n-decane (φ ) 0.5). γ is the mass fraction of surfactant in the total mixture, i.e., γ ) mC/(mA + mB + mC), where A, B, and C denote water, oil, and surfactant, respectively. The well-known “fish” (big circles) is shown for water-ndecane-C10E4; for water-n-decane-C12E4 only the “fish tail” (big circles), i.e., the one-phase region (1), is shown. The increase in efficiency when replacing C10E4 by C12E4 is accompanied by the formation of an LR phase (small circles) at low surfactant concentrations. 2, 2, and 3 are two- and three-phase regions explained in the text.
as PEPX-PEOY, where X and Y are the molar mass of the blocks in kg mol-1)6,7 are similar to the alkyl polyglycol ether surfactants CiEj, differing from these by the branched nature of the hydrophobic block and, of course, in the larger overall molar mass. Replacing small amounts of surfactant by these polymers, one observes a dramatic increase of the efficiency. Recent theoretical developments8-10 explain the origin of the efficiency boosting as effect on the saddlesplay modulus, because the polymers are anchored at the interfaces.11 Whether or not the increase in efficiency is accompanied by the formation of lyotropic mesophases such as the LR phase can be controlled by the nature of the block copolymer. It was found that symmetric block copolymers with equal molar masses of the two blocks stabilize highly ordered phases in the same way as the low molecular CiEj surfactants. On the other hand, asymmetric block copolymers have a destabilizing effect on the LR phase. These and related phase studies will be published elsewhere. In the present Letter we examine some interesting aspects of lyotropic mesophases formed in a quaternary system with a symmetric copolymer, namely, water-ndecane-C10E4-PEP5-PEO5. Usually, a more or less broad LR phase is observed in these kinds of systems. However, in the present case, several additional regions of unidentified lyotropic mesophases have been observed. To clarify their microstructure, 2H NMR spectroscopy was employed.12 The quadrupole coupling of the deuterium nuclei is not averaged out if the rotational motion of the water molecules is anisotropic as, for example, in an anisotropic (6) Allgaier, J.; Poppe, A.; Willner, L.; Richter, D. Macromolecules 1997, 30, 1582. (7) Poppe, A.; Willner, L.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, 7462. (8) Lipowsky R. Colloids Surf., A 1997, 128, 255. (9) Hiergeist, C.; Indrani, V. A.; Lipowsky, R. Europhys. Lett. 1996, 36, 491. (10) Lipowsky R. Encycl. Appl. Phys. 1998, 23, 199. (11) Yang, B.-S.; Lal, J.; Kohn, J.; Huang, J. S.; Russel, W. B.; Prud’homme, R. K. Langmuir 2001, 17, 6692. (12) Davis, J. H. Biochim. Biophys. Acta 1983, 737, 117.
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Figure 2. Same section as in Figure 1 for the quaternary system water-n-decane-C10E4-PEP5-PEO5 at φ ) 0.5 and δ ) 0.158, where δ ) mD/(mC + mD) is the mass fraction of polymer in the surfactant/polymer mixture. Three different regions of lyotropic mesophases are shown, with H1 (down triangles) being the normal hexagonal, LR (small circles) the lamellar, and H2 (up triangles) the reverse hexagonal phase. The H2 phase coexists with an excess water phase (w). Along the vertical line (γ ) 0.20) 2H NMR spectra were recorded.
lyotropic mesophase. The preferred orientation of the water molecules leads to a line splitting of the 2H NMR spectrum. In the following the phase diagram as well as NMR spectra of the system water-n-decane-C10E4PEP5-PEO5 are shown. We demonstrate how detailed information on the microstructure of the mesophases and on the number and type of coexisting phases in the different regions of the phase diagram can be extracted. II. Phase Behavior The phase diagrams were determined in a water bath with temperature control to (0.02 K. The samples were weighed into test tubes and sealed. The masses of the components water, oil, surfactant, and polymer are denoted mA, mB, mC, and mD, respectively. The composition is given by the mass fraction of oil in the water plus oil mixture, R ) mB/(mA + mB), the overall mass fraction of surfactant or of the surfactant/polymer mixture, γ ) (mC + mD)/(mA + mB + mC + mD), and the mass fraction of polymer in the surfactant/polymer mixture, δ ) mD/(mC + mD). At constant composition the temperature was varied and the occurring phases were characterized by visual inspection in transmitted light. Crossed polarizers were used to detect the presence of anisotropic phases. The typical uncertainty for the phase boundaries is (0.05 K. The phase diagram of the system H2O-n-decaneC10E4-PEP5-PEO5, shown in Figure 2, was determined at R ) 0.422 (corresponding to equal volume fractions of water and n-decane, i.e., φ ) 0.5) and δ ) 0.158 (compare with the phase diagram of the polymer-free (δ ) 0) base system plotted in Figure 1). The symmetric block copolymer has a molar mass of 10650, where the mass fraction of the hydrophobic block is about 0.446.5 In a previous paper5 phase diagrams for δ ) 0.015, 0.050, and 0.119 were shown focusing on surfactant concentrations around γ˜ . Exchanging 15.8 wt % surfactant by the amphiphilic block copolymer, one observes three pronounced effects on the phase diagram of the base system. First, the substitution leads to a dramatic increase in the efficiency of the system, i.e., to a reduction of γ˜ from γ˜ ) 0.132 to γ˜ < 0.03. At these low
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concentrations of the surfactant/polymer mixture, the structures are very large. Consequently, light is strongly scattered and a distinction between a one- and a twophase region becomes increasingly difficult by visual inspection so that the exact value of γ˜ was not determined. Second, the efficiency enhancement by means of polymer addition is obtained without shifting the hydrophilelipophile balance temperature, T ˜ , which is about 30 °C. This is different from the efficiency enhancement obtained from a substitution of C10E4 by C12E4 (cf. Figure 1). Third, not only was a broad LR phase formed, but also two additional anisotropic regions surrounding the LR phase were identified. Clearly, visual inspection of macroscopic test tubes is not the way to determine unambiguously the structure of the anisotropic phase. Furthermore, the high viscosity and turbidity of certain mesophases make it very difficult to decide whether the anisotropic phase is a twophase or multiphase region. To clarify the nature of the lyotropic mesophases and the number of coexisting phases, 2H NMR measurements were carried out. For that purpose, H2O was replaced by D2O, which led to the wellknown decrease of the phase boundaries by 2-3 K.13 III. Structure of the Mesophases 2
The H NMR measurements were carried out with a Bruker MSL 300 spectrometer at a deuterium resonance frequency of 46.07 MHz. The samples were filled in 4 cm long glass tubes of 5 mm diameter, which were sealed with a Teflon plug and epoxy glue. The tubes were inserted in a home-built goniometer probe which allowed us to rotate the samples about the tube axis, which is perpendicular to the static magnetic field. Spectra were obtained with a quadrupole echo pulse sequence; typically 16 scans were averaged. For the NMR measurements a single sample composition was judiciously chosen in order to obtain maximum information. The possibility to reach the different phases by simply changing the temperature guarantees identical overall sample composition. A temperature scan from 21 to 39 °C was performed at γ ) 0.20 (indicated in Figure 2, full vertical line) with equilibration times of at least 1 h for each temperature. Note that for the NMR measurements water was replaced by D2O so that the phase boundaries indicated in Figure 2 are shifted by 2 °C toward lower temperatures. The temperature control of the goniometer was only (0.5 K, so care had to be taken to reach the proper phase. To obtain well-aligned lyotropic phases, the sample was preequilibrated in the isotropic phase at 23 °C before cooling down to 21 °C. Slow cooling from an isotropic to an anisotropic phase is often sufficient to achieve a uniform alignment of the anisotropic phase by the magnetic field. In Figure 3 characteristic spectra along the chosen line of constant γ are shown. The splitting of the spectra indicates anisotropic mesophases, whereas a single peak results for isotropic phases. The evolution of the phases is clearly seen: An anisotropic single phase is formed at 21 °C which becomes isotropic at 23 °C via a two-phase region (spectrum not shown). The spectra at 24 and 30 °C monitor the lower and upper two-phase regions between the anisotropic phase in the middle of the phase diagram (25-29 °C) and the surrounding isotropic phases (23 and 34 °C). The anisotropic phase formed at high temperatures always coexists with an isotropic phase as can be seen in the spectrum at 36.5 °C. An anisotropic single phase was not observed. With (13) Strey, R.; Glatter, O.; Schubert, K. V.; Kaler, E. W. J. Chem. Phys. 1996, 105, 1175.
Figure 3. 2H NMR spectra of the system D2O-n-decaneC10E4-PEP5-PEO5 at φ ) 0.5, δ ) 0.158, and γ ) 0.20 (see Figure 2) as a function of the temperature. For anisotropic phases a quadrupolar splitting ∆ν is observed (spectra at 21 and 27 °C), whereas isotropic phases give a single peak (spectra at 23, 34, and 39 °C). At 24, 30, and 36.5 °C an anisotropic and an isotropic phase coexist. Note that the full phase sequence is shifted by -2 K compared to Figure 2 because of the exchange of H2O by D2O. The isotropic oil-in-water (o/w) and water-in-oil (w/o) microemulsions are denoted by L1 and L2, respectively.
this sequence of spectra the number and nature (anisotropic or isotropic) of the phases could be identified. In particular, the information about the two-phase regions complements the results of the macroscopic visual inspection. From the latter we know, for example, that a broad birefringent regime exists between 25 and 35 °C. The NMR measurements led to the result that not the pure anisotropic phase (its temperature range is only about 5 K) but rather the adjacent two-phase regions cause such a broad birefringent region. However, the quadrupole splitting ∆ν does not reveal the structure of the anisotropic phase. The differences in ∆ν seen in Figure 3 are not very pronounced and could simply be due to temperature changes. To elucidate the structure a rotation experiment, in which the line shape is measured in dependence of the sample orientation with respect to the magnetic field, was carried out. For perfectly aligned lyotropic mesophases the NMR spectra are welldefined doublets (see Figure 3, spectra at 21 and 27 °C), where ∆ν is given by
∆ν ) 3/4 δ(3 cos2 θ - 1)
(1)
θ is the angle between the external magnetic field and the director (axis of symmetry) of the corresponding phase, δ ) e2qQ/h (with e the elementary charge, h Planck’s constant, eq the anisotropy of the averaged electric field gradient, and eQ the nuclear quadrupole moment) is the quadrupole coupling constant averaged over the molecular motions of the water molecules. Because of the negative anisotropy of the diamagnetic susceptibility of aliphatic surfactants, the molecules prefer to align with their long axis perpendicular to the magnetic field. In a hexagonal phase this results in an orientation of the director (hexatic axis) parallel to the magnetic field, whereas in a lamellar phase the director (layer normal) aligns perpendicularly to the external magnetic field. In the latter case, the magnetic alignment does not yield a macroscopically uniform orientation in the sample but leads to many domains with a distribution of their directors in the plane perpendicular to the magnetic field. According to eq 1 ∆ν depends on the orientation of the director. This relation-
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Figure 4. 2H NMR spectra of the sample at 21 °C as a function of the rotation angle, θ0. The reduction of the splitting ∆ν by 50% upon a sample rotation by 90° is evidence of a hexagonal structure.
ship is used to obtain information about the phase structure of the sample by means of a rotation experiment. Changing the sample orientation by rotating it by 90° about an axis perpendicular to the magnetic field, one observes that the splitting ∆ν of the hexagonal phase decreases by 50%, whereas for the lamellar phase a more complicated spectrum corresponding to the superposition of doublets according to the distribution of directors in the sample results. This spectrum extends over twice the range of frequencies compared to the initial spectrum of the aligned lamellar sample and has maximum intensities at frequencies corresponding to θ ) 90° and 0°.14 Note that the rotation has to be fast enough to prevent a realignment, i.e., a “relaxation”, of the sample’s director back to the starting point. The rotation experiment carried out at 21 °C for the presented system is shown in Figure 4. Changing the sample orientation from its initial value, defined as θ0 ) 0° (spectrum at the bottom of Figure 4), to θ0 ) 90° (second spectrum from bottom) leads to a decrease of the absolute splitting |∆ν| from 250 to 125 Hz. A further change of the sample orientation θ0 leads to a continuous change of the splitting as shown in Figure 4. At θ0 ) 125° (or its equivalent of 55°, which is close to the magic angle of 54.7°) the splitting vanishes. From the observed rotation pattern we can conclude that θ ) θ0, in other words, that the director in the initial magnetically aligned sample (θ0 ) 0) is parallel to the magnetic field, which means that at T ) 21 °C the phase under investigation is hexagonal. A reduction of |∆ν| by 50% upon a sample rotation by 90° was also found at T ) 36.5 °C (not shown) in the hightemperature anisotropic phase which is hexagonal as well. In contrast, a rotation of the sample by 90° at T ) 25 °C (not shown) leads to a spectrum that extends over 400 Hz compared to |∆ν| ) 200 Hz for the line doublet of the magnetically aligned sample. This is evidence of a lamellar phase. (14) Spiess, H. W. In Developments in Oriented Polymers; Ward, I. M., Ed.; Applied Science Publishers Ltd.: Barking, Essex, 1982; Vol. 1.
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In summary one can say that the lamellar phase in the middle of the phase diagram is surrounded by two hexagonal phases. This observation is in accordance with the findings for comparable systems15-18 and with the temperature-dependent curvature of nonionic amphiphilic films,3 which predicts the sequence normal hexagonal (H1)-lamellar-reverse hexagonal (H2) upon increasing temperature. In contrast to the normal H1 phase and the lamellar phase, a homogeneous phase region was not found for the reverse H2 phase. (This observation is not surprising as we investigated only a very limited part of the whole phase diagram.) Although it is obvious that the isotropic phases are continuously connected to bicontinuous microemulsions (cf. Figure 2), their structure is still an open question. In some parts of the diagram extremely viscous states were encountered together with optical isotropy so that the existence of bicontinuous cubic lyotropic mesophases cannot be excluded. IV. Conclusions The phase behavior of the system water-n-decaneC10E4-PEP5-PEO5 at φ ) 0.5 and δ ) 0.158 was investigated with a special focus on the characterization of the lyotropic mesophases. Three different regions of anisotropic mesophases were identified, the extension and structure of which were investigated by 2H NMR. For that purpose, a temperature scan at a constant γ was performed. At the chosen γ ) 0.20 the different anisotropic regions are clearly separated by isotropic regions, which was confirmed by the NMR measurements. The alignment of the samples in a magnetic field reveals that the broad lamellar phase is surrounded by two hexagonal phases. In accordance with the temperature-dependent curvature of nonionic amphiphiles, the low-temperature phase is identified as a normal hexagonal H1 phase, whereas the high-temperature phase is a reverse H2 phase. Consistent with the general observation that structuring diminishes with increasing temperature, the H2 phase is not a single phase but coexists with an isotropic phase, presumably excess water. The demonstration of this coexistence makes 2H NMR measurements such a versatile tool for the investigation of phases and phase equilibria. Because of the high viscosity of the lyotropic mesophases, macroscopic phase separation may take days, whereas for the NMR measurements the microscopic phase separation is sufficient. Acknowledgment. The authors wish to thank Britta Jakobs and Thomas Sottmann for fruitful cooperation, emphasizing that parts of the phase diagrams were measured by Britta Jakobs. Financial support by the Fonds der Chemischen Industrie and the Ministerium fu¨ r Wissenschaft und Forschung des Landes NRW is gratefully acknowledged. LA0201725 (15) Fukuda, K.; Olsson, U.; Wu¨rz, U. Langmuir 1994, 10, 3222. (16) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (17) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. 1998, 102, 1149. (18) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. 1998, 102, 831.
