Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized

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General Research

Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized Hydrophobic Ionic Liquid-Based Microemulsions and the Effect of n-Alcohols Rongrong Wang, Zhenyu Feng, Wei Jin, and Xirong Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03766 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Industrial & Engineering Chemistry Research

Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized Hydrophobic Ionic Liquid-Based Microemulsions and the Effect of n-Alcohols Rongrong Wang, Zhenyu Feng, Wei Jin, and Xirong Huang* Key Laboratory of Colloid and Interface Chemistry of the Education Ministry of China, Shandong University, Jinan 250100, China.

ABSTRACT In

this

work,

the

fishlike

phase

diagram

of

the

H2O/[Bmim][AOT]/[Bmim][PF6]/n-alcohol as a function of temperature (T) and the mass fraction of [Bmim][AOT] (with or without n-alcohol) in the total mixture (γ) has been observed for the first time at several mass ratios of [Bmim][PF6] to H2O () and with different n-alcohols. The larger area of the three-phase region occurs at   0.500, and the resulting fish-shapes are similar to each other. For a given , a temperature scan (from lower to higher) at several γ values reveals that the present system forms an upper phase microemulsion first and then a lower phase microemulsion. The formation of hydrophobic ionic liquid-in-water (HIL/W) microemulsion at low temperature and water-in-hydrophobic ionic liquid (W/HIL) microemulsion at high temperature was confirmed by DLS and SAXS techniques. Here, the phase sequence occurred during the temperature scan is opposite to that of a classic H2O/NaAOT/Oil system. At the lower

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temperature, the H-bonding interaction is considered to be the main driving force for the aggregation; at the higher temperature, however, the main driving force may be the hydrophobic interaction. n-Alcohols with medium/long alkyl chain have a great influence on the fish-tail coordinates of the present systems. Compared with the ternary system without alcohol, the addition of n-alcohols (C4 ~ C8) decreases the phase inversion temperature (𝑇) and the surfactant efficiency. With the increase of the alkyl chain length of n-alcohols, however, the decrement in 𝑇 become smaller due to the increase of the interfacial rigidity. A comparison of these results with those obtained for the H2O/NaAOT/Oil system indicates that there are some similarities and also some differences, depending on the relative density, polarity or hydrophobicity among the HIL, oil and n-alcohols. The above insight into the phase behavior of the present HIL-based system helps to formulate biocompatible HIL-based AOT-stabilized microemulsions which are important templates for the biosynthesis of conducting polymers.

1.

INTRODUCTION

Microemulsions consisting mainly of oil, water and surfactant are thermodynamically stable

systems.

They

are

macroscopically

homogeneous

but

microscopically

heterogeneous. Microemulsions can be divided into three categories, i.e., oil-in-water (O/W), bicontinuous (O/W/O) and water-in-oil (W/O) microemulsions. Actually O/W/O bicontinuous microemulsion is an intermediate phase during the phase transition from O/W to W/O or from W/O to O/W type microemulsion.1,


2

O/W/O bicontinuous

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microemulsions have some unique properties, such as ultra-low interfacial tension, large interfacial area, and high solubilization capacity. These properties have been exploited in the fields of (bio)chemical separation, (bio)chemical catalysis, nanomaterials synthesis. 3-9

Room temperature ionic liquids (RTILs) are molten salts consisting of organic cation and organic (or inorganic) anion with the melting point below 100 °C.10 RTILs have many unique properties, such as low vapor pressure, high chemical/thermal stability, and good ability to dissolve a variety of organic and inorganic substances (via the combination of their ions of different kinds, i.e., the designability of RTILs).11 Compared with the traditional molecular organic solvents, RTILs could be regarded as “green solvents”.12, 13 In recent decade, many attempts have been made to formulate RTIL-based droplet-type microemulsions.14-28 Relatively speaking, less attention has been paid to the hydrophobic ionic liquid (HIL)-based bicontinuous microemulsions,29-31 especially those with the phase inversion temperature being close to the optimum temperature of an enzyme. To formulate an HIL-based bicontinuous microemulsion suitable for the expression of an enzyme activity, it is necessary to study the phase behavior of HIL/Surfactant/H2O systems. Based on the effects of multiple parameters on the phase behavior of polyoxyethylene-type nonionic surfactant (CnEm)/buffer/[Cnmim][PF6] system, we constructed several HIL-based bicontinuous microemulsions by tuning the phase inversion temperature of the ternary systems with the parameters studied. It has



been demonstrated that these systems are, to some extent, compatible with laccase.32 Compared with nonionic surfactants, ionic surfactants are rarely tried to stabilize the HIL-based bicontinuous microemulsions,33 especially the anionic surfactants. Due to the importance of the template of anionic surfactant aggregates to the performance of biosynthesized conducting polymers,34,

35

it is necessary to construct the anionic

surfactant-stabilized HIL-based bicontinuous microemulsion. In the present work, the phase behavior of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-alcohol system has been studied. Some interesting phenomena which are quite different from those in classical water-oil systems are observed and rationalized based on the relative density, polarity or hydrophobicity among the HILs, oils and n-alcohols. To the best of our knowledge, this is the first report of its kind for anionic surfactants. The insight into the phase behavior of the present pseudo-ternary system helps to formulate biocompatible AOT-stabilized HIL-based microemusions for biosynthesizing conducting polymers.

2.

EXPERIMENTAL SECTION 2.1. Materials. Sodium 1,4-bis-2-ethylhexylsulfosuccinate (NaAOT,  97%) was

purchased from Sigma-Aldrich. 1-Butyl-3-methylimidazolium chloride ([Bmim][Cl], > 98%) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6], > 98%) were purchased from TCI Co. Ltd., China. n-Butanol, n-pentanol, n-hexanol, n-octanol and dichloromethane were purchased from Sinopharm Chemical Reagent Co. Ltd., China. D2O was purchased from Qingdao Tenglong Microwave Co. Ltd., China. All chemical


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reagents were of analytical grade. Ultrapure water (18.25 MΩ cm) was used throughout the experiments. 2.2. Synthesis and Characterization of [Bmim][AOT]. The ionic surfactant [Bmim][AOT] was prepared according to the literature.36-38 A brief procedure is as follows. Equimolar amounts of [Bmim][Cl] and NaAOT were mixed in dichloromethane and stirred at room temperature for 12 h, followed by filtration to remove the precipitate (NaCl). The resulting [Bmim][AOT] in dichloromethane solution was washed with water several times to completely remove residual NaCl (the aqueous layer gives clear solution even with the addition of excess 1 M AgNO3). After that, dichloromethane and a little of water were removed in a rotary evaporator, resulting in the product [Bmim][AOT]. Prior to use, the product was dried under vacuum for 48 h. The water content in the product was analysed by a Carl-Fisher titration method and found to be less than 0.3 wt%. The product was also characterized by 1H NMR and HR-MS spectroscopy. The 1H NMR spectrum of the product was recorded on a Bruker Avance 500 MHz NMR spectrometer with D2O as external standard. As shown in Figure S1, no extra peak in 1H NMR ascertained the high purity of the product. Figure S2 is the mass spectrum of the product which was recorded on an Agilent 6510 Q-TOF mass spectrometer with an ESI source, indicating no other anion was present in the product. 2.3. Phase behavior study. The parameters used to describe the composition of a pseudo-ternary H2O/HIL/surfactant (alcohol) system are defined as follows: The mass fraction of the HIL () in the mixture of the HIL and water is given by:



α =

mHIL mHIL + mH2O

The mass fraction of the alcohol in the surfactant mixture () is given by: δ =

malcohol msurfactant + malcohol

The mass fraction of the surfactant (with or without alcohol) in the total mixture (γ) is given by: γ =

msurfactant + malcohol msurfactant + malcohol + mHIL + mH2O

For a given binary mixture of H2O and HIL ( was set), a known amount (weighed using a balance with an accuracy of  0.1 mg) of the surfactant [Bmim][AOT] (with or without n-alcohol) was added, and the resulting mixture was then equilibrated at a temperature for a period of time (in a thermostatic bath with an accuracy of  0.1 °C). After equilibrium, the number of coexisting phases was visually determined. For a given ternary system, the phase number of the system was changed with the temperature set for equilibration (the temperature was adjusted in steps of 1 °C). For a given , γ was varied by diluting the original system with the known amounts of HIL and water, and then the above strategy was used to determine the phase number of the resulting system at different temperatures. Based on the temperature dependent phase number of systems at different γ values, the T-γ fishlike phase diagram of the system could be determined. The parameters used to describe the T-γ fishlike phase diagram are defined as


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follows: Tl and Tu are defined as the minimum temperature of the lower boundary and the maximum temperature of upper boundary of the three-phase region, respectively. γ0 is the lowest surfactant concentration at which a middle phase appears, and T0 is the corresponding temperature at the fish head point.

γ

is the lowest surfactant

concentration at which Winsor IV phase is formed, and 𝑇 is the corresponding phase inversion temperature. 2.4. Dynamic Light Scattering (DLS) Measurement. The average particle size and the size distribution of the droplet-type microemulsion were measured on a Malvern Nano ZS instrument with a 4 mW He-Ne laser ( = 632.8 nm). The scattering light was collected at a scattering angle of 173°. Prior to measurement, all sample solutions were filtrated through a membrane filter with a pore size of 0.45 µm and fully equilibrated at the corresponding temperature. The resulting correlation function was analyzed using the CONTIN method to obtain the diffusion coefficient (D) of the droplet,39 and then the Stokes-Einstein equation (dh = kBT/3πηD, where kB, T and η have their usual meanings) was used to calculate the apparent hydrodynamic diameter (dh). The viscosity and the refractive index of systems were determined on a Brookfield DV-II Pro viscometer and a WYA-2W Abbe refractometer, respectively. 2.5. Small Angle X-ray Scattering (SAXS) Measurement. SAXS experiments were performed on SAXSess mc2 small-angle X-ray scattering instrument (Anton Paar，Austria). Prior to measurement, the sample was transferred into a quartz capillaries with a diameter of 1 mm, followed by thermo equilibration in a temperature control unit (Anton-Paar



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TCS 120). The sample was irradiated with the X-ray (Cu Kα，λ = 0.1542 nm) for 1 h. The data collected was corrected against the background scattering from the capillary and pure

[Bmim][PF6].

Finally

the

program

GIFT

(generalized

indirect

Fourier

transformation) was used to fit the scattering curve (more details can be found in literature40) and calculate the pair distance distribution function (PDDF) (p(r) ~ r curve) of the sample. The value of r at which p(r) reaches to zero in the higher-r regime determines the maximum dimension of the droplet.

3.

RESULTS AND DISCUSSION 3.1.

Phase

Behavior

of

the

H2O/[Bmim][AOT]/[Bmim][PF6]/n-Butanol

Pseudo-Ternary System. 3.1.1. Phase Transition of the System with Temperature. The T-γ fishlike phase diagrams of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-butanol (δ = 0.100) pseudo-ternary system

at different α values were shown in Figure 1(A-E). The system at α = 0.500 was

taken as an example to discuss the phase behavior as a function of temperature.


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Figure 1. The T- fishlike phase diagrams of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-butanol (δ = 0.100) pseudo-ternary system at α = 0.200 (A), 0.300 (B), 0.400 (C), 0.500 (D), and 0.650 (E), respectively.

Figure 2 shows the photographs of the system (α = 0.500, γ = 0.308) taken at several specific temperatures during the temperature scan (the samples are identified by a cross (), see Figure. 1). As is seen in Figure 2, at 16.0 °C, the system coexists in two phases, and the volume of the upper phase (water-rich phase) is significantly larger than the lower phase (HIL-rich phase); at 45.0 °C, the system is in three phases; at 68.0 °C, two


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phases coexist, but the volume of the lower phase (HIL-rich phase) is significantly larger than the upper phase (water-rich phase). Based on the phase transition with temperature, a preliminary conclusion could be drawn that an HIL/W-type microemulsion is formed at the low temperature, and a W/HIL-type microemulsion is formed at the high temperature. A similar phenomenon was observed for the pseudo-ternary system at different α values.

Figure 2. Photographs of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-butanol (α = 0.500, δ = 0.100, γ = 0.308) pseudo-ternary system at 16.0 °C (2a), 45.0 °C (2b) and 68.0 °C (2c), respectively.

In order to demonstrate that a droplet-type microemulsion is formed in the upper phase of Figure 2a and the lower phase of Figure 2c, DLS measurements were performed for the samples taken from the upper phase or the lower phase of their respective equilibrium systems. It can be seen from Figure 3 that the droplet-type microstructure does exist in upper phase of the system at 16.0 °C and the lower phase of the system at 68.0 °C with average particle diameters of 8.1 nm and 3.7 nm, respectively.



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Figure 3. DLS curves of the upper phase (water-rich phase) at 16.0 °C (3a) and the lower phase (HIL-rich phase) at 68.0 °C (3b) of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-butanol pseudo-ternary system (α = 0.500, δ = 0.100, γ = 0.308).

For further demonstration that the lower phase at 68.0 °C forms a W/HIL-type microemulsion, SAXS technique was also used to investigate the variation of the particle size of the formed reverse micelle with the amount of solubilized water w0 ( w0 is defined as

the

molar

ratio

of

H2O

to

[Bmim][AOT])

in

pseudo-binary

[Bmim][AOT]/[Bmim][PF6]/n-butanol system (δ = 0.100, I = 1.350 (I is defined as the mass ratio of [Bmim][PF6] to [Bmim][AOT]/n-butanol)) (see Figure 4). For comparison,


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the results from both SAXS and DLS techniques are shown in Figure 5. It is seen that at a given w0, both techniques result in a comparable droplet size,24,

41

and that the size

increases with the increase of w0, exhibiting a well-known "swelling" phenomenon. The above results support the conclusion that the above system does form a W/HIL-type microemulsion at 68.0 °C.

Figure 4. The SAXS (4a) and p (r) (4b) curves of the [Bmim][AOT]/[Bmim][PF6]/n-butanol (δ = 0.100, I = 1.350) reverse micelle system at different water contents at 68.0 °C. For better visibility, I(q) in Figure 4a for w0=1.5，2.0 and 2.5 are multiplied by the factors 1.1, 1.2 and 1.3.



Figure 5. The droplet size of the [Bmim][AOT]/[Bmim][PF6]/n-butanol (δ = 0.100, I = 1.350) reverse micelle system as a function of w0 at 68.0 °C (■: SAXS data and □: DLS data).

For the present H2O/[Bmim][AOT]/[Bmim][PF6]/n-butanol pseudo-ternary system, at the lower temperature the ionic surfactant [Bmim][AOT] is found to aggregate preferentially in the aqueous phase, forming an HIL/W-type microemulsion; For the traditional H2O/NaAOT/Oil system,42,

43

however, a W/O-type microemulsion is

preferentially formed in the oil phase at the lower temperature. This phenomenon can be explained as follows. At the lower temperature, NaAOT is more soluble in the traditional oil phase than in water phase, and the hydrophobic interactions make NaAOT assemble preferentially in the oil phase, forming a Winsor Ⅱ microemulsion. In the present system, [Bmim][AOT] has greater monomeric solubility in both the HIL phase and in water phase; however, the ability of [Bmim][AOT] to aggregate in water phase is enhanced due to the replacement of counterion,36-38 and the driving force for the aggregation in the HIL phase is weakened due to the decreased solvophobic effect of the surfactant


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headgroups,44, 45 causing [Bmim][AOT] to aggregate preferentially in the aqueous phase to form a Winsor Ⅰ microemulsion. At the higher temperature, the hydrogen bonds between the surfactant headgroup and water, which favors the formation of the HIL/W-type microemulsion at the lower temperature, are somewhat destroyed, resulting in an increase in the dehydration of the hydrophilic headgroups, and thus a decrease in the curvature of the interfacial layer according to the stacking parameter theory.46 Further increase of the temperature will eventually lead to a phase inversion of the system, forming a W/HIL-type microemulsion. The above-mentioned phase inversion phenomenon can also be explained based on the change of hydrophilicity/lipophilicity of the interface with temperature. 3.1.2. Variation of the Area of Three-Phase Region with . Table 1 shows the characteristic parameters of the fishlike phase diagrams at different α values. As far as the coordinates of the fish-head points are concerned, it is seen that T0 changes little with α; however, γ0 varies greatly with α. The fact that γ0 rises with increasing α could be ascribed to the high monomeric solubility of [Bmim][AOT] in the HIL.30 The larger the α value, the more HIL in the system, the more [Bmim][AOT] is needed to saturate the HIL phase with the monomers, and the larger the γ0 value. As for the coordinates of the fish-tail points, Table 1 shows that with the increase of α, 𝑇 increases continuously, while γ does not change greatly (a significant change occurs at α > 0.500). This changing trend is similar in nature to that of the traditional oil-water system.47 It can be explained as follows: with increasing α, the system contains more HIL; in order to



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achieve the phase inversion from HIL/W to W/HIL microemulsion, more surfactants should be expelled from the aqueous phase to the HIL phase to saturate the HIL phase, so a higher temperature is needed to destroy the hydrogen bonds between the surfactant headgroups and water. In terms of the surfactant efficiency, the present system is less efficient than the traditional NaAOT stabilized oil-water system.42, 48 The area of the three-phase region is proportional to  γ and T. As shown in Table 1, the larger area of the three-phase region occurs at α ≤ 0.500 (the resulting fish-shapes are similar to each other). This may be due to the good miscibility of the HIL with [Bmim][AOT]. In the 1-phase region of the present system, no liquid crystal phase was observed. In the traditional H2O/NaAOT/Oil system, however, a liquid crystal phase usually appears near the upper boundary of the corresponding 1-phase region on its fishlike phase diagram.48

Table 1. Characteristic Parameters of the Fishlike Phase Diagrams of the Pseudo-Ternary H2O/[Bmim][AOT]/[Bmim][PF6]/n-Butanol (δ = 0.100) System at Different α Values*

α

γ0

T0/°C

γ

𝑇/°C

γ

T/°C

0.200

0.09

47

0.55

33

0.46

41

0.300

0.10

45

0.57

39

0.46

42

0.400

0.11

46

0.58

40

0.46

40

0.500

0.12

47

0.56

46

0.44

43


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0.650 *

0.21

46

0.50

50

0.29

36

 γ ( γ = γ - γ0) and T (T = Tu - Tl) are the maximum extension of the surfactant concentration range and the temperature range of the three-phase region.

3.2.

Effect

of

Alcohols

on

the

Fish-Tail

Coordinates

of

the

H2O/[Bmim][AOT]/[Bmim][PF6] System. In a traditional surfactant/oil/water system, the addition of n-alcohol will change the spontaneous curvature and elasticity of the surfactant interfacial film. The phase behavior of the microemulsion could be effectively regulated by varying the alkyl chain length of n-alcohols and their concentration.49-51 In the present system, HIL was used to replace the traditional oil phase. In order to understand the effect of co-alcohol on the phase behavior of the HIL-based microemulsion, the influence of n-alcohols with different concentrations or alkyl chain lengths on the phase behavior of the H2O/[Bmim][AOT]/[Bmim][PF6] system was investigated. Since the coordinates of the fish-tail point (γ, 𝑇) are crucial for the preparation of bicontinuous microemulsions, the following discussion will be focused on the effect of n-alcohols on the fish-tail coordinates. 3.2.1. Effect of the Concentration of n-Butanol. Figure 6 shows that with increasing the concentration of n-butanol (δ = 0 ~ 0.200), 𝑇 decreases but γ increases gradually. A possible explanation is that as the concentration of n-butanol increases, the interfacial film has more partitioned n-butanol, which reduces the repulsion between the surfactant headgroups and the interfacial curvature; in other words, the addition of n-butanol makes



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the interface more hydrophobic and the phase inversion of the system easier. Here the concentration effect of n-butanol on 𝑇 is analogous to that in traditional oil-water system. For γ, however, it shows an opposite effect.48, 50, 51 This is caused mainly by the polarity difference between the HIL and the traditional oil phase (the polarity of [Bmim][PF6] is close to that of ethanol).52, 53 The addition of n-butanol makes the HIL phase less polar but more hydrophobic, so it becomes more difficult for the HIL/W-type microemulsion to solubilize the HIL phase, thereby resulting in a larger γ value.

Figure 6. Effects of the concentration of n-butanol on the fishlike phase diagrams (fish tail part) of the H2O/[Bmim][AOT]/[Bmim][PF6] (α = 0.500) system.

3.2.2. Effect of the Alkyl Chain Length of n-Alcohols. Compared with the system without n-alcohols, as shown in Figure 7, the addition of n-alcohol (C4 ~ C8) generally makes 𝑇 decrease and γ increase (as explained in Section 3.2.1). The decrement in 𝑇, however, becomes smaller with the increase of alkyl chain length of n-alcohols, indicating that for long-alkyl-chain alcohol, the rigidity of the interface has exerted more influence on 𝑇 than the hydrophobicity of the interface.49,


50

This is reason why the

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system containing n-octanol has higher 𝑇 than that of the system without n-alcohols. Similarly, the presence of long-alkyl-chain alcohol will make the HIL phase less polar but more hydrophobic, so the ability of the HIL/W-type microemulsion to solubilize the HIL is weakened, the γ value of the system thus increases.

Figure 7. Effect of the alkyl chain length of n-alcohols on fishlike phase diagrams (fish tail part) of the H2O/[Bmim][AOT]/[Bmim][PF6] (α = 0.500, δ = 0.200) system.

4.

CONCLUSIONS

The phase behavior of the H2O/[Bmim][AOT]/[Bmim][PF6]/n-alcohol pseudo-ternary system was investigated as a function of temperature and the mass fraction of [Bmim][AOT] (with or without alcohol) in the total mixture. Compared to NaAOT, [Bmim][AOT] has higher monomeric solubility in [Bmim][PF6] due to replacement of the counterion, which, together with the high polarity of the HIL, weakens the driving force for [Bmim][AOT] aggregation in the HIL phase. This feature, along with the



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enhanced [Bmim][AOT] aggregation in aqueous solution, results in a phase sequence opposite to that in a traditional H2O/NaAOT/Oil system when the temperature scan was performed. The existence of [Bmim][AOT]-(n-butanol)-stabilized HIL/W and W/HIL droplet-type microemulsions has been confirmed by DLS and SAXS techniques. n-Alcohols with medium/long alkyl chain have a great influence on the fish-tail coordinates of the present HIL-based system. Compared with the ternary system without alcohol, the addition of n-alcohol (C4 ~ C8) decreases the phase inversion temperature (𝑇) and the surfactant efficiency. With the increase of the alkyl chain length of n-alcohols, however, the decrement in 𝑇 become smaller due to the increase of the interfacial rigidity. A comparison of these results with those obtained for the H2O/NaAOT/Oil system indicates that there are some similarities and also some differences, depending on the relative density, polarity or hydrophobicity among the HIL, oil and n-alcohols. The present

study

helps

to

formulate

biocompatible

HIL-based

AOT-stabilized

microemulsions which are important templates for the biosynthesis of conducting polymers.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 1H

NMR spectrum of [Bmim][AOT] (Figure S1) and ESI()MS spectrum of [Bmim][AOT]


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(Figure S2)

■ AUTHOR INFORMATION Corresponding Author *Tel.: +86 531 88365433. E-mail: [email protected] ORCID Xirong Huang: 0000-0002-7531-6283 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS We are grateful for the financial supports from the Fundamental Research Funds of Shandong University (2015CJ005), the Provincial Key R & D Project of Shandong (2015GSF121035) and the National Natural Science Foundation of China (21773143). ■ REFERENCES (1) Guering, P.; Lindman, B. Droplet and Bicontinuous Structures in Microemulsions from Multicomponent Self-Diffusion Measurements. Langmuir 1985, 1 (4), 464-468. (2) Kogan, A.; Shalev, D. E.; Raviv, U.; Aserin, A.; Garti, N. Formation and Characterization of Ordered Bicontinuous Microemulsions. J. Phys. Chem. B 2009, 113 (31), 10669-10678. (3) Solanki, J. N.; Murthy, Z. V. P. Controlled Size Silver Nanoparticles Synthesis with Water-in-Oil Microemulsion Method: A Topical Review. Ind. Eng. Chem. Res. 2011, 50



(22), 12311-12323. (4) Pogrzeba, T.; Schmidt, M.; Milojevic, N.; Urban, C.; Illner, M.; Repke, J.-U.; Schomäcker, R. Understanding the Role of Nonionic Surfactants during Catalysis in Microemulsion Systems on the Example of Rhodium-Catalyzed Hydroformylation. Ind. Eng. Chem. Res. 2017, 56 (36), 9934-9941. (5) Schwarze, M.; Pogrzeba, T.; Volovych, I.; Schomäcker, R. Microemulsion Systems for Catalytic Reactions and Processes. Catal. Sci. Technol. 2015, 5 (1), 24-33. (6) Sathishkumar, M.; Jayabalan, R.; Mun, S. P.; Yun, S. E. Role of Bicontinuous Microemulsion in the Rapid Enzymatic Hydrolysis of (R,S)-Ketoprofen Ethyl Ester in a Micro-Reactor. Bioresour. Technol. 2010, 101 (20), 7834-7840. (7) Subinya, M.; Steudle, A. K.; Nestl, B.; Nebel, B.; Hauer, B.; Stubenrauch, C.; Engelskirchen, S. Physicochemical Aspects of Lipase B from Candida Antarctica in Bicontinuous Microemulsions. Langmuir 2014, 30 (11), 2993-3000. (8) Negro, E.; Latsuzbaia, R.; Koper, G. J. M. Bicontinuous Microemulsions for High Yield Wet Synthesis of Ultrafine Platinum Nanoparticles: Effect of Precursors and Kinetics. Langmuir 2014, 30 (28), 8300-8307. (9) Yu, X.; Sun, Y.; Xue, L.; Huang, X.; Qu, Y. Strategies for Improving the Catalytic Performance of an Enzyme in Ionic Liquids. Top. Catal. 2014, 57 (10), 923-934. (10) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99 (8), 2071-2084. (11) Binetti, E.; Panniello, A.; Triggiani, L.; Tommasi, R.; Agostiano, A.; Curri, M. L.; Striccoli, M. Spectroscopic Study on Imidazolium-Based Ionic Liquids: Effect of Alkyl Chain Length and Anion. J. Phys. Chem. B 2012, 116 (11), 3512-3518. (12) Earle, M. J.; Seddon, K. R. Ionic Liquids. Green Solvents for the Future. Pure. Appl. Chem. 2000, 72 (7), 1391-1398. (13) Rogers, R. D.; Seddon, K. R. Ionic Liquids: Solvents of the Future? Science 2003, 302 (5646), 792-793.


Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Ren, H.-R.; Liang, X.-D.; Zhou, D.; Yin, J.-Z. Study on the Phase Behavior and Molecular Dynamics Simulation of a Supercritical Carbon Dioxide Microemulsion Containing Ionic Liquid. Ind. Eng. Chem. Res. 2017, 56 (13), 3733-3739. (15) Zheng, W.; Hao, X.; Zhao, L.; Sun, W. Controllable Preparation of Nanoscale Metal–Organic Frameworks by Ionic Liquid Microemulsions. Ind. Eng. Chem. Res. 2017, 56 (20), 5899-5905. (16) Yan, K.; Sun, Y.; Huang, X. Effect of the Alkyl Chain Length of a Hydrophobic Ionic Liquid (IL) as an Oil Phase on the Phase Behavior and the Microstructure of H2O/IL/Nonionic Polyoxyethylene Surfactant Ternary Systems. RSC Adv. 2014, 4 (61), 32363-32370. (17) Bai, T.; Ge, R.; Gao, Y.; Chai, J.; Slattery, J. M. The Effect of Water on the Microstructure and Properties of Benzene/[Bmim][AOT]/[Bmim][BF4] Microemulsions. Phys. Chem. Chem. Phys. 2013, 15 (44), 19301-19311. (18) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Microemulsions with Ionic Liquid Polar Domains. Phys. Chem. Chem. Phys. 2004, 6 (11), 2914-2916. (19) Thater, J. C.; Gérard, V.; Stubenrauch, C. Microemulsions with the Ionic Liquid Ethylammonium Nitrate: Phase Behavior, Composition, and Microstructure. Langmuir 2014, 30 (28), 8283-8289. (20) Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. Designing a New Strategy for the Formation of IL-in-Oil Microemulsions. J. Phys. Chem. B 2012, 116 (9), 2850-2855. (21) Atkin, R.; Warr, G. G. Phase Behavior and Microstructure of Microemulsions with a Room-Temperature Ionic Liquid as the Polar Phase. J. Phys. Chem. B 2007, 111 (31), 9309-9316. (22) Moniruzzaman, M.; Kamiya, N.; Nakashima, K.; Goto, M. Formation of Reverse Micelles in a Room-Temperature Ionic Liquid. Chemphyschem 2008, 9 (5), 689-692. (23) Rai, R.; Pandey, S. Evidence of Water-in-Ionic Liquid Microemulsion Formation by



Page 24 of 28

Nonionic Surfactant Brij-35. Langmuir 2014, 30 (34), 10156-10160. (24) Kusano, T.; Fujii, K.; Hashimoto, K.; Shibayama, M. Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids. Langmuir 2014, 30 (40), 11890-11896. (25) Li, N.; Zhang, S.; Ma, H.; Zheng, L. Role of Solubilized Water in Micelles Formed by Triton X-100 in 1-Butyl-3-methylimidazolium Ionic Liquids. Langmuir 2010, 26 (12), 9315-9320. (26) Gao, Y.; Li, N.; Zheng, L.; Zhao, X.; Zhang, S.; Han, B.; Hou, W.; Li, G. A Cyclic Voltammetric Technique for the Detection of Micro-Regions of BmimPF6/Tween 20/H2O

Microemulsions

and

their

Performance

Characterization

by

UV-Vis

Spectroscopy. Green Chem. 2006, 8 (1), 43-49. (27)

Li,

Q.;

Huang,

X.

Bis(2-ethyl-1-hexyl)sulfosuccinate

Formation Stabilized

of

1-Butyl-3-methylimidazolium

Water-in-1-Butyl-3-methylimidazolium

Bis(trifluoromethanesulfonyl)imide Microemulsion and the Effects of Additives. J. Solution Chem. 2017, 46 (9), 1792-1804. (28) Kaur, M.; Singh, G.; Kumar, S.; Navnidhi; Kang, T. S. Thermally Stable Microemulsions Comprising Imidazolium Based Surface Active Ionic Liquids, Non-polar Ionic Liquid and Ethylene Glycol as Polar Phase. J. Colloid Interface Sci. 2018, 511, 344-354. (29) Porada, J. H.; Mansueto, M.; Laschat, S.; Stubenrauch, C. Microemulsions with Novel Hydrophobic Ionic Liquids. Soft Matter 2011, 7 (15), 6805-6810. (30) Anjum, N.; Guedeau-Boudeville, M.-A.; Stubenrauch, C.; Mourchid, A. Phase Behavior and Microstructure of Microemulsions Containing the Hydrophobic Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2009, 113 (1), 239-244. (31) Porada, J. H.; Zauser, D.; Feucht, B.; Stubenrauch, C. Tailored Ionic Liquid-Based Surfactants for the Formation of Microemulsions with Water and a Hydrophobic Ionic


Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Liquid. Soft Matter 2016, 12 (30), 6352-6356. (32) Yu, X.; Li, Q.; Wang, M.; Du, N.; Huang, X. Study on the Catalytic Performance of Laccase in the Hydrophobic Ionic Liquid-Based Bicontinuous Microemulsion Stabilized by Polyoxyethylene-Type Nonionic Surfactants. Soft Matter 2016, 12 (6), 1713-1720. (33) Wang, M.; Du, N.; Zhong, Y.; Huang, X. Additive Effects on the Phase Behavior of Cationic Surfactant ([C16mim]Br) Stabilized Hydrophobic Ionic Liquid Based Middle-Phase Microemulsions. J. Chem. Eng. Data 2017, 62 (2), 878-884. (34) Junker, K.; Zandomeneghi, G.; Guo, Z.; Kissner, R.; Ishikawa, T.; Kohlbrecher, J.; Walde, P. Mechanistic Aspects of the Horseradish Peroxidase-Catalysed Polymerisation of Aniline in the Presence of AOT Vesicles as Templates. RSC Adv. 2012, 2 (16), 6478-6495. (35) Zou, F.; Xue, L.; Yu, X.; Li, Y.; Zhao, Y.; Lu, L.; Huang, X.; Qu, Y. One Step Biosynthesis of Chiral, Conducting and Water Soluble Polyaniline in AOT Micellar Solution. Colloids Surf. A: Physiochem. Eng. Aspects 2013, 429, 38-43. (36) Cheng, N.; Ma, X.; Sheng, X.; Wang, T.; Wang, R.; Jiao, J.; Yu, L. Aggregation Behavior of Anionic Surface Active Ionic Liquids with Double Hydrocarbon Chains in Aqueous Solution: Experimental and Theoretical Investigations. Colloids Surf. A: Physiochem. Eng. Aspects 2014, 453, 53-61. (37) Wu, J.; Yin, T.; Shi, S.; Shen, W. Aggregation Behaviours of Surface-Active Ionic Liquids 1-Alkyl-3-methylimidazolium Bis(2-ethylhexyl)sulfosuccinate. Aust. J. Chem. 2016, 69 (11), 1254-1260. (38) Rao, K. S.; Gehlot, P. S.; Trivedi, T. J.; Kumar, A. Self-assembly of New Surface Active Ionic Liquids Based on Aerosol-OT in Aqueous Media. J. Colloid Interface Sci. 2014, 428, 267-275. (39) Ju, R. T. C.; Frank, C. W.; Gast, A. P. CONTIN Analysis of Colloidal Aggregates. Langmuir 1992, 8 (9), 2165-2171. (40) Tong, K.; Zhao, C.; Sun, Z.; Sun, D. Formation of Concentrated Nanoemulsion by



W/O Microemulsion Dilution Method: Biodiesel, Tween 80, and Water System. ACS Sustainable Chem. Eng. 2015, 3 (12), 3299-3306. (41) Tomšič, M.; Bešter-Rogač, M.; Jamnik, A.; Kunz, W.; Touraud, D.; Bergmann, A.; Glatter, O. Ternary Systems of Nonionic Surfactant Brij 35, Water and Various Simple Alcohols: Structural Investigations by Small-Angle X-ray Scattering and Dynamic Light Scattering. J. Colloid Interface Sci. 2006, 294 (1), 194-211. (42) Kahlweit, M.; Strey, R.; Schomaecker, R.; Haase, D. General Patterns of the Phase Behavior of Mixtures of Water, Nonpolar Solvents, Amphiphiles, and Electrolytes. Langmuir 1989, 5 (2), 305-315. (43) Kunieda, H.; Shinoda, K. Solution Behavior and Hydrophile-Lipophile Balance Temperature in the Aerosol OT-Isooctane-Brine System: Correlation between Microemulsions and Ultralow Interfacial Tensions. J. Colloid Interface Sci. 1980, 75 (2), 601-606. (44) Greaves, T. L.; Drummond, C. J. Ionic Liquids as Amphiphile Self-Assembly Media. Chem. Soc. Rev. 2008, 37 (8), 1709-1726. (45) Smirnova, N. A.; Safonova, E. A. Micellization in Solutions of Ionic Liquids. Colloid J. 2012, 74 (2), 254-265. (46) Mitchell, D. J.; Ninham, B. W. Micelles, Vesicles and Microemulsions. J. Chem. Soc., Faraday Trans. 2 1981, 77 (4), 601-629. (47) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. On Microemulsion Phase Behavior and the Monomeric Solubility of Surfactant. Phys. Chem. Chem. Phys. 1999, 1 (18), 4299-4306. (48) Sager, W. F. C. Systematic Study on the Influence of Impurities on the Phase Behavior of Sodium Bis(2-ethylhexyl) Sulfosuccinate Microemulsions. Langmuir 1998, 14 (22), 6385-6395. (49) Leung, R.; Shah, D. O. Solubilization and Phase Equilibria of Water-in-Oil Microemulsions: II. Effects of Alcohols, Oils, and Salinity on Single-Chain Surfactant


Page 26 of 28

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Systems. J. Colloid Interface Sci. 1987, 120 (2), 330-344. (50) Mathew, D. S.; Juang, R.-S. Role of Alcohols in the Formation of Inverse Microemulsions and Back Extraction of Proteins/Enzymes in a Reverse Micellar System. Sep. Purif. Technol. 2007, 53 (3), 199-215. (51) Kahlweit, M.; Strey, R.; Busse, G. Microemulsions: A Qualitative Thermodynamic Approach. J. Phys. Chem. 1990, 94 (10), 3881-3894. (52) Baker, S. N.; Baker, G. A.; Bright, F. V. Temperature-Dependent Microscopic Solvent Properties of 'Dry' and 'Wet' 1-Butyl-3-methylimidazolium Hexafluorophosphate: Correlation with (30) and Kamlet-Taft Polarity Scales. Green Chem. 2002, 4 (2), 165-169. (53) Poole, C. F. Chromatographic and Spectroscopic Methods for the Determination of Solvent Properties of Room TemperatureIonic Liquids. J. Chromatogr. A 2004, 1037 (1), 49-82.

Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized Hydrophobic Ionic Liquid-Based Microemulsions and the Effect of n-Alcohols



Rongrong Wang, Zhenyu Feng, Wei Jin, and Xirong Huang* Key Laboratory of Colloid and Interface Chemistry of the Education Ministry of China, Shandong University, Jinan 250100, China. Graphic Abstract:

For a given , a temperature scan at several  values reveals that at lower temperature an HIL/W microemulsion is formed, while at higher temperature a W/HIL microemulsion is formed.


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Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized

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