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Phase behavior of epoxidized soybean oil-based ionic liquid microemulsions: Effects of ionic liquids, surfactants, and co-surfactants
Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 500–505
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Phase behavior of epoxidized soybean oil-based ionic liquid microemulsions: Effects of ionic liquids, surfactants, and co-surfactants Aili Wang ∗ , Li Chen, Fan Xu, Zongcheng Yan School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The phase behavior of ESBO-based ionic liquid microemulsions was investigated. • Ionic liquids, surfactants and cosurfactants influenced the phase behavior significantly. • ESBO-based ionic liquid microemulsions exhibit potential in biolubricant application.
a r t i c l e
i n f o
Article history: Received 6 January 2015 Received in revised form 4 May 2015 Accepted 10 May 2015 Available online 10 June 2015 Keywords: Ionic liquid microemulsions Epoxidized soybean oil Phase behavior Phase-forming capacity
a b s t r a c t Epoxidized soybean oil (ESBO)-based ionic liquid microemulsions are promising alternatives for petroleum-based lubricants. This study presents the phase behavior of these microemulsions through phase manifestation, and the areas of the single-phase domain were accordingly calculated to illustrate the phase-forming capacities of the designed microemulsions. Effects of ionic liquid anions and cations, surfactant and co-surfactant chain lengths on the phase behavior, and phase-forming capacities of ESBObased ionic liquid microemulsions were investigated. Results showed that the phase-forming capacities of the ESBO-based ionic liquid microemulsions with different ionic liquid anions and cations showed the following sequence: Tf2 N− -based > PF6 − -based > BF4 − -based, OMIM+ -based > HMIM+ -based > BMIM+ based > EMIM+ -based. Given the presence of ionic liquid–ESBO amphiphilic balance in the designed systems, the ESBO–surfactant micelles achieved maximum solubilization capacity for 1-butyl-3-methylimidazolium tetrafluoroborate when the surfactant had approximately eight ethoxylated groups. In addition, ESBO-based microemulsion containing n-hexanol showed a higher phase-forming capacity than that containing n-butanol, and n-octanol caused a different phase behavior for the ESBO-based microemulsion because of its oilier nature than a co-surfactant. © 2015 Published by Elsevier B.V.
∗ Corresponding author. Fax: +86 20 87111109. E-mail address: [email protected] (A. Wang). http://dx.doi.org/10.1016/j.colsurfa.2015.05.042 0927-7757/© 2015 Published by Elsevier B.V.
A. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 500–505
1. Introduction
2. Experimental
Non-aqueous ionic liquid microemulsions, which comprise a system of oil, ionic liquid, surfactant, and co-surfactant (added optionally) [1], provide numerous benefits in various applications, such as a reaction medium for nanomaterials, solubilizing agent for drug molecules, and direct utilization as potential bioproducts [2–5]. In the past decades, ionic liquid microemulsions have been increasingly investigated. The interfacial composition, thermodynamic properties, and structural parameters for certain ionic liquid microemulsions have been explored through dilution method [6]. Prior to the investigation of ionic liquid microemulsions, the phase-forming capacity of each component, which is expressed by the areas of the single-phase domain (SME ) in pseudo ternary phase diagrams, should be determined [7]. Moreover, the microstructures and properties of the ionic liquid microemulsions are substantially influenced by the nature and concentration ratio of the components [8]. Accordingly, determining the effects of polar phase structure, surfactant and co-surfactant on the phase behavior of ionic liquid microemulsions is important. Vegetable oil has a notable role in a sustainable industrial development because of their renewable and biodegradable nature [9]. We presented in our previous study a successful formulation of microemulsions with vegetable oil as the continuous phase and with room-temperature ionic liquid as the polar phase. We showed that the ionic liquid microemulsification of vegetable oil effectively reduced the high viscosity of the oil and improved its friction-reduction property [5]. Traditional vegetable oils with lubricating feature, such as castor, jatropha, and soybean, have been confirmed to be capable of producing isotropic microemulsions with ionic liquids, and the phase behavior of vegetable-oil based ionic liquid microemulsions have been previously reported [10]. Epoxidized vegetable oils, which are biodegradable and have the potential as biolubricants, have recently attracted much attention [11–14]. Exploring efficient and environment-friendly alternative resources is essential given the decreasing fossil fuel resources and worsening environmental conditions. Ionic liquid microemulsions containing epoxidized soybean oil (ESBO) may be beneficial as lubricants because of their unique features. We have previously shown that ESBO-based ionic liquid microemulsions could be successfully produced. In addition, the influences of Km and temperature on the phase behavior of ESBO-based ionic liquid microemulisons had been initially investigated [15]. In this study, we further investigated the effects of the ionic liquid structure, surfactant type and co-surfactant chain length on the phase behavior of ESBO-based ionic liquid microemulsions. 1-Ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF4 ]), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4 ]), 1-hexyl-3-methyl-imidazolium tetrafluoroborate ([HMIM][BF4 ]), and 1-octyl-3-methyl-imidazolium tetrafluoroborate ([OMIM][BF4 ]) were used as ionic liquid phases to investigate the influence of ionic liquid cations on the phase behavior of ESBO-based ionic liquid microemulsions. 1-Butylhexafluorophosphate ([BMIM][PF6 ]) 3-methyl-imidazolium and 1-butyl-3-methyl -imidazolium bis[(trifluoro-methyl) sulfonyl]imide ([BMIM][Tf2 N]), together with [BMIM][BF4 ], were used to determine the influence of ionic liquid anions. Triton X series nonionic surfactants with five (Triton X-45), eight (Triton X-114), and ten (TX-100) oxyethylene (OE) groups were investigated. Cosurfactants with different chain lengths were n-butanol, n-hexanol, and n-octanol. The phase behavior of the aforementioned ESBObased microemulsions was explored through pseudo-ternary phase diagrams.
2.1. Materials
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ESBO (>99 wt%) was purchased from Sigma–Aldrich (Shanghai, China). The ESBO physicochemical properties had been presented in our previous study [15]. All the ionic liquids, including [EMIM][BF4 ] (>99 wt%), [BMIM][BF4 ] (>99 wt%), [HMIM][BF4 ] (>99 wt%), [OMIM][BF4 ] (>99 wt%), [BMIM][PF6 ] (>99 wt%) and [BMIM][Tf2 N] (>99 wt%) were provided by the Center of Green Chemistry and Catalysis at the Lanzhou Institute of Chemical Physics (Lanzhou, China). n-Butanol (>99 wt%), nhexanol(>99 wt%), n-octanol (>99 wt%) and TX-100 (>99 wt%) were purchased from Kermel (Tianjin, China). TX-45 (>99 wt%) and TX114 (>99 wt%) were bought from J&K Scientific (Guangzhou, China) and Aladdin (Shanghai, China), respectively. 2.2. Methods Pseudo-ternary phase diagrams of ESBO, ionic liquid, surfactant, and co-surfactant were constructed through titration and direct observation. Mixtures of the ESBO and surfactants/co-surfactants with varying mass ratios from 1:9 to 9:1 were prepared in a series of stoppered test tubes. The samples were placed in a thermostatic water bath at 298 ± 0.5 K for 10 min, and then titrated with the ionic liquid under moderate agitation. The ionic liquid volumes that caused the solutions to become turbid from clear transparent were noted to determine the phase boundaries. In each plotted phase diagram, the upper part of the phase boundary represented a single-phase region (microemulsion, 1ϕ), and the lower part was a two-phase region (2ϕ). Compositions in the phase diagrams were in weight fractions. Corresponding values of the SME were calculated using AutoCAD software. Each set of experiment was repeated thrice, and the average value obtained was used for data processing and analysis. 3. Results and discussion 3.1. Effect of ionic liquids Ionic liquids with BF4 − , PF6 − , or Tf2 N− as anions have been previously reported as excellent lubricants [16]. Thus, these ionic liquids were used to investigate ESBO-based ionic liquid microemulsions. [BMIM][PF6 ] and [BMIM][Tf2 N] were used as the polar phases of ESBO-based microemulsions, and TX-100 and n-butanol, with Km at 4:1 were used as the surfactant and co-surfactant, respectively. Pseudo-ternary diagrams of ESBObased microemulsions with different ionic liquid phases at 298 ± 0.5 K are illustrated in Fig. 1a and b. The phase diagram of [BMIM][BF4 ]-based microemulsion under the same condition has already been presented in our published work [15]. The SME values of the above systems are listed in Fig. 2 to illustrate their phase-forming capacities. The phase-forming capacity evidently followed the succeeding sequence: ESBO-based microemulsion containing [BMIM][Tf2 N] > ESBO-based microemulsion containing [BMIM][PF6 ] > ESBO-based microemulsion containing [BMIM][BF4 ]. In the investigation of the phase formations of ionic liquid microemulsions, the hydrogen bond between the surfactant OEs and imidazolium ring of the ionic liquid was identified as the driving force [17], and the hydrogen bond strength was positively correlated with the electronegativity of the OE units and imidazolium ring [18]. With similar surfactant, the ESBO-based microemulsion containing [BMIM][BF4 ] showed the lowest phaseforming capacity because [BMIM][BF4 ] has less fluorine atoms
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Fig. 1. Pseudo-ternary phase diagrams of ESBO-based ionic liquid microemulsions using different ionic liquids at 298 ± 0.5 K with Km at 4:1. [Ionic liquids are (a) [BMIM][PF6 ] and (b) [BMIM][Tf2 N]].
Fig. 2. Effect of ionic liquid anions on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
compared with [BMIM][Tf2 N] and [BMIM][PF6 ]. Meanwhile, the ESBO-based microemulsion containing [BMIM][Tf2 N] showed a higher phase-forming capacity compared with that containing [BMIM][PF6 ] because nitrogen atom is more electronegative than phosphorus [19]. The ionic liquid chain lengths of the cations remarkably influence the phase behavior of the ESBO-based microemulsions. The pseudo-ternary phase diagrams of the ESBO-based microemul-
Fig. 3. Pseudo-ternary phase diagrams of ESBO-based ionic liquid microemulsions using different ionic liquids at 298 ± 0.5 K with Km at 4:1. [Ionic liquids are (a) [EMIM][BF4 ], (b) [HMIM][BF4 ], and (c) [OMIM][BF4 ]].
sions containing [EMIM][BF4 ], [HMIM][BF4 ], and [OMIM][BF4 ] at 298 ± 0.5 K are illustrated in Fig. 3a–c, respectively. TX-100 and nbutanol, with a constant Km of 4:1, were used as surfactant and co-surfactant, respectively. The calculated SME values of the different aforementioned systems are shown in Fig. 4. Results showed that the phase-forming capacities of ESBO-based microemulsions
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Fig. 4. Effect of ionic liquid cations on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1
increased gradually with increasing chain length of the ionic liquids. This phenomenon could be attributed to the surfactant effect of certain ionic liquids. According to Qiu et al., [1] ionic liquids comprising a charged hydrophilic head group and a hydrophobic tail are considered good surfactant candidates in forming micelles and microemulsions. A hydrophobic tail with long chain length has high higher surfactant ability and is more solubilized inside ESBO. 3.2. Effect of surfactants OE groups can enhance the solubilization capacity of microemulsions, and hence, most basic studies on non-aqueous ionic liquid microemulsions have utilized nonionic surfactants, such as TX-100 and Brij-35. Thus, the effect of the number of OE groups (NOE ) undoubtedly has an important role in the phase behavior of ionic liquid microemulsions. Pseudo-ternary phase diagrams of ESBO-[BMIM][BF4 ]-n-butanol-TX-45 (NOE ≈ 5) and ESBO-[BMIM][BF4 ]-n-butanol-TX-114 (≈8) microemulsions at 298 ± 0.5K, with constant Km of 4:1, are shown in Fig. 5a and b, respectively. Single-phase areas of the above microemulsions compared with the systems with TX-100 (NOE ≈ 10) are plotted in Fig. 6. Results illustrated that the ESBO-surfactant micelles achieved the maximum solubilization capacity for [BMIM][BF4 ] when NOE ≈ 8, whereas the minimum solubilization capacity was obtained when NOE ≈ 10. These solubilization effects could be attributed to the [BMIM][BF4 ]-ESBO amphiphilic balance of the surfactant existing in ionic liquid microemulsion systems [10]. The packing symmetry between the surfactant and water-oil phases has been reported to control the phase-forming capacity of traditional water-oil microemulsion [20]. ESBO-based ionic liquid microemulsions similarly have the largest monophasic region because the amphiphilic balance of TX-114 is the most suitable value for ESBO-[BMIM][BF4 ] microemulsions investigated.
Fig. 5. Pseudo-ternary phase diagrams ESBO-based ionic liquid microemulsions using different surfactants at 298 ± 0.5 K with Km at 4:1. [Surfactants are (a) TX-45 and (b) TX-114].
3.3. Effect of co-surfactants In microemulsions, the co-surfactant can reduce the surface tension and fluidize the interfacial surfactant film as well as provide the proper hydrophile-lipophile balance in the interfacial layer [21,22]. Short chain n-alkanols, such n-butanol, n-hexanol, and n-octanol, are the most widely used co-surfactants, and the solubilization of water and phase behavior of microemulsions substantially depend on the chain length of n-alkanols [23,24]. For the ESBO-based ionic liquid microemulsions, the chain length of the co-surfactants had considerable effects on the phase behavior of the microemulsion systems investigated. Pseudoternary phase diagrams of ESBO/(TX-100+n-hexanol)/[BMIM][BF4 ] and ESBO/(TX-100+n-octanol)/[BMIM][BF4 ] systems at 298 ± 0.5 K with Km at 4:1 are shown in Fig. 7a and b, respectively. An
Fig. 6. Effect of surfactant NOE on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
intuitive comparison of the SME values of ESBO-based microemulsions with different co-surfactants is presented in Fig. 8. Results showed that the phase-forming capacity of the aforementioned microemulsions have the following order: ESBO-based microemulsion containing n-butanol < ESBO-based microemulsion containing n-hexanol > ESBO-based microemulsion containing n-octanol. This
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microemulsion containing n-octanol showed a different phase behavior and presented an unexpected phase-forming capacity. 4. Conclusions Phase behavior studies on ESBO-based ionic liquid microemulsions were performed by plotting pseudo-ternary phase diagrams. Corresponding SME values were calculated to illustrate the effects of ionic liquids, surfactants, and co-surfactants on the phaseforming capacities of the designed systems. Results showed that the phase-forming capacities of the ESBO-based ionic liquid microemulsions with varying anions have the following sequence: ESBO-based microemulsion containing [BMIM][Tf2 N] > ESBObased microemulsion containing [BMIM][PF6 ] > ESBO-based microemulsion containing [BMIM][BF4 ]. This order could be attributed to the electronegativity of the imidazolium ring donated by associated atoms. Meanwhile, through the surfactant effect of certain ionic liquids, the phase-forming capacity of the designed microemulsions with different cations followed the sequence: ESBO-based microemulsion containing [OMIM][BF4 ] > ESBObased microemulsion containing [HMIM][BF4 ] > ESBO-based microemulsion containing [BMIM][BF4 ] > ESBO-based microemulsion containing [EMIM][BF4 ]. The ESBO-surfactant micelles achieved maximum solubilization capacity for [BMIM][BF4 ] when NOE ≈ 8 because of the appropriate ionic liquid-ESBO amphiphilic balance existing in the designed systems. In addition, ESBO-based microemulsion containing n-hexanol had a higher phase-forming capacity than that containing n-butanol because the spontaneity of the microemulsification increased with increasing co-surfactant chain length when the chain length was shorter than seven. However, when the chain length of n-alkanol was eight, the n-alkanol behaved more similar to oil than a co-surfactant. This study provided useful information for ESBO-based ionic liquid microemulsions for their application as biolubricant basestocks.
Fig. 7. Pseudo-ternary phase diagram of ESBO-based ionic liquid microemulsions using different co-surfactants at 298 ± 0.5 K with Km at 4:1.
Acknowledgements This research was supported by the National Natural Science Foundation of China (21376088), Project of Production, Education and Research, Guangdong Province and Ministry of Education (2012B09100063, 2012A090300015), and the Fundamental Research Funds for the Central Universities (2013ZZ0071). The authors would also gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology. References
Fig. 8. Effect of co-surfactant on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
regular change was consistent with our previous study, in which castor oil was used as the non-polar phase [25]. The ESBO-based microemulsion containing n-hexanol had a higher phase-forming capacity than that containing n-butanol because the spontaneity of the microemulsification increased with increasing co-surfactant chain length when the chain length was shorter than seven. When the chain length of n-alkanol was eight, the n-alkanol behaved more similar to oil than a co-surfactant [24]. Overall, ESBO-based
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Phase behavior of epoxidized soybean oil-based ionic liquid microemulsions: Effects of ionic liquids, surfactants, and co-surfactants Aili Wang ∗ , Li Chen, Fan Xu, Zongcheng Yan School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The phase behavior of ESBO-based ionic liquid microemulsions was investigated. • Ionic liquids, surfactants and cosurfactants influenced the phase behavior significantly. • ESBO-based ionic liquid microemulsions exhibit potential in biolubricant application.
a r t i c l e
i n f o
Article history: Received 6 January 2015 Received in revised form 4 May 2015 Accepted 10 May 2015 Available online 10 June 2015 Keywords: Ionic liquid microemulsions Epoxidized soybean oil Phase behavior Phase-forming capacity
a b s t r a c t Epoxidized soybean oil (ESBO)-based ionic liquid microemulsions are promising alternatives for petroleum-based lubricants. This study presents the phase behavior of these microemulsions through phase manifestation, and the areas of the single-phase domain were accordingly calculated to illustrate the phase-forming capacities of the designed microemulsions. Effects of ionic liquid anions and cations, surfactant and co-surfactant chain lengths on the phase behavior, and phase-forming capacities of ESBObased ionic liquid microemulsions were investigated. Results showed that the phase-forming capacities of the ESBO-based ionic liquid microemulsions with different ionic liquid anions and cations showed the following sequence: Tf2 N− -based > PF6 − -based > BF4 − -based, OMIM+ -based > HMIM+ -based > BMIM+ based > EMIM+ -based. Given the presence of ionic liquid–ESBO amphiphilic balance in the designed systems, the ESBO–surfactant micelles achieved maximum solubilization capacity for 1-butyl-3-methylimidazolium tetrafluoroborate when the surfactant had approximately eight ethoxylated groups. In addition, ESBO-based microemulsion containing n-hexanol showed a higher phase-forming capacity than that containing n-butanol, and n-octanol caused a different phase behavior for the ESBO-based microemulsion because of its oilier nature than a co-surfactant. © 2015 Published by Elsevier B.V.
∗ Corresponding author. Fax: +86 20 87111109. E-mail address: [email protected] (A. Wang). http://dx.doi.org/10.1016/j.colsurfa.2015.05.042 0927-7757/© 2015 Published by Elsevier B.V.
A. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 500–505
1. Introduction
2. Experimental
Non-aqueous ionic liquid microemulsions, which comprise a system of oil, ionic liquid, surfactant, and co-surfactant (added optionally) [1], provide numerous benefits in various applications, such as a reaction medium for nanomaterials, solubilizing agent for drug molecules, and direct utilization as potential bioproducts [2–5]. In the past decades, ionic liquid microemulsions have been increasingly investigated. The interfacial composition, thermodynamic properties, and structural parameters for certain ionic liquid microemulsions have been explored through dilution method [6]. Prior to the investigation of ionic liquid microemulsions, the phase-forming capacity of each component, which is expressed by the areas of the single-phase domain (SME ) in pseudo ternary phase diagrams, should be determined [7]. Moreover, the microstructures and properties of the ionic liquid microemulsions are substantially influenced by the nature and concentration ratio of the components [8]. Accordingly, determining the effects of polar phase structure, surfactant and co-surfactant on the phase behavior of ionic liquid microemulsions is important. Vegetable oil has a notable role in a sustainable industrial development because of their renewable and biodegradable nature [9]. We presented in our previous study a successful formulation of microemulsions with vegetable oil as the continuous phase and with room-temperature ionic liquid as the polar phase. We showed that the ionic liquid microemulsification of vegetable oil effectively reduced the high viscosity of the oil and improved its friction-reduction property [5]. Traditional vegetable oils with lubricating feature, such as castor, jatropha, and soybean, have been confirmed to be capable of producing isotropic microemulsions with ionic liquids, and the phase behavior of vegetable-oil based ionic liquid microemulsions have been previously reported [10]. Epoxidized vegetable oils, which are biodegradable and have the potential as biolubricants, have recently attracted much attention [11–14]. Exploring efficient and environment-friendly alternative resources is essential given the decreasing fossil fuel resources and worsening environmental conditions. Ionic liquid microemulsions containing epoxidized soybean oil (ESBO) may be beneficial as lubricants because of their unique features. We have previously shown that ESBO-based ionic liquid microemulsions could be successfully produced. In addition, the influences of Km and temperature on the phase behavior of ESBO-based ionic liquid microemulisons had been initially investigated [15]. In this study, we further investigated the effects of the ionic liquid structure, surfactant type and co-surfactant chain length on the phase behavior of ESBO-based ionic liquid microemulsions. 1-Ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF4 ]), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4 ]), 1-hexyl-3-methyl-imidazolium tetrafluoroborate ([HMIM][BF4 ]), and 1-octyl-3-methyl-imidazolium tetrafluoroborate ([OMIM][BF4 ]) were used as ionic liquid phases to investigate the influence of ionic liquid cations on the phase behavior of ESBO-based ionic liquid microemulsions. 1-Butylhexafluorophosphate ([BMIM][PF6 ]) 3-methyl-imidazolium and 1-butyl-3-methyl -imidazolium bis[(trifluoro-methyl) sulfonyl]imide ([BMIM][Tf2 N]), together with [BMIM][BF4 ], were used to determine the influence of ionic liquid anions. Triton X series nonionic surfactants with five (Triton X-45), eight (Triton X-114), and ten (TX-100) oxyethylene (OE) groups were investigated. Cosurfactants with different chain lengths were n-butanol, n-hexanol, and n-octanol. The phase behavior of the aforementioned ESBObased microemulsions was explored through pseudo-ternary phase diagrams.
2.1. Materials
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ESBO (>99 wt%) was purchased from Sigma–Aldrich (Shanghai, China). The ESBO physicochemical properties had been presented in our previous study [15]. All the ionic liquids, including [EMIM][BF4 ] (>99 wt%), [BMIM][BF4 ] (>99 wt%), [HMIM][BF4 ] (>99 wt%), [OMIM][BF4 ] (>99 wt%), [BMIM][PF6 ] (>99 wt%) and [BMIM][Tf2 N] (>99 wt%) were provided by the Center of Green Chemistry and Catalysis at the Lanzhou Institute of Chemical Physics (Lanzhou, China). n-Butanol (>99 wt%), nhexanol(>99 wt%), n-octanol (>99 wt%) and TX-100 (>99 wt%) were purchased from Kermel (Tianjin, China). TX-45 (>99 wt%) and TX114 (>99 wt%) were bought from J&K Scientific (Guangzhou, China) and Aladdin (Shanghai, China), respectively. 2.2. Methods Pseudo-ternary phase diagrams of ESBO, ionic liquid, surfactant, and co-surfactant were constructed through titration and direct observation. Mixtures of the ESBO and surfactants/co-surfactants with varying mass ratios from 1:9 to 9:1 were prepared in a series of stoppered test tubes. The samples were placed in a thermostatic water bath at 298 ± 0.5 K for 10 min, and then titrated with the ionic liquid under moderate agitation. The ionic liquid volumes that caused the solutions to become turbid from clear transparent were noted to determine the phase boundaries. In each plotted phase diagram, the upper part of the phase boundary represented a single-phase region (microemulsion, 1ϕ), and the lower part was a two-phase region (2ϕ). Compositions in the phase diagrams were in weight fractions. Corresponding values of the SME were calculated using AutoCAD software. Each set of experiment was repeated thrice, and the average value obtained was used for data processing and analysis. 3. Results and discussion 3.1. Effect of ionic liquids Ionic liquids with BF4 − , PF6 − , or Tf2 N− as anions have been previously reported as excellent lubricants [16]. Thus, these ionic liquids were used to investigate ESBO-based ionic liquid microemulsions. [BMIM][PF6 ] and [BMIM][Tf2 N] were used as the polar phases of ESBO-based microemulsions, and TX-100 and n-butanol, with Km at 4:1 were used as the surfactant and co-surfactant, respectively. Pseudo-ternary diagrams of ESBObased microemulsions with different ionic liquid phases at 298 ± 0.5 K are illustrated in Fig. 1a and b. The phase diagram of [BMIM][BF4 ]-based microemulsion under the same condition has already been presented in our published work [15]. The SME values of the above systems are listed in Fig. 2 to illustrate their phase-forming capacities. The phase-forming capacity evidently followed the succeeding sequence: ESBO-based microemulsion containing [BMIM][Tf2 N] > ESBO-based microemulsion containing [BMIM][PF6 ] > ESBO-based microemulsion containing [BMIM][BF4 ]. In the investigation of the phase formations of ionic liquid microemulsions, the hydrogen bond between the surfactant OEs and imidazolium ring of the ionic liquid was identified as the driving force [17], and the hydrogen bond strength was positively correlated with the electronegativity of the OE units and imidazolium ring [18]. With similar surfactant, the ESBO-based microemulsion containing [BMIM][BF4 ] showed the lowest phaseforming capacity because [BMIM][BF4 ] has less fluorine atoms
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Fig. 1. Pseudo-ternary phase diagrams of ESBO-based ionic liquid microemulsions using different ionic liquids at 298 ± 0.5 K with Km at 4:1. [Ionic liquids are (a) [BMIM][PF6 ] and (b) [BMIM][Tf2 N]].
Fig. 2. Effect of ionic liquid anions on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
compared with [BMIM][Tf2 N] and [BMIM][PF6 ]. Meanwhile, the ESBO-based microemulsion containing [BMIM][Tf2 N] showed a higher phase-forming capacity compared with that containing [BMIM][PF6 ] because nitrogen atom is more electronegative than phosphorus [19]. The ionic liquid chain lengths of the cations remarkably influence the phase behavior of the ESBO-based microemulsions. The pseudo-ternary phase diagrams of the ESBO-based microemul-
Fig. 3. Pseudo-ternary phase diagrams of ESBO-based ionic liquid microemulsions using different ionic liquids at 298 ± 0.5 K with Km at 4:1. [Ionic liquids are (a) [EMIM][BF4 ], (b) [HMIM][BF4 ], and (c) [OMIM][BF4 ]].
sions containing [EMIM][BF4 ], [HMIM][BF4 ], and [OMIM][BF4 ] at 298 ± 0.5 K are illustrated in Fig. 3a–c, respectively. TX-100 and nbutanol, with a constant Km of 4:1, were used as surfactant and co-surfactant, respectively. The calculated SME values of the different aforementioned systems are shown in Fig. 4. Results showed that the phase-forming capacities of ESBO-based microemulsions
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Fig. 4. Effect of ionic liquid cations on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1
increased gradually with increasing chain length of the ionic liquids. This phenomenon could be attributed to the surfactant effect of certain ionic liquids. According to Qiu et al., [1] ionic liquids comprising a charged hydrophilic head group and a hydrophobic tail are considered good surfactant candidates in forming micelles and microemulsions. A hydrophobic tail with long chain length has high higher surfactant ability and is more solubilized inside ESBO. 3.2. Effect of surfactants OE groups can enhance the solubilization capacity of microemulsions, and hence, most basic studies on non-aqueous ionic liquid microemulsions have utilized nonionic surfactants, such as TX-100 and Brij-35. Thus, the effect of the number of OE groups (NOE ) undoubtedly has an important role in the phase behavior of ionic liquid microemulsions. Pseudo-ternary phase diagrams of ESBO-[BMIM][BF4 ]-n-butanol-TX-45 (NOE ≈ 5) and ESBO-[BMIM][BF4 ]-n-butanol-TX-114 (≈8) microemulsions at 298 ± 0.5K, with constant Km of 4:1, are shown in Fig. 5a and b, respectively. Single-phase areas of the above microemulsions compared with the systems with TX-100 (NOE ≈ 10) are plotted in Fig. 6. Results illustrated that the ESBO-surfactant micelles achieved the maximum solubilization capacity for [BMIM][BF4 ] when NOE ≈ 8, whereas the minimum solubilization capacity was obtained when NOE ≈ 10. These solubilization effects could be attributed to the [BMIM][BF4 ]-ESBO amphiphilic balance of the surfactant existing in ionic liquid microemulsion systems [10]. The packing symmetry between the surfactant and water-oil phases has been reported to control the phase-forming capacity of traditional water-oil microemulsion [20]. ESBO-based ionic liquid microemulsions similarly have the largest monophasic region because the amphiphilic balance of TX-114 is the most suitable value for ESBO-[BMIM][BF4 ] microemulsions investigated.
Fig. 5. Pseudo-ternary phase diagrams ESBO-based ionic liquid microemulsions using different surfactants at 298 ± 0.5 K with Km at 4:1. [Surfactants are (a) TX-45 and (b) TX-114].
3.3. Effect of co-surfactants In microemulsions, the co-surfactant can reduce the surface tension and fluidize the interfacial surfactant film as well as provide the proper hydrophile-lipophile balance in the interfacial layer [21,22]. Short chain n-alkanols, such n-butanol, n-hexanol, and n-octanol, are the most widely used co-surfactants, and the solubilization of water and phase behavior of microemulsions substantially depend on the chain length of n-alkanols [23,24]. For the ESBO-based ionic liquid microemulsions, the chain length of the co-surfactants had considerable effects on the phase behavior of the microemulsion systems investigated. Pseudoternary phase diagrams of ESBO/(TX-100+n-hexanol)/[BMIM][BF4 ] and ESBO/(TX-100+n-octanol)/[BMIM][BF4 ] systems at 298 ± 0.5 K with Km at 4:1 are shown in Fig. 7a and b, respectively. An
Fig. 6. Effect of surfactant NOE on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
intuitive comparison of the SME values of ESBO-based microemulsions with different co-surfactants is presented in Fig. 8. Results showed that the phase-forming capacity of the aforementioned microemulsions have the following order: ESBO-based microemulsion containing n-butanol < ESBO-based microemulsion containing n-hexanol > ESBO-based microemulsion containing n-octanol. This
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microemulsion containing n-octanol showed a different phase behavior and presented an unexpected phase-forming capacity. 4. Conclusions Phase behavior studies on ESBO-based ionic liquid microemulsions were performed by plotting pseudo-ternary phase diagrams. Corresponding SME values were calculated to illustrate the effects of ionic liquids, surfactants, and co-surfactants on the phaseforming capacities of the designed systems. Results showed that the phase-forming capacities of the ESBO-based ionic liquid microemulsions with varying anions have the following sequence: ESBO-based microemulsion containing [BMIM][Tf2 N] > ESBObased microemulsion containing [BMIM][PF6 ] > ESBO-based microemulsion containing [BMIM][BF4 ]. This order could be attributed to the electronegativity of the imidazolium ring donated by associated atoms. Meanwhile, through the surfactant effect of certain ionic liquids, the phase-forming capacity of the designed microemulsions with different cations followed the sequence: ESBO-based microemulsion containing [OMIM][BF4 ] > ESBObased microemulsion containing [HMIM][BF4 ] > ESBO-based microemulsion containing [BMIM][BF4 ] > ESBO-based microemulsion containing [EMIM][BF4 ]. The ESBO-surfactant micelles achieved maximum solubilization capacity for [BMIM][BF4 ] when NOE ≈ 8 because of the appropriate ionic liquid-ESBO amphiphilic balance existing in the designed systems. In addition, ESBO-based microemulsion containing n-hexanol had a higher phase-forming capacity than that containing n-butanol because the spontaneity of the microemulsification increased with increasing co-surfactant chain length when the chain length was shorter than seven. However, when the chain length of n-alkanol was eight, the n-alkanol behaved more similar to oil than a co-surfactant. This study provided useful information for ESBO-based ionic liquid microemulsions for their application as biolubricant basestocks.
Fig. 7. Pseudo-ternary phase diagram of ESBO-based ionic liquid microemulsions using different co-surfactants at 298 ± 0.5 K with Km at 4:1.
Acknowledgements This research was supported by the National Natural Science Foundation of China (21376088), Project of Production, Education and Research, Guangdong Province and Ministry of Education (2012B09100063, 2012A090300015), and the Fundamental Research Funds for the Central Universities (2013ZZ0071). The authors would also gratefully acknowledge the support from the Guangdong Provincial Laboratory of Green Chemical Technology. References
Fig. 8. Effect of co-surfactant on SME of ESBO-based microemulsions at 298 ± 0.5 K with Km at 4:1.
regular change was consistent with our previous study, in which castor oil was used as the non-polar phase [25]. The ESBO-based microemulsion containing n-hexanol had a higher phase-forming capacity than that containing n-butanol because the spontaneity of the microemulsification increased with increasing co-surfactant chain length when the chain length was shorter than seven. When the chain length of n-alkanol was eight, the n-alkanol behaved more similar to oil than a co-surfactant [24]. Overall, ESBO-based
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