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Surface Tension Measurements of N-Alkylimidazolium Ionic Liquids George Law and Philip R. Watson* Department of Chemistry, Oregon State University, Corvallis, Oregon 973331-4003 Received April 30, 2001. In Final Form: June 22, 2001 We have measured the surface tensions, with use of a ring tensiometer, of a series of closely related ionic liquids [Cnmim][X] where [Cnmim] represents the 1-CnH2n+1-3-methylimidazolium cation (n ) 4 [bmim], 8 [omim], and 12 [C12mim]) and X ) PF6-, BF4-, Cl-, or Br-. The values of surface tension span an unusually wide range for compounds of such similar structure, from 45 mJ/m2 ([bmim][PF6]) to 24 mJ/m2 ([C12mim][PF6]) at 336 K. They all show a linear variation of surface tension with temperature allowing a separation of the surface excess entropy and energy components. The surface excess quantities are alkyl chain length-dependent; both the surface excess entropy and energy decrease as the alkyl chain in the 1-position of the cation is lengthened for a particular anion. For a common cation, a reduction in the surface excess entropy occurs with decreasing anion size. A similar effect occurs in the surface excess energy when the anion decreases in size for the shorter alkyl chain compounds. The wide range of values of the surface properties for these materials probably reflects changes in surface orientation of the cation.
Introduction Room-temperature ionic liquids (ILs), based on the N-alkylimidazolium cation1-3 (Figure 1), are rapidly gaining interest as replacements for traditional organic solvents used in chemical processes.4-7 These environmentally friendly solvents have many useful properties: They have the ability to dissolve an enormous range of inorganic, organic, and polymeric materials at very high concentrations, are noncorrosive, and have low viscosities and no significant vapor pressures.8-10 Their versatility has driven increasing interest in using them in multiphasic homogeneous catalytic reactions3,11-17 where one phase is chosen to dissolve the catalyst and be immiscible, and the second phase contains the reactant and products. Such catalysis is believed to occur at the interface between the IL and the overlying organic phase, and should be dependent on access of the catalyst to the surface and the * To whom correspondence correspondence should be addressed. E-mail:
[email protected]. (1) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351-356. (2) Seddon, K. R. Molten Salt Forum 1998, 5-6, 53-62. (3) Welton, T. Chem. Rev. 1999, 99, 2071-2083. (4) Holbrey, J. D.; Seddon, K. R. Clean Product Processes 1999, 1, 223-236. (5) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 13911398. (6) Wasserscheid, P.; Keim, W. Angew. Chem. Int. Ed. Engl. 2000, 39, 3772-3789. (7) Rooney, D. W.; Seddon, K. R. In Handbook of Solvents; Wypych, G., Ed.; ChemTech: Toronto, 2000; Vol. Publishing, pp 1459-1484. (8) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221-227. (9) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133-2140. (10) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem 1998, 8, 2627-2636. (11) Herrmann, W. A.; Bo¨hm, V. P. W. J. Organomet. Chem. 1999, 572, 141-145. (12) Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon, K. R. Org. Lett. 1999, 1, 997-1000. (13) Olivier, H.; Chauvin, Y. Chem. Ind. 1996, 68, 249-263. (14) Chauvin, Y.; Olivier-Bourbigou, H. CHEMTECH 1995, 25, 2630. (15) Cull, S. G.; Holbrey, J. D.; Vargas-More, V.; Seddon, K. R.; Lye, G. J. Biotechnol. Bioeng. 2000, 69, 227-233. (16) Erbeldinger, M.; Mesiano, A. J.; Russel, A. J. Biotechnol. Prog. 2000, 16, 1129-1131. (17) Lau, R. M.; Rantwijk, F. v.; Seddon, K. R.; Sheldon, R. A. Org. Lett. 2000, 2, 4189-4191.
Figure 1. The generic structure of room-temperature ionic liquids [Cnmim][X] based on the substituted imidazolium cation: R1 = methyl (typically); R2 = alkyl; X- = BF4-, PF6-, halide-.
transfer of material across the interface. A clearer understanding of the mechanisms behind the catalysis process requires examination of the surface properties of the ionic liquids. A knowledge of the surface tension (γ) of ILs is clearly of some importance, yet, to our knowledge, none have been measured. We report here measurements of the surface tensions of a variety of 1-alkyl-3-methylimidazolium ILs as a function of temperature. Parallel experiments aimed at establishing the composition and orientation at the surface of ionic liquids using direct recoil spectrometry (DRS) are underway in this laboratory18,19 and we draw on some of these results. Experimental Section Equilibrium measurements of the apparent surface tension were performed with use of a DuNuoy Tensiometer with a platinum ring with a mean circumference of 5.992 cm, and a ring/wire radius ratio of 53.6. A 10-mL sample in a 50-mL beaker was determined to be sufficient to completely wet the probe and to prevent its interaction with the liquid meniscus. The beaker was placed inside an aluminum heating cell that sat on top of the tensiometer sample table. The cell allowed for rapid heating/ cooling of the sample and insulated the sample from variations in the laboratory environment. The sample temperature was regulated by pumping thermostated water through holes bored in the cell and was monitored by an alumel-chromel thermocouple. Each sample was allowed to attain thermal equilibrium for 5-10 min at each temperature setting and at each measurement. The temperature was stable to better than 1 °C. The apparent surface tensions from the scaled reading of the tensiometer were corrected to obtain the true surface tension (18) Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429-8434. (19) Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Phys. Chem. Chem. Phys. 2001, 3, 2879-2885.
10.1021/la010629v CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001
Surface Tension of N-Alkylimidazolium Ionic Liquid Table 1. Ionic Liquids Used in This Study cation
anion
mp (°C)
[bmim]
[PF6] [BF4] [PF6] [BF4] [Cl] [Br] [PF6] [BF4]
-61 -81 -70 -80 -82 -a 50 39
[omim]
[C12mim]
a No meltting point is observed from this material. The glass transition point is -48.7 °C.
Langmuir, Vol. 17, No. 20, 2001 6139 Table 2. Values of the Surface Tension γ (mJ/m2) at 336 K, Surface Excess Entropy Ss (mJ/m2 K), and Surface Excess Energy Es (mJ/m2)a ionic liquid
Ss ) -dγ/dt (mJ/m2 K)
Es (mJ/m2)
γ (mJ/m2) at 336 K
[bmim][PF6] [bmim][BF4] [omim][PF6] [omim][BF4] [omim][Br] [omim][Cl] [C12mim][PF6] [C12mim][BF4]
0.0783 0.0572 0.0634 0.0551 0.0853 0.0592 0.0552 0.0433
69.2 57.6 54.2 47.9 60.7 50.4 40.0 39.8
42.9 38.4 32.8 29.8 32.0 30.5 23.6 25.2
a This temperature was chosen as one where all the compounds exist as liquids.
Figure 2. Variation of measured values of surface tension γ (mJ/m2) as a function of temperature for the ionic liquids used in this study and listed in Table 1. with use of the methods described by Zuidema and Waters.20 The surface tension values obtained were accurate to 0.1 mJ/m2. Ionic liquids of the general form [Cnmim][X] where [Cnmim] represents the 1-CnH2n+1-3-methylimidazolium cation (n ) 4 [bmim], 8 [omim], and 12 [C12mim]) and X ) PF6-, BF4-, Cl-, or Br- were provided by Prof. K. R. Seddon, Queen’s University, Belfast, Northern Ireland (see Table 1). Most of the samples were used as received. The hygroscopic [omim] halides were vacuum-dried several times at temperatures above their respective melting points before use. These latter samples were transferred directly to a preheated sample container (∼60 °C), and the γ variations were determined at above this temperature. The data for [C12mim][PF6] were obtained at above its melting point (Tm ∼ 50 °C), and for [C12mim][BF4] data were obtained at above both the clearing (Tc ∼ 29 °C) and melting (Tm ∼ 39 °C) points.
Results Plots of γ(T) for the ionic liquids are shown in Figure 2. As expected, the surface tension decreases linearly with temperature for all the liquids examined here. Extrapolation of the surface tensions for different cations of a common anion shows that the γ(T) plots converge slowly to high temperatures. The range of surface tension values (20) Zuidema, H. H.; Waters, G. W. Ind. Eng. Chem. Anal. Ed. 1941, 13, 312-313.
exhibited by structurally similar homologous ILs is unusually high. For example, the surface tensions of the [Cnmim][PF6] ILs vary by more than 20 mJ/m2 as n changes from 4 to 12. We can compare this approximate doubling of the surface tension between [C12mim][PF6] and [bmim][PF6] with the much smaller variance (and in the opposite direction) for n-alcohols (at 333 K γ values for butanol and decanol are 21.8 and 25.9 mJ/m2, respectively) and triglycerides (at 333 K γ values for glyceroltributyrate and glyceroltridodecanoate are 28.1 and 29.4 mJ/m2, respectively).22 In general, for [Cnmim] ILs containing the same anion, γ diminishes with increasing alkyl chain length. For example, for ILs with a PF6- anion at 363 K (see Table 2), as the alkyl substituent in the cation increases in size, the measured surface tension decreases from a value of 42.9 mJ/m2 for [bmim] (C4) to 32.8 mJ/m2 for [omim] (C8) to 23.6 mJ/m2 for C12. A similar variation is observed for the BF4- series of ILs. For a fixed cation, in general, the compound with the larger anion has the higher surface tension (at the same temperature). Thus, at 336 K (see Table 2) the surface tension of [bmim][PF6] is higher than that of [bmim][BF4] and that of [omim][Br] is higher than that of [omim][Cl]. However, in [C12mim][PF6] and [C12mim][BF4], the compound having the larger anion exhibits a slightly lower surface tension. The surface tensions of the ILs studied here seem to be well described as a linearly decreasing function of temperature
γ(T) ) a - bT
(1)
where we can identify the slope b with the (assumed temperature-independent) surface excess entropy (Ss) and intercept a with the surface excess energy (Es).22 These quantities were calculated for each ionic liquid and are listed in Table 2 and displayed in Figures 3 and 4. The surface excess quantities are alkyl chain length-dependent; both the surface excess entropy and energy decrease as the alkyl chain in the 1-position of the cation is lengthened for a particular anion. For a common cation, a reduction in the surface excess entropy occurs with decreasing anion size (Figure 3). A similar effect occurs for the surface excess energy (Figure 4), although, for the longest C12 alkyl chain, the surface energies for both the small BF4- and the large PF6- anions are essentially equal. (21) Birdi, K. S. Surface Tension and Interfacial Tension of Liquids. In Handbook of Surface and Colloid Chemsitry; Birdi, K. S., Ed.; CRC Press: Boca Raton, 1997; Chapter 3. (22) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, UK, 2000; Vol. III, Liquid-Fluid Interfaces.
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Figure 3. Variation of the surface excess entropy Ss (mJ/m2K) as a function of the alkyl chain length n in ionic liquids [Cnmim][X].
Figure 4. Variation of surface excess energy Es (mJ/m2) as a function of the alkyl chain length n in ionic liquids [Cnmim][X].
Discussion The principle of independent surface action, originally formulated by Langmuir,23 has often been used to describe the microscopic structure of a liquid surface. According to this principle, each part of a molecule possesses a local surface free energy, and therefore the measured surface tension should correspond to the part of the molecule that is actually present at the interface. The most familiar example is that of methanol. The measured roomtemperature surface tension of methanol is 22.5 mJ/m2, comparable with that of an n-alkane (typically 16-27 mJ/ m2) and much lower than that of water (75.8 mJ/m2), and is attributed to the methanol molecules being oriented in the surface with the methyl group uppermost.24 Recent measurements on liquid crystals25 support the validity of (23) Langmuir, I. In Phenomena, Atoms and Molecules; Philosophical Library, New York 1950, p 72. (24) MacRitchie, F. Chemistry at Interfaces; Academic Press: San Diego, 1990.
Law and Watson
this principle and indicate that surface tension value “is strongly determined within a submolecular length scale from the free surface”. Other work in this laboratory with DRS18,19 has shown that both the anion and cation are present at the surface of these ILs and both should contribute to the surface free energy. To apply the principle of independent action to ILs we would need to know the values of the surface tension for the anion and cation independently, which, of course, is not feasible. We can, however, note that the surface tension of imidazole, extrapolated to 333 K, is 40.3 mJ/ m2, 26 whereas those of symmetric inorganic main-group halides tend to be much lower (at 333 K, γ for PCl3 is 23.5 and for SiCl4 is 14.8 mJ/m2).22 Fluorination also tends to lower the surface tension substantially.25 However, we should not jump to the conclusion that the surfaces of ILs with high γ values are cation dominated and those with low values are anion dominated for several reasons. The need to maintain electroneutrality makes it unlikely that the surface could completely consist of one type of ion. Also, given the much larger size of the cation than the anion in ILs of the type considered here, we might expect the overall surface tension to primarily reflect the properties of the cation, modified by those of the smaller anion. Furthermore, as mentioned earlier, DRS measurements are consistent with a sharing of the surface between anion and cation. The data in Figure 2 and Table 2 show that increasing the size of the cation, by lengthening the chain size of the 1-alkyl substituent, for a common anion results in a lowering of the surface tension. For the highly symmetrical anions used here, anion orientation cannot be a factor in the change in the surface tension, and variation in surface properties are probably caused by the cations, particularly in differences in surface orientation. Such changes are suggested by DRS experiments.19 Comparing absolute surface tension data between different molecules is complicated by the fact that surface tension is a free energy that is temperature-dependent. It is perhaps more appropriate to compare the temperature-independent excess surface quantities. The values of these quantities for the ILs measured here are compared with those for some neutral organic liquids, n-alkanes, and heterocycles22 in Figure 5. The ionic liquid [bmim][PF6] lies close in the plot to the position of imidazole. This molecule itself exhibits a similar surface energy to the other heterocycles pyrrole and quinoline, but a much lower value of the surface free entropy. In fact, imidazole is unusual in this respect in the context of organic molecules in general, which usually have surface entropies greater than 0.075 mJ/m2K. The fluorine-containing ILs other than [bmim][PF6] all have lower surface entropies than imidazole, and they decrease with alkyl chain length (Figure 4). Such a decrease in surface entropy with chain length is also seen for n-alkanes and is usually taken to indicate an increased degree of surface orientation.22 The fluorine-containing ILs exhibit surface energies similar to those of n-alkanes, at the low end of the range for organic liquids. Although ILs consist of ions, they are large and well-separated, and we might expect the Coulombic forces to be relatively weak. Comparing ILs with the same alkyl chain length (Figure 4), we observe a drop in the surface energy for a smaller anion (BF4- vs PF6-). The difference in surface energies decreases as the chain length increases and is essentially absent for the (25) Mach, P.; Huang, C. C.; Stoebe, T.; Wedell, E. D.; Nguyen, T.; Jeu, W. H. d.; Guittard, F.; Naciri, J.; Shashidhar, R.; Clark, N.; Jiang, I. M.; Kao, F. J.; Liu, H.; Nohira, H. Langmuir 1998, 14, 4330-4341. (26) Hofmann, K. Imidazole and Its Derivatives; Interscience: New York, 1953; Vol. 1.
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is probably a result of increased van der Waals interactions between the chains. The principal influence on the surface properties of these ionic liquids seems to be the structure and surface orientation of the cation. We hope that future parallel experiments with direct recoil spectrometry will allow us to correlate surface tension measurements with preferred cation orientations. Conclusions
Figure 5. Plot of the relationship of the surface excess entropy Ss (mJ/m2K) and surface excess energy Es (mJ/m2) for the ionic liquids used in this study and literature values for neutral organic liquids taken from the compilation of Lyklema,22 except for imidazole.26
dodecyl chain length. This may be an indication that as the chain length increases any influence of the anion on the surface energy is rapidly quenched, and for [C12mim] species the surface energy is dominated by the cation and
The surface tensions, measured in a ring tensiometer, of a series of closely related 1-alkyl-3-methylimidazolium ionic liquids span an unusually wide range. They range, at 336 K, from values as high as 45 mJ/m2 ([bmim][PF6]) to as low as 24 mJ/m2 ([C12mim][PF6]). They all show a linear variation of surface tension with temperature allowing a separation of the surface excess entropy and energy components. The surface excess quantities are alkyl chain length-dependent; both the surface excess entropy and energy decrease as the alkyl chain in the 1-position of the cation is lengthened for a particular anion. For a common cation, a reduction in the surface excess entropy occurs with decreasing anion size. A similar effect occurs in the surface excess energy when changing the anion for the shorter alkyl chain compounds. Acknowledgment. We thank Professor Kenneth Seddon, QUILL Centre, Queen’s University, Belfast, Northern Ireland for kindly providing the samples of ionic liquids used in this study. LA010629V
