J. Phys. Chem. B 2009, 113, 983–988
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Novel Gel Phase Formed by Mixing a Cationic Surfactive Ionic Liquid C16mimCl and an Anionic Surfactant SDS in Aqueous Solution Yurong Zhao, Xiao Chen,* Bo Jing, Xudong Wang, and Fumin Ma Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, 250100, People’s Republic of China ReceiVed: October 13, 2008; ReVised Manuscript ReceiVed: NoVember 14, 2008
The phase behavior of a catanionic system composed by a cationic surfactive ionic liquid (IL), 1-hexadecyl3-methylimidazolium chloride ([C16mim]Cl), an anionic sodium dodecyl sulfate (SDS), and water has been investigated. A novel gel phase with quite high water content can be fabricated showing similar rheological properties to vesicles usually formed in traditional catanionic systems. The lamellar structure could also be constructed in SDS-rich region. Both the hydrophobic interaction of alkyl chains and interactions between oppositely charged head groups play important roles for the gel formation. Such a facile method to form gels directly from the catanionic system at relative low surfactant concentrations is novel, which should be related to the specific molecular structure of imidazolium ILs. The obtained results are expected to be helpful for better understanding of catanionic systems. Introduction In recent years, ionic liquids (ILs) composed of 1-alkyl-3methylimidazolium salts ([Cnmim]+, where n is the alkyl chain length) have attracted much attention of scientists for their wide use in the fields of catalysis, organic synthesis, electrochemistry, liquid/liquid extraction, and material preparation.1-5 For short alkyl chains, such ILs are often used as nonaqueous solvent, where micelles from traditional surfactants or block copolymers could be formed.6-10 Either lyotropic liquid crystals or vesicles have been observed in [Bmim][PF6]11,12 or ethylammonium nitrate.13 In addition, the microemulsion systems including ILs have also had much attention paid to them.14-18 Meanwhile, as a kind of salt with alkyl chains, their aggregation properties in water are another extensively studied subject. The micellar aggregates of [Bmim][BF4], [C8mim]I, and [C8mim]Cl in water are investigated.19 It has been reported that [Bmim][C8SO4] can also form micelles by themselves in the aqueous solution.20 Different characterization techniques including surface tension,21,22 electric conductivity,21-27 fluorescent spectroscopy,24,27 small angle neutron scattering (SANS),21 volumetric study,24,26 rheology,28 and freeze-fracture transmission electron microscope (FF-TEM)28 have been used to thoroughly explore the micellization behavior of [Cnmim]+Br- (n ) 2-12) in aqueous solution. The lamellar (LR) and hexagonal (H1) lyotropic liquid crystals could be produced from [Cnmim]X in water by themselves or with alcohol and p-xylene.29-33 This is not surprising for such “surface active”22,28 or “surfactant-like”34 imidazolium ILs with long hydrophobic chains because of their similar molecular structures to the traditional cationic surfactants. As for the catanionic surfactants, their self-assembly and phase behavior in aqueous solution have been extensively studied since the first catanionic system with the composition of cetyl trimethylammonium tosylate (CTAT), sodium dodecyl benzene sulfonate (SDBS), and water was reported by Kaler et al. in 1989.35 As a kind of classical cationic surfactants and a counterpart to long chained imidazolium ILs, the alkyltrim* To whom correspondence should be addressed. E-mail: xchen@ sdu.edu.cn. Fax: +86-531-88564464. Tel: +86-531-88365420.
Figure 1. The gel and lamellar regions formed in the ternary system composed of SDS, [C16mim]Cl, and water at relative high water concentrations (g90%).
ethylammonium salts are well known to cause abundant phase behaviors in their aqueous solutions when combined with anionic surfactant.35-39 In such formed catanionic systems, micelles, vesicles, and lamellar phases can usually be found. Then, what about these surfactive imidazolium ILs? Their strongly π-conjugated ring structures may result in novel selfassemblies to show different structural characters from those made by the conventional cationic surfactants. However, to the best of our knowledge, the phase behavior of such catanionic system composed by surfactive ILs and anion surfactants in water has never been reported. With this in mind, a ternary system composed of [C16mim]Cl, SDS, and water has been investigated in this paper. The particular focus is paid on the gel phase formed at quite high water concentrations (g90%, weight fraction, thereinafter). Such a study should be helpful for better understanding of the interactions inside the imidazolium ILs composed catanionic systems. Experimental Section Materials. [C16mim]Cl was prepared according to the procedures reported previously.40,41 The compound 1-methylimidazole and an excess amount of 1-hexadecyl chloride were mixed in a flask, refluxed at 90 °C for 24 h, and then cooled to
10.1021/jp809048u CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
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Figure 2. Photographs for the gel phase formed at the SDS to [C16mim]Cl ratio of 4:6 and total surfactant concentrations ranging from 3 to 10%. From left to right, the total surfactant concentration is incremental.
Figure 3. (a) The strain sweep line for a system with the composition of 2% SDS, 3% [C16mim]Cl, and 95% water. (b) The frequency sweep line for the same sample in which G′, G′′, and (|η*|) are as a function of the frequency. T ) 25.0 ( 0.1 °C.
Figure 4. Comparison of the G′ values versus frequency at different total surfactant concentrations but the same ratio of SDS to [C16mim]Cl. T ) 25.0 ( 0.1 °C.
room temperature. The product (a white waxy solid) was purified by recrystallizing the mixture in fresh tetrahydrofuran (THF) at least three times and then dried under vacuum condition for 48 h. The product purity was ascertained by surface tension measurement and 1H NMR (nuclear magnetic resonance) spectrum in D2O. SDS was purchased from Sigma and cetyl trimethyl ammonium chloride (CTAC) was from Alfa Aesar. The other reagents were products of TCI (Shanghai) Development Co., Ltd. All of them were used without further purification. Water was triply distilled. Sample Preparation. Samples were prepared by mixing stock solutions of [C16mim]Cl and SDS at various weight ratios and different total surfactant concentrations. After sealing, they were vortex mixed and then equilibrated in a thermostat at 25 °C for at least one month before further investigations.
Characterization. Photographs of sample birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WV-CP460). Rheology measurements were performed on a Haake RS75 rheometer equipped with a DC50 temperature controller (water circulating bath). A cone-plate fixture (Ti, radius 20 mm, cone angle 1°) was used in our experiment. The distance between the sensor and the cone plate was adjusted to 52 µm for all measurements. The temperature was kept at 25 ( 0.1 °C. Frequency sweep measurements were performed in the linear viscoelastic region. This region was determined from strain sweep measurement in which the stress (σ) was varied but the frequency was kept at 1.0 Hz. Differential scanning calorimetry (DSC) measurements were carried out on a DSC822e thermal analysis system (MettlerToledo, Switzerland). Samples of about 12 mg were placed and then analyzed in a hermetically sealed aluminum pan. The linear heating rate of 5 °C /min was employed on all samples over the temperature range from 5 to 60 °C under nitrogen at a flow rate of 50 mL min-1. Results and Discussion Like the traditional catanionic systems, there also exist abundant phase behaviors in our investigated system. Among all the obtained aggregate structures, the novel gel and lamellar phases (LR) are specially focused here. Their distribution regions have been displayed in the phase diagram of the ternary system composed of SDS, [C16mim]Cl, and water as shown in Figure 1. The Gel Phase Formation. As we know, the macroscopic manifestation of a successful gelation is the absence of observable gravitational flow upon inversion of the test tube. On the
Novel Gel Phase from C16mimCl and SDS mixture
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Figure 5. Polarized optical micrographs of the gel phase at the same SDS to [C16mim]Cl ratio of 4:6 but different total surfactant concentrations: (a) 5, (b) 6, (c) 7, and (d) 8%.
Figure 6. A simple sketch map for the gel formation process.
basis of this approach, the approximate region for the gel phase is determined. As can be seen from Figure 1, such a gel phase exists in between the weight ratio of SDS to [C16mim]Cl ranging from 3.4:6.6 to 4.5:5.5 with a total surfactant concentration no less than 2.7%. The visual photographs of the gel formation at a certain ratio of SDS to [C16mim]Cl of 4:6 and different total surfactant concentrations (ranging from 3 to 10%) are presented in Figure 2. It should be noted that the gel phase produced in our imidazolium ILs containing system exhibits certain characters somewhat different from the usual ones. It shows a little sky-blue appearance as well as a less viscous and dilute solutionlike feeling when touching it. Meanwhile, the gel phase formed here is of quite high water content, that is, it could be formed at total surfactant concentration as low as 2.7%. Such advantages may expand the applications of catanionic systems in some fields with special requirement of great water content. Rheology Measurements. The macro properties of obtained gels at various ratios and different total surfactant concentrations were characterized by rheology measurements. Because all the gel samples were found to show similar rheological behaviors,
Figure 7. DSC curves for the samples at different total surfactant concentrations (from 3 to 9%) but the same SDS to [C16mim]Cl ratio of 4:6.
one typical system with a composition of 2% SDS, 3% [C16mim]Cl, and 95% water is chosen here as an example for
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Figure 8. Representative polarized optical micrographs of the lamellar phase with a ratio of SDS to [C16mim]Cl of 6.5:3.5. (a,b) These panels correspond to the systems with total surfactant concentration of 8 and 10%, respectively.
Figure 9. Photographs for the parallel system samples. They are ternary systems with anionic SDS being substituted by SDBS (a), SL (b), SLS (c), and SOS (f), or with cationic [C16mim]Cl being substituted by CTAC (d), and [C8mim]Cl (e).
Figure 10. The simulated structures for different catanionic complexes. (a-d) Structures are respectively formed by [C16mim]Cl with SDBS (a), SL (b), SLS (c), and SDS (d). (e) Structure is formed by CTAC and SDS.
expatiation. In the rheology experiments, the strain sweep measurement was first carried out to determine the linear viscoelastic region. As shown in Figure 3a, there is a wide plateau for both the storage modulus (G′) and loss modulus (G′′) at the stress value (σ) in the strain sweep line. σ ) 9 Pa is chosen for the frequency sweep measurement on the same
system with the obtained result shown in Figure 3b. As can be seen from Figure 3b, over the frequency range from 0.1 to 10 Hz, both G′ and G′′ increase slowly and are nearly frequencyindependent at certain narrow regions. What’s more, G′ is higher than G′′. The complex viscosity (|η*|) also exhibits a linear decrease with a slope of -1. All these results indicate a
Novel Gel Phase from C16mimCl and SDS mixture viscoelastic nature for the produced gels in our system, which is in agreement with previous reports on the gels formed in other systems.42-45 However, a further investigation shows that the values of G′ and G′′ here are much smaller than those of some gel phases reported previously,42-48 or only about 1% of those for polymer gels.46-48 Though much smaller, the values of G′ and G′′ here are still somewhat higher (about ten times or more) than those for vesicles phases formed in traditional catanionic systems.12,43,49-51 A possible explanation should be related to the quite high water content in our gel, which is much similar to the vesicle phases mentioned above. Besides, as shown in Figure 4, the values of G′ and G′′ seem have no quantitative relation with the total surfactant concentration if the ratio of SDS to [C16mim]Cl is kept constant. This may be attributed to the not quite ordered structures or the tanglesome arrangement of catanionic complexes in obtained gels at relative low surfactant concentrations. Structure Characterization of the Gel Phase. The polarized optical microscopy (POM) observation can provide further information on the produced gel structure. As can be seen from the images for several representative samples shown in Figure 5, certain threadlike textures in Figure 5a and some scattered Maltese crosses in Figure 5b-d are present, which are all typical for the lamellar phase. However, detailed investigation shows that all these textures are not much clear and sometimes exhibit local bright areas like that in Figure 5b, which may reflect the nature of less ordered gel structure as referred above. The lamellar structure self-assembled in single [C16mim]Cl powder has been reported by Smarsly et al.31 In our case, the observed marbling texture may be resulted from the selfassembled structure either by [C16mim]Cl itself, or by catanionic complexes. But the solution of [C16mim]Cl is dilute and isotropic, which could not lead to any anisotropic self-aggregates by itself at room temperature. Therefore, the marbling texture seems much more reasonable to be resulted from the aggregate of catanionic complexes. A simple sketch map for the process of the gel phase formation is given in Figure 6. Sol-Gel Transition. As an important parameter, the sol-gel transition temperature has been ascertained by DSC measurement. From Figure 7 of DSC curves for several typical samples, it can be seen that all systems exhibit two endothermic peaks respectively at about 14.5 and 38.9 °C. However, for solutions with only SDS or [C16mim]Cl, no peak at around 14.5 °C can be detected. Besides, the heat flow for the peaks at this temperature is rather small. The first peak in the curves may thus be attributed to the chain melting of catanionic samples, as was reported for other catanionic systems.52 As for the second peak, it is just corresponding to the sol-gel transition temperature and such a transition process is reversible as can be confirmed by the usual heating-cooling cycle. When the gel sample is heated to a definite temperature, interactions between the tail groups may weaken to a certain extent, which will result in the softening of the catanionic complex or the breakage of the network formed by them, and then lead to the sol-gel transition. The Lamellar Phase Formation. Except for the gel phase, there also exist lamellar phases at SDS-rich region as shown in Figure 1, which was determined by POM measurements. The samples in this region are all transparent and somewhat viscous. Representative photographs from POM observations are shown in Figure 8. As usually observed from the conventional lamellar phases,53 both marbling and stripe textures can be obtained here, indicating the formation of lamellar structures. It is noted that the total surfactant concentration needed to form this lamellar
J. Phys. Chem. B, Vol. 113, No. 4, 2009 987 phase is much lower than that of a conventional catanionic system composed of CTAT, SDBS, and water,35,37 but more comparable to the one with the composition of sodium laurate (SL), dodecyl trimethyl ammonium bromide (DTAB), and water.53 This may imply that the lamellar structure formed in the present system has little difference than that of conventional ones. Formation Mechanism of the Gel Phase. What factors bring about the new characteristics to our gel phase? To answer this question, a series of parallel systems have been investigated for contrast. First, in surfactants with different head groups but the same hydrophobic chains, such as the anionic SDBS, SL, and sodium dodecyl sulfonate (denoted as SLS), the cationic CTAC is chosen to replace only one corresponding surfactant in the ternary system each time. No gel phase could be observed in all such parallel systems, as can be seen from their sample appearances shown in Figure 9a-d. Therefore, it can be concluded that although the headgroup structures of both [C16mim]Cl and SDS are of great importance for the formation of the novel gel phase, not a single one can be omitted. One possible reason for this may be that, the complex formed between the [C16mim]Cl and SDS is much more different from others. In order to validate this assumption, molecular dynamics simulations for both the mostly investigated system and the parallel ones with different head groups were carried out. Such a simulation is on molecular level and can present us the intuitionistic structure of the optimized molecules, which is different from the MesoDyn simulation we have referred to before.54 Each complex formed by different catanionic surfactants was first geometrically optimized in vacuum with the rms gradient of 0.1 kcal/mol and then followed by a 50 ps (first 30 and then 20 ps) molecular dynamics simulation with the time step size of 0.001 ps at the temperature of 298 K. The obtained result was shown in Figure 10, from which the great difference between the headgroup of SDS/C16mimCl complex and those of the parallel ones with different anionic surfactants can be clearly seen in the directions pointed by the arrows, that is the imidazolium ring in C16mimCl/SDS complex is almost in a plane, but that in the other complexes are all contorted to certain extent. Even though no other obvious difference except for the different cationic ions can be observed between the head groups of SDS/C16mimCl and SDS/CTAC complexes, the tails of them have been proved to curve in dissimilar way, as can be seen in Figure 10d,e. Such differences may all prove our conjecture, that is, the complex formed by the [C16mim]Cl and SDS is not the same as that formed by other catanionic surfactants. Then, surfactants with the same head groups but different alkyl chains are also chosen for our parallel experiments. The corresponding sample appearances are also shown in Figure 9e,f. The obtained results indicate that, [Cnmim]Cl cannot form gels with SDS in aqueous solution until the carbon number n reaches 14. Besides, the ternary system composed of [Cnmim]Cl, sodium octyl sulfate (SOS), and water was still unable to form the gel phase, even the value of n reaches 16. This observation could definitely reflect the important role of solvophobic interaction played in the gel phase formation. Conclusions The phase behavior of a catanionic system with the composition of [C16mim]Cl, SDS, and water is studied in this work. A novel gel phase with relative low surfactant concentrations is formed and has been characterized by many techniques. Results from rheology and POM measurements show that the gel phase formed in the present system may be less ordered and has
988 J. Phys. Chem. B, Vol. 113, No. 4, 2009 analogous rheological properties than that of classical vesicles owing to the higher water content. The simulated results have certified that the complex formed by the [C16mim]Cl and SDS is dissimilar to that formed by the parallel catanionic surfactants with different head groups. Besides, the solvophobic interaction is also proved to play a very important role for the gel formation. Even though the detailed structure of the gel phase is still unclear, the novel gel phase presented here is very interesting and of great importance, which had broken up the conventional phase behavior of catanionic systems and may help us better understand the special structure of the surface-active surfactant [Cnmim]Cl. Acknowledgment. We are thankful for the financial supports from the National Natural Science Foundation of China (20573066, 20773080) and Natural Science Fund of Shandong Province (Y2005B18). References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Holbrey, J. D.; Seddon, K. R. Clean Prod. Proc. 1999, 1, 223. (3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (4) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Z. Catal. Today 2002, 74, 157. (5) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106. (6) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 19, 2444. (7) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745. (8) He, Y. Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666. (9) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de Lauth-Viguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99. (10) Wu, J. P.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y.; Inoue, T. Langmuir 2008, 24, 9314. (11) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C.; Sun, Z. W. Chem. Commun. 2004, 24, 2840. (12) Hao, J. C.; Song, A. X.; Wang, J. Z.; Chen, X.; Zhuang, W. C.; Shi, F.; Zhou, F.; Liu, W. M. Chem.sEur. J. 2005, 11, 3936. (13) Zhang, G. D.; Chen, X.; Zhao, Y. R.; Ma, F. M.; Jing, B.; Qiu, H. Y. J. Phys. Chem. B 2008, 112, 6578. (14) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (15) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z.; Shen, D.; Li, Z. H.; Du, J. M.; Hou, W. G.; Zhang, G. Y. Langmuir 2005, 21, 5681. (16) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (17) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X.; Hou, W. G.; Li, G. Z. Green Chem. 2006, 8, 43. (18) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. ChemPhysChem 2006, 7, 1554. (19) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 2191. (20) Miskolczy, Z.; Sebok-Nagy, K.; Biczo´k, L.; Go¨ktu¨rk, S. Chem. Phys. Lett. 2004, 400, 296.
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