Langmuir 2005, 21, 4931-4937
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Hexagonal Liquid Crystalline Phases Formed in Ternary Systems of Brij 97-Water-Ionic Liquids Zhongni Wang,*,† Feng Liu,† Yanan Gao,‡ Wenchang Zhuang,† Limei Xu,† Buxing Han,‡ Ganzuo Li,*,† and Gaoyong Zhang† Key Laboratory for Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China, and Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received January 30, 2005. In Final Form: March 11, 2005 Phase diagrams of two ionic liquids: hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (bmim-PF6) and relatively hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4) in aqueous solutions of Brij 97 were determined at 25 °C. Two hexagonal liquid crystalline phases formed in bmim-PF6- and bmim-BF4-containing ternary systems were investigated by means of small-angle X-ray scattering (SAXS) and rheological techniques, with comparison of composition and temperature effects. From analysis of the SAXS data, bmim-PF6 is dominantly penetrated between the oxyethylene chains of surfactant molecules, whereas bmim-BF4 is mainly located in the water layer of hexagonal phases. The strength of the network of hexagonal phase formed in the Brij 97/water/bmim-BF4 system is appreciably stronger than that of the Brij 97/water/bmim-PF6 system, indicated by the smaller area of the surfactant molecule at the interface and the higher moduli (G′, G′′). Temperature has a converse effect on the lattice parameters of the two hexagonal phases.
Introduction With the growing challenge of environmentally benign chemical processing, room-temperature ionic liquids (ILs) have emerged during the past decade. ILs are molten salts, whose chemical and physical properties can be tailored by judicious selection of cation, anion, and substituents. They have a negligible vapor pressure, are not flammable, and are stable from below ambient up to 300 °C or more. These properties of ILs make them highly desirable in many reactions of industrial importance.1 ILs have been studied extensively for potential uses as electrolytes in batteries and fuel cells due to the wide electrochemical window, high conductivity, wide operation temperature range, and low dielectric constant.2 In addition, recent reports have considered ILs for gas separation3 and IL/ sc-CO2 for product extraction and separation.4 In the chemistry of ILs, however, the one major issue that remains to be explored is the formation of supramolecular assemblies. Preliminary investigations of ILs containing aggregates such as micelles, vesicles, emulsions, and liquid crystallines are of great interest and, thus, could enhance IL applications. ILs based on the 1-alkyl-3-methylimidazolium cation (Cnmim) possess an inherent amphiphilicity. In recent years, surfactant actions in such ILs received much attention. Surface tension measurements of a series of Cnmim-X have been done by Law and Watson.5 Bowers et al. reported the aggregates formation of ILs in aqueous * To whom correspondence should be addressed. E-mail:
[email protected] (Z.W.);
[email protected] (G.L.). † Shandong University. ‡ Chinese Academy of Sciences. (1) Welton, T. Chem. Rev. 1999, 99, 2071. (2) Wang, P.; Zakeeruddion, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (3) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315. (4) Blanchard, L. A.; Hancu, A.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 28. (5) Law, G.; Watson, P. R. Langmuir 2001, 17, 6138.
solutions.6 The group of Davis, Jr., demonstrated that imidazolium cations with attached long fluorous tails appear to self-aggregate within imidazolium-based ILs.7 Lamellar liquid-crystalline formation in concentrated aqueous solutions of 1-decyl-3-methylimidazolium bromide (C10min-Br) was investigated by Firestone and coworkers.8 Studies on self-assemblies obtained from conventional surfactants in ILs have also been reported. Micellar aggregates in two ILs, bmim-Cl and bmim-PF6, upon addition of different surfactants have been presented by Anderson et al.9 At almost the same time, the aggregation behavior of Brij 35, Brij 700, Tween 20, and Triton X-100 has been investigated within low-viscosity emim-Tf2N by Fletcher and Pandey.10 The Kimizuka group observed the vesicle formation by dispersing suitably designed glycolipid or dialkyldimethylammonium bromide in ethercontaining ILs.11 A microemulsion of Triton X-100/bmimBF4/cyclohexane was prepared and characterized by Gao and Han et al.12 Lyotropic liquid crystalline formations of surfactants in IL were also investigated.13 Nevertheless, research on supramolecular assemblies formed in surfactant/water/IL ternary systems is extremely scarce. Reddy’s group investigated micelle formation of sodium dodecyl sulfate (SDS) in aqueous solutions of a variety of ILs, indicating that the critical micelle concentration of (6) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 2491. (7) Merrigan, T. L.; Bates, E. D.; Dorman, S. C.; Davis, J. H., Jr. Chem. Commun. 2000, 2051. (8) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258. (9) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (10) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (11) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759. (b) Nakashima, T.; Kimizuka, N. Chem. Lett. 2002, 31, 1018. (12) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (13) Wang, L.; Chen, X.; Chai, Y.; Hao, J.; Sui, Z.; Zhuang, W.; Sun, Z. Chem. Commun. 2004, 2840.
10.1021/la050266p CCC: $30.25 © 2005 American Chemical Society Published on Web 04/12/2005
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Figure 1. Molecular structures of ILs and the surfactant investigated.
SDS is correlated with the nature of the alkyl groups in the ILs.14 Solubilization of bmim-PF6 in the lamellar liquid crystal formed by concentrated Laureth 4 aqueous solution was studied by Friberg et al.15 using the small-angle X-ray scattering (SAXS) method. The less intensive investigations on such kinds of ternary systems are certainly unfavorable to the development of the studies on ILcontaining supramolecular assemblies and their applications. Most of all, it is not clear how the IL structure affects the phase behavior and properties of surfactant/IL/water systems. In this work, we choose two typical ILs: hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4), to determine the phase diagrams of IL/water/Brij 97 ternary systems in comparison. Brij 97, oleyl polyoxyethylene (10) ether, is a pharmaceutically accepted surfactant. For years, microemulsions formed by Brij 97 have been studied as drug delivery systems;16 however, there are only limited investigations on the liquid crystals for such long-chain surfactants. To compare with the lamellar phase formed in Laureth 4/bmim-PF6/water system reported previously,15 a large domain of a different liquid crystal phase-hexagonal phase was found and studied in Brij 97/water/IL systems in this investigation. The aim of our investigation is to characterize the hexagonal liquid crystals formed in Brij 97/water/bmim-BF4 and Brij 97/water/bmim-PF6 systems and to find any new properties that they share. SAXS, rheological methods, and polarizing optical microscopy were applied to investigate the textures, microstructures, and dynamic rheological properties of the hexagonal liquid crystals in comparison. Some interesting results were obtained. Experimental Section Materials. The nonionic Brij 97 of purity >99% was purchased from Sigma-Aldrich and used without further purification. ILs bmim-PF6 and bmim-BF4 were prepared using the procedures reported by other authors.17 The residual chloride in the ILs was less than 0.002 mol/L, which was determined by the apparatus and procedures previously described.18 Their molecular structures are shown in Figure 1. Phase Diagram Determination. Preweighed mixtures of Brij 97 and IL, with weight ratios varying from 0:10 to 10:0, were well-stirred at a temperature of about 60-70 °C to be homogenized; then, the water phase was added sequentially and the samples were mixed using a vortex mixer and repeated centrifugation. Samples inside and outside the phase boundary line were stored in a thermostat for 1 week for equilibration. Phase (14) Beyaz, A.; Oh, W. S.; Reddy, V. P. Colloids Surf., B 2004, 35, 119. (15) Friberg, S. E.; Yin, Q.; Pavel, F.; Mackay, R. A.; Holbrey, J. D.; Seddon, K. R.; Aikens, P. A. J. Dispsion Sci. Technol. 2000, 21, 185. (16) Warisnoicharoen, W.; Lansley, A. B.; Lawrence, M. J. Int. J. Pharm. 2000, 198, 7. (17) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; de Souza, R. F.; Fulmer, S. L.; Richardson, D. P.; Smith, T. E.; Wolff, S. Org. Synth. 2002, 79, 236. (18) Zhang, J. M.; Yang, C. H.; Hou, Z. S.; Han, B. X. New J. Chem. 2003, 27, 333.
Wang et al. equilibria were determined by visual observation of the samples in normal light and also observed between a cross polarizer for anisotropy. The types of liquid crystals were identified by means of a polarized optical microscopy technique and SAXS. Polarization Microscopy. A HUND WETZLAR H500 polarization microscope with a maximum magnification of 1000 was applied in the microscopic observation. The relevant pictures were digitalized using a charge-coupled device camera and the proper computer hardware. SAXS. The lattice spacing of the liquid crystals was measured using SAXS, performed on a HMBG-SAX X-ray small angle system (Austria) with Ni-filtered Cu KR radiation (0.154 nm) operating at 50 kV and 40 mA. The sample-to-detector distance was 277 cm. Except as noted, the temperature was kept at 25.0 ( 0.1°C. The relative position of the SAXS peaks on the scattering vector (q) axis was used to determine the structure of the liquid crystal phase. For hexagonal structure the first three peaks obey the relationship of 1/x3/2. Calculations of Aggregate Dimensions. Calculations of the hexagonal structure dimensions were made using relations between the volume fraction of Brij 97 and the lattice parameter obtained from SAXS. The density of Brij 97 is 1.00 g/mL according to the manufacturer. The densities of bmim-PF6 and bmim-BF4 are 1.37 and 1.15 g/mL, respectively.17 The volume of the apolar tail of the Brij 97 molecule was estimated as 500 Å3 using vL ) 27(m - 1 - 2ncis) + 55 + 40ncis,19 whereas 1158.6 Å3 was used for the molecular volume of Brij 97 by taking the volume of the oxyethylene (EO) group as 64.4 Å3 and that of the hydroxyl group as 14.6 Å3 according to the literature data on C12EOn.20 Rheological Measurements. Rheological measurements were performed with a HAAKE RS 75 rheometer. A cone-plate sensor was used, with a diameter of 35 mm, and a cone angle of 2°. The sample thickness in the middle of the sensor was 0.105 mm. Samples were kept in saturated water vapor for the whole time of the measurements. Because the rheological properties of such systems are dependent on the shear deformation history,21 the sample was gently inserted onto the top of the plate of the sensor, and then the plate was slowly elevated to its measuring position with constant velocity. The sample squeezed out from the sensor system was then gently removed. Measurements were carried out after a period of 10 min to allow for the stress relaxation. The maximum permitted deviation in temperature was 0.1 °C during measurements. Frequency sweep measurements were performed at a constant stress of 10 Pa, which was found to be in the linear viscoelastic domain in all the cases, where the amplitude of the deformations is very low. The frequency varied from 0.04 to 100 rad‚s-1. Before the rheological measurements, the samples were repeatedly centrifuged with 3000 rpm to remove bubbles and then were kept undisturbed for at least 6 days to get the complete buildup of liquid crystalline structures.
Results and Discussion Phase Behavior. The phase diagram of the Brij 97/ bmim-PF6/water system at 25 °C is presented in Figure 2a. The phase diagram has four single-phase regions, two isotropic solution phases (L1 and L2), one anisotropic lamellar liquid crystalline phase (LR), and one anisotropic hexagonal liquid crystalline phase (H1). The isotropic phase L2 is the liquid solution of surfactant and IL dissolving water up to 14 wt %. The other isotropic solution phase L1 is present between 0 and 32 wt % Brij 97, dissolving 5.5 wt % of bmim-PF6. A lamellar texture is formed between 10 and 33 wt % water solubilizing 10.7 wt % of bmim-PF6. Greater amounts of water in the surfactant give a hexagonal liquid crystal containing water between 16 and 69 wt %. This liquid crystal solubilizes IL (bmim-PF6) to a maximum of 31 wt %. On increase of (19) Malcolmson, C.; Lawrence, M. J. Colloids Surf., B 1995, 4, 97. (20) Kunieda, H.; Kabir, H.; Aramaki, K.; Shigeta, K. J. Mol. Liq. 2001, 90, 157. (21) Dimitrova, G. T.; Tadros, Th. F.; Luckham, P. F. Langmuir 1995, 11, 1101.
Liquid Crystalline Phases in Brij 97-Water-IL
Figure 2. Phase diagrams for (a) Brij 97/bmim-PF6/water and (b) Brij 97/bmim-BF4/water at 25 °C. L1, isotropic solution (water-rich); L2, isotropic solution (Brij 97-rich); LR, lamellar liquid crystal phase; H1, hexagonal liquid crystal phase.
bmim-PF6 concentration, the hexagonal liquid crystal phase turns from transparent to gray-turbid in appearance. Above 31 wt %, a distinct phase separation between two single phases is observed. The top phase is stiff, grayturbid, and birefringent, and the bottom phase is always transparent and liquid. As the IL concentration increases, the volume of the liquid phase grows. Figure 2b shows the phase diagram of the Brij 97/bmimBF4/water system with three single phases and their equilibrium. A lamellar liquid crystal phase (LR) between 17 and 23 wt % water is observed. Around medium surfactant concentration, a single hexagonal liquid crystalline phase (H1) is formed between 17 and 67 wt % water, solubilizing bmim-BF4 up to 34 wt %. This phase is always transparent and stiffer in appearance than the H1 phase formed in the above bmim-PF6 system. Also different from the bmim-PF6 ternary system, a H1/L biphasic area is not found. An isotropic solution of surfactant with water and the hydrophilic IL (bmim-BF4) covers the other area of the whole diagram. The boundary of the different type of isotropic phase is not identified in this work. To compare the phase behavior of the Brij 97/bmimBF4/water system with that of the Brij 97/bmim-PF6/water system, a large area of hexagonal liquid crystal phase exists in both systems. This phenomenon can be explained by use of the surfactant packing parameter Rp.22 Rp is defined as vL/(aS lc), where aS is the area occupied at the interface by the surfactant headgroup, lc is the hydrophobic chain length, and vL is the volume of the hydrophobic tail. The packing parameter is the fundamental geometric (22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1975, 1526.
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Figure 3. Textures of hexagonal liquid crystals formed by the (a) Brij 97/bmim-PF6/water and (b) Brij 97/bmim-BF4/water systems with the composition of 48/12/40 wt %, at 25 °C.
quantity for some possible aggregation shapes.22 For example, critical conditions for the formation of spherical, cylindrical, bilayer, and inverted structures are Rp e 1/3, 1/3 e Rp e 1/2, 1/2 e Rp e 1, and Rp g 1, respectively. For Brij 97 surfactant, aS was estimated as 48.9 Å2 by using aS ) 1.56n + 33.32.19 The hydrocarbon chain length was evaluated as 22.7 Å by dividing 500 Å3 (the volume of an oleyl chain) by 22.0 Å2 (the cross-sectional area per chain of the close-packed hydrocarbon chains in a liquid state).20 The packing parameter [vL/(aS lc)] of Brij 97, therefore, is to be 0.45, which makes it easy to form a cylindrical structure. Representative polarizing optical microscopy patterns of hexagonal phases are shown in Figure 3. A giant fanlike texture and a nongeometric texture were produced in bmim-PF6- and bmim-BF4-containing hexagonal phases, respectively. Hereafter, further investigations on the cylindrical hexagonal phases formed in the two systems are made comparatively. Component Effect on Hexagonal Liquid Crystals. SAXS. The hexagonal phase consists of infinitely long cylinder-like aggregates packed in a hexagonal array and separated by a continuous water region. SAXS measurements were performed on the hexagonal liquid crystal samples chosen in Figure 2. Representative SAXS spectra are presented in Figure 4. The X-ray diffraction patterns clearly identified the phase as a classical hexagonal structure, for which three reflections corresponding to the (100), (110), and (200) planes were assigned, showing a good agreement with Miller indexes according to
qhk )
4π xh2 + k2 + hk x R 3
(1)
where qhk is the scattering vector observed in the SAXS spectra and R is the lattice parameter, which represents the distance from the center of one cylinder to another,
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Figure 4. SAXS patterns for the hexagonal phases the same as those presented in Figure 3, at 25 °C.
including the total diameter of the cylinder and the thickness of the water layer. The lattice parameter R is calculated from the first peak position by R ) (4π/x3)/ q100. From the lattice parameter and volume fraction, φL, of the hydrophobic part of the surfactant in the system, some characteristic parameters are deduced:20 The radius, dH, of the hydrophobic part of the cylinders in the hexagonal structures was calculated using
x
x3φL 2π
dH ) R
(2) Figure 5. Characteristic structural parameters as a function of water content. (a) Lattice parameter R for the bmim-PF6 system (down triangle), bmim-BF4 system (up triangle), and Brij 97/water system (circle); (b) thickness of the water layer dW (hollow) and effective cross-sectional area per surfactant molecule at the hydrophile-lipophile interface aS (filled) for the bmim-PF6 system (up triangle), bmim-BF4 system (down triangle), and binary system (circle).
The distance between the cylinders, dW, is given by the relation
dW ) R - 2dH
(3)
The effective cross-sectional area per surfactant at the interface, aS, is calculated by the following equation:
aS )
2vL dH
The results of parameters R, dH, dW, and aS are summarized in Table 1. The changes in R, dW, and aS along the water dilution line at the 8:2 Brij 97/bmim-PF6 (or bmim-BF4) mass ratio are illustrated in Figure 5. In comparison, the results of the hexagonal phase on the
(4)
where vL is the volume of the apolar part of one surfactant molecule.
Table 1. SAXS Data for Hexagonal Liquid Crystalline Phases bmim-BF4 system
surf:IL (w/w)
H2O (wt %)
8:2 8:2 8:2 8:2
20.0 30.0 40.0 50.0 30.0 30.0 30.0
bmim-PF6 system
8:2 8:2 8:2 8:2
1:0 1:0 1:0
14.0 19.6 23.9
20.0 30.0 40.0 50.0 30.0 30.0 30.0
Brij 97/H2O system
IL (wt %)
40.0 48.0 60.0
14.0 19.6 23.9
φL
R (Å)
dW (Å)
dH (Å)
aS (Å2)
0.282 0.246 0.211 0.175
65.3 68.7 73.7 80.1
28.9 33.0 38.2 44.9
18.2 17.9 17.8 17.6
55.0 55.9 56.3 56.8 55.9 ( 0.9
0.246 0.223 0.205
68.7 69.7 70.2
32.9 35.1 36.8
17.9 17.3 16.7
55.9 57.8 59.8
0.289 0.251 0.214 0.177
62.2 66.2 71.9 77.4
27.1 31.4 37.0 43.2
17.5 17.4 17.5 17.1
57.0 57.5 57.2 58.4 57.7 ( 0.7
0.251 0.230 0.213
66.2 67.2 67.7
31.4 33.4 34.9
17.4 16.9 16.4
57.5 59.2 61.0
0.259 0.224 0.173
72.5 77.4 85.4
33.8 38.9 48.1
19.4 19.3 18.6
51.6 51.9 53.7
Liquid Crystalline Phases in Brij 97-Water-IL
surfactant axis when no IL is added are shown together in Figure 5. As shown in Figure 5a, the values of the lattice parameter are different as the water content is changed. The lattice parameter increases linearly with increase in water content. The slope of the Brij 97/H2O system is changed significantly by employing an IL. Moreover, at the same water content, the R increases when adding bmim-BF4 while decreases when adding bmim-PF6. For example, at the water content of 40%, the values of R for bmim-BF4 system, Brij 97/H2O system, and bmim-PF6 system are 73.3, 72.5, and 71.9 Å, respectively (see Table 1). From Figure 5b, at a constant water content, the thickness of the continuous water layer, the values of dW are in the order of bmim-BF4 system > bmim-PF6 system > Brij 97/H2O system, while the values of aS are in order of bmim-PF6 system > bmim-BF4 system > Brij 97/H2O system. All these differences should be related to the ways that bmim-PF6 or bmim-BF4 penetrate through the hexagonal phase. In previous studies, first both bmim-BF4 and bmimPF6 are immiscible with oils such as hexane,23 decane, and vice versa.15 Friberg has also reported that bmimPF6 is localized within the polar part of the layered structure of a lamellar phase.15 Therefore, these ILs will not be solubilized in the nonpolar part of the surfactant though solvatophobic interaction may exist between IL and the hydrophobic tail of the surfactant.9 Second, hydrogen bonds can occur between the EO unit and the PF6- or BF4- anion. Interaction also exists between the cation moiety (-N+) of ILs and the lone pairs on oxygen atoms of the EO groups.9,10,13 From the above results, therefore, it is clear that both bmim-BF4 and bmim-PF6 are solubilized in the hydrophilic domain of the hexagonal phases. Though bmim-BF4 + water has an upper critical solution temperature of 278 K,6 bmim-BF4 is completely miscible with water at room temperature.6,23 Considering these together with the SAXS results, it is reasonable to conclude that bmim-PF6 is dominantly penetrated between the EO chains of Brij 97 molecules which causes an increase in aS, whereas bmimBF4 is localized in the water layer of the hexagonal phases and resulting in an increase in dW. Also from Table 1 and Figure 5, at different water contents, the values of aS are fairly constant at 55.9 ( 0.9 for the bmim-BF4 system and 57.7 ( 0.7 for the bmim-PF6 system, respectively. This fact suggests that the overall structure of the interfacial region is largely unaffected by the water content over this range. To make further investigations, SAXS measurements were also performed on the IL-containing hexagonal phases at a constant water content of 30 wt %. The data of R, dH, dW, and aS at varied bmim-PF6 or bmim-BF4 concentrations are presented together in Table 1. With an increase in the concentration of IL or decrease in the surfactant concentration, the aS values clearly increase for both hexagonal phases as shown in Table 1. Considering that aS usually changes very slightly with the surfactant concentration when the type of liquid crystal is unchanged,24 this increase in aS values reflects the enhanced solubilization of either bmim-PF6 or bmim-BF4 between the polar groups of surfactants. It is also noted that, at all dH, the values of the radius of the cylinder are considerably shorter than the all-trans chain length 22.7 Å. For example, with water content (23) Dupont, J.; Consorti, C. S.; Spencer, J. J. Braz. Chem. Soc. 2000, 11, 337. (24) Mele, S.; Ninham, B. W.; Monduzzi, M. J. Phys. Chem. B 2004, 108, 17751.
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Figure 6. Storage (square) and loss (circle) moduli as a function of angular frequency (a) for 56/14/30 (filled) and 46/24/30 (hollow) of the Brij 97/bmim-BF4/water system and (b) for 50/ 20/30 of the Brij 97/bmim-BF4/water system (filled) and Brij 97/bmim-PF6/water system (hollow).
changes from 40 to 50%, dH is 17.5-17.1 for the bmimPF6 system, 17.8-17.6 for bmim-BF4 system, and 19.419.3 for Brij 97/water binary system, respectively. That is, the hydrophobic chain of Brij 97 shrinks, and it is shrunk more considerably in IL-containing hexagonal phases than in the Brij 97/water one. The hydration of the EO unit and the penetration of ILs between the EO groups result in an increase in the occupied area by the headgroup (larger aS). The occupied area results from a balance of forces, the interfacial free energy favors a small area, and the conformational entropy favors a larger area. Therefore, the larger the area, the more shrunken is the alkyl chain (smaller lc). It should be emphasized that the aS values of the bmim-BF4 system are always smaller than that of the bmim-PF6 system at the same composition. In other words, surfactant molecules are packed more densely in the bmim-BF4 system. This is consistent with the higher moduli determined in the following rheological measurement. Rheological Behavior. More information on the network structure of the hexagonal phases can be obtained from oscillatory shear frequency sweep measurements. The frequency dependence of the storage modulus G′ and loss modulus G′′ for several liquid crystalline samples is demonstrated in Figure 6. From the results, it can be clearly seen that the moduli G′ and G′′ of all investigated samples show traits of the general Maxwell model.25 The values of the dynamic moduli increase with increasing frequency with different (25) Cordobes, F.; Munoz, J.; Gallegos, C. J. Colloid Interface Sci. 1997, 187, 401.
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Figure 7. SAXS patterns as a function of temperature for 40/10/50 of the (a) Brij 97/bmim-PF6/water system (LP) and (b) Brij 97/bmim-BF4/water system (LB). The numeric 1-10 at the temperature axis account for 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 °C, respectively.
Figure 8. Lattice spacing R, determined from the primary (100) peak position as a function of temperature for LP (square) and LB (circle).
slopes. At low frequencies, G′′ > G′, showing viscous behavior, and at higher frequencies, G′ > G′′, where the elastic response dominates. This means that the hexagonal samples exhibit viscoelastic behavior.26 As Figure 6a illustrates, at constant water content, values of G′ decrease with increase in the bmim-BF4 concentration from 14 to 24 wt %. We also noticed that the crossover frequency (26) Durreschmidt, T.; Hoffmann, H. Colloid Polym. Sci. 2001, 279, 1005.
Wang et al.
Figure 9. Storage (filled) and loss (hollow) moduli as a function of temperature (a) for LP at 10 °C (square), 25 °C (up triangle), and 40 °C (circle) and (b) for LB at 15 °C (square), 25 °C (up triangle), and 55 °C (circle).
increase dramatically with this change, which reflects that the network strength was decreased and the internal structure becomes less stable.27 Figure 6b shows the rheograms of bmim-PF6- and bmim-BF4-formed liquid crystals in comparison. At a wide frequency region, the moduli of the bmim-BF4 system are clearly larger than those of the bmim-PF6 system, which is in agreement with its smaller aS value shown above. One may note that the viscoelastic profiles of G′ and G′′ for the two systems follow the same pattern; however, at higher frequency the loss modulus (G′′) of the bmim-BF4 system turned out to be lower than that of the bmim-PF6 system. This probably implies that at higher frequencies the large amount of free IL molecules in the bmim-BF4 system facilitate a viscous flow of the sample, which makes the viscous response decreased. Temperature Effect on Hexagonal Liquid Crystals. The effect of temperature on the hexagonal mesophases is also investigated. Figure 7 show the SAXS patterns for two hexagonal liquid crystals, LB and LP from 5 °C to 50 °C at a step of 5 °C. LB and LP belong to the bmim-BF4 and bmim-PF6 systems, respectively, with the same Brij 97/IL/water composition of 40/10/50 (wt %). It is noted that the two samples show reflection profiles of a hexagonal structure in the whole temperature region (27) Galatanu, A. N.; Chronakis, I. S.; Anghel, D. F.; Khan, A. Langmuir 2000, 16, 4922.
Liquid Crystalline Phases in Brij 97-Water-IL
investigated. The calculated values of the lattice parameter (R) as a function of temperature are plotted in Figure 8. It is worth noting that the lattice spacing for the hexagonal phase, either LB or LP, is constant at relatively high temperatures, and the primary peak intensity saturates as well, indicating a stabilization of order in the hexagonal phases. At lower temperature, however, the lattice parameters of the two hexagonal phases show temperature dependence in different ways. For sample LB, the lattice spacing decreases gradually when temperature decreases from 25 to 5 °C. It is easily understood that the slow down in the Brownian movement of molecules with decrease in temperature can cause a reduction in the distance between adjoining rodlike micelles. Conversely, the fact that the lattice spacing for sample LP shows a slow increase as temperature decreases from 35 to 5 °C seems puzzling. Because bmim-PF6 is dominantly penetrated between the surfactant molecules in the hexagonal phase, the hydrophobic butyl group in the cation moiety tends to push bmim-PF6 molecules to cooperate with the surfactant in forming interfaces, thus, making the interfacial film more flexible. Therefore, the reasonable explanation for the increase in R could be the increasing order and lateral compaction of surfactant molecules with decrease in temperature, leading to further straightening of the hydrophobic tails and the effective increase in the cylinder radius.28 Investigation of the rheological response of LB and LP with change in temperature is illustrated in Figure 9. As can be observed, temperature has a significant effect on the values of the moduli, G′ and G′′, increase with decreasing temperature, for both samples. This indicates a higher strength of the hexagonal phase network. The crossover frequency is a measure of how easily the network (28) Petrov, P. G.; Ahir, S. V.; Terentjev, E. M. Langmuir 2002, 18, 9133.
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can be formed. It is noteworthy that the crossover frequency decreases with temperature, and it is not experimentally obtained at the lowest temperature (10 °C) for the LP sample. According to Cordobes, this phenomenon is explained by the appearance of a wider “plateau” region, implying enhanced elastic characteristics, as a consequence of stronger interaction among rodshaped aggregates at low temperature.25 This is consistent with results deduced from SAXS data that the network is more ordered with decrease in temperature. Summary The work reported here has demonstrated that the hexagonal liquid crystalline phases formed in aqueous solutions of Brij 97 by solubilizing hydrophobic IL bmimPF6 and hydrophilic IL bmim-BF4 exhibit appreciably different properties. bmim-PF6 is dominantly located between the EO part of surfactant, while bmim-BF4 is mostly solubilized in the water area. The lattice parameter of bmim-BF4 containing liquid crystal phase is clearly larger than that of bmim-PF6 containing one. Decrease in temperature results in a decrease in lattice spacing for the bmim-BF4 system but an increase in lattice spacing for the bmim-PF6 system. Surfactant molecules are packed more tightly in the bmim-BF4 system than in the bmimPF6 system. The former has a smaller aS value and higher moduli (G′ and G′′) values. We have also noticed that the bmim-BF4-containing hexagonal phase has a higher melting point, and further study on the phase transition of the two IL-containing hexagonal liquid crystals using SAXS, 2H NMR, differential scanning calorimetry, and rheological techniques is in progress. Acknowledgment. Support of this work by the Natural Science Fund Foundation of China (Grant 30370945) and the Fund Foundation of Guizhou Provincial Governor (Qian, 2001-6) is gratefully acknowledged. LA050266P
