Phase Behavior of Vegetable Oil-Based Ionic Liquid Microemulsions

Feb 18, 2014 - Moreover, a larger single phase region area of vegetable oil-based .... the naked eye, and the corresponding composition was remarked a...
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Phase Behavior of Vegetable Oil-Based Ionic Liquid Microemulsions Aili Wang, Li Chen, Dongyu Jiang, and Zongcheng Yan* School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

ABSTRACT: This study presents the formation and phase behavior of vegetable oil-based ionic liquid microemulsions (ILMs). Castor oil, jatropha oil, and soybean oil were used as oil phases of the ILMs respectively. The effects of the mass ratio of surfactant to cosurfactant (Km) and temperature on the phase behavior of ILMs were investigated by pseudo ternary phase diagrams. The results indicated that the homogeneous and stable microemulsions composed of target vegetable oils, ionic liquid, Triton X-100, and n-butanol could form at ambient condition successfully. The size of a single-phase region area was in a sequence: castor oil-based ILMs > jatropha oil-based ILMs > soybean oil-based ILMs. Each vegetable oil-based ILMs corresponded to an optimum Km since the ionic liquid-oil amphiphilic balance existed in ILM systems. A maximum single phase region area of each system was observed when Km = 1:0, 2:1, and 2:1, respectively. Moreover, a larger single phase region area of vegetable oil-based ILMs could be obtained by increasing the temperature.

1. INTRODUCTION Vegetable oils continue to attract public interest with their unique properties such as renewability and biodegradability and are widely used as raw materials in the food, medical, cosmetic, biolubricant, and biofuel industries.1−4 Nowadays, due to the energy crisis and environmental issues, it is meaningful to pursue alternate approaches to utilize vegetable oils.5,6 Microemulsion systems are thermodynamically stable, homogeneous, optically isotropic solutions, consisting of oil, water, and surfactants (cosurfactants).7,8 Ascribing to its typical and distinctive properties, microemulsions have attracted a great deal of attention in different fields of application such as pharmaceutical, cosmetical, and agrochemical formulations over the last few decades.9−11 Triglycerides, as the primary components of vegetable oils, have been used as the oil phase in microemulsions. Many researchers have provided substantial information on the formulation, phase diagrams, and microstructure of triglyceride-based microemulsions. Jeirani et al.12 presented the phase behavior study of triglyceride microemulsions. The results showed that those microemulsions could be used effectively in enhanced oil recovery. Bragato et al.13 reported canola oil-based microemulsions using carboxylate-based extended surfactants, and the designed reverse micelle microemulsions have been evaluated as an alternative method of reducing vegetable oil viscosity. Klossek et al.14 discussed the influence of chemical structures of renewable feedstock oils on the domains of existence and the nanostructures of microemulsions. © 2014 American Chemical Society

Currently, the ionic liquid microemulsion (ILM) system has attracted expanding interest, where ionic liquids (ILs) were used as oil substitutes, water substitutes, cosurfactants (additives), and surfactants.15 ILs, with outstanding properties such as low volatility, nonflammable, negligible vapor pressure, and excellent chemical and thermal stability, have been applied in various areas.16 These IL-based microemulsions were found to have potential use in the synthesis of metal nanomaterials,17 biodiesel production,18 and enzymatic reactions.19 Besides, the nearby “water-free” ionic liquid-based microemulsions, with an amount of water < 5 %, seemed to be of special interest for noncorrosive lubricants.7 Herein, ILMs containing vegetable oils may have some unknown properties and some potential new applications owing to the unique features of the ILs and microemulsions. The necessary basis for the discussion on the properties of vegetable oil-based ILM systems is constituted by the complex characteristics of the phase behavior. To the best of our knowledge, studies about the phase behavior of vegetable oilbased ILMs are rather scarce. The aim of the research, presented here, was to investigate the phase formation regularity of vegetable oils/surfactants (cosurfactants)/ILs microemulsions in detail. The pseudoternary phase diagrams were plotted to analysis the phase behavior of each ILM at Received: June 24, 2013 Accepted: January 21, 2014 Published: February 18, 2014 666

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Table 1. Main Fatty Acid Compositions of Target Oils (wt %)a ricinoleic

linoleic

oleic

stearic

palmitic

linolenic

C18:1

C18:2

C18:1

C18:0

C16:0

C18:3

1.0 ± 0.1

1.0 ± 0.1

0.3 ± 0.1

6.9 ± 0.1

14.2 ± 0.1

N/A

4.2 ± 0.1

6.5 ± 0.1

2.0 ± 0.1

Castor Oil 89.5 ± 0.1

4.2 ± 0.1

3.0 ± 0.1 Jatropha Oil

N/A

34.3 ± 0.1

43.1 ± 0.1 Soybean Oil

N/A a

50 ± 0.1

32 ± 0.1

Provided by material suppliers.

Figure 1. Pseudoternary phase diagrams of vegetable oil-based ILMs at 298 (± 0.5) K with Km of 2:1. (Vegetable oil is castor oil (a), jatropha oil (b), and soybean oil (c)).

obtained from Tianjin Kermel Chemical Reagent Science and Technology Co. Ltd., China. Before used, TX-100 was vacuumdried at 70 °C for 6 h to remove excess water. Jatropha oil (with mass fraction purity > 0.99) was provided by ShuYang East Lake Oils Co. Ltd., China. Soybean oil (with mass fraction purity > 0.99) was provided by Sigma-Aldrich. The main fatty acid compositions of three oils are shown in Table 1. [BMIM][BF4] (with mass fraction purity > 0.99) was provided by Centre of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, China.

different temperatures and different mass ratios of surfactant to cosurfactant (Km). The vegetable oils employed in the research were castor oil, jatropha oil, and soybean oil. 1-Butyl-3methylimidazolium tetrafluoroborate ([BMIM][BF4]) was the ionic liquid phase. The nonionic surfactant TritonX-100 (TX100) was used as a surfactant and n-butanol as a cosurfactant.

2. EXPERIMENTAL SECTION 2.1. Materials. The castor oil (with mass fraction purity > 0.99), TX-100 (with mass fraction purity > 0.99), and n-butanol (with mass fraction purity > 0.99) used in this work were 667

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2.2. Methods. The most useful parameter to characterize microemulsions is the size of the microemulsion region area in the phase diagram. The phase diagrams of microemulsions consisted of [BMIM][BF4], TX-100, n-butanol, and vegetable oils were determined by direct observation. In a typical experiment, different proportions of the surfactant−cosurfactant mixture to vegetable oil were prepared in 10 % steps between 0 and 100 wt % of the vegetable oil. Then titrating the mixture by [BMIM][BF4] under moderate agitation. The corresponding values of the single phase region area were calculated by AUTOCAD software. The temperature was controlled by a thermostat water bath. The phase transitions from clear transparent solution to turbid solution were observed through the naked eye, and the corresponding composition was remarked as the phase boundary. Each set of experiments was repeated three times, and the average of the values obtained was used for data processing and analysis.

3. RESULTS AND DISCUSSION 3.1. Formation of Vegetable Oil-Based ILMs. The formation possibility of microemulsions, consist of ionic liquid and vegetable oil, was the premise and key of the research. Herein, four-component systems including vegetable oil as the oil phase, TX-100 as the surfactant, n-butanol as a cosurfactant, and [BMIM][BF4] as the ionic liquid phase were formed at 298 (± 0.5) K. The mass ratio of surfactant to cosurfactant was kept constant at 2:1. The pseudo-ternary phase diagrams of different kind of vegetable oil-based ILMs are illustrated in Figure1a−c, representing castor oil-based, jatropha oil-based, and soybean oil-based ILM, respectively. In each phase diagram, the upper part of the phase boundary represented a single-phase part region (microemulsion region area, 1ϕ), whereas the lower part was a two phase region (2ϕ). The compositions in the phase diagrams were represented in weight fractions. In addition, the values of the single phase region area are presented in Figure 2.

Figure 3. Effect of Km on ternary phase diagrams of castor oil/TX-100 + n-butanol/[BMIM][BF4] microemulsion systems at 298 (± 0.5) K.

systems composed of castor oil, [BMIM][BF4], TX-100, or TX100/n-butanol. It deduced a comparison of the systems with Km of 1:0, 2:1, 1:1, 1:2, and 1:4 at 298 (± 0.5) K. These results showed that the phase diagrams of vegetable oil-based ILMs were quite different depending on Km. The single-phase region areas decreased with an increasing amount of n-butanol. The calculated single-phase region areas are plotted in Figure 4.

Figure 4. Single-phase region areas of castor oil/TX-100−n-butanol/ [BMIM][BF4] with different Km at 298 (± 0.5) K.

Vegetable oils are mixture esters of fatty acid with glycerol, and the unique structures of triglyceride molecules contribute to their complicated phase behavior.20 Therefore, many attempts devoted to forming vegetable oil microemulsions at ambient conditions without co-oil or cosurfactant have been unsuccessful so far.21−23 In traditional water−oil microemulsions, cosurfactants could change the surface activity and HLB of surfactants, participate in a micelle, and adjust the polarity of water and oil.24 The influence of cosurfactant on petroleum hydrocarbon-based ILMs was already studied in more detail, and a magnification of single-phase region with the aid of n-butanol, as a cosurfactant, was shown.25 However, in castor oil-based ILM systems, the isotropic microemulsion could form successfully in absence of n-butanol, and the single-

Figure 2. Single-phase region areas of different vegetable oil-based ILMs at 298 (± 0.5) K with Km at 2:1).

The results showed that the target vegetable oils could be used as components of ionic liquid microemulsions successfully, and the size of the microemulsion region area followed the sequence: castor oil-based ILMs > jatropha oil-based ILMs > soybean oil-based ILMs. 3.2. Effect of Km on Phase Behavior of Vegetable OilBased ILMs. The mass ratio between surfactant and cosurfactant (Km) could significantly impact the phase behavior of ILMs. Figure 3 shows the pseudo ternary phase diagrams of 668

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in Figure 6 for better understanding. The major triglycerides in jatropha oil are oleic and linoleic (see Table 1), and their molecular structures lead to poor solubilization by surfactant at ambient conditions. Therefore, isotropic microemulsions containing jatropha oil and ionic liquid [BMIM][BF4] could not form due to high interfacial tension of TX-100 within systems. As n-butanol was added, the flexibility of microemulsions and spontaneous mean curvature H0 changed.26 When Km was 1:2, the amphiphilic balance value of surfactant and cosurfactant was most close to the value of jatropha oil; thus the jatropha oil-TX-100 micelles achieved the maximum solubilization capacity for [BMIM][BF4]. After the n-butanol distributed in the oil-ionic liquid interfacial film became saturated, the extra n-butanol penetrated to the inside of jatropha oil/ionic liquid, leading to a broken balance of interfacial film and a decreased microemulsion region area. The effects of n-butanol on the soybean oil-based ILMs at 298 (± 0.5) K were observed in the present study. Figure 7

phase region area of castor oil-based ionic liquid microemulsions become broadest without n-butanol. The reason for this could be attributed to the molecular structure of main fatty acid esters among castor oil. From Table 1, ricinoleic is the major triglyceride in castor oil. Since the hydroxyl of ricinoleic molecular, castor oil is soluble with TX-100, producing low interfacial tension at conditions without cosurfactant. As the cosurfactant concentration increased, a noticeable decrease in the sing-phase region of the samples could be observed. The reason for this could be attributed to the ionic liquid-castor oil amphiphilic balance existed in ILM systems, which is similar to HLB. The balance value of castor oil is close to the value of TX-100. The function of cosurfactant was to alter the surface activity and the ionic liquid-vegetable oil amphiphilic balance value of surfactant, thus affecting the phase behavior of the system and decreasing the solubilization of [BMIM][BF4]. The pseudo-ternary phase diagrams of systems composed of jatropha oil, [BMIM][BF4], TX-100, and n-butanol are shown in Figure 5. The calculated single-phase region areas are shown

Figure 7. Effect of Km on ternary phase diagrams of soybean oil/TX100/[BMIM][BF4] microemulsion systems at 298 (± 0.5) K. Figure 5. Effect of Km on ternary phase diagrams of jatropha oil/TX100/[BMIM][BF4] microemulsion systems at 298 (± 0.5) K.

illustrates the ternary phase diagrams of systems composed of soybean oil, [BMIM][BF4], TX-100, and n-butanol. Figure 8 shows the calculated single-phase region areas. Similar to

Figure 6. Single-phase region areas of jatropha oil/TX-100−nbutanol/[BMIM][BF4] with different Km at 298 (± 0.5) K.

Figure 8. Single-phase region areas of soybean oil/TX-100−nbutanol/[BMIM][BF4] with different Km at 298 (± 0.5) K. 669

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butanol, and the single-phase region area of castor oil-based ionic liquid microemulsions become broadest without nbutanol. The maximum single phase region area of jatropha and soybean oil-based systems were observed when Km = 2:1 similarly. Moreover, due to the dehydration of the oxyethylene group of nonionic surfactant TX-100 at higher temperatures, single-phase region areas of each vegetable oil-based system grew up with increasing temperature.

jatropha oil, the major triglycerides in soybean oil are oleic and linoleic (see Table 1); the largest microemulsion region area of soybean oil-based ILMs were obtained with a Km of 1:2. Based on the results, it can be concluded that each vegetable oil-based ILM has its own amphiphilic balance, which is quite different due to the molecular structure of their major triglyceride. In addition to this, before reaching the optimum Km, it is easier to obtain an isotropic single phase with increasing cosurfactant. 3.3. Effect of Temperature on Phase Behavior of Vegetable Oil-Based ILMs. Temperature is important to the formation of microemulsion systems.27 The single-phase region areas of castor oil/TX-100/[BMIM][BF4], jatropha oil/TX100−n-butanol (1:2)/[BMIM][BF4], and soybean oil/TX100−n-butanol (1:2)/[BMIM][BF4] at a range of 298 (± 0.5) K to 353 (± 0.5) K are represented in Figure 9. The results

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel./fax: +86-2087111109. Funding

The authors gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology, and the financial support of Project of Production, Education and Research, Guangdong Province and Ministry of Education (nos. 2012B09100063, 2012A090300015). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 9. Effect of temperature on single-phase region areas of vegetable oil-based ILMs.

showed that the microemulsion region areas of vegetable oilbased system grew up with the increase of temperature. This is most likely ascribed to the hydrophilic enhancement of nonionic surfactant TX-100 at higher temperatures.28 Since the surfactant micelles in vegetable oil generally increase, it is possible to incorporate more polar material in the palisade layer.

4. CONCLUSIONS In the present contribution, castor oil, jatropha oil, and soybean oil were used as the oil phase to form isotropic ILMs respectively. The phase diagrams of microemulsions consisted of [BMIM][BF4], TX-100, n-butanol, and vegetable oils at 298 (± 0.5) K, keeping Km constant at 2:1, were determined by direct observation. The results showed that the homogeneous and stable microemulsions composed of target vegetable oils, ionic liquid, TX-100, and n-butanol could form at ambient condition successfully, and the size of the microemulsion region area was in the sequence: castor oil-based ILM > jatropha oilbased ILM > soybean oil-based ILM. The effect of Km on phase behavior of vegetable oil-based ILMs was investigated, illustrating that there was an ionic liquid−vegetable oil amphiphilic balance existing in ILM systems, and the molecular structures of major triglycerides in different vegetable oils were attributed to the optimum amphiphilic balance value. Especially, in castor oil-based ILM systems, the isotropic microemulsion could form successfully in the absence of n670

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