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Surfactant-free microemulsions of 1-butyl-3-methylimidazolium hexafluorophosphate, diethylammonium formate, and water jie xu, Jiaxin Song, Huanhuan Deng, and Wanguo Hou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00974 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Surfactant-free microemulsions of 1-butyl-3-methylimidazolium hexafluorophosphate, diethylammonium formate, and water

Jie Xu, * a Jiaxin Song, a

a

Huanhuan Deng, a Wanguo Hou * b

State Key Laboratory Base of Eco-chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, P.R. China;

b

Key Laboratory of Colloid and Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, P.R. China

Supporting Information

* To whom correspondence should be addressed Email: [email protected] Telephone: +86-0531-88564750 Fax: +86-0531-88564750 Email: [email protected] Telephone: +86-0532-84023654 Fax: +86-0532-84023927

Running title: Surfactant-free microemulsions of bmimPF6, DEAF, and water

ACS Paragon Plus Environment

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ABSTRACT Surfactant-free microemulsions (SFMEs) are a unique kind of microemulsions, which form from immiscible fluids (i.e., oil and water phases) in the presence of amphi-solvents rather than traditional surfactants. In comparison with traditional surfactant-based microemulsions (SBMEs), SFMEs have received much less attention, and the current understanding of the unique system is very limited. Herein, we report a SFME consisting of the hydrophobic ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), the protic IL diethylammonium formate (DEAF), and water, in which the bmimPF6 and DEAF are used as the oil phase and amphi-solvent,

respectively.

Three

kinds

of

microstructures,

namely,

water-in-bmimPF6 (W/IL), bicontinuous (BC), and bmimPF6-in-water (IL/W), are identified for the SFME, using cyclic voltammetry, cryo-TEM, and DLS techniques. Especially, the volumetric and surface free energy properties of the SFME are investigated by excess molar volume (VmE) and surface tension (γ) measurements, and they are found to be similar to those of SBMEs. Discontinuous changes in VmE and γ with the system compositions are observed as the system microstructures change, which can be used to identify the structural transition of SFMEs. We think this study provides a better understanding of SFME features.

Keywords: Surfactant-free microemulsion; Ionic liquids; Amphi-solvent; Excess molar volume; Surface tension


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Introduction Microemulsions are one kind of colloid systems that have attracted much attention because

of

their

fundamental

and

practical

significance.[1−5]

Commonly,

microemulsions are defined as thermodynamically stable and optically isotropic transparent dispersions formed by at least two immiscible fluids (generally a polar and a nonpolar component) and a surfactant (or an amphiphilic compound),[6] and the surfactant is believed to be a necessary component of microemulsions.[7] Indeed, most of the microemulsions reported are formed in the presence of surfactants and sometimes with co-surfactants. However, it has been found that, with the participation of an “amphi-solvent” instead of traditional surfactants, microemulsions can also form from two immiscible fluids.

[8−14]

The so-called amphi-solvent, an amphiphilic

substance but not a traditional surfactant, is a solvent being completely or at least partially miscible with both the two immiscible fluids. [14] It cannot form micelles in a bulk solution or ordered films at a water-oil interface, lacking features of traditional surfactants. Such microemulsions formed in the absence of traditional surfactants are termed “surfactant-free microemulsions” (SFMEs). [14] Similar to traditional surfactant-based microemulsions (SBMEs), [15−18] the ternary phase diagram is commonly used to examine the formation of SFMEs from a ternary mixture consisting of a polar component (or water phase), a nonpolar component (oil phase), and an amphi-solvent. If there is a single-phase region in the ternary phase diagram, it is very possible to form SFMEs in the single-phase region.

[13−14]

So far,

the formation of SFMEs has been identified in about 20 surfactant-free ternary mixture systems.

[14]

The structures, properties, and the formation mechanism of

SFMEs have been investigated.

[19−31, 32, 33, 34]

It has been preliminarily demonstrated

that the structures and properties of SFMEs are similar to those of traditional SBMEs


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to some extent.

[14]

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For instance, similar to SBMEs,

[15−18]

SFMEs can exhibit three

structures, namely, oil-in-water (O/W), bicontinuous (BC), and water-in-oil (W/O), and the three kinds of structures can be translated into each other along with the change in the composition of systems.

[19−25, 31]

Especially, the formation and

structures of SFMEs have been confirmed by molecular dynamics simulations.

[32−34]

However, SFMEs have received much less attention in comparison with SBMEs.

[32]

Over the past decades, the more than 10000 papers dealing with SBMEs have been reported in the literature, [22] while only a few (less than 100) papers deal with SFMEs. [14]

Consequently, the current understanding of SFMEs is still very limited, and many

issues about SFMEs thus need to be studied. It is desirable to find more SFME systems and to widely investigate the feature of SFMEs. Volumetric and surface free energy (or surface tension) properties of traditional SBMEs have been widely investigated.

[35−38]

The two thermodynamic properties can

provide information about the interaction among components of microemulsions. [35−38]

In addition, they are sensitive to the microstructures of microemulsions, thereby

can be used to identify the structural transition of microemulsions induced by changes of their composition and environmental conditions. been published by Lara et al.

[10]

[35−40]

So far, only one report has

on the volumetric properties of SFMEs that consist

of benzene, i-propanol, and water; the volumetric results suggest that the SFMEs exhibit the O/W, BC, and W/O microstructures. However, there have been no reports on the surface tension properties of SFMEs. It is essential to systematically understand the thermodynamic properties of SFMEs. Recently, room temperature ionic liquid (IL)-containing SBMEs have drawn much interest.

[41, 42]

This is because ILs as a neoteric solvent have unique features

like non-volatility, nonflammability, thermal stability, and tunable solvent power,


[18,

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24, 25]

which can render IL microemulsions with excellent performance such as

high-temperature stability,

[43]

green synthesis of materials.

thus with special potential applications such as in the

[41, 42]

[24, 25, 31]

Our previous work

demonstrated that ILs

can also be used to construct SFMEs, in which the ILs can be used as the oil phase, water phase or amphi-solvent. Three IL SFME systems have been indetified in the ternary mixtures of toluene / ethanol / 1-butyl-3-methylimidazolium tetrafluoroborate (bminPF4, a hydrophilic IL, as the water phase), hexafluorophosphate

(bminPF6,

a

hydrophobic

N,N-dimethylformamide (DMF) / water,

[25]

[24]

1-butyl-3-methylimidazolium

IL,

as

the

oil

phase)

/

and bmimPF6 (oil phase) / propylamine

nitrate (PAN, a protic IL, as the amphi-solvent) / water, [31] respectively. To construct more IL-containng SFME systems and to further understant their features are of fundamental and practical importance. In this paper, we report a new IL-containing SFME sytem that consists of bminPF6, diethylammonium formate (DEAF, a protic IL), and water, in which bminPF6 and DEAF are used as the oil phase and amphi-solvent, respectively. Three microstructures, namely, bminPF6 (O)/W, BC, and W/bminPF6 (O), and their transition are identified using cyclic voltammetry, cryogenic transmission electron microscopy (cryo-TEM), and dynamic light scattering (DLS) techniques. Especially, the thermodynamic properties excess molar volume (VmE) and surface tension (γ, or interfacial tension between gas (air)-phase and microemulsion-phase) of the SFME are investigated, Similar to the case of traditional SBMEs,

[35−40]

the two

thermodynamic parameters can reflect the structural transition of SFMEs. To the best of our knowledge, this is the first report on the volumetric and surface tension properties of IL-containing SFMEs. This study provides a better understanding of SFME features. Such SFMEs may have specific applications such as in material


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synthesis, reaction engineering, and separation, deduced from their IL-containing and surfactant-free natures.

EXPERIMENTAL Materials BmimPF6 (its molecular structure is shown in Figure S1 in the Supporting Information, SI), purchased from Nanjing Duodian Reagents Co., China, was dried to constant weight at 50 °C under vacuum before use. DEAF was synthesized in our laboratory according to the literature

[44]

(for details, see the SI). No miscellaneous

peaks being detected by 1H NMR (see Figure S2, SI) indicate that the obtained DEAF has a high purity. Potassium ferricyanide (K3Fe(CN)6, AR grade) was purchased from Tianjin Chemical Reagents Co., China, and used as received. Ultrapure water with a resistivity of 18.25 MΩ·cm was obtained using an AFZ-1000-U purification system (Chongqing Ever Young Enterprises Development Co. Ltd, China). Phase diagram construction The phase diagram of the bmimPF6/DEAF/water ternary system was constructed at 25.0 ± 0.2 °C using the cloud point technique by titration with DEAF as follows. A mixture with the desired volume ratio of bmimPF6 to water (RB/W) was prepared in a dry test-tube. An appropriate volume of DEAF was added under magnetic stirring. The phase boundary was determined by observing the transition from turbidity to transparency or vice versa. Repeating this procedure for other RB/W values allowed the phase diagram to be established. The entire procedures were repeated three times for average values. The component content was expressed as the volume fraction (f) in the ternary phase diagram. Cyclic voltammetry measurement


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Cyclic voltammetry measurements were performed with a three-electrode cell, which consisted of a glass-carbon working electrode (electrode area 0.07 cm2), a Ag|AgCl reference electrode, and a platinum wire counter electrode, on a CHI model 660D electrochemical work station (Shanghai Chenhua Instrument Factory, China). The space between the adjacent electrodes was 2.0 cm. Before each measurement, the working electrode was polished using a 0.05 mm aluminum oxide slurry and rinsed with ultrapure water. The electrode was ultra-sonicated for approximately 2 min in ultrapure water before use. The potential was scanned from 0.4 V to −0.4 V, with a sweep rate range of 20–100 mV·s−1. All experiments were carried out at 25.0 ± 0.2 °C under a nitrogen atmosphere to avoid oxygen effects. K3Fe(CN)6 was selected as the electroactive probe, with a fixed concentration of 0.65 g·L−1. Based on the peak current data obtained at various sweep rates, the diffusion coefficient of the electroactive probe, Dp (m2·s−1), in a microemulsion can be estimated from the Randles-Sevcik equation: [45] 1/2

 F 3ne3 Dpν  ip = 0.477 Ac    RT 

(1)

where ip is the peak current for a redox-active reversible system, A is the area of the working electrode, c is the concentration of the electroactive probe, R is the gas constant, T is the absolute temperature, F is the Faraday constant, ne is the number of electrons involved in oxidation or reduction, and v is the sweep rate. Based on the Randles-Sevcik equation, ip will linearly increase with v1/2 for a given electrode and constant electroactive probe concentration, and the Dp of the electroactive probe can be calculated from the slope of the ip–v1/2 linear plot. Measurement of surface tension


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Surface tensions (γ) of the microemulsions were measured by a platinum plate method on a JK99C automatic tensiometer (Shanghai Zhongchen Digital Technology Equipment Co. Ltd, China). The platinum plate was thoroughly cleaned with ethanol and flame-dried before each measurement. All detections were performed for 150 s at 25.0 ± 0.2 °C. Measurement of density Densities (ρ) of test samples were measured at 25 ± 0.2 °C suing a DMA-5000 M vibrating tube densimeter (Anton Paar, Austria). The measured ρ values were used to calculate the values of excess molar volumes (VmE) with the following equations: [46]

∑x M i

V = E m

i

ρ

i

−∑ i

xi M i

ρi

(2)

where Mi and ρi are the molecular weight and density of the component i in its pure state, xi is the mole fraction of the component i in the mixture, and ρ is the density of the mixture. Cryogenic transmission electron microscopy observation The microstructure of the microemulsions was observed by cryogenic transmission electron microscopy (cryo-TEM). The cryo-TEM samples were prepared at 25 °C and 95% relative humidity in a controlled-environment vitrification system (Cryoplunge TM3, USA). A 4 µL aliquot of sample was loaded onto a carbon-coated copper grid. The excess solution was then blotted off with filter paper, producing a thin film suspended on the mesh holes. After about 5 s, the sample-loaded grid was quickly put into liquid ethane (cooled by liquid nitrogen). The vitrified sample was transferred to a cryogenic specimen holder (Gatan 626), and observed at ca. −174 °C with an accelerating voltage of 120 kV on a JEM-1400 TEM (JEOL, Japan). The


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images were recorded on a Gatan multiscan CCD and processed with Digital Micrograph. Dynamic light scattering measurement Dynamic light scattering (DLS) measurements were performed with a 4 mW He-Ne laser (λ = 632.8 nm) at 25.0 ± 0.2 °C on a Nano ZS90 instrument (Malvern, England). All of the scattering photons were collected at a 90° scattering angle. The viscosity and refractive index measurements were carried out at 25.0 ± 0.2 °C on a rotational rheometer (RheolabQC, Anton Paar, Austria) and an Abbe refractometer (WAY-2S, Shanghai Instrument Physical Optics Instrument Co. Ltd, China), respectively.

Results and discussion Phase behavior of bmimPF6/DEAF/water system Figure 1 shows the ternary phase diagram of the bmimPF6/DEAF/water system at 25 ± 0.2 °C, in which the component content in the system is expressed as the volume fraction. Two evident regions are observed in the diagram, a single-phase region (blank region) and a multiphase region (gray region). The compositions in the single-phase region are optically isotropic and transparent, while those in the multiphase region are turbid under stirring and break quickly into two phases when left to stand. The single-phase region occupies about 28% area of the total, which is lower than that of bmimPF6/PAN/water system (~35%),

[31]

suggesting that

amphi-solvents have obviously influence on the single-phase area for a given oil-water system, similar to the case of surfactants. Based on previous work, [8−11, 19−30, 32, 34]

SFMEs are most likely formed in the single-phase region, which was confirmed

by subsequent characterizations.


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Figure 1. Phase diagram of bmimPF6/DEAF/water ternary system at 25 ± 0.2 °C. Blank and gray regions represent single-phase and multiphase regions, respectively. I, II, and III represent water-in-bmimPF6

(W/IL),

bicontinuous

(BC),

and

bmimPF6-in-water (IL/W) subregions, respectively. Samples a, b, c1, and c2 were chosen for cryo-TEM and DLS measurements.

Microstructure of microemulsion As shown in Figure 1, the single-phase region extends from oil-rich to water-rich regions, which is suitable for studying the structural transition of the microemulsion. [31]

Usually, the W/O and O/W structures form in oil-rich and water-rich regions,

respectively, and the BC structure can form with comparable contents of water and oil. [19, 23, 24, 29]

To identify the formation of microemulsions and to determine their

structural transition for the bmimPF6/DEAF/water ternary system studied here, cyclic voltammetry, DLS, and cryo-TEM measurements were performed over the single-phase region.

Cyclic voltammetry.


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Cyclic voltammetry is a widely used technique to study the microstructures and structural transition of microemulsions, by measuring the diffusion coefficients (Dp) of electroactive probes located in the microemulsions.

[17, 18, 47, 48]

When an

electroactive probe is completely solubilized in dispersed droplets, a lower Dp value will be obtained compared with that in a continuous phase because the probe diffuses with the droplets. In this case, the obtained Dp value corresponds to the apparent diffusion coefficient of the droplets.

[47, 48]

In this study, K3Fe(CN)6 was used as the

electroactive probe, and is expected to preferentially probe the water environment because of its limited solubility in bmimBF6. Owing to the fact that only the diffusion-controlled electrochemical charge transfer is suitable for the Dp measurement, [49] the anodic peak current ip were first measured by varying v for two systems in the single-phase region, with the bmimPF6/DEAF/water volume ratios (RB/D/W) of 0.15/0.65/0.20 and 0.06/0.70/0.24, respectively. Plots of ip versus v1/2 for the two systems are straight lines passing through zero (Figure S3, SI), indicating that the electron transport of the electrode reaction in the microemulsions is diffusion controlled. [18, 50] Therefore, K3Fe(CN)6 is a suitable electroactive probe for the studied microemulsions. The Dp of K3Fe(CN)6 in the bmimPF6/DEAF/water ternary system were determined over the single-phase region, at constant DEAF volume fractions (fD) but varying water volume fractions (fw). Typical changes in Dp as a function of the fw for fD = 0.65 and 0.70 are shown in Figure 2. Similar results were obtained for fD = 0.60, and 0.75 (Figures S4 A and B, SI). The Dp monotonically goes up with increasing fw within the entire single-phase region. However, the whole curve can be divided into three successive stages: an initially slow increase, a subsequently quick increase, and a finally slow increase again, as indicated in Figure 2. This is similar to the literature


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reports on SFMEs.

[24, 25, 31]

Based on previous work,

[18, 24, 25, 31, 45]

the discontinuous

change in Dp can be attributed to the change in the microstructure of the K3Fe(CN)6-located environment, namely, a microemulsion forms in the ternary system and its microstructure changes with variation in fw. 5

4.0 3.5

B

2 -1

2.5

DP(10 m ⋅s

3.0

-10

)

4

)

2 -1

A 3

-10

DP(10 m ⋅s

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

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2 1 BC W/IL IL/W 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

fW

2.0 1.5 1.0 0.00

W/IL 0.05

0.10

BC 0.15

fW

IL/W 0.20

0.25

0.30

Figure 2. Diffusion coefficient of K3Fe(CN)6 in the bmimPF6/DEAF/water ternary system with fD = (A) 0.65 and (B) 0.70 as a function of fw. The K3Fe(CN)6 concentration is 0.65 g·L−1. At low fw (e.g., < 0.1 for fD = 0.65), the relatively low Dp value suggests formation of a W/IL microemulsion, corresponding to the diffusion of water droplets in the bmimBF6 continuous phase. The gradual increase in Dp may be attributed to the decrease in viscosity

[51]

and an enlargement of the water droplet size.

[52]

At

intermediate fw (e.g., 0.1 < fw < 0.25 for fD = 0.65), the more rapid elevation of Dp implies a microstructure different from the W/IL microstructure, suggesting formation of a BC microstructure. [18, 24, 25, 31, 45, 47, 48, 52] At high fw (e.g., > 0.25 for fD = 0.65), the relatively high Dp value sees a IL/W microemulsion, corresponding to the diffusion of the probe in the continuous water phase. These results demonstrate a structural transition of the microemulsion occurs from W/IL via BC to IL/W with increasing fw. Notably, only one breakpoint appears for the ternary system with fD = 0.80 (Figure S4C, SI), suggesting only a transition from the W/IL to BC structure occurs.


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Furthermore, there are no obvious breakpoints for the ternary system with fD = 0.85 (Figure S4D, SI), suggesting no phase transition occurs along the fixed fD line. The

above results demonstrate that a microemulsion forms in

the

bmimPF6/DEAF/water ternary system and the microemulsion exhibits three types of microstructures, namely, W/IL, BC, and IL/W. These are similar to the case of bmimPF6/DMF/water or bmimPF6/PAN/water systems.

[25, 31]

Three subregions

corresponding to the three microstructures can be identified based on the cyclic voltammetry data along with the VmE and γ data (see Sections 3.3 and 3.4), as marked in Figure 1. Cryo-TEM and DLS studies To identify the three different microstructures, cryo-TEM observations were performed for four samples with RB/D/W of 0.30/0.65/0.05, 0.15/0.70/0.15, 0.05/0.60/0.35, and 0.05/0.55/0.40, marked as a, b, c1, and c2, respectively, in Figure 1. The samples a and b fall in the W/IL and BC subregions, respectively, and the samples c1 and c2 all in the IL/W subregion. The cryo-TEM images of the four samples are shown in Figure 3 (and Figure S5, SI). Spherical droplets are observed for the samples a, c1, and c2, indicating there exist discrete droplets dispersed in the continuous phase. The average diameters of the discrete droplets for the samples a, c1, and c2 are measured to be ~110, 90, and 80 nm, respectively. On the contrary, a sponge-like structure instead of discrete droplets is observed for the sample b, corresponding to both bmimPF6 and water as continuous phases. Similar results were obtained

for

oleic

acid/n-propanol/water,

[23]

toluene/ethanol/bminPF4,

bminPF6/DMF/water, [25] and bminPF6/PAN/water systems.

[31]

[24]

These images directly

confirm the formation of microemulsions in the bmimPF6/DEAF/water system and the presence of W/IL, BC, and IL/W microstructures in the microemulsions.


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Figure 3. Cryo-TEM images of samples a, b, c1, and c2 as marked in Figure 1.

DLS measurements were also carried out to determine the droplet sizes of the W/IL and IL/W SFMEs. The results (Figure S6, SI) show that the dh values of the samples a, c1, and c2 are 98, 88, and 68 nm, respectively, close to those observed by cryo-TEM. Density and excess molar volume of microemulsion Density (ρ) and excess molar volumes (VmE) of a mixture are related to its internal structures and the interactions between the component molecules [46, 53, 54] The volumetric parameter measurements for a microemulsion system can provide information of the microstructure and structural transition of the microemulsion. 35−40]

[10,

To understand the nature of SFMEs, the volumetric properties of the

bmimPF6/DEAF/water microemulsion were determined in this work. It is worth to note that the bmimPF6-phase and water-phase in the SFMEs actually consist of


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bmimPF6-DEAF and water-DEAF binary solutions, respectively, owing to the fact that DEAF is miscible with both bmimPF6 and water. Therefore, the ρ values of the water/DEAF and bmimPF6/DEAF binary solutions were first measured (Tables S1 and S2, SI). Figure 4 shows the changes in VmE values, obtained from the ρ values based on Eq. (2), for the two binary solutions as a function of fD. For the bmimPF6/DEAF binary solution, positive VmE values are observed in the fD range of 0–0.4, with a maximum appearing at fD ~0.2; over this fD range, the VmE is about zero. On the contrary, negative VmE values are observed for the water/DEAF binary solution over the whole fD range (0 < fD 0.25 for fD = 0.65), namely, for the W/IL and IL/W microemulsions, the quick increase in γ with fw is easily understood, which results from the higher γ value of water (or water/DEAF phase) than bmimPF6 (or bmimPF6/DEAF phase). However, in the intermediate fw range (e.g., 0.12 < fw < 0.25 for fD = 0.65), namely, for the BC microemulsion, the reasons for the relatively little change in γ with fw are not well clear. A possible reason for this is that, in the BC microstructure, both the bmimPF6 and water phases are


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probably in an uninterruptible conversion between discrete and continuous modes, which causes the γ to be insensitive to the change in water content. Similar phenomena were observed in measurements of fluorescence emission and UV-visible absorption spectra using pyrene and methyl orange as the probes, respectively, for bmimPF6/DMF/water or bmimPF6/PAN/water systems. [25, 31] Subregions of single-phase microemulsion region As mentioned above, the SFME consisting of bmimPF6, DEAF, and water exhibits the similar volumetric and surface tension properties to traditional SBMEs. The VmE and γ data determined for the SFME system can be used to identify the structural transition of the microemulsion, similar to the Dp data obtained by cyclic voltammetry measurements. The boundaries between W/IL and BC and between BC and IL/W subregions identified by the three techniques are almost consistent with each other. The uncertainty in locating the boundaries is estimated to be less than 8%, based on the Dp, VmE, and γ data. The microstructures and structural transition observed here are similar to those for other SFMEs [23−25, 31] and traditional IL SBME systems. [41, 45, 52]

Conclusion This paper demonstrates that SFMEs can form from a ternary mixture of the hydrophobic IL bmimPF6 (oil phase), the protic IL DEAF (amphi-solvent), and water. Similar to traditional SBMEs, the SFMEs exhibit three kinds of microstructures, i.e., W/IL, BC, and IL/W, depending on the composition of the ternary mixture. Especially, the volumetric and surface tension (or surface free energy) properties of the SFME are found to be similar to those of traditional SBMEs. The VmE and γ measurements can be used to identify the microstructures and structural transition of SFMEs. We think that this study provides a better understanding of SFME features. The SFME


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constructed here may have specific applications in material preparation, reaction engineering, and separation because of its IL-containing and surfactant-free natures.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ?????? Synthesis and 1H NMR spectrum of DEAF, molecular structure of bmimPF6, scan-rate dependence of anodic peak current for microemulsions, diffusion coefficient of K3Fe(CN)6 in microemulsions, cryo-TEM images, size distributions, and surface tension of microemulsions, and density and excess molar volume data. (PDF)

AUTHOR INFORMATION

Corresponding Authors Email: [email protected] Email: [email protected] ORCID ID Jie Xu: 0000-0002-4201-1897 Wanguo Hou: 0000-0003-1655-3593

Notes: The authors declare no competing financial interest.

Acknowledgements


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This work was supported by the National Natural Science Foundation of China (No. 21403121 and 21573133), and the foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, China (No. 201405).


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TABLE OF CONTENTS (TOC) GRAPHIC DEAF 0.0 1.0 0.2

IL

/W

BC

/I L

0.4

0.8 W

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0.6

0.6

0.4

0.8 1.0 0.0

0.2 0.0 0.2

0.4

0.6

0.8

H 2O

1.0

bmimPF 6

Surfactant-free microemulsions (SFMEs) can form from mixtures of the hydrophobic IL bmimPF6, the protic IL DEAF, and water. The volumetric and surface free energy properties of the SFMEs are similar to those of traditional surfactant-based microemulsions (SBMEs).


Surfactant-Free Microemulsions of 1-Butyl-3 ... - ACS Publications

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