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Dielectric Insight into the Microcosmic Behavior of Ionic Liquid Based Self-Assembly——Microemulsions/Micelles Cancan Zhang, Haixia Rao, and Kongshuang Zhao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03024 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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The Journal of Physical Chemistry

Dielectric Insight into the Microcosmic Behavior of Ionic Liquid based Self-Assembly——Microemulsions/Micelles Cancan Zhang, Haixia Rao, Kongshuang Zhao* College of Chemistry, Beijing Normal University, Beijing 100875, China E-mail address: [email protected] Abstract: Dielectric relaxation spectra of ([Bmim][BF4]/TX-100/P-xylene) microemulsions and ([Bmim][BF4]/TX-100) micelles were measured. A specific dielectric relaxation changing with the concentration of ILs was observed in the range of 106~108Hz. Combined dielectric parameters with the Einstein displacement equation and Bruggeman’s effective-medium approximation, the interaction between [Bmim][BF4] and TX-100 in microemulsions/micelles was presented: Due to the electrostatic interaction and van der Waals' force, [Bmim][BF4] is bound around the PEO chains of TX-100, and once added electric field, ions of [Bmim][BF4] will move along the PEO chain. The dependence of dielectric and phase parameters such as relaxation time, permittivity and volume fraction on the mass fraction of ILs presents an evidence for our proposals about the transition of both systems with the increase of ILs content. In addition, it was confirmed that percolation is a unique phenomenon in microemulsions and the percolation mechanism here belongs to static percolation. The transition process of micelles with the change of ILs content is presented from the dielectric view. Keywords: ionic interaction, percolation.

liquid

microemulsions/micelles,

dielectric

relaxation,

1. Introduction Since it was prepared for the first time in 1914, ionic liquids (ILs) have always been the focus of attention on account of their distinctive characteristics, such as high thermal and electrochemical stability, high conductivity, non-flammability and designability1-3. ILs can be used to substitute traditional toxic and volatile organic solvents in many applications, and consequently as alleged "green solvents"4, 5, which has extensive applications in various fields including chemical synthesis6, catalysis7, 8, separation technology9, 10, biodegradation11, 12, electrochemistry13, 14 and so forth. However, the intermiscibility of many ILs and non-polar solvents is poor, which limits the development of ILs to wider application. The introduction of surfactants makes the self-assembly (such as micelles15, 16, microemulsions17) in ILs come true, which not only overcomes that limitation, but enriches the traditional ordered

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molecular aggregates. Therefore, this kind of system, with the advantages of both ILs and ordered microstructure, has recently become a research hotspot18, 19, and the research on their microstructure transition and characteristics can be of sustained interest for their various applications20. It's the first time that Reinsborough and Bloom reported the amphiphilic self-assembled system, the molten salt of pyridinium chloride with melting point 146℃ 21 . The first use of ILs in self-assembled systems was the formation of micelles in ethylammonium nitrate (EAN) reported by Evans et al in 1980s22. With more and more new reports about amphiphilic assembly ILs, researchers have an increasing interest in the field23-25. While it is 2004, Han Buxing group17, who utilized hydrophilic ILs replacing water in traditional microemulsions, prepared IL microemulsions ([Bmim][BF4]/TX-100/cyclohexane) for the first time. In addition, they still presented three microregions of the IL microemulsions, namely IL/O, bicontinuous (B.C.) and O/IL, under the help of electrical conductivity meter, dynamic light scattering (DLS) and freeze etching electron microscopy (FF-TEM). Apart from that, they also obtained another IL microemulsions water/TX-100/[Bmim][PF6] using hydrophobic IL [Bmim][PF6] as oil phase, and similarly three microareas were divided by cyclic voltammetry26. Since the aggregation behavior of such systems has an important impact on their application in synthesis, catalysis, and separation19, 27-30, many technologies have been used to insight into them, including small angle neutron scattering (SANS )31, ultraviolet visible spectroscopy (UV-Vis)26, and fluorescent probe technology32 and so on. For example, for the IL micelles: with surface tension technique and near infrared spectrum technology, Tran et al33 obtained the CMC of various surfactants including CTAB, SDS, Triton X-100, Brij-35, Brij-700, Tween-20, SB-12 and SB3-10. Gao et al15 firstly used surface tensiometer and 1H NMR to study the aggregation behavior of nonionic surfactant TX-100 in imidazolium ILs ([Bmin][PF6], [Bmim][BF4]). And they found that there is a strong interaction between the positively charged imidazole ring of ILs and the negatively charged ethylene oxide group (EO) of PEO chains on the TX-100 molecules. What's more, their results also showed that the size of micelles TX-100 formed in ILs ([Bmim][BF4] and [Bmin][PF6]) is far larger than that of spherical micelles in traditional micelles, and attributed the formation of micelles to the low hydrophobicity of TX-100 in ILs compared to aqueous system. For IL microemulsions: through studying IL microemulsions ([Bmim][BF4]/TX-100/cyclohexane) by SANS, Eastoe et al31 found that with the increase of [Bmim][BF4] content, the droplet volume of microemulsions increased, and showed swelling phenomenon similar with the traditional micellar. In addition, Sarkar et al32 found that the polarity of system increased as the increasing content of [Bmim][BF4] by fluorescence probe. However, for the complex systems like IL microemulsions and micelles, it is still an opening hotspot and the study of their microstructures and characteristics will provide important reference for their wider application. Especially with the advent of molecular motor, the understanding of the


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self-assembly process is increasingly significant. Dielectric spectroscopy can provide information reflecting the microstructure, interfacial properties and ion migration by measuring the response of tested system to the electric field in the broadband. It has been successfully applied to the study of the traditional micelles and microemulsions systems for a long time34-38, and in recent years, has also been extended to the binary and ternary systems with ILs involved, especially the latter, namely, the IL microemulsions systems39-43. Among them, Schröder et al44 ascribed the two relaxations to ions and water rotation respectively by the experimental and computational analysis of the dielectric spectroscopy. Hunger et al45 revealed the orientation of cations and anions of ILs by analyzing the binary system of 1-N-ethyl-3-N-methylimidazolium ethylsulfate ([emim][EtSO4]) and Dichloromethane (DCM). In 2011, our group42 first applied dielectric spectroscopy to the micelles and microemulsion systems formed by hydrophobic ILs. The interaction between them and the percolation mechanism of microemulsions were also given through dielectric analysis. After that, Chen et al46 also reported the dielectric properties of water/TX-100/[Bmim][PF6] microemulsions whose oil phase was replaced by hydrophobic ILs, and systematically discussed the relaxation mechanism of the system. Most recently, our group39 has made a comparative research on the percolation mechanism of IL microemulsions, and found that hydrophobicity of ILs has an important effect on percolation mechanism of microemulsions. In order to further understand the self-assembly process and physicochemical properties of the IL self-assembly systems like ILs microemulsions and micelles, this research chooses the representative systems as the research target, namely [Bmim][BF4]/TX-100/P-xylene (microemulsion system) and [Bmim][BF4]/TX-100 (micelle system) which both have the involvement of hydrophilic ILs [Bmim][BF4] and nonionic surfactant TX-100. With dielectric measurement, we managed to distinguish micelle and microemulsion systems from the dielectric perspective. Apart from that, the transition process of microemulsions and micelles with the change of IL contents, as well as the interaction between surfactants and ILs is given, which will provide many effective reference for their wider application.

2. Experimental section 2.1. Materials [Bmim][BF4] (purity>99.2%) was purchased from Shanghai Cheng Jie Chemical Co. LTD., China, in which the residual chloride was less than 800ppm and the water content was less than 1000ppm. TX-100 (p-(1, 1, 3, 3-tetramethylbutyl) phenoxypolyoxyethyleneglycol) and P-xylene were purchased from Amresco Chemical Inc. (America) and Beijing Chemical Work, respectively, reagent grade. The chemical structures of them are shown in Fig. 1.



[Bmim][BF4]

TX-100 Fig. 1 The chemical structures of [Bmim][BF4], TX-100 and P-xylene.

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P-xylene

Fig. 2 shows the isothermal phase diagram of [Bmim][BF4]/TX-100/P-xylene.47 There are different colors differentiate the two phase and one phase with different microareas including IL/O, bicontinuous (B.C.) and O/IL. Apart from that, the experimental paths are also shown in Fig. 2: the signs ● denote the tested phase points of binary micelle system [Bmim][BF4]/TX-100 with TX-100 concentration ranging from 0 to 100 wt%. The symbols ○ represent the series of microemulsions where the mass ratio of [Bmim][BF4]/P-xylene is changed when the mass ratio of TX-100 is fixed at 50 wt%. The experimental path of microemulsions spans all the three microphases including IL/O, B.C. and O/IL, which aims at obtaining the parameters that can illustrate the microstructure transition of the system. Experiment was carried out along the paths in Fig. 2 and the total mass of each phase point was fixed at 4g.

Fig. 2 The isothermal phase diagram of [Bmim][BF4], TX-100, and P-xylene, and the experimental paths represented by different signs: path 1: the mass ratio of [Bmim][BF4]/P-xylene is changed when the mass ratio of TX-100 is fixed at 50 wt%. Path 2: the mass ratio of TX-100 and [Bmim][BF4] is increased from 0 to 100 wt%.

2.2. Dielectric measurements The 4294A Precision Impedance Analyzer from Agilent Technologies (Made in Japan) was used to carry out the dielectric measurements from 40 Hz to 110 MHz. The dielectric measurement cell with concentrically cylindrical platinum electrodes was connected to the impedance analyzer by a 1607E Spring Clip fixture (Aglient Technologies, Made in Japan). The capacitance Cx and conductance Gx can be directly obtained by measurement. And the capacitance and conductance of specimen Cs and Gs can be calculated by correcting the original data Cx and Gx. Then, the permittivity ε


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and conductivity κ can be calculated by the equation ε=Cs/Cl and κ=Gsε0/Cl (Cl is the cell constant and ε0 is permittivity of vacuum ( ε 0 = 8.8541× 10−12 F • m −1 ))

3. Results and discussion 3.1. Different dielectric behaviors of micelles/microemulsions Fig. 3a and b show the dielectric spectroscopy of binary micelles and ternary microemulsions. It can be seen that there is an obvious relaxation in the range of 106~108 Hz for both systems. The frequency and intensity of relaxation change with the increase of IL contents, which indicates there may be some change for the internal structure of aggregates caused by the change of the composition in the system. Using the Cole-Cole model equation (equation (1)) containing a relaxation term (i=1) to fit the data obtained, the best fitting results are represented as the solid line in Fig. 3.

ε ∗ = ε ' − jε " = ε h + ∑ i

εl − εh β 1 + ( jωτ i )

(1)

Where i denotes the number of relaxation, ε ∗ is the complex permittivity. ε ' and ε " are the real and imaginary part of permittivity, and the latter is also called dielectric loss and can be expressed with ε " = (κ − κ l ) / ωε 0 , κl

is the

low-frequency limit of conductivity. ε l and ε h are the low- and high-frequency limit of permittivity, f 0 is the characteristic frequency of relaxation, τ = ( 2π f 0 )

−1

is the relaxation time, β (0 < β ≤ 1) is a parameter related to the distribution of relaxation time, j =

−1 .

Fig. 3 The dielectric spectroscopy of each experimental specimen for (a) [Bmim][BF4]/TX-100



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micelles and (b) [Bmim][BF4]/TX-100/P-xylene microemulsions.

Fig. 4 (a) and (b) are obtained by plotting the dielectric increment (∆ and relaxation time ( against the ILs content for both systems. It can be seen that the dielectric parameters of the micelles (red lines and circles) are larger than those of the microemulsions (blue lines and squares) at the measured concentration range of ILs. In addition, with the change of ILs content, the dielectric parameters of both systems all appear inflection points, but the trend and inflection points are completely different, which indicates that the addition of p-xylene has an important influence on the system. It shows that the dielectric parameters can be used to distinguish micelles and microemulsions and be used as an indicator of the microstructural transition which will be discussed in the next section.

Fig. 4 The dependence of dielectric parameters (dielectric increment (a) and relaxation time (b)) on the mass fraction of ILs for [Bmim][BF4]/TX-100 micelles and [Bmim][BF4]/TX-100/P-xylene microemulsions.

3.2. The phase transition of micelles/microemulsions In order to present the specific microstructure transition process happened in both tested systems, Hanai equation based on the Bruggeman’s effective-medium approximation has been used to the dielectric parameters we obtained. It is a classic theoretical equation that can be used to calculate and discuss the phase parameters (including the permittivity and conductivity of spherical droplet interior ( ε i , κi ) and exterior ( , , and the volume fraction (φ) of droplets) of micelles and microemulsions from the dielectric parameters (where κ h

is high-frequency

conductivity). Therefore, according to the following equations ((2) and (3)), the phase parameters of both tested systems can be calculated and the results are listed in Table S2 (the permittivity of [Bmim][BF4] is fixed at 11.241).


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εi = ε a

(2ε a + ε h )φ − 2(ε a − ε h ) (2ε a + ε h )φ + (ε a − ε h )

(2)

κi = κ a

(2κ a + κ l )φ − 2(κ a − κ l ) (2κ a + κ h )φ + (κ a − κ h )

(3)

Fig. 5a shows the volume fractions of droplet in [Bmim][BF4]/TX-100 or [Bmim][BF4]/TX-100/P-xylene change with the increase of TX-100 and ILs. It is obvious that there are three regions whether it is in micelles or microemulsions system: for micelles, when wTX-100 is less than 0.2, φ closes to 1. At this moment, there are just a few surfactants and they are in twos or threes dispersed in ILs bulk, so the volume of ILs can be approximated as the volume of micelles, namely the volume of bulk. When the content of the surfactants reaches a certain value (0.2), the micelles are formed in the system. But the micelles are a little looser due to the relatively less surfactants. With the increase of surfactants, micelles gradually become denser, accordingly, decreases little by little. When the content of surfactants is over 0.76, the increase of surfactants content is dominant, and the micelle volume can be regarded as the volume of surfactants contained in the system. It is worth mentioning that the concentration partition here is nearly consistent with that of the dependence of ∆ and on ILs (Fig. 4), which both give the evidence for the changes of microstructure in the system.

Fig. 5 The dependence of microaggregates volume fraction (φ) on the surfactants or ILs content in (a) the micelles system ([Bmim][BF4]/TX-100) and (b) microemulsions system ([Bmim][BF4]/TX-100/P-xylene).

For the microemulsions (Fig. 5b), the volume fraction (φ) of dispersed droplets grows with the increase of the ILs overall. According to the increasing amplitude, the dependence of φ on the ILs could be divided into three regions by two inflection points (wIL=0.1 and 0.2). What’s more, these inflection points are identical with those in the phase diagram (Fig. 2). This is a corroboration for the credibility of such dielectric analysis method, and it also confirms the important role of component content on the microstructure of system. From Fig. 5b, it can be seen that when wIL is



less than 0.1, the φ rapidly increases with wIL. It is reasonable that with the addition of ILs, they are gradually accumulated in the drops of IL/O, which swells the ILs droplets and then increases the volume of microaggragates. When wIL is in the range of 0.1-0.2, the volume of microphase changes slowly. This segment is B.C., that is, ILs continuous phase and oil continuous phase formed (see the inset in Fig. 5b). Therefore, φ is not sensitive to the change of wIL. When wIL reaches more than 0.2, it can be seen that φ experiences a sudden jump and then stably maintains at about 0.7 (Fig. 5b). Changing from B.C to O/IL requires the reorientation and reassociation of surfactants, which causes φ increase. After the recombination of surfactants, the O/IL formed. Then, the added ILs was dispersed in the bulk phase, which has no obvious effect on φ.

3.3. The interaction of surfactants and ILs Due to the presence of anions and cations which are involved in the formation of microphase interface, it will be much helpful for understanding the microstructure of such systems if we can explicate the interaction between ILs and surfactants. For this reason, we try to make clear the interaction mechanism by using the Einstein displacement equation to calculate the motion distance of the ions in the system. According to Einstein displacement equation, the time τ that a relaxation process needs can be expressed as follows: τ= x

2

/ 2 D−

(4)

Where D is diffusion coefficient and can be calculated from equation (5), x is the distance of ions motion.

D = ( RT / F 2 ) ( λ / ν

)

(5)

Where R, F and v are gas constant, Faraday constant and the ionic charge, respectively, λ is the molar conductivity of the ion infinite dilution--here is that of [BF4]- and λ− ([BF4]-，298 K)=4.20 S m-1M-1 can be calculated based on the formula of the independent ionic motion law (equation (6)): Λ 0m = ν + λ+ + ν − λ−

(6)

Where Λ 0m is limit molar conductivity of [bmim][BF4], and Λ 0m ([bmim][BF4]， 298K)48=9.37 S m-1M-1， λ+ ([bmim]+，298K)49=5.17 S m-1M-1. Putting the values of each parameters in the equation (5), the infinite dilution diffusion coefficient of [BF4]- can be calculated: D−∞ ([BF4]-，298 K)=1.12×10-9 m2s-1.


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Therefore, replacing the values of τ and D in the equation (4), the distance of ions motion x can be obtained (see Table 1). Table 1 The distance of ions motion ( x ) for (a) the micelle system ([Bmim][BF4]/TX-100) and (b) microemulsion system ([Bmim][BF4]/TX-100/p-xylene)

a

w [Bmim][BF4] 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05

x /nm 2.689833 2.626633 2.626633 2.677312 2.689833 2.718823 2.767815 2.886659 2.948084 3.041579 3.195997 3.336585 3.535421 3.789248 4.171906 7.350429 13.41581 31.6851 72.15219

b

w[Bmim][BF4] 0.015 0.032 0.045 0.061 0.075 0.089 0.104 0.119 0.14 0.151 0.168 0.183 0.194 0.206 0.248 0.303 0.347 0.401 0.453

x /nm 3.865333 3.687384 3.588872 3.576367 3.389867 3.268761 3.160759 2.963241 2.731154 2.600923 2.45471 2.366432 2.328261 2.313785 2.385288 2.445567 2.526658 2.668932 3.167838

From Table 1, it can be seen that except that several values are exceptionally large up to dozens of nanometers when the content of ILs is very little (≤ 0.15), others are all approximately at around 2-3nm. According to the literature, the length value of PEO chains is < x >=20 Å50, namely 2.0nm. Accordingly, there is reason to believe that the relaxation is caused by the fluctuations of the IL anions along the PEO chains in the barrier layer of TX-100 (see the broken circle in Fig. 6): in both systems, because the Van der Waals' force from PEO chains will bind the cations [Bmim]+ to distribute around the PEO chains so that the diffusibility of [Bmim]+ becomes smaller. And the anions [BF4]- are bound near the PEO chains by the electrostatic interaction from cations [Bmim]+, but the interaction is relatively weak, so [BF4]- still can be “free” to move and have diffusion coefficient D. Once the electric field is added, the anions will fluctuate with the electric field in order to balance the distribution along the PEO chain. For the abnormal values of micelles (when wIL0.15, which indicates the system has a larger polarity when wIL≤0.15. And the ordered arrangement of several adjacent PEO chains will cause a large polarity which also provides an evidence for the rationality of the proposed mechanism above.

Fig. 6 The interaction model of TX-100 and [Bmim][BF4]. "⨁" represents the [Bmim]+ and "⊖" represents the [BF4]- in dotted circle.

Based on the model that the micellar particles with volume fraction Φ and conductivity disperse in the medium with the conductivity , the equation (7) (Hanai derived from the Bruggeman’s effective-medium approximation) was used to calculate the numbers of ILs bound nearby the PEO chains:

 κ -κp   κa 1/3     = 1 − Φ  κa -κp   κ 

(7)

Because the TX-100 is dissolved in hydrophilic [Bmim][BF4] ILs, the micelles they formed meet the conditions of , , so the equation (7) can be simplified to:  κ     κa 

2/3

= 1− Φ

(8)

Since [Bmim][BF4] is a hydrophilic ILs, the hydrophilic PEO chains of TX-100 will be directed to the external of micelles, as shown in Fig. 6. It is known from the previous discussion that the strong Van der Waals interaction between PEO chains and [Bmim]+ makes PEO chains associate with a certain amount of [Bmim]+, which changes the effective volume fraction of TX-100 from Φ to γΦ, γ is the volume ratio of micelles bound with cations and bare micelles, so the equation (8) can be changed to:


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κ     κa 

2/3

= 1 − γΦ

(9)

Put the experimental values of the DC conductivity of the system into (κ/κa)2/3 and then plot (κ/κa)2/3 against Φ as shown in Fig. 7. From Fig. 7, it can be clearly seen that when Φ≤48%, (κ/κa)2/3 showed a linear relationship with Φ, indicating that TX-100 in ILs exists in the form of spherical micelles at this moment. γ =1.43 can be obtained from the slope of the linear relationship in Fig. 7.

Fig. 7 The dependence of (κ/κa)2/3 on volume fraction (Φ). As above mentioned, γ is the volume ratio of micelles bound with cations and bare micelles, then, the values of γ-1 is the volume ratio of cations [Bmim]+ and TX-100, and it can be calculated by the equation (10):

γ - 1=

ν c cac Mc νTX cTX MTX

(10)

Where V, c and M are volume, molar concentration and relative molecular mass, respectively. TX, c and ac individually denote TX-100, cations in bulk and cations bound with TX-100. So the number of cations bound with each TX-100 can be calculated by the equation (11):

nac =

cac ν M =（γ - 1） TX TX cTX ν c Mc

(11)

Putting MTX=646.85, νTX=0.9756 nm3, νc=0.218 nm3, Mc=139.21 and γ=1.43 into equation (11), nac=9.0 can be calculated, which indicates that each TX-100 is bound with nine [Bmim]+. Compared with traditional micelles (TX-100/water), it is due to the volume effect of ILs that the number of cations bound on each TX-100 in IL micelles is less than that in water solution (20~30)51, 52, which can also be confirmed



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by the report other methods concluded47.

3.4. The percolation in microemulsions The percolation phenomenon indicates the transformation process of microstructure of microemulsions, and usually is an important physicochemical characteristic of microemulsions. As the red dots shown in Fig. 8a, conductivity has a significant jump in the range of 0.075-0.2--the B.C. of [Bmim][BF4]/TX-100/P-xylene, which is more obvious in Fig. 8b. Compared with that of [Bmim][BF4]/TX-100 micelles (black dots) in Fig. 8a, there is a reason to believe that percolation is a unique phenomenon for microemulsions.

Fig. 8 The comparison diagram (a) of the dependence of DC conductivity on wIL for [Bmim][BF4]/TX-100 micelles (black dots), [Bmim][BF4]/TX-100/P-xylene microemulsion (red dots); the dependence of dκ/dwIL on wIL (b).

From Fig. 8, the so-called percolation phenomenon can be seen, namely a rapid increase in B.C.. At present, there are two theories about the explanation of percolation. One is the static percolation theory53: percolation is raised by the two-channel of oil and water formed in B.C. which accelerates the migration of ions. The other is dynamic percolation theory54, 55: near the percolation threshold, a continuous rearrangement of percolation clusters is formed due to the interaction between the droplets, and ions constantly collide, move and exchange through the percolation clusters, resulting in the occurrence of percolation. No matter which kind of theory, it is predicted that the conductivity data of the hydrophilic phase ([Bmim][BF4] in this work) will follow the scaling relation when the concentration is below or above the percolation concentration threshold:

κl ∝ C p − C κl ∝ C − C p

−s

µ

Cp > C Cp < C


(12) (13)

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Where the values of s is predicted to be 0.753 and 1.254 for static and dynamic percolation, respectively; while the values of µ are 1.955, 56 for both theories.

Fig. 9 The dependence of DC conductivity on |wILp-wIL| when wIL is below (○) or above (●) the percolation threshold wILp.

Fig. 9 shows the dependence of and |wILp-wIL|, in which the hollow and solid data represent the data below and above the percolation threshold, respectively. The shadow part (wIL:7.5%~14.0%) shows the scaling relation meets s=0.63 from the slope, which is close to the predicted value 0.7 of the static percolation theory. To a certain extent, it indicates that it is the static percolation process that occurs in microemulsions, that is, ions are transferred through the formed continuous phase like a pipeline (as the inset in Fig. 9). Other data in Fig. 9 (non-shaded region) cannot be described by dynamic or static percolation theory. It may be mainly due to the fact that ILs, which act as hydrophilic phases in IL microemulsions, possess high conductivity, which makes it not fully satisfy with the hypothesis of percolation theory when the content of ILs is too high. From the above conclusion: the high conductivity of [Bmim][BF4] makes the microemulsions percolation process be not easy to detect compared with that in water. However, the dependence of the DC conductivity on the concentration of ILs near the percolation threshold is consistent with that of the static percolation theory predicted, which gives an evidence for the existence of the percolation process and bicontinuous microregion.

4. Conclusion The microstructure transition of micelles and microemulsions with ILs involved was investigated by dielectric spectroscopy and the relaxation mechanism is ascribed to the directional motion of anions of the ILs along the PEO chains of TX-100. Based on the Bruggeman’s effective-medium approximation, it is calculated that each TX-100 is bound with nine [Bmim]+ cations, which is far less than the number (20~30) of water bound on each TX-100, indicating that the volume effect of ILs have an important effect on their microstructure. In addition, the dependence of the dielectric parameters on the ILs content shows the different experience of the formation of the



micelles and microemulsions, which provides a credible reference for their application in different occasions and proves that dielectric spectroscopy can be used as a powerful “indicator” to distinguish micelles and microemulsions. According to the concentration dependence of conductivity combined with percolation theory, the percolation process of microemulsions in this work tends to be static percolation when wIL is in the range of 7.5~14.0%. And the existence of bicontinuous microphase is confirmed from the dielectric perspective, which will be helpful for the effective application (especially drug delivery, catalysis and synthesis) of this kind of systems.

Conflicts of interest There are no conflicts of interest to declare.

Supporting Information The dielectric parameters obtained by fitting and the phase parameters are included in the Supporting Information.

Acknowledgements Financial support for this work from the National Natural Science Foundation of China (No. 21673002, 21173025, 21473012) is gratefully acknowledged.

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TOC Graphic

In this article, it was confirmed that percolation is a unique phenomenon in microemulsions by dielectric analysis and the percolation of [Bmim][BF4]/TX-100/P-xylene is attributed to static percolation. The transition process of micelles with the change of ILs content is presented from the dielectric view. In addition, combined dielectric parameters (obtained by dielectric relaxation spectroscopy) with the Einstein displacement equation and Bruggeman’s effective-medium approximation (Fig. a and b), the interaction between [Bmim][BF4] and TX-100 in microemulsions/micelles was presented (Fig. c): Due to electrostatic interaction and van der Waals' force, [Bmim][BF4] is bound around the PEO chains of TX-100, and once added electric field, ions of [Bmim][BF4] will move along the PEO chain.


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Micelles

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