Letter pubs.acs.org/Langmuir
Chemical Propulsion Using Ionic Liquids Shigeki Tsuchitani,*,† Nobuhiro Takagi,‡ Kunitomo Kikuchi,† and Hirobumi Miki† †
Faculty of Systems Engineering, Department of Opto-mechatronics, and ‡Graduate School of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan S Supporting Information *
ABSTRACT: Chemical propulsion generates motion by directly converting locally stored chemical energy into mechanical energy. Here, we describe chemically driven autonomous motion generated by using imidazolium-based ionic liquids on a water surface. From measurements of the driving force of a locomotor loaded with an ionic liquid and observations of convection on the water surface originating from the ionic liquid container of the locomotor, the driving mechanism of the motion is found to be due to the Marangoni effect that arises from the anisotropic distribution of ionic liquids on the water surface. The maximum driving force and the force-generation duration are determined by the surface activity of the ionic liquid and the solubility of the ionic liquid in water, respectively. Because of the special properties of ionic liquids, a chemical locomotor driven by ionic liquids is promising for realizing autonomous micromachines and nanomachines that are safe and environmentally friendly.
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INTRODUCTION Micromachines and nanomachines that propel autonomously in liquid require a highly energy efficient driving mechanism because the energy source and the driving mechanism are housed in a small body. In addition, with decreasing machine size, the frictional (viscous) force between the machine and the surrounding medium becomes more significant than the inertial force because of the decrease in the Reynolds number.1 Chemical propulsion generates motion by directly converting locally stored chemical energy into mechanical energy. It is thus a promising mechanism for propelling autonomous micromachines and nanomachines especially driven in liquid. The autonomous motion of a micro-object is induced in a free solid or a liquid surface with a surface tension gradient, which gives rise to mass transfer along the interface between two fluids (the Marangoni effect).2−10 Such a surface tension gradient can be generated chemically,3,4,7−10 thermally,2,6 or electrochemically5 on a free solid or a liquid surface. In the chemical Marangoni effect, the anisotropic distribution of the surface-active molecular layer on surfaces around objects induces a surface tension difference. Alcohols,7 camphor,8−10 camphanic acid,11,12 and soaps13 are well-known surface-active materials that induce autonomous motion on a water surface. The molecules of these materials are amphiphilic and thus alter the surface tension of a gas/water interface on adsorption at the interface. Ionic liquids are organic electrolytes that are liquids at room temperature.14−18 From the viewpoint of interfacial chemistry, almost all ionic liquids (but especially those with long hydrocarbon chains) have amphiphilic structures.19 Thus, like surfactants, they modify the surface tension of water when they are adsorbed on an air/water interface.19−23 Consequently, ionic liquids are expected to induce autonomous motion on an aqueous surface. © 2013 American Chemical Society
In this study, we investigate chemical propulsion using ionic liquids as a fuel and discuss how the kind of ionic liquid affects the driving performance. Ionic liquids have special properties such as negligible volatility, nonflammability, and thermal and chemical stability. These properties are desirable as fuels in practical chemical locomotors from the viewpoints of safety and environmental considerations. In addition, the physical and chemical properties of ionic liquids are easily tunable by synthetic means. For these reasons, chemical propulsion processes using ionic liquids are more suitable as the driving mechanism than those using common surfactants.
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EXPERIMENTAL SECTION
To evaluate the effects of the surface tension of water on the characteristics of autonomous motion, we observed the motion on aqueous solutions of neutral surfactant Triton X-100 (Sigma-Aldrich, St. Louis, MO). The surface tension of water was measured with a surface tensiometer (Kyowa Interface Science Co., Ltd., DY-300, Saitama, Japan). We evaluated the time variation of the driving force of autonomous motion using various ionic liquids as fuels. Figure 1 shows plane and side photographs of a locomotor (rotator) used to measure the driving force. It was formed by a 0.3-mm-thick polystyrene film with a cavity (capacity 16 μL) that functioned as a container for the ionic liquid and a nozzle to enable the ionic liquid to spurt onto the aqueous surface. Because the density of polystyrene (1.03−1.06 g/cm3) is almost equal to that of water, the rotator can float on an aqueous surface and rotate about a pivot hole. A rod at the end of the rotator contacts a force sensor to measure the driving force. Received: November 7, 2012 Revised: February 6, 2013 Published: February 11, 2013 2799
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Figure 1. (A) Plane and (B) side photographs of the locomotor (rotator). It is a 0.3-mm-thick polystyrene film with a cavity (capacity 16 μL) that acts as a container for an ionic liquid and a nozzle for spurting the ionic liquid on an aqueous surface. It can rotate about the pivot hole. The rotator was floated on water (100 mL) in a water tray (180 × 130 mm2) and rotated about an axis (needle). The rod (Figure 1) of the rotator pushes a cantilever beam (polystyrene; spring constant: 0.032 N/m) whose displacement was measured by a laser displacement sensor (Keyence Co., Ltd., LK-G85, Osaka, Japan) and converted into the driving force at the rod. We used three ionic liquids as the fuels for the locomotor: 1-buthyl3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-buthyl3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), and 1-ethyl3-methylimidazolium tetrafluoroborate ([EMIM][BF4]). [BMIM][PF6] and [BMIM][BF4] have the same cation, and [EMIM][BF4] and [BMIM][BF4] have the same anion. Because [PF6]− is more hydrophobic than [BF4]−, [BMIM][PF6] is immiscible with water. In contrast, [BMIM][BF4] and [EMIM][BF4] are water-soluble. Because [EMIM][BF4] has a shorter alkyl chain (ethyl chain) in the cation than that (butyl chain) in [BMIM][BF4], [EMIM][BF4] is more hydrophilic than [BMIM][BF4]. All ionic liquids were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan).
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Figure 2. Time-variation curves for the driving forces at the rod of rotators loaded with 10 μL of the three ionic liquids. (A) [BMIM][PF6], (B) [BMIM][BF4], and (C) [EMIM][BF4]. Measurements were performed three or four times for each ionic liquid.
RESULTS AND DISCUSSION When a rectangular chip of polypropylene (lateral dimensions, 7 × 3.5 mm2; thickness, ∼0.2 mm) with a droplet (∼20 μL) of [BMIM][PF6] deposited on one end was placed on a water surface in a Petri dish, it moved in a straight line with a velocity of ∼90 mm/s before colliding with the dish wall. The motion was directed away from the edge closest to where [BMIM][PF6] was deposited. During this motion, a very thin layer and small ripples were visible on the aqueous surface. When neutral surfactant Triton X-100 was added to water (concentration 1.1 μM), the chip moved with a velocity of ∼60%. However, no motion was observed at concentrations above 2.3 μM. In the former and latter cases, the surface tensions of the aqueous solution of Triton X-100 were 66 and 60 mN/m, respectively. Therefore, the threshold surface tension value for the self-motion lies between these two values. Figure 2 shows time variation curves for the driving forces at the rod of the rotators loaded with the three ionic liquids (volume 10 μL). For each ionic liquid, the measurement was performed three or four times. The rotator with [BMIM][PF6] behaved very differently from those with [BMIM][BF4] and [EMIM][BF4]. The initial peak with a width of ∼0.2 s in each curve of Figure 2 was caused by the force sensor (the cantilever beam) colliding with the rotator. The maximum driving force at the rod (Fr) was thus defined as the maximum force after that initial peak. Table 1 lists the driving forces Fr and the force-
Table 1. Maximum Driving Forces at the Rod (Fr) and Ionic Liquid Container (Fc), Driving Forces F Calculated by Equation 1, and Force-Generation Duration (Td) of Rotators with the Three Ionic Liquidsa ionic liquid
Fr (mN)
Fc (mN)
F (mN)
Td (s)
[BMIM][PF6] [BMIM][BF4] [EMIM][BF4]
0.070−0.075 0.149−0.169 0.080−0.089
0.090−0.096 0.191−0.216 0.102−0.114
0.144 0.140 0.092
258−404 42−56 9−23
a
Fr and Td were derived from the time-variation curves of the driving force at the rod in Figure 2. Fc was derived to satisfy eq 2.
generation durations Td of rotators with the three ionic liquids. The value range corresponds to the results of the measurements shown in Figure 2. The force-generation duration differs remarkably for the three ionic liquids. The driving time was longest in the rotator with hydrophobic ionic liquid [BMIM][PF6], followed by that with [BMIM][BF4]. The rotator with [BMIM][BF4] generated a larger driving force than those with [BMIM][PF6] and [EMIM][BF4]. 2800
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When the rotator was placed on an aqueous surface, it floated such that its top surface was almost level with the aqueous surface. The time evolution of the driving force was sensitive to the initial shape of the ionic liquid in the container and the initial contact state of the ionic liquid with water. Prior to the driving force measurements, the ionic liquids coated on all the surfaces of the container. When [EMIM][BF4] and [BMIM][BF4] were used, the ionic liquid was immediately covered with water as soon as the rotator was placed on the aqueous surface and the ionic liquid at the entrance of the container made contact with water because of their high water solubility. In contrast, when the rotator with water-immiscible [BMIM][PF6] was placed on the aqueous surface only the ionic liquid at the entrance of the container initially made contact with water and the water advanced to the inside of the container with time. In all cases, the water eventually entered the container so that the level of the aqueous surface became the same as that of the upper surface of the rotator. To observe the phenomena that occur on the aqueous surface near the ionic liquid container during force generation, mica powder (average particle size 24 μm) was sprinkled on the aqueous surface. After the ionic liquid in the container made contact with the aqueous surface, the powder particles were swept away from the container, indicating spreading of a thin film of the ionic liquid on the aqueous surface. Figure 3 shows
The area without powder particles is formed by convection around the container; particles on both sides of the container flowed away from the container because of outflow convection. An analysis of the moving images estimated the maximum flow rate of the regular convection current just outside this area at approximately ∼200 mm/s. In the initial stages of spreading of the ionic liquid (Figure 3A,C), the area without powder particles for the rotator with [BMIM][BF4] was larger, more circular, and shrank more rapidly than that of the rotator with [BMIM][PF6]. This indicates that, in the initial stages, convection during the spreading of [BMIM][BF4] is more intense and isotropic than that for [BMIM][PF6] and that it decays faster. When [BMIM][BF4] began spreading, the area without powder particles expanded at a speed of 170−180 mm/s. When the spreading slowed, regular convection currents flowing from the container perpendicular to the rotator were formed. In the case of the rotator with [EMIM][BF4], the situation was almost the same except that the spreading time was shorter. We now discuss the mechanism of the driving force. The observations of the spreading of the thin ionic liquid film and the accompanying lateral convection of water (Figure 3) strongly imply that the main force-generation mechanism is the chemical Marangoni effect. Furthermore, the present experimental finding of the transition from continuous to no motion on the addition of a surfactant (second paragraph of this section) has also been reported for the self-motion of a camphanic acid disk on an aqueous phase,11 which occurs as a result of Marangoni flow induced by the surface tension gradient and depends on the camphanic acid concentration of the aqueous surface. As mentioned above, from the viewpoint of interfacial chemistry, almost all ionic liquids (especially those with long hydrocarbon chains) have amphiphilic structures.19 Thus, they (including [BMIM][PF6], [BMIM][BF4], and [EMIM][BF4]) are expected to modify the surface tension of water when they are adsorbed on an air/water interface.19−23 The surface-active properties of ionic liquids can be controlled by varying the chemical structures of the cations and anions. Table 2 lists the surface tensions of aqueous solutions of the three ionic liquids at relatively low concentrations.22,23 [BMIM][BF4] has the highest surface activity of the three ionic liquids, and those of [BMIM][PF6] and [EMIM][BF4] are almost the same as and lower than that of [BMIM][BF4], respectively.
Figure 3. Photographs indicating the dispersion state of mica powder on a water surface in the (A, C) initial and (B, D) final stages whereas ionic liquids (A, B) [BMIM][PF6] and (C, D) [BMIM][BF4]) in the container spread on the water surface. In each photograph, powder particle movement in areas with mica powder indicates lateral water convection.
Table 2. Surface Tensions of Aqueous Solutions of the Three Ionic Liquids at Relatively Low Concentrations and Surface Tensions of the Pure Ionic Liquids surface tension of aqueous solution ionic liquid
photographs that indicate the dispersion state of the mica powder on the aqueous surface in the initial and final stages of the spreading of the ionic liquid ([BMIM][PF6] in Figure 3A,B and [BMIM][BF4] in Figure 3C,D) deposited in the container on the aqueous surface. In all of the photographs, there is an area on the aqueous surface near the container that has no mica powder. Movement of the powder particles was observed outside this area, indicating the lateral convection of water. In the case of [BMIM][PF6], regular convection currents flowing from the container perpendicular to the rotator were observed.
[BMIM] [PF6] [BMIM] [BF4] [EMIM] [BF4]
concentration (mol/L)
surface tension (mN/m)
surface tension of pure ionic liquid (mN/m)
0.95
60.922,a
44.124
0.925 1.610 0.661 1.301
46.923 46.023 61.923 58.223
44.824 54.425
a
The surface tension was obtained from surface tension versus concentration curves of an aqueous solution of [BMIM][PF6] at pH 2 (Figure 4 of ref 22). 2801
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density of this butyl group reduces the surface tension of the solution.21 The anisotropic distribution of the cation on the aqueous surface around the container causes a surface tension gradient, which gives rise to Marangoni-driven spreading of the cation toward the rear of the container. Water-immiscible [BMIM][PF6] (water solubility 2.0 wt %26) gradually dissolves in water from the container entrance to the inner regions. As mentioned above, [BMIM][PF6] dissolves in the aqueous phase forming an anion−cation pair rather than being adsorbed on the aqueous surface by the effect of the voluminous and hydrophobic anion.22 As a result, [BMIM]+ will spread by Marangoni convection in the vicinity of the aqueous surface accompanying [PF6]−. The dissolution rates of the ionic liquids in water determine the force-generation duration. Because [EMIM][BF4] is more hydrophilic and dissolves in water more readily than [BMIM][BF4], the rotator with [EMIM][BF4] is driven for a shorter period than that with [BMIM][BF4]. In this model, the driving force F is generated by the surface tension difference of the water on either side of the rotator (i.e., the difference between the surface tension of pure water (γw) in front of the rotator and that of water on which cations are adsorbed (γs) in the container). Thus, the force can be approximately estimated by
In the case of hydrophobic [BMIM][PF6], the adsorption of [BMIM]+ on an aqueous surface and its self-aggregation in an aqueous solution are strongly affected by the presence of the voluminous and hydrophobic anion ([PF6]−).22 Because of ion pair formation in the bulk solution, [BMIM][PF6] has a lower surface activity22 than [BMIM][BF4]. In other words, [BMIM][PF6] reduces the surface tension less than the other two ionic liquids because it dissolves in the aqueous phase, forming an anion−cation pair rather than adsorbing on the aqueous surface. As for the mixture of water and [BMIM][BF4], with increasing ionic liquid concentration the surface tension of the mixture decreases abruptly from that of pure water (72.8 mN/m at 20 °C) to ∼40 mN/m at 0.76 mol/L because the surface is initially mainly covered by cations ([BMIM]+) that have an amphiphilic structure.21 In the case of [EMIM][BF4], because [EMIM]+ has a shorter hydrocarbon chain (ethyl group) than that (butyl group) in [BMIM]+, [EMIM][BF4] is more hydrophilic and so is less surface-active than [BMIM][BF4]. In the mixture of water and [EMIM][BF4], the surface tension of the mixture decreases more gradually with increasing ionic liquid concentration than in the case of [BMIM][BF4] (i.e., from the value of pure water to ∼54 mN/m at 3.7 mol/L).23 Figure 4 schematically depicts the generation model of the driving force in the rotator loaded with ionic liquid. Because
F = (γw − γs)w
(1)
where w is the width (5 mm) of the container entrance. When γs is equal to the surface tension of the pure ionic liquid (Table 2), the driving forces F of the rotators with [BMIM][PF6], [BMIM][BF4], and [EMIM][BF4] are calculated to be 0.144, 0.140, and 0.092 mN, respectively, as shown in Table 1. When the main force-generation source is the container of the ionic liquid (i.e., the water pressure by counter water flows that circulate in the water tray and return to the rotator is negligible), the maximum generation force Fc at the position of the ionic liquid container is estimated (Table 1). Here, Fc was derived so that the moments due to Fr (the force acting on the rod from the force sensor) and Fc about the pivot hole were equal to satisfy the equation Figure 4. Driving force generation model of a rotator loaded with an ionic liquid. The driving force is generated by the surface tension difference between the pure water (γw) in front of the rotator and the water in the container that contains cations (γs). (A) The ionic liquid is [BMIM][PF6]. Because [BMIM][PF6] is water-immiscible, when the rotator is placed on an aqueous surface, only the ionic liquid at the entrance of the container is in contact with pure water during the initial stages. Water moves into the container over time. Dissociated cations [BMIM]+ are generated where [BMIM][PF6] contacts pure water; they are adsorbed on the aqueous surface and then spread. (B) The ionic liquid is [BMIM][BF4] or [EMIM][BF4]. Because these ionic liquids are water-soluble, when the rotator is placed on an aqueous surface, the ionic liquid in the container is immediately covered with water. [BMIM]+ or [EMIM]+ dissociated cations are adsorbed on the aqueous surface and then spread.
Frdr = Fcdc
(2)
where dr and dc are the distances of the rod and the container from the pivot hole, respectively. In the rotators with [BMIM][BF4] and [EMIM][BF4], F is calculated to be 65−73% and 81−90% of the maximum driving force Fc, respectively. In contrast, in the rotator with [BMIM][PF6], F is calculated to be 150−160% of the maximum driving force Fc. For [BMIM][BF4] and [EMIM][BF4], the surface tensions of their aqueous solutions reach those of pure ionic liquids with increasing molar fraction of the ionic liquid in the miscibility range.23 As for [BMIM][PF6], although it has almost the same surface tension as [BMIM][BF4] as a pure ionic liquid, the rotator with [BMIM][PF6] has a lower driving force than [BMIM][BF4] because of its lower surface activity. In addition, the driving force was found to decrease with increasing water temperature. The average maximum driving forces Fr of the rotator with [BMIM][PF6] at 20 and 60 °C were 96 and 82% of that at 3 °C, respectively. This result can be largely explained by the temperature dependence of the surface tension difference between water and [BMIM][PF6] (γw − γs). The surface tensions of water at 3, 20, and 60 °C are 75.2, 72.8,
[BMIM][BF4] and [EMIM][BF4] are hydrophilic, when the ionic liquids at the entrance of the container come into contact with water they are immediately covered with water and dissolve in the aqueous phase. Cations [BMIM]+ and [EMIM]+ with amphiphilic structures rise to the aqueous surface and are adsorbed. In the case of [BMIM][BF4], the butyl chain of [BMIM]+ is the least polar part of the cation- and anioncontaining molecule, and an increase in the surface number 2802
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and 66.2 mN/m,27 respectively, whereas those of [BMIM][PF6] are 43.4, 43.0, and 42.0 mN/m,28 respectively. Next, we observed the free rotary motion of the rotators with the three ionic liquids and measured their rotational speeds. The average speeds of the rotators with [BMIM][PF6] and [BMIM][BF4] (volume 10 μL) were 43.1 and 59.1 rpm in the 1st rotational cycle and 49.6 and 30.2 rpm in the 10th rotational cycle, respectively. The rotators with [BMIM][BF4] had a higher initial rotational speed than that with [BMIM][PF6] and reduced their speed more rapidly. These results well reflect the time variations for the driving force shown in Figure 2. In the case of [EMIM][BF4], the average speed decreased rapidly from 44.1 rpm in the first rotational cycle to 26.7 rpm in the third rotational cycle. From a practical viewpoint, the effects of an ionic liquid’s water content on the rotator motion are important because it is known that the water content affects the surface tension of ionic liquids. In the case of [BMIM][PF6], because the presence of a small amount of water reduces the electrostatic attraction between ions, the surface tension of [BMIM][PF6] decreases with increasing water content and has a minimum at a mole fraction of ∼0.10.24 Therefore, in this water content range, the driving force of the rotator using [BMIM][PF6] would increase with increasing water content. At higher water content, the driving force would decrease because of the increase in the surface tension of [BMIM][PF6].24 In contrast, in the cases of hydrophilic [BMIM][BF4] and [EMIM][BF4], the effects of the water on their surface tension are smaller up to a mole fraction of 0.8 or 0.9.21,23 Finally, we estimate the propulsion power of the rotator, which is calculated by the product of the moment about the rotational axis and the rotational speed. In the rotator with [BMIM][PF6] (volume 10 μL), the speeds of the initial regular rotation (from the first to fifth rotational cycle) were found to be 46.8 and 49.1 rpm in two separate measurements. If we assume that the driving force during the rotation is equal to that when the rotation is constrained, the maximum propulsion power of the rotator is calculated to be 110 μW by using the values of the maximum driving force (Table 1).
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Corresponding Author
*Tel: +81-73-457-8146. Fax: +81-73-457-8201. E-mail:
[email protected]. Author Contributions
This manuscript was written through the contributions of all of the authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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REFERENCES
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CONCLUSIONS Autonomous motion when an imidazolium-based ionic liquid was used as a fuel was observed on a water surface. From measurements of the driving force of a rotator loaded with ionic liquids and observations of the lateral convection on a water surface starting from the ionic liquid container of the rotator, we conclude that the driving force of the autonomous motion is caused by the Marangoni effect due to the anisotropic distribution of the cations of the imidazolium-based ionic liquids adsorbed on the water surface. The maximum driving force of the motion and the force-generation duration are respectively determined by the surface activity of the ionic liquid and the solubility of the ionic liquid in water. Because of the special properties of ionic liquids, Marangonidriven chemical propulsion using ionic liquids has the potential to realize a chemical driving mechanism that is safe and environmentally friendly.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
S Supporting Information *
Interfacial structure between the ionic liquid in the container and water during propulsion. Free rotary motion of the rotator and spreading of the ionic liquids on an aqueous surface. This 2803
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(19) Aratono, M.; Shimamoto, K.; Onohara, A.; Murakami, D.; Tanida, H.; Watanabe, I.; Ozeki, T.; Matsubara, H.; Takiue, T. Adsorption of 1-Decyl-3-methylimidazolium Bromide and Solvation Structure of Bromide at the Air/Water Interface. Anal. Sci. 2008, 24, 1279−1283. (20) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T. Surface Adsorption and Micelle Formation of Surface Active Ionic Liquids in Aqueous Solution. Langmuir 2007, 23, 4178−4182. (21) Sung, J.; Jeon, Y.; Kim, D.; Iwahashi, T.; Seki, K.; Iimori, T.; Ouchi, Y. Gibbs Monolayer of Ionic Liquid+H2O Mixtures Studied by Surface Tension Measurement and Sum-Frequency Generation Spectroscopy. Colloids Surf., A 2006, 284−285, 84−88. (22) Modaressi, A.; Sifaoui, H.; Mielcarz, M.; Domańska, U.; Rogalski, M. Influence of the Molecular Structure on the Aggregation of Imidazolium Ionic Liquids in Aqueous Solutions. Colloids Surf., A 2007, 302, 181−185. (23) Rilo, E.; Pico, J.; García-Garabal, S.; Varela, L. M.; Cabeza, O. Density and Surface Tension in Binary Mixtures of CnMIM-BF4 Ionic Liquids with Water and Ethanol. Fluid Phase Equilib. 2009, 285, 83− 89. (24) Freire, M. G.; Carvalho, P. J.; Fernandes, A. M.; Marrucho, I. M.; Queimada, A. J.; J. A. P. Coutinho, J. A. P. Surface Tensions of Imidazolium Based Ionic Liquids: Anion, Cation, Temperature and Water Effect. J. Colloid Interface Sci. 2007, 314, 621−630. (25) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Structure and Properties of New Ionic Liquids Based on Alkyl- and Alkenyltrifluoroborates. ChemPhysChem 2005, 6, 1324−1332. (26) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solution Thermodynamics of Imidazolium-Based Ionic Liquids and Water. J. Phys. Chem. B 2001, 105, 10942−10949. (27) Vargaftik, N. B.; Volkov, B. N.; Voljak, L. D. International Tables of the Surface Tension of Water. J. Phys. Chem. Data 1983, 12, 817−820. (28) Halka, V.; Tsekov, R.; Freyland, W. Peculiarity of the Liquid/ Vapour Interface of an Ionic Liquid: Study of Surface Tension and Viscoelasticity of Liquid BMImPF6 at Various Temperatures. Phys. Chem. Chem. Phys. 2005, 7, 2038−2043.
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