Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids

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Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids between Micelle and Vesicle by CO2 Yunlei Shi, Dazhen Xiong, Huiyong Wang, Yang Zhao, and Jianji Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01167 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids between Micelle and Vesicle by CO2 Yunlei Shi, Dazhen Xiong, Huiyong Wang,* Yang Zhao, and Jianji Wang* Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China E-mail: [email protected]

ABSTRACT: The creation of CO2 responsive materials that undergo structural transition between micelle and vesicle is of great importance from both theoretical and practical points of view. In this work, we have developed a series of CO2 responsive

single-tailed

amphiphilic

ionic

liquids

(ILs)

composed

of

N-alkyl-N-methyldiethanolamine cation [CnMDEA]+ (n = 8, 10, 12, 14, 16, 18) and 2-pyrrolidinone [2-Pyr]- anion. The aggregation behavior and self-assembly structures of the ILs in aqueous solution have been investigated by electrical conductivity, surface tension, dynamic light scattering, cryogenic transmission electron microscopy, small angle X-ray scattering, and nuclear magnetic resonance spectroscopy. For the first time, CO2 driven reversible switching of self-assembly between spherical micelle and unilamellar vesicle is found for [CnMDEA][2-Pyr] (n=16, 18) in aqueous solutions at 20 °C and atmospheric pressure. It is shown that the mechanism behind the reversible micelle to vesicle transition involves the formation of carbamate anion from the reaction between [2-Pyr]- and CO2. 1. INTRODUCTION Stimuli-responsive materials have attracted increasing interest in a variety of areas, such as drug delivery,1 sensing,2 coating,3 and catalysis.4 To date, pH,5 temperature,6 light,7 redox agents,8 and CO29 etc. have been used as external stimuli for 1

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stimuli-responsive materials. Among these external stimuli, CO2 is cheap, abundant, non-toxic and non-flammable. Additionally, it can be readily removed by N2 or air bubbling. Thus, CO2-responsive materials have received particular attention due to the advantages of using CO2 as a trigger.10 Ionic liquids (ILs) have found widespread applications in many areas of chemistry because of their unique physicochemical properties and tunable nature.11,12 The inherent amphiphilic character of long chain ILs makes them to have surface-active property that is often superior to conventional ionic surfactants.13 An ionic liquid surfactant is just an ionic surfactant melt at around room temperature, and thus it has emerged as a novel class of liquid surfactants. In the past decade, many studies showed that long chain imidazolium and pyridinium ILs could form micelle or micelle-like self-assembling structures in aqueous solution.14,15,16 However, until 2013, unilamellar and multilamellar vesicles formed by single-tailed amphiphilic ILs, such as 1-alkyl-3-methylimidazolium bromides [Cnmim]Br (n = 10, 12, 14) and 1-dodecyl-3-methylimidazolium β-naphthalene sulfonate ([C12mim][Nsa]) had been discovered in aqueous solutions without any additives.17,18 Considering the great promise in pharmaceutical and tissue engineering areas, in particular as vehicles for the targeted delivery of therapeutic agents,19 reversible structural transition of self-assembly between micelle and vesicle has been also explored recently under external stimuli. In this context, Wang et al20 investigated the reversible structural transition between micelle and vesicle of [Cnmim]X (n = 12 and 14, X = [C6H4COOKCOO], [C6H3OHCOOSO3Na] and [C6H4COOSO3Na]) in water triggered by solution pH, while Shi et al21 studied the reversible

structural

transition

between

4-butylazobenzene-4'-hexyloxytrimethylammonium

micelle

and

trifluoroacetate

vesicle

of

(BHATfO)

triggered by UV/visible light. To the best of our knowledge, no work has been reported for such a structural transition of an ionic liquid surfactant induced by CO2 at room temperature and atmospheric pressure. Herein, a novel class of CO2 responsive ILs was developed through carefully design of chemical structures of the cations and anions. These ILs were composed of 2


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N-alkyl-N-methyldiethanolamine cation [CnMDEA]+ (n = 8, 10, 12, 14, 16, 18) and 2-pyrrolidinone anion [2-Pyr]- (Scheme 1). CO2-driven reversible structural transitions of the ILs aggregates were investigated by conductivity, surface tension, dynamic light scattering (DLS), cryogenic transmission electron microscopy (Cryo-TEM), small angle X-ray scattering (SAXS), and

13

C nuclear magnetic

resonance (13C NMR). A possible mechanism was proposed for this reversible structural transition.

Scheme 1. Chemical structures of anions and cations of the ILs studied in this work

2. EXPERIMENTAL SECTION 2.1. Materials. 1-Bromooctane (C8H17Br, 98 wt%), 1-bromodecane (C10H21Br, 98 wt%), 1-bromododecane (C12H25Br, 98 wt%), 1-bromotetradecane (C14H29Br, 98 wt%), 1-bromohexadecane (C16H33Br, 98 wt%), 1-bromooctadecane (C18H37Br, 98 wt%) 2-pyrrolidinone (2-Pyr, 99 wt%) and anion exchange resin (Amersep 900 OH) were purchased from Alfa Aesar. N-methyldiethanolamine (MDEA, 99 wt%) was acquired from Aladdin. Deuterium oxide (D2O, 99.9% in D atom) was purchased from Sigma-Aldrich. CO2 (Praxair, SFC grade, 99.998 vol%) and N2 (Praxair, 99.9993 vol%) were used as received. 2.2. Preparation of the ILs. [C8MDEA][Br], [C10MDEA][Br], [C12MDEA][Br], [C14MDEA][Br], [C16MDEA][Br] and [C18MDEA][Br] were prepared and purified by similar procedures. Taking [C8MDEA][Br] as an example, MDEA and alkyl bromide (C8H17Br) were mixed at a molar ratio of 1.0:1.2 (MDEA : C8H17Br) in absolute ethanol, and the mixture was refluxed for 5 days at 70 °С with magnetic stirring. Then the solvent was removed by rotary evaporation, and the product ([C8MDEA][Br]) was obtained by extraction with petroleum ether, and then dried under vacuum for 72 h at 50 °С. Then, a solution of [C8MDEA][OH] in the mixed solvent of ethanol and water (1:1 volume ratio) was obtained from [C8MDEA][Br] by using anion-exchange 3


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method. Equimolar 2-pyrrolidinone (2-Pyr) was added to the solution, and the mixture was then stirred at room temperature for 24 h. After that, water and ethanol were removed by rotary evaporation. Thus obtained IL was dried for at least 48 h under vacuum at 50 °С before use. The other ILs were prepared in the similar procedures. The chemical structures of these ILs were confirmed by NMR and IR spectroscopy. The residual bromide content in the ILs was measured with the procedures described by Seddon et al.22 It was shown that the bromide content was less than 0.02 wt%. The purity (expressed in mole percent) of the ILs was determined by

1

H NMR

spectroscopy according to the method described by Xu et al.,23 and 98.3%, 97.3%, 99.1%, 98.8%, 98.8%, 99.5% were found for [C8MDEA][2-Pyr], [C10MDEA][2-Pyr], [C12MDEA][2-Pyr], [C14MDEA][2-Pyr], [C16MDEA][2-Pyr] and [C18MDEA][2-Pyr], respectively. 2.3. Measurements of 1H NMR,

13

C NMR, IR spectrum, melting point and

thermal decomposition temperature. 1H NMR and 13C NMR spectra were recorded on a Bruker spectrometer (400 MHz) in D2O. IR spectra were measured by using a Perkin-Elmer FTIR-400 spectrometer with universal ATR sampling accessory. Glass transition temperature (Tg) and melting point (Tm) were measured with a Netzsch 204F1 differential scanning calorimeter (DSC). Thermal decomposition temperature (Td) was determined with a Netzsch Sta 449 C thermal analyzer. Each experiment was performed in triplicate, and average values of the glass transition temperature, melting point, and thermal decomposition temperature were reported in Table 1. The uncertainties in the glass transition temperature (melting point) and thermal decomposition temperature were estimated to be 2 °C and 5 °C, respectively. 2.4. Measurements of critical micelle concentration of the ILs in water. Conductivity measurements were performed by using a Wayne-Kerr 6430B Auto Balance Bridge to determine critical micelle concentration (CMC) of the ILs in aqeous solutions. The conductance cell was equipped with a water circulating jacket, and the temperature was controlled at 20.00 ± 0.05 °С with a HAAKE V26 thermostat (Thermo Electron, Germany). The cell was initially calibrated with aqueous KCl solutions at different concentrations, and a cell constant of 1.0127 cm-1 was obtained. 4


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For the determination of CMC of each IL in water, a certain amount of water was transferred into the cell, and a known amount of concentrated aqueous solution of the IL was titrated into the cell by a microsyringe under magnetic stirring. After the equilibrium was arrived, the solution conductivity was determined. The uncertainty in conductivity was estimated to be 4.2%, and the uncertainty in CMC values determined from conductivity was listed in Table 2. Surface tension measurements by a DCA 315 tensiometer (Cahn Instruments) using a platinum plate (20 × 15 × 0.127 mm3) were also carried out to determine CMC of the ILs in aqeous solutions. The temperature around the sample cell was maintained at 20.0 ±0.1°C by circulating water from a HAAKE DC30-K20 temperature thermostat (Thermo Electron, Germany). The tensiometer was calibrated with double distilled water as described by the manufacturer. The platinum plate was rinsed with water and ethanol and then flame cleaned in an alcohol burner to eliminate any contaminants before each measurement. For each sample, three independent sets of immersion detachment cycles were measured in order to obtain an average surface tension value. The uncertainty of the surface tension measurements was estimated to be about ± 0.1 mJ·m−2. Typically, it took 0.5-1 h to reach equilibrium before the measurement. 2.5. Measurements of dynamic light scattering (DLS). All sample solutions were filtered through a 0.22 µm hydrophilic PVDF membrane filter and then kept at the measuring temperature for 12 h before determination. DLS measurements were performed on a Nano ZS-90 particle size analyzer (Malvern, United Kingdom) equipped with a solid-state He-Ne (4.0 mW) laser operating at λ = 633 nm. All the samples were measured at a scattering angle of 90° and 20.0 ± 0.1 °С. Each experiment was carried out in triplicate, and the uncertainty in DLS was estimated to be 4 %. 2.6.

Measurements

of

cryogenic

transmission

electron

microscopy

(Cryo-TEM). Cryo-TEM samples were prepared utilizing a custom-built chamber. After fixation of the TEM grid in the preparation chamber, a 5 µL of solution sample was placed on a carbon-coated holey film supported by a copper grid at room 5


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temperature. After the excess of solution was blotted away to form a thin liquid film, the grid was quenched rapidly into liquid ethane cooled by liquid nitrogen (-175 °C), and then the specimen stored in liquid nitrogen was transferred into a JEOL JEM1400 cryo-microscope. The acceleration voltage was 200 kV, and the working temperature was kept approximately at -175 °C. The images were recorded digitally with a charge-coupled device camera under low-dose conditions. 2.7. Measurements of small angle X-ray scattering (SAXS). SAXS experiments were performed on a SAXSpace small angle X-ray scattering instrument (Anton Paar GmbH) equipped with a Kratky block-collimation system at 20.0 ± 0.1 °C. The samples were held in a quartz capillary placed in a stainless steel tank. The X-ray was generated using sealed-tube X-ray generator with Cu target operating at 50 kV and 40 mA, and the wavelength was 0.1542 nm. The X-ray intensities were recorded on an imaging-plate detection system with a pixel size of 42.3 × 42.3 µm2. 3. RESULTS AND DISCUSSION 3.1. Thermal properties of the ILs and their aggregation behavior in water. These

ILs

were

synthesized

by

acid-base

neutralization

between

hydroxy-functionalized quaternary ammonium hydroxide and 2-Pyr neutral molecules according to the procedures described in the literature.24 Their chemical structures were confirmed by 1H NMR,

13

C NMR and IR spectroscopy, and the exact spectral

data were shown in Supporting Information. Table 1 shows glass transition temperature (Tg), melting point (Tm) and thermal decomposition temperature (Td) of these ILs. It was found that [C8MDEA][2-Pyr] and [C10MDEA][2-Pyr] exhibited only glass transition temperature, and the other ILs had a melting point below 42 °C. This suggests that all the ILs are room temperature ILs. In addition, the thermal decomposition temperature of these ILs was in the range from 156 to 174 °C, and the effect of the alkyl chain length of the cations was not significant.

6


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Table 1. Glass transition temperature (Tg) / melting point (Tm) and thermal decomposition temperature (Td) of the ILs IL

Tg / °C

[C8MDEA][2-Pyr]

-55

156

[C10MDEA][2-Pyr]

-62

165

Tm / °C

Td / °C

[C12MDEA][2-Pyr]

-40

168

[C14MDEA][2-Pyr]

0

174

[C16MDEA][2-Pyr]

9

165

[C18MDEA][2-Pyr]

41

162

Experimental conductivities (κ) for aqueous solutions of [CnMDEA][2-Pyr] (n = 8, 10, 12, 14, 16, 18) at 20 °C were shown in Figure 1 and Figure S1 as a function of IL concentrations. As can be seen from Figure 1, a characteristic shape of curve was observed for each IL, which exhibited typical behavior with two linear fragments, and the concentration at which the two linear fragments intersect was assigned to the CMC.25 Thus obtained CMC values were listed in Table 2. In addition, the CMC values of these ILs were also determined through the plots of surface tension versus IL concentrations,20 and the results were also included in Table 2. Generally, the CMC values determined by surface tension were in reasonable agreement with those determined by conductivity. It can be seen from Table 2 that the CMC values of [CnMDEA][2-Pyr]

(n = 8, 10, 12, 14, 16, 18) significantly decreased with the

increase of alkyl chain length of the cations due to the enhanced hydrophobic interaction of the ILs in water.26 Indeed, a linear relationship was found between the logarithm of the CMC and the number of carbon atoms (nc) in the alkyl chains of cations: log(CMC) = 1.623 - 0.298nc, and the values of intercept and slope of this linear equation were very close to those of the imidazolium ILs reported by Wang et al.27 This indicates that the contribution per -CH2 group in the alkyl chains of cations to CMC did not depend on the cationic structure and the anionic type.

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Figure 1. Concentration dependence of the conductivity for [C16MDEA][2-Pyr] in aqueous solutions. Table 2. CMC values of the ILs in aqueous solutions determined by conductivity and surface tension at 20 °C CMC / mol·kg-1

IL

Conductivity

Surface tension

[C8MDEA][2-Pyr]

(1.6±0.4)×10

-1

(7.5±1.0)×10-2

[C10MDEA][2-Pyr]

(5.6±0.8)×10-2

(2.2±0.6)×10-2

[C12MDEA][2-Pyr]

(1.3±0.4)×10-2

(7.6±1.3)×10-3

[C14MDEA][2-Pyr]

(2.2±0.7)×10-3

(1.0±0.9)×10-3

[C16MDEA][2-Pyr]

(5.1±1.3)×10-4

(4.8±1.2)×10-4

[C18MDEA][2-Pyr]

(2.7±1.2)×10-4

(2.4±1.1)×10-4

3.2. CO2-driven structural transition between micelle and vesicle. Generally, turbid solution indicates the formation of larger particles, aggregates or vesicles. The turbidity changes of aqueous [CnMDEA][2-Pyr] (n = 8, 10, 12, 14, 16, 18) solutions in the presence of CO2 were determined by UV-vis spectroscopy. It was found that after bubbling of CO2, the turbidity of aqueous [CnMDEA][2-Pyr] (n = 8, 10, 12, 14) solutions was not changed, while the turbidity of aqueous [C16MDEA][2-Pyr] and [C18MDEA][2-Pyr] solutions exhibited significant change. This indicates the formation of larger aggregates or vesicles in aqueous [CnMDEA][2-Pyr] (n = 16, 18) solutions. In order to check the possible size change, the effect of CO2 bubbling on the size of self-assembly of [C16MDEA][2-Pyr] in water was investigated at 20 °C by DLS measurements. It was shown that before and after CO2 bubbling, no obvious size change was observed for the self-assemblies of this IL at low concentration (0.008 8


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mol·kg-1), but a remarkable change was found at high concentration (0.016, and 0.08 mol·kg-1) of the IL. Thus the concentration of 0.016 mol·kg-1 was selected to study the structural differentiation of [C16MDEA][2-Pyr] with and without CO2. As can be seen from Figure 2, before CO2 bubbling, the diameter of the self-assembly of [C16MDEA][2-Pyr] was about 2.9 nm. However, after CO2 was bubbled through the aqueous solution, the self-assembly grew enormously, and its diameter was increased to 99.7 nm. Such a remarkable change in the size of self-assemblies may reflect the structural differentiation of [C16MDEA][2-Pyr] due to the bubbling of CO2.28 Similar phenomenon was observed in aqueous [C18MDEA][2-Pyr]. However, no obvious size change was observed for the self-assemblies of [CnMDEA][2-Pyr] (n=8, 10, 12, 14) before and after CO2 bubbling (Figure S3). To visualize the structural transition of self-assemblies of [CnMDEA][2-Pyr] (n=16, 18) before and after CO2 bubbling at 20 °C, cryo-TEM was used to observe the structures of the IL self-assemblies in aqueous solution. It was found that the self-assembly of [CnMDEA][2-Pyr] (n=16, 18) was spherical micelles in aqueous solution at the IL concentration of 0.016 and 0.008 mol·kg-1, respectively (Figure 3a and S4a), while the spherical micelles vanished and the unilamellar vesicles were formed after bubbling of CO2 (Figure 3b and S4b). The vesicles in aqueous [C16MDEA][2-Pyr] solution were stable for at least three weeks in this case. When CO2 was removed by bubbling of N2, the vesicles were returned to spherical micelles again (Figure 3c). To the best of our knowledge, this is the first example for the reversible transition between micelle and vesicle of IL surfactants in aqueous solution triggered by CO2 at room temperature and atmospheric pressure.

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Figure 2. The diameter of self-assemblies of [C16MDEA][2-Pyr] in aqueous solution at the IL concentration of 0.016 mol·kg-1 and 20 oC before and after CO2 bubbling.

Figure 3. Cryo-TEM images of the self-assemblies of [C16MDEA][2-Pyr] in aqueous solutions at the IL concentration of 0.016 mol·kg-1: (a) before CO2 bubbling; (b) after CO2 bubbling; and (c) after N2 bubbling.

To confirm this structural transition and acquire the related structural parameters, SAXS measurements were performed for [C16MDEA][2-Pyr] (0.016 mol·kg-1) and [C18MDEA][2-Pyr] (0.008 mol·kg-1) before and after CO2 bubbling. Figure 4 and Figure S5 show the SAXS patterns obtained for such IL surfactants in aqueous solutions. Several fitting models were attempted to reproduce the experimental data, and the best fits for the curves in Figure 4a / Figure S5a and Figure 4b/Figure S5b were found by using a spherical micelle model and a unilamellar vesicle model,20 respectively. The vesicle diameter, shell thickness, and diameter polydispersity for [C16MDEA][2-Pyr] were found to be 90.1 ± 5.6 nm, 5.0 ± 1.3 nm, and 0.16 ± 0.03, respectively. For [C18MDEA][2-Pyr], they were 100.1 ± 4.3 nm, 4.5 ± 1.1 nm, and 0.10 ± 0.02, respectively.

10


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Figure 4. SAXS data for aqueous [C16MDEA][2-Pyr] solution (0.016 mol·kg-1) before and after CO2 bubbling. The line was fitted by using a spherical micelle model and a unilamellar vesicle model: (a) before CO2 bubbling; (b) after CO2 bubbling.

3.3. Possible mechanism for CO2-driven structural transition of the ILs. Considering the fact that the ILs surfactants investigated in this work have hydroxy-functionalized quaternary ammonium cations and N-containing heterocyclic anion [2-Pyr]-, it is likely that the structural transition of the self-assemblies induced by CO2 may be ascribed to the reaction of [2-Pyr]- anion with CO2 in aqueous solution. To verify this speculation, 13C NMR spectra were determined for [C16MDEA][2-Pyr], as an example, in D2O before and after CO2 bubbling. It was found that after bubbling of CO2, two new signals appeared at 157.2 and 160.2 ppm in the 13C NMR spectra of the IL in aqueous solution (Figure 5). The signal at 157.2 ppm was assigned to N-linked carbonyl carbon of the carbamate ([2-PyrCO2]-) formed from the reaction between CO2 and [2-Pyr]- anion, and the chemical shift value and signal intensity were very close to those reported previously.29,30 It was reported that [2-Pyr]anion-functionalized protic ILs were excellent for the rapid and reversible capture of CO2 with equimolar CO2 absorption,31 which indicates a strong interaction of the [2-Pyr]- anion with CO2. However, the observed intensity of the signal at 157.2 ppm was quite small, this is possibly due to the relatively long spin-lattice relaxation time (T1) and no nuclear overhauser effect (NOE) for the N-linked carbonyl carbon of carbamate.32 Additionally, the signal at 160.2 ppm could be ascribed to the carbonyl carbon of bicarbonate generated from the reaction of [2-Pyr]- anion and CO2 with water.33,34,35 On the other hand, the peak at 182.7 ppm was ascribed to carbonyl carbon of [2-Pyr]- anion,36 and after CO2 bubbling, this peak was split into two peaks 11


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and moved upfield to 181.8 and 181.4 ppm due to the reaction of [2-Pyr]- anion with CO2. The signals at 181.4 and 181.8 ppm could be, respectively, assigned to carbonyl carbon of the pyrrole ring in [2-PyrCO2]- anion and the neutral molecule 2-Pyr formed from the reaction of [2-Pyr]- anion and CO2 with water. In addition, it is not possible for the hydroxy-functionalized quaternary ammonium cation to interact chemically with CO2. Therefore, the reactions of CO2 with anions of the ILs generated ammonium salts (bicarbonate and carbamate), resulting in the increase of ionic strength in aqueous solution.

Figure 5.

13

C NMR spectra of [C16MDEA][2-Pyr] in D2O before CO2 bubbling, after CO2

bubbling, and then after N2 bubbling.

On the other hand, it was found that pH value of aqueous IL solutions decreased from 10.5 to 5.6 when the reaction equilibrium was reached after CO2 bubbling. We wanted to know whether the structural transition of the IL self-assemblies was caused by the increase of ionic strength or the decrease of the pH value of the aqueous solutions. Firstly, NaCl, NaHCO3 and Na2CO3 were chosen to examine the effect of ionic strength on the size of [C16MDEA][2-Pyr] self-assembly at the IL concentration of 0.016 mol· kg-1. The DLS results showed that the change in the size of the self-assemblies of the IL was very small with the increase of electrolyte concentrations up to 0.5 mol·kg-1, which was about thirty times of the IL concentration (see Figure S6). Thus the structural transition in the IL self-assemblies 12


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cannot be ascribed to the increase of ionic strength. In addition, to clarify the pH effect on the size of [C16MDEA][2-Pyr] self-assembly, a very small amount of hydrochloric acid was added into aqueous [C16MDEA][2-Pyr] solution (in the absence of CO2) to change solution pH value to 5.0, 6.0, 7.0, 8.0 and 9.0, respectively. It was shown that the pH value change also did not noticeably affect the size of the IL self-assembly (see Figure S7). This finding suggests that the change in pH value caused by CO2 bubbling was also not the reason for the structural transition from micelle to vesicle. As mentioned above, the reaction between anion of the IL and CO2 can lead to the formation of neutral molecules 2-Pyr and carbamate ([2-PyrCO2]-) in aqueous solution. It is important to know whether or not the structural transition of the IL self-assemblies was caused by 2-Pyr or carbamate ([2-PyrCO2]-). DLS measurements showed that the change in the size of self-assemblies for the IL was very small after equimolar 2-Pyr was added into aqueous [C16MDEA][2-Pyr] solution (see Figure S8). This result indicates that the structural transition in the IL self-assemblies cannot be originated from the formation of neutral molecule 2-Pyr. Therefore, it is likely that the structural transition from micelle to vesicle was related to the formation of carbamate ([2-PyrCO2]-). Thus, we tried to examine the effect of carbamate ([2-PyrCO2]-) on the size of the IL self-assemblies by an independent experiment. Unfortunately, we failed to do this because we are not able to obtain any carbamate. It is known that molecular packing parameter P is a classical theoretical parameter widely used to explain, rationalize and even predict molecular self-assembly in surfactant solutions.37 This parameter is defined as P = V/AL, where V is the volume of the surfactant tail, L is the tail length, and A is the surface area of the hydrophilic head unit at the hydrophobic-hydrophilic interface. With the increase of P value, spherical (0 ˂ P ˂ 1/3), cylindrical (1/3 ˂ P ˂ 1/2), and planar (1/2 ˂ P ˂ 1) structures such as vesicles would be formed.38 As previously reported, the P parameter is still working for the prediction of the self-assembly structure of IL surfactant systems. Actually, the prediction for the self-assembly structures by using P values is only approximate, especially when its value is close to or slightly greater than 1/3 and 1/2, 13


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respectively, it should be careful to determine the self-assembly structures. The P values of [CnMDEA][2-Pyr] (n=8, 10, 12, 14, 16, 18) in aqueous solutions before and after CO2 bubbling were calculated and the results were shown in Table 3. The details for the calculation of P values were given in the Supporting Information. It can be seen from Table 3 that the self-assembly structures of the ILs predicted by the molecular packing parameter is in agreement with those observed by using TEM. For [CnMDEA][2-Pyr], the A values of the ILs in water decreased after CO2 bubbling, which leads to the increase of P values. This result suggests that compared with [2-Pyr]- anion, the electrostatic repulsion between the IL headgroups was screened more efficiently by carbamate ([2-PyrCO2]-) due to its higher local charge density. This would make the hydrocarbon chains of the ILs to be packed more tightly and lead to the formation of vesicles when the P value of the ILs was increased enough to be larger than 0.5. On the other hand, the ability of carbamate ([2-PyrCO2]-) to screen the electrostatic repulsion between the IL headgroups becomes stronger with the increase of alkyl chain length of the ILs, resulting in smaller A value and larger P value (> 0.5) for [CnMDEA][2-Pyr] (n = 16, 18) in water. Thus, a transition from micelle to vesicle was achieved for the ILs with long alkyl chain. Based on the above analysis, the vesicle formation in aqueous [CnMDEA][2-Pyr] (n = 16, 18) solutions by CO2 bubbling could be ascribed to the stronger shielding ability of carbamate ([2-PyrCO2]-) for electrostatic repulsion between the IL headgroups and the long alkyl chain of their cations. Table 3 The values of A and molecular packing parameter P at 20 oC IL [C8MDEA][2-Pyr] [C10MDEA][2-Pyr] [C12MDEA][2-Pyr] [C14MDEA][2-Pyr] [C16MDEA][2-Pyr] [C18MDEA][2-Pyr]

A(nm2) a

0.879 0.693a 0.681a 0.582a 0.436b 0.543a 0.327b 0.525a 0.325b

P

0.237a 0.302a 0.308a 0.362a 0.482b 0.388a 0.645b 0.402a 0.649b

a, before CO2 bubbling; b, after CO2 bubbling 14


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3.4. Reversibility of the structural transition. The reversibility for the structural transition of the self-assemblies between spherical micelle and unilamellar vesicle was evaluated by conductivity determination of aqueous [C16MDEA][2-Pyr] at the concentration of 0.016 mol·kg-1 and 20 oC upon bubbling and removal of CO2 over three cycles (Figure 6). It was shown that upon CO2 bubbling, solution conductivity of [C16MDEA][2-Pyr] increased from 22.4 to 31.3 µS·cm-1, indicating that the IL reacted with CO2 in aqueous solution and generated extra hydrophilic electrolytes.10 However, the original low conductivity was restored upon bubbling with N2 to remove CO2. In addition, the self-assembly structure of this IL was examined by DLS after each bubbling and removal of CO2. It was noted that the diameters of the micelles and vesicles after N2 and CO2 bubbling, respectively, were kept at about 3 nm and 100 nm after several cycles. This confirms the good reversibility of the structural transition of the IL self-assemblies. Furthermore, the reversibility of the structural transition was also verified by 13C NMR spectroscopy. It was shown that when [C16MDEA][2-Pyr] –D2O-CO2 system was heated for 30 min at 65°C with N2 bubbling, the signals at 157.2, 160.2, 181.4, and 181.8 ppm in 13C NMR spectrum of the system disappeared (Figure 5). This result suggests that the products generated from the reaction between the anion of the IL and CO2 in aqueous solution vanished, and the structural transition of the IL self-assemblies was completely reversible. These results also indicate that the role of the carbamate anion is decisive in the observed structural transition of the IL self-assemblies in aqueous solutions.

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Figure 6. The variations in the conductivity of [C16MDEA][2-Pyr] (0.016 mol·kg-1) upon alternatively bubbling of CO2 and N2 at 20 °C. Before measurements, a small amount of DMSO was added into the aqueous IL solution to provide good defoaming effect because a lot of bubbles came up when CO2 was bubbled through the solution.

4. CONCLUSIONS We designed and developed a novel class of single-tailed amphiphilic ionic liquids composed of N-alkyl-N-methyldiethanolamine cation [CnMDEA]+ (n=8, 10, 12, 14, 16, 18) and 2-pyrrolidinone [2-Pyr]- anion. It was found that among these ILs, [CnMDEA][2-Pyr] (n = 16, 18) exhibited interesting structural transition of the self-assembly from spherical micelle to unilamellar vesicle in aqueous solutions upon bubbling of CO2 at 20 °C and atmospheric pressure. Such a transition is reversible upon alternatively bubbling and removal of CO2. This structural transition induced by CO2 is ascribed to the formation of carbamate anion from the reaction between anion of the ILs and CO2. To the best of our knowledge, this is the first example to report CO2-driven reversible structural transition of ILs surfactants between micelles and vesicles. These ILs may be of significant importance in advanced functional materials preparation, drug delivery and controlled release. ASSOCIATED CONTENT Supporting Information 1

H NMR,

13

C NMR and IR data for the ILs, the concentration dependence of the

conductivity for [CnMDEA][2-Pyr] (n=8, 10, 12, 14, 18) and surface tension for [CnMDEA][2-Pyr] (n=8, 10, 12, 14, 16, 18), the change in diameter of self-assemblies of [CnMDEA][2-Pyr] (n=8, 10, 12, 14, 18) in aqueous solutions before and after CO2 bubbling, Cryo-TEM images of self-assemblies of [C18MDEA][2-Pyr] (0.008 mol·kg-1) in aqueous solutions before and after CO2 bubbling, the effect of solution pH value, and addition of electrolytes and neutral molecule 2-Pyr on the diameter of self-assemblies of [C16MDEA][2-Pyr], and the details for the calculation of P values. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 16


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*E-mail: [email protected] (J.W.). *E-mail: [email protected] (H.W.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grant No. 21273062 and 21133009), the Program for Innovative Research Team

in

Science

and

Technology

in

University

of

Henan

Province

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TOC Graphic

The first example is reported for CO2-driven reversible structural transition between micelle and vesicle of some ILs surfactants at room temperature and atmospheric pressure.

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Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids

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