Article pubs.acs.org/jced
Comparison of the Phase Behavior and Thermodynamic Properties between Ionic Liquid−Oil and Water−Oil Microemulsion Systems Jinling Chai,* Lei Xu, Wei Liu, and Meili Zhu College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan, Shandong 250014, China S Supporting Information *
ABSTRACT: The phase behavior and thermodynamic properties of IL/O microemulsions (ionic liquid dispersed in oil) containing 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) + pentan-1-ol + octane + 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and W/O microemulsions (water dispersed in oil) containing [C12mim]Br + pentan-1-ol + octane + water were studied and compared. The area of the single-phase domain in [bmim][BF4] microemulsion systems is larger than that in water-containing systems. Both the mole fractions of the nalkyl alcohol in the oil phase Xoa and the mole fractions of the n-alkyl alcohol at the interfacial layer Xia in IL/O microemulsion systems are always larger than Xoa and Xia in W/O o systems, respectively. IL/O systems have smaller absolute values of the free enthalpy values −ΔGo→i than those in W/O systems. In the IL/O microemulsion systems, a larger number of cosurfactants at the interfacial layer facilitates the formation of a smaller droplet IL/O microemulsion. The effects of n-alkyl alcohols, alkanes, salinity, and temperature on the interfacial composition and the structural parameters of the IL/O and W/O microemulsions were also investigated and discussed.
1. INTRODUCTION Microemulsions are transparent, isotropic, and thermodynamically stable dispersions of otherwise immiscible water and oil.1−3 The microheterogeneity of such dispersions make them useful in biological and technological applications.4,5 Surfactant and cosurfactant (short chain lipophilic n-alkyl alcohol) molecules constitute the interfacial layer to control the bending elasticity of the interfacial layer offering stability to the dispersions6−8 and affect the droplet dimension of the microemulsion droplets. Ionic liquids (ILs), which are liquid organic salts at or close to room temperatures, have wide applications in chemical reactions,9,10 electrochemistry,11,12 separations,13 and material synthesis.14 In microemulsion systems, ILs can be used as a substitute for water or organic solvent to prepare IL microemulsions. A hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) can replace water and form a nonaqueous IL microemulsion system with adequate surfactant. Freeze-fracture electron microscopy (FFEM) indicates that the shape of the microemulsion droplets is the same as “classic” droplets of water-in-oil microemulsions.15 Gao et al. investigated the effect of polyvinylpyrrolidone on the microstructure of the [bmim][BF4] + TritonX-100 + cyclohexane microemulsion system.16 Rabe and Koetz studied the phase behavior and properties of microemulsion systems consisting of methylbenzene + pentan-1-ol + hexadecyl-trimethyl-ammonium bromide (CTAB) + ionic liquid 1-ethyl-3-methylimida© 2012 American Chemical Society
zolium-ethylsulfate (EMIM-EtOSO3) and 1-ethyl-3-methylimidazolium-hexylsulfate (EMIM-HexOSO3).17 The phase behavior and characteristics of ionic liquid based microemulsions containing 1-octyl-3-methylimidazolium chloride ([Omim]Cl) or 1-butyl-3-methylimidazolium hexaflourophosphate [bmim][PF6] + water were reported.18 These IL based microemulsions were found to have potential use in the synthesis of metal nanomaterials, biological extraction, and enzymatic reactions.19 IL-based microemulsion systems are of current interest.20−22 IL-in-oil (IL/O) nonaqueous microemulsions have attracted much attention from theoretical viewpoint and have wide application prospects in the fields of solar energy conversion, semiconductors, microcolloids, and cosmetics, and so forth.23,24 There seems to be distinct advantage of nonaqueous microemulsion systems over the aqueous systems.25 It is of great importance to compare the phase behavior, structure parameters, and thermodynamic properties of these IL microemulsions with the corresponding aqueous systems. A dilution method of W/O microemulsions was extensively used to determine the structural parameters and thermodynamic properties of W/O microemulsions.26,27 With the dilution method, we have studied the effects of different aqueous components on the properties of ionic liquids [Cnmim]Br based W/O microemulsion systems.28 In this Received: January 12, 2012 Accepted: July 18, 2012 Published: August 2, 2012 2394
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paper, [bmim][BF4] was used as a substitute for water to form a nonaqueous IL microemulsion system. Their structural parameters and thermodynamic properties were investigated using the above dilution method, and a comparison with the microemulsion system containing water was also made.
2. EXPERIMENTAL SECTION Materials. 1-Dodecyl-3-methylimidazolium bromide ([C12mim]Br) and 1-butyl-3-methylimidazolium bromide ([bmim]Br) were synthesized and purified according to the literature.29−31 1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) was obtained by ion exchange of [bmim]Br and potassium tetrafluoroborate in distilled water. The purities of the two products were checked using 1H NMR spectroscopy. The products were dried in vacuum for 2 days prior to use. Hexane, octane, decane, dodecane, butan-1-ol, pentan-1-ol, and hexan-1-ol (with mass fraction purity > 0.99) were all of analytical reagent (AR) grade and were purchased from Alfa Aesar Johnson Matthey, USA. All of these chemicals were used without further purification. NaCl (with mass fraction purity > 0.99) was AR grade. Doubly distilled water of conductivity less than 3 μS·cm−1 was used in the experiments. Methods. Construction of Pseudoternary Phase Diagram. Calculated amounts of [bmim][BF4] (or water), surfactant and different amounts of oil were taken in sealed test tubes and kept in a thermostatic water bath at 303 K (uncertainty ± 0.1 K). The cosurfactant n-alkyl alcohol was then added to all the mixtures from a microsyringe until the system became clear. The clear points would indicate the formation of single-phase systems. The uncertainty of all samples weighed above was ± 0.0001 g. The same procedure was repeated for 2 to 3 times for each mixture, and an average of these results was taken for the construction of phase diagrams.32 Dilution Experiments. An appropriate amount of surfactant [C12mim]Br (0.676 mmol) and different amounts of [bmim][BF4] (or water) and oil were mixed in sealed test tubes, shaken vigorously in a vortex mixer, and then kept in a thermostatted water bath at the desired temperatures (uncertainty ± 0.1 K) to attain equilibrium. N-Alkyl alcohol was added gradually in small intervals into the tube from a microsyringe until a clear system appeared. A calculated small amount of oil was then added into the tube, and the system was back to cloudy again. This mixture was again titrated with n-alkyl alcohol until a clear system appeared again. This procedure was repeated for several times in the same test tube, recording the volumes of the n-alkyl alcohol added after each time. The entire experiment was then repeated for a second time to ensure accuracy, and the average values obtained were used for data processing and analysis.
Figure 1. Pseudoternary phase diagram of the systems [C12mim]Br + pentan-1-ol + octane + hydrophilic components at fixed value of R (0.342) at 303 K. R = m(hydrophilic components)/m([C12mim]Br) = 0.342. Hydrophilic components: ▽, pure water; ○, [bmim][BF4] aqueous solution (the mass ratio of [bmim][BF4] = 0.05); △, NaCl aqueous solution (the mass ratio of NaCl = 0.05); □, [bmim][BF4] (100 ppm of water contained).
aqueous solution (the mass ratio of NaCl = 0.05) < pure [bmim][BF4] (100 ppm of water contained). [Bmim]BF4 is an “organic solvent”. There are interactions present between the most hydrophobic part of the imidazolium ion, namely, the n-butyl chain with the carbon chain of octane molecules. Furthermore, [bmim][BF4] is less polar than water (the ETN scale of polarity33 of [bmim][BF4] is estimated to be 0.673, while H2O is 1.000). It is conducive for the formation of the single-phase IL/O microemulsion. Therefore, the IL/O microemulsion system has a larger domain with a single phase. The larger single-phase domain of the IL/O microemulsion system may be beneficial for its use as a medium to prepare porous or hollow nanomaterials by hydrolysis reactions.34 Dilution Experiments of the IL/O and W/O Microemulsion Systems. The theoretical consideration of the dilution method and the related equations to calculate the thermodynamic and structural parameters of the respective IL/ O (or W/O) microemulsion systems are represented in Supporting Information. The dilution experiments were conducted for the systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (or water). Figure 2 shows the graph of na/ns versus no/ns for the IL/O and W/O microemulsion systems, where na is the total number of moles of n-alkyl alcohol, ns the number of moles of surfactant, and no the number of moles of oil.
3. RESULTS AND DISCUSSION Pseudoternary Phase Diagram. Figure 1 depicts the quasi-ternary phase diagrams for the systems [C12mim]Br + pentan-1-ol + octane + hydrophilic components ([bmim][BF4]) + H2O + NaCl, the mass ratio of hydrophilic components to surfactant R = m(hydrophilic components)/ m([C12mim]Br) = 0.342) at 303 K in a two-dimensional Gibbs triangle. The single phase region (1Φ) contains IL/O (or W/ O) type microemulsions. In Figure 1, different hydrophilic phases are used. The magnitudes of areas of the single-phase domain are in ascending order as follows: pure water < [bmim][BF4] aqueous solution (the mass ratio of [bmim][BF4] = 0.05) < NaCl
Figure 2. Plots of na/ns vs no/ns for systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (a) or H2O (b) at 303 K. R [= mIL (or mw)/ms; the mass ratio of hydrophilic components to surfactant]: ■, 0.137; ●, 0.342; ▲, 0.683; ▼, 1.024; ⧫, 1.366. 2395
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Table 1. Interfacial Composition and Structural Parameters for IL/O (or W/O) Microemulsion Systems [C12mim]Br (0.676 mmol) + Pentan-1-ol + Octane + [bmim][BF4] or H2O with Different R [= mIL(or mw)/ms] Values at 303 K (Uncertainty Limits: noa /no ± 0.001, nia/ns ± 0.02, and nia ± 0.001) R
noa /no
nia/ns
0.137 0.342 0.683 1.024 1.366
0.031 0.061 0.123 0.196 0.288
1.60 1.93 2.40 3.00 3.82
0.137 0.342 0.683 1.024 1.366
0.043 0.054 0.092 0.107 0.116
1.41 2.01 2.24 2.31 2.48
104 ns/mol
104 nia/mol
Re/nm
[C12mim]Br (0.676 mmol) + Pentan-1-ol + Octane + [bmim][BF4] 6.76 10.8 0.58 6.75 13.0 0.91 6.76 16.2 1.40 6.76 20.3 1.70 6.75 25.8 1.98 [C12mim]Br (0.676 mmol) + Pentan-1-ol + Octane + H2O 6.76 9.50 0.57 6.76 13.6 0.93 6.76 15.1 1.81 6.76 15.6 2.32 6.76 16.8 2.91
According to ref 28 and the theoretical consideration in the S1 section of the Supporting Information, the values of ns (the number of moles of the surfactant), nia (the number of moles of the n-alkyl alcohol in the interfacial layer), Re (the effective radius of the water pool; Re = (Vd/VH2O(VIL))1/3Rw), Nd (the total number of droplets in the system; Nd = 3Vd/4πRe3), N̅ s (the average aggregation number of the surfactant; N̅ s = nsNA/ Nd), and N̅ a (the average aggregation number of n-alkyl alcohols; N̅ a = niaNA/Nd) were all calculated from Figure 2 and are listed in Table 1. For the two microemulsion systems containing [bmim][BF4] or water as the hydrophilic phase, the mole fraction of the nalkyl alcohol in the oil phase Xoa (= noa /(no + noa )), the mole fraction of the n-alkyl alcohol in the interfacial layer Xia (= nia/(ns o + nia)), and the free enthalpy −ΔGo→i (= −RT ln K = −RT ln (Xia/Xoa )) were calculated from Table 1. The radius of the water pool Rw were obtained according to ref 28 and the theoretical consideration presented in the Supporting Information. All parameters calculated were compared in Figure 3. o for the It can be seen from Figure 3 that Xoa , Xia, and −ΔGo→i [bmim][BF4] and water microemulsion systems have the same pattern of change as R increases. The hydrophilic property of the system is strengthened as R increases. Therefore, more n-alkyl alcohol is needed to adjust the hydrophile−lipophile balance of the microemulsion, resulting in the increase in nia values. As R increases, the volumes of droplets expand, resulting in a decrease in the interface curvature. Hence, the molecular geometry of the surfactants was changed. This weakens the trend of pentan-1-ol transferring from the oil to the interfacial layer and also increases the mole fractions of the n-alkyl alcohol in oil phase Xoa .35 It was evident from Figure 3 that the magnitudes of both Xoa and Xia in [bmim][BF4] microemulsion systems are always larger than Xao and Xai in H2O microemulsion systems, respectively, while [bmim][BF4] systems have smaller absolute o than that in H2O values of the free enthalpy values −ΔGo→i microemulsion systems. This indicates that more cosurfactants were required to form an IL/O microemulsion than that of the traditional W/O microemulsion. o indicate weaker The lower absolute values of −ΔGo→i interactions at the interfacial layer of the IL/O microemulsion, which agrees well with the degree of spontaneity of the transfer process of n-alkyl alcohol from the continuous oil phase to the interfacial layer.
10−18 Nd
N̅ s
N̅ a
18.9 6.72 2.69 1.98 1.64
2 6 15 21 25
3 12 36 62 94
18.0 6.62 1.37 0.78 0.49
2 6 30 52 83
3 12 66 120 207
Figure 3. Comparison of the parameters of Xoa (a), Xia (b), −ΔGoo→i (c), and Rw (d) for the two microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (●) and H2O (○) with different R values.
For the W/O microemulsion system [C12mim]Br + pentan1-ol + octane + water, water molecules will form hydrogen bonds with both imidazolium cations of [C12mim]Br and pentan-1-ol molecules. The hydrogen bonds strengthen the interfacial layer and thus increase the stability of the microemulsion. For the IL/O microemulsion system 2396
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Effects of the carbon chain length of n-alkyl alcohols (A) and o alkanes (B) on the parameters of Xoa , Xia, −ΔGo→i , and Rw for the two microemulsion systems containing [bmim][BF4] or water were plotted and compared in Figure 5. Figure 5 indicates that the effects of the chain lengths of both n-alkyl alcohols and alkanes on four parameters of Xoa , Xia, o −ΔGo→i , and Rw are similar in IL/O and W/O microemulsion systems. As the carbon chain length of the n-alkyl alcohol increases, the values of Xoa and Xia decrease and −ΔGoo→i values increase. Following the increase in the size of the droplet of the IL/O and W/O microemulsion, there was a notable decrease in the total number of the droplet Nd. The n-alkyl alcohol is mainly distributed between the interfacial layer and oil phase of microemulsion systems. It acts as both the cosurfactant (at interfacial layer) and cosolvent (in oil phase) in microemulsions. The main effect of n-alkyl alcohol molecules is to change the hydrophilicity of the amphiphilic mixture. The n-alkyl alcohols with longer chains are more able to change the hydrophilicity, making the amphiphilic mixture more hydrophobic or less hydrophilic. This facilitates the formation of a microemulsion, and therefore, less n-alkyl alcohol is needed to balance the interfacial layer, resulting in the o values. In addition, smaller values of Xia, Xoa , and larger −ΔGo→i the radii of the microemulsion drops would increase due to the increase in the carbon chain length of the n-alkyl alcohol molecules. It also can be seen from Figure 5 that the pattern of change of all of the parameters resulting from different carbon chain lengths of alkanes is contrary to the pattern of change resulting from different carbon chain lengths of n-alkyl alcohols. This phenomenon can be explained in terms of the penetrating ability of alkane molecules into the interfacial layer. It is favorable for the alkane molecules with short carbon chain lengths to penetrate the interfacial layer,37 resulting in an interfacial layer that tends to be convex to the oil. This facilitates the change of the curvature of the interfacial layer, and therefore, less n-alkyl alcohol is needed to balance the hydrophile−lipophile property of the interfacial layer.38 Effect of Temperature on the IL/O and W/O Microemulsions. Figure 6 shows the effect of temperature on the dilution parameters of the IL/O and W/O microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (a) or H2O (b). The interfacial composition and structural parameters were obtained from Figure 6 and listed in the S3 section of Supporting Information. The effect of temperature on the parameters of Xoa , Xia, o −ΔGo→i , and Rw for above IL/O and W/O microemulsion systems were plotted and compared in Figure 7. It can be seen from Figure 7 that the mole fractions of n-alkyl alcohol at the interfacial layer (Xia) for both the IL/O and W/O microemulsion systems increase as temperature increases. It can be explained that the interactions at the interfacial layer of the microemulsion systems decrease as temperature increases. As such, more n-alkyl alcohol is needed to balance the interfacial layer. In addition, the mole fractions of the n-alkyl alcohol in the oil phase (Xoa ) would decrease for the IL/O microemulsion systems but would increase for the W/O microemulsion systems as temperature increases. As temperature increases, the radii of the microemulsion droplets decrease. A larger number of cosurfactant molecules at the interfacial layer would facilitate the formation of smaller
[C12mim]Br + pentan-1-ol + octane + [bmim][BF4], the interactions between [bmim][BF4] and the interfacial layer are mainly electrostatic in nature, and there also exist interactions between the carbon chains. As these interactions are weak, more n-alkyl alcohol molecules are required together with surfactant to form IL/O microemulsions. To maintain distribution equilibrium of n-alkyl alcohol between the oil phase and the interfacial layer, more n-alkyl alcohol molecules are solubilized in the oil. From Figure 3d, it can be seen that smaller IL droplets (smaller Rw and Re) were obtained in the IL/O microemulsion systems. Therefore, the total number of the droplets (Nd) would increase, while the average aggregation number of the surfactant (N̅ s) and the average aggregation number of n-alkyl alcohols (N̅ a) decrease as the droplet size decreases. That is, the larger number of cosurfactants at the interfacial layer in the IL/ O microemulsion systems was conducive for forming a smaller droplet IL/O microemulsion.36 Effects of n-Alkyl Alcohols and Alkanes on the Interfacial Composition and the Structural Parameters. Figure 4 shows the effects of n-alkyl alcohols (A) and alkanes (B) on the graphs of na/ns versus no/ns for IL/O and W/O microemulsion systems [C12mim]Br (0.676 mmol) + n-alkyl alcohol + alkane + [bmim][BF4] (a) or H2O (b). The related parameters were obtained from Figure 4 and are listed in S2 section of the Supporting Information.
Figure 4. Effects of n-alkyl alcohols (A) and alkanes (B) on the na/ns vs no/ns relationships for IL/O and W/O microemulsion systems with R = 0.342 at 303 K. (A) [C12mim]Br (0.676 mmol) + n-alkyl alcohol + octane + hydrophilic phase ([bmim][BF4] (a), H2O (b)). ■, butan-1ol; ●, pentan-1-ol; ▲, hexan-1-ol; (B) [C12mim]Br (0.676 mmol) + pentan-1-ol + alkane + hydrophilic phase ([bmim][BF4] (a), H2O (b)). ■, hexane; ●, octane; ▲, decane; ▼, dodecane. 2397
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Figure 5. Effects of the chain lengths (n) of n-alkyl alcohols (A) and alkanes (B) on the parameters of Xoa (a), Xia (b), −ΔGoo→i (c), and Rw (d) for the microemulsion systems, A: [C12mim]Br (0.676 mmol) + n-alkyl alcohol + octane + [bmim][BF4] (●) or H2O (○); B: [C12mim]Br (0.676 mmol) + pentan-1-ol + alkanes + [bmim][BF4] (●) or H2O (○).
All of the interfacial composition and structural parameters of o Xoa , Xia, K, −ΔGo→i , Re, Rw, Nd, N̅ s, N̅ a were calculated and represented in the S4 section of the Supporting Information. The effect of NaCl concentration on the parameters of Xoa , i o , and Rw for the W/O microemulsion systems were Xa, −ΔGo→i plotted in Figure 9. Figure 9 indicates that the values of Xoa and Xia would decrease, but the value of −ΔGoo→i would increase with increasing NaCl concentrations. The radii of the droplets Rw would increase by a small extent. This can be explained by the salting-out effect. The salt dissolves into the water pool in the core of the microemulsion droplets and compresses the hydrophilic group of [C12mim]Br molecules. This results in the [C12mim]Br molecules becoming less hydrophilic. Thus less pentan-1-ol was required to form stable W/O microemulsions. Hence, Xia values as well as Xoa values would decrease as salt contents increase for the microemulsion systems.39 The effect of NaCl concentrations on the interfacial composition and structural parameters for IL/O microemulsion systems was also studied, and no significant effect was found. It was reported40,41 that there are strong contacts from the [BF4]− anion to all of the imidazolium ring protons without selectivity. An individual cation is surrounded by more than one anion, which eliminates the hydrogen bonding as the primary source of the interactions between the anion and cation in [bmim][BF4] molecules. The salt cannot dissolve in [bmim][BF4]. Thus, it does not show any effect on the related parameters for IL/O microemulsion systems.
Figure 6. Effect of temperature on the graph of na/ns vs no/ns for IL/O (a) and W/O (b) microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (a), H2O (b). R = 0.342; temperature: ■, 303 K; ●, 313 K; ▲, 323 K; ▼, 333 K.
droplets of microemulsions. This agrees well with the abovementioned results. Effect of NaCl Concentrations on the IL/O and W/O Microemulsions. W/O type of microemulsion systems were widely used as templates to the synthesis of nanoparticles by mixing two reactants, such as inorganic salts in microemulsions. NaCl solution was used as an example of salts to investigate the effect of inorganic salts on the properties of W/O and IL/O microemulsions. Figure 8 shows the dilution curves of the W/O microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1ol + octane + NaCl solution with R = 0.342. 2398
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Figure 9. Effect of NaCl concentrations (w) on the parameters of Xoa (a), Xia (b), −ΔGoo→i (c), and Rw (d) for the W/O microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + NaCl solution.
Figure 7. Comparison of the effect of temperature on the parameters of Xoa (a), Xia (b), −ΔGoo→i (c), and Rw (d) for IL/O (●) and W/O (○) microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + [bmim][BF4] (●) or H2O (○).
mass ratio of NaCl = 0.05) < pure [bmim][BF4] (100 ppm of water contained). Both Xoa and Xia in [bmim][BF4] microemulsion systems are always larger than Xoa and Xia in H2O systems, respectively. The formation of IL/O microemulsion was less stable, and the droplets of IL/O microemulsions were smaller than the conventional W/O microemulsions. As the carbon chain length of the n-alkyl alcohol increases, the formation of IL/O microemulsion was increasingly stable. However, the pattern of change of all the parameters resulting from different carbon chain lengths of alkanes is in the contrary to the pattern of change resulting from different carbon chain lengths of n-alkyl alcohols. Salinity has no effect on the interfacial composition and structural parameters for IL/O microemulsion systems. At higher temperatures, the IL/O microemulsion was less stable, while the W/O microemulsion was more stable. The radii of the IL/O and W/O microemulsion droplets would decrease as a result.
Figure 8. Effect of NaCl concentrations (w) on the graph of na/ns vs no/ns for W/O microemulsion systems [C12mim]Br (0.676 mmol) + pentan-1-ol + octane + NaCl solution at 303 K. w (NaCl): ■, 0; ●, 0.025; ▲, 0.050; ▼, 0.075; ⧫, 0.100.
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4. CONCLUSIONS The phase behavior and thermodynamic properties of IL/O microemulsion systems containing [C12mim]Br + [bmim][BF4] + pentan-1-ol + octane was studied and compared with that of W/O systems containing [C12mim]Br + water + pentan1-ol + octane. When different hydrophilic phases are used, the magnitudes of areas of the single-phase domain are in ascending order as follows: pure water < [bmim][BF4] aqueous solution (the mass ratio of [bmim][BF4] = 0.05) < NaCl aqueous solution (the
ASSOCIATED CONTENT
S Supporting Information *
Theoretical considerations, effects of n-alkyl alcohols and alkanes, effect of temperature, and effect of salinity. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 2399
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This work was supported by Natural Science Foundation of Shandong Province of China (ZR2009BM036). Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/je3000537 | J. Chem. Eng. Data 2012, 57, 2394−2400
