pH triggered self-assembly structural transition of ionic liquids in aqueous solutions: smart use of pH-responsive additives

Abstract

The creation of smart self-assembling fluids that undergo a morphological transition in response to a specific pH value can allow for the enhanced accumulation of drug delivery agents. In this work, we developed a series of pH-responsive fluids composed of 1-alkyl-3-methylimidazolium bromide [C_nmim]Br (n = 12, 14) and one of the pH-responsive hydrotropes of potassium hydrogen phthalate ([C₆H₄COOKCOOH]), sodium sulfosalicylate ([C₆H₃OHCOOHSO₃Na]), or m-carboxylbenzenesulfonate sodium ([C₆H₄COOHSO₃Na]). The self-assembled structures of these ILs in aqueous hydrotrope solutions were investigated by surface tension, dynamic light scattering, cryogenic-transmission electron microscopy, small-angle X-ray scattering, polarized optical microscopy, and nuclear magnetic resonance spectroscopy. It was found that the ionic liquids, [C_nmim]Br (n = 12, 14), could self-assemble into vesicles with the addition of the hydrotrope, and a reversible transition between spherical micelles and vesicles was observed with the change of solution pH value. The transition in the self-assembled structures of the ILs is suggested to be driven by the change in the molecular structure and hydrophilicity/hydrophobicity of the hydrotrope.

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Introduction

Amphiphilic molecules such as lipids spontaneously self-assemble into organized structures such as micelles and vesicles in solutions. These organized structures have found broad applications ranging from catalysis,^1,2 preparation of novel materials³ to biopharmaceutics.⁴ The ability to control the self-assembly behavior of amphiphilic molecules has the potential to significantly enhance their performance by tuning the catalyst microenvironment, switching the activity of the nanoscale device, or triggering payload release selectively at the target. Thus far, a number of amphiphilic molecule systems have been developed, which are responsive to stimuli such as ultrasound,⁵ light,^6,7 electricity,⁸ temperature^9–12 or solution pH.^13–17 Particularly, pH responsive self-assembling systems have attracted great attention from scientists in the application of drug delivery because of the numerous pH gradients that exist in both normal and pathophysiological states.¹⁸ Up to now, a large number of pH responsive vesicle systems have been explored. Usually, these systems are mainly constituted from polymer, phospholipids, catanionic surfactants, and mixture of cationic and anionic surfactants with carboxylic acid or amino groups.^19–27 Such systems offer the advantage of controllable pH-responsive vesicle through a similar strategy, that is, incorporating pH-responsive moieties into amphiphiles covalently through organic synthesis.

However, more effort should be made to overcome the disadvantages of this strategy before these responsive vesicles can be applied to industrial process. First, this strategy relies on novel synthesis of amphiphiles with pH-responsive functional groups, which requires a time-consuming and low-yield synthetic procedure. This may greatly weaken the application of these responsive vesicles. Second, a given pH-responsive vesicle can only be modulated within a suitable pH range. In principle, this pH range is mainly determined by the pH-responsive group attached to the amphiphile. For any applications, it is necessary to prepare different pH-responsive vesicles with different pH ranges. Consequently, a series of organic syntheses should be conducted to incorporate different pH-responsive functional groups to amphiphiles. Based on this consideration, it is of importance to develop a simple and effective approach for preparing pH-responsive vesicles based on small organic molecules with various pH ranges without complicated organic synthesis. In this context, a new strategy for the preparation of pH-responsive vesicles by incorporating pH-responsive moieties non-covalently to amphiphiles is suggested.

Room temperature ionic liquids (ILs) are composed of organic cations and inorganic or organic anions, which are liquid at temperatures below 100 °C. The most interesting property of ILs is that their structures and properties can be efficiently and easily tailored by a judicious design of cations and anions structures. This makes ILs potentially useful as “designer solvents”.²⁸ Previous investigations had shown that some of the ILs hold inherent amphiphilic character and can form self-assembly structures such as micelle and micelle-like in water.^29–34 In the presence of oppositely charged additives, single-chain ionic liquids surfactants can self-assemble into vesicles spontaneously in aqueous solution.^35–39 These additives may be a conventional surfactant, an IL or a salt. In some of these mixed systems, reversible transition from micelle to vesicle have been observed by changing the content of the additive added.^38–40 For example, single-chain ionic liquid surfactant [C₁₂mim]Br can form vesicle with the addition of 1-butyl-3-methylimidazolium 2-naphthalenesulfonate in aqueous solution, which was driven by the π–π interaction between the imidazolium headgroup of [C₁₂mim]Br and naphthalenesulfonate anion of the added IL.⁴¹ Rao et al.⁴² reported that NaBr can induce the transition from micelle to vesicle for the ionic liquids with dodecylbenzenesulfonate as anion and 1-butyltrimethylammonium, 1-butyl-3-methylimidazolium or 1-butylpyridinium as cation in aqueous solution. Yan et al.⁴³ constructed light-responsive wormlike micelle consisting of ionic liquid surfactant N-methyl-N-cetylpyrrolidinium bromide (C₁₆MPBr) and sodium (4-phenylazo-phenoxy)-acetate (AzoNa) in water. However, to the best of our knowledge, there are no reports on the formation of pH-responsive vesicles and structural transition for single-chain ionic liquids surfactants in the presence of hydrotrope, which is an amphiphilic compound and its hydrophobic moiety is smaller than the typical surfactants, e.g., the number of methylene units in their alkyl chain does not exceed 7. It is expected that pH-responsive vesicles can be formed in the mixtures of [C_nmim]Br (n = 12, 14) ionic liquid surfactant and commercially available hydrotropes with different pH-responsive groups (such as –COOH, –NH₂, and –ArOH), because these hydrotropes can strongly bind to the imidazolium ring non-covalently by electrostatic attraction, π–π interaction and hydrophobic effect, which greatly change the self-assembly structure of ILs in solution.³²

In this work, pH-responsive vesicles were designed with commonly used ionic liquids [C_nmim]Br (n = 12, 14) and commercially available hydrotropes such as potassium hydrogen phthalate ([C₆H₄COOKCOOH]), sodium sulfosalicylate ([C₆H₃OHCOOHSO₃Na]) and m-carboxylbenzenesulfonate sodium ([C₆H₄COOHSO₃Na]). The effect of solution pH value on the size and morphology (micelle or vesicle) of the ILs self-assembly was investigated by surface tension, dynamic light scattering (DLS), cryogenic-transmission electron microscopy (Cryo-TEM), small-angle X-ray scattering (SAXS), polarized optical microscopy and nuclear magnetic resonance (NMR) spectroscopy. And then, possible mechanism for the self-assembling structural changes was analyzed from the variation in molecular structure and hydrophilicity/hydrophobicity of the hydrotropes with solution pH value.

Materials and methods

Chemicals

1-Methylimidazole (99%) was acquired from Shanghai Chem. Co.; 1-bromobutane (99%), 1-bromohexane (99%), 1-bromooctane (99%), 1-bromodecane (99%), 1-bromododecane (99%) and 1-bromotetradecane (99%) were purchased from Alfa Aeser. Potassium hydrogen phthalate (99%), sodium sulfosalicylate (99%) and m-carboxylbenzenesulfonate sodium (99%) were all obtained from Alfa Aeser. These chemicals were used as received. [C_nmim]Br (n = 12, 14) were prepared by using the procedures described in the literature.⁴⁴

Stock solutions of the ILs (around 0.03 mol L⁻¹) were prepared and the test samples were obtained by successive dilution of the stock solutions. Samples with required pH values were prepared by adding a small amount of hydrochloric acid or sodium hydroxide into aqueous solutions. pH measurements were conducted by using a Rex model PHSJ-4F digital pH meter (Leici, China).

¹H NMR spectra of the ILs in deuterated water were obtained at 25 °C on a Bruker Avance-400 NMR spectrometer operating at 400.13 MHz, and TMS was used as an internal standard. The solution pH was adjusted by adding deuterated hydrochloric acid (DCl) or deuterated sodium hydroxide (NaOD).

Surface tensions of aqueous ILs solutions were determined by a DCA 315 tensiometer (Cahn Instruments) with a platinum plate (20 × 15 × 0.127 mm³) at 25.0 °C and different solution pH values. The temperature around the sample cell was controlled by circulating water from a HAAKE DC30-K20 thermostat (Thermo Electron, Germany), and the temperature was maintained to be within ±0.1 °C. All measurements were repeated three times to allow the determination of an average surface tension value. The tensiometer was calibrated with double distilled water according to the method provided by the manufacturer, and the uncertainty of the surface tension data was estimated to be around ±0.1 mJ m⁻².

The DLS measurements were performed using a laser light scattering photometer (Nano-ZS90, Malvern, U. K) at 25.0 °C and 90° scattering angle. The light source was a solid-state He–Ne laser operated at 633 nm with maximum output power of 4.0 mW. A 0.22 μm hydrophilic PVDF membrane filter was used to filter all sample solutions. At least three measurements were carried out for each solution, and the reproducibility of self-assembly sizes from DLS data was found to be within ±3%.

For the Cryo-TEM measurements, 5 μL solutions were loaded on a TEM grid by a micropipette, and a thin film was produced by blotting off the redundant liquid with two pieces of filter papers. After the excess of solution was blotted away to form a thin liquid film, the grid was immediately plunged into liquid ethane cooled by liquid nitrogen (−175 °C). The samples were maintained at approximately −175 °C and imaged in a transmission electron microscope (JEOL JEM1400) at an accelerating voltage of 200 kV under low dose conditions.

SAXS experiments were carried out on a SAXSpace small-angle X-ray scattering instrument (Anton Paar, Austria, Cu-Kα), equipped with a Kratky block-collimation system at 25 °C. The incident X-ray wavelength was 0.154 nm. The X-ray intensities were recorded by an imaging-plate detection system with a pixel size of 42.3 × 42.3 μm². Polarized optical microscopy observation was performed by a Motic B2 polarized optical microscope to determine the lyotropic liquid crystals phase of solutions.

Results and discussion

The effect of solution pH on the critical aggregation concentrations of the ILs in aqueous solution

Fig. 1 shows the plots of solution surface tension of [C₁₂mim]Br versus its concentrations in the presence of 30 mM of [C₆H₄COOKCOOH] at different solution pH values. The critical aggregation concentrations (CACs) of the IL in aqueous [C₆H₄COOKCOOH] solutions were determined through the second derivative of surface tension versus IL concentrations. It is noted from Fig. 1 that the effect of solution pH value on CAC values of [C₁₂mim]Br is negligible within the experimental uncertainty. Similar phenomena have been observed for the other mixtures of hydrotropes and ionic liquids. Therefore, we determined the CAC values of the ILs in aqueous solution at the hydrotrope concentration of 30 mM and pH 7.0. The CAC values thus obtained are included in Table 1. It is known that the salt affects the micelle behavior of surfactants in water in two different ways, salt-out effect and salt-in effect. When the added salt can reduce the electrostatic repulsion among the surfactant head groups and therefore decrease CAC of ionic surfactants, the effect of salt is called as salt-out. On the contrary, the effect of salt is called as salt-in. It can be seen from Table 1 that the addition of hydrotrope decreases CAC values of the ILs in water, suggesting that the hydrotropes have salt-out effect on the ILs in water. For a given cation, the CAC values of ILs decrease in the order: [C₆H₃OHCOOSO₃Na]⁻ < [C₆H₄COOSO₃Na]⁻ < [C₆H₄COOKCOO]⁻. This suggests that the ability of these anions to promote self-assembly structure formation follows the order: [C₆H₃OHCOOSO₃Na]⁻ > [C₆H₄COOSO₃Na]⁻ > [C₆H₄COOKCOO]⁻.

Fig. 1 Variation of surface tension of aqueous [C₁₂mim]Br solutions in the presence of 30 mM [C₆H₄COOKCOOH] as a function of IL concentrations at given solution pH values.

Table 1 Critical Aggregation Concentration (CAC) of [C_nmim]Br (n = 12, 14) in aqueous solutions in the presence of 30 mM hydrotrope at pH 7.0 and 298.2 K

IL	Hydrotrope	CAC/mol L^−1
[C12mim]Br	[C6H4COOKCOOH]	1.6 × 10^−4
	[C6H3OHCOOHSO3Na]	1.1 × 10^−4
	[C6H4COOHSO3Na]	7.4 × 10^−5
[C14mim]Br	[C6H4COOKCOOH]	7.3 × 10^−5
	[C6H3OHCOOHSO3Na]	5.4 × 10^−5
	[C6H4COOHSO3Na]	1.3 × 10^−5

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pH switching for the transition between micelle and vesicle of the ILs in the presence of hydrotrope

Fig. 2 shows the turbidity change with solution pH value for solutions of 10 mM [C₁₂mim]Br in the presence of different concentrations of [C₆H₄COOKCOOH] (10, 20, 30 mM) determined by UV-vis measurements. It can be seen from Fig. 2 that with the increase of solution pH value, solution turbidity firstly increased and then decreased, leading to a maximum at a particular pH value. This suggests that self-assembly structure of the ILs changes with the variety of solution pH. Similar situations were found for [C_nmim]Br (n = 12, 14) in the presence of other hydrotropes. The pH value corresponding to the maximum of turbidity of aqueous [C₁₂mim]Br in the presence of [C₆H₄COOKCOOH], [C₆H₃OHCOOHSO₃Na] or [C₆H₄COOHSO₃Na] was 4.1, 3.5 and 2.0, respectively. Interestingly, these turbid solutions were stable for a few weeks at the pH values mentioned. This is probably an indicative for the formation of vesicles. To verify this speculation, structures of aqueous solution of [C_nmim]Br (n = 12, 14) in the presence of hydrotropes were investigated by Cryo-TEM technique. It was found that for our bluish sample, unilamellar vesicles were really observed. Fig. 3b shows the unilamellar vesicles of [C₁₄mim]Br in the presence of 30 mM [C₆H₄COOKCOO] at the IL concentration of 10 mM and pH 4.1. To further confirm the formation of vesicle, small-angle X-ray scattering measurements were also performed. Fig. 4 shows the SAXS patterns obtained at 10 mM [C₁₄mim]Br, pH 4.1 and 25 °C. The experimental data were well fitted by a vesicle model with a single diffuse lamellar shell.⁴⁵ The structure of the shell was simulated by assuming a Gaussian profile for the excess electronic density.⁴⁵ Then, the related vesicle diameter, shell thickness and diameter polydispersity were found to be 90.3 ± 10.2 nm, 9.9 ± 1.1 nm and 0.17 ± 0.03, respectively. The fitting in Fig. 4 is in good agreement with unilamellar vesicles reported by Villa and his co-workers from SAXS measurements.⁴⁵

Fig. 2 Variation of turbidity of aqueous 10 mM [C₁₂mim]Br solutions in the presence of different concentrations of [C₆H₄COOKCOOH] as a function of solution pH values: ■ 10 mM; ● 20 mM; ▲30 mM.

Fig. 3 Cryo-TEM images for the aggregates of [C₁₄mim]Br in aqueous 30 mM [C₆H₄COOKCOOH] solution at the IL concentration of 10 mM and different pH values: (a) 1.36, (b) 4.10, and (c) 5.70.

Fig. 4 SAXS data for [C₁₄mim]Br in aqueous 30 mM [C₆H₄COOKCOOH] solution at the IL concentration of 10 mM, pH 4.1 and 25 °C. The line was fitted by using a vesicle model.

It is known that Na⁺ or K⁺ and Br⁻ were existed in aqueous ionic liquid solutions in combination with hydrotropes. Thus one may wonder if the vesicle formation was resulted from the salting effect of NaBr or KBr? To clarify this point, NaBr and KBr were used to examine the effect of electrolyte addition on the aggregate structure and size of [C₁₄mim]Br at IL concentration of 10 mM. From DLS and Cryo-TEM measurements, it was found that with the increase of NaBr or KBr concentrations, the size of the ILs aggregates increased gradually, but turbid solutions and the vesicle structure were not observed at NaBr or KBr concentration up to 1 mol L⁻¹. This indicates that the vesicle formation cannot be ascribed to the salting effect of NaBr or KBr.

To the best of our knowledge, this is the first observation for conventional single-chain ionic liquid surfactants (not the functionalized ones) to self-assemble into stable vesicles induced by solution pH change in the presence of hydrotropes. For example, with the change of solution pH, self-assembly structure of traditional alkyltrimethylammonium surfactants in aqueous solutions can only change from spherical micelle to wormlike micelle in the presence of [C₆H₄COOKCOOH], [C₆H₃OHCOOHSO₃Na] or [C₆H₄COOHSO₃Na].⁴⁶ Obviously, the difference in self-assembly structure of our ionic liquid surfactants with traditional ionic surfactants is ascribed to the unique structure of imidazolium cation of the ILs. Compared with the conventional alkyltrimethylammonium surfactants, cations of the imidazolium ILs have less headgroup repulsion due to lower electron density on the headgroup and favorable π–π interaction between the headgroups and the aromatic ring of hydrotropes,⁴⁶ thus resulting in the enhanced stack of headgroups. As a result, the vesicle was formed more easily for IL surfactants than for conventional alkyltrimethylammonium surfactants. In addition, spherical micelle was observed by Cryo-TEM in the lower turbidity solution containing 10 mM [C₁₄mim]Br and 30 mM [C₆H₄COOKCOOH] at pH values of 1.36 and 5.70 (see Fig. 3a and c), which was also confirmed by SAXS measurements (see Fig. S1–S2 in ESI†). This means that when solution pH value increased from 1.36 and 4.10 to 5.70, the self-assembly morphology underwent the transition from spherical micelle to vesicle and then to spherical micelle again. The transition between micelle and vesicle was reversible for at least four cycles by the alternative addition of several drops of aqueous HCl or NaOH solutions to switch pH value between 1.36 and 5.70. Furthermore, morphology of the vesicles was kept unchanged during the cycles according to TEM and SAXS measurements. It was also found that the vesicle and micelle formed from the ILs in aqueous [C₆H₄COOKCOOH] solution were quite stable after a few weeks storage at room temperature. These results suggest that the vesicle and micelle are thermodynamically stable and their transition takes place under thermodynamic equilibrium.

DLS measurement was further performed to determine the effect of pH value on the size of self-assembly structures. It is clearly indicated that with increasing solution pH value, size of [C₁₄mim]Br aggregates firstly increased and then decreased, and leading to a maximum size at a particular pH value (see Fig. 5). Even a small pH change could induce a remarkable difference in the aggregate size. For example, when pH value varied from 1.01 to 4.10, diameter of the aggregates changed from 20.1 to 90.2 nm in aqueous solution containing 10 mM [C₁₄mim]Br and 30 mM [C₆H₄COOKCOOH]. In addition, in order to evaluate the stability of the vesicle and micelle during the cycle, reversible change of the size of [C₁₄mim]Br aggregates in aqueous 30 mM [C₆H₄COOKCOOH] solution was also detected by DLS as a function of solution pH in four cycles at the IL concentration of 10 mM (see Fig. 6). It can be seen that the size of vesicle and micelle was kept unaltered during the cycles, suggesting again that self-assembly structure of the ILs was highly stable.

Fig. 5 The variation of the aggregate diameter of [C₁₄mim]Br in aqueous 30 mM [C₆H₄COOKCOOH] solutions as a function of pH value at the IL concentration of 10 mM.

Fig. 6 The reversible change of the size of [C₁₄mim]Br aggregates in aqueous 30 mM [C₆H₄COOKCOOH] solution with solution pH value at the IL concentration of 10 mM.

Temperature has an important effect on the aggregate structure of ILs in aqueous solution. Therefore, the change of aggregate structures of [C₁₄mim]Br in aqueous solution containing 10 mM of [C₁₄mim]Br and 30 mM of [C₆H₄COOKCOOH] at pH 1.36, 4.10 and 5.70 were investigated by DLS, SAXS and polarized optical microscopy in the temperatures range from 5 to 45 °C. It can be seen from Fig. 7 that the size of [C₁₄mim]Br aggregate decreased with the increase of temperature in the studied systems. However, for the sample at pH 4.10, the scattering peaks in the SAXS curves (Fig. 8) disappeared gradually as the temperature increases from 5 to 45 °C, indicating the change of [C₁₄mim]Br aggregate structure from vesicle to micelle. In addition, from SAXS and polarized optical microscopy measurements, the lyotropic liquid crystals of all the studied systems were not observed with the decrease of temperature from 45 to 5 °C.

Fig. 7 The size change of [C₁₄mim]Br aggregates with temperature in aqueous solutions containing 10 mM of IL and 30 mM of [C₆H₄COOKCOOH] at different pH values.

Fig. 8 SAXS curves for [C₁₄mim]Br in aqueous solution containing 10 mM of IL and 30 mM of [C₆H₄COOKCOOH] at pH 4.1 as a function of temperature.

Possible mechanism for pH induced structural transition of the ILs in the presence of hydrotrope

The hydrotropes ([C₆H₄COOKCOOH], [C₆H₃OHCOOHSO₃Na] and [C₆H₄COOHSO₃Na]) investigated here have pH-responsive groups, such as carboxylic, phenolic or sulfonic groups, for [C₆H₄COOHCOOH], [C₆H₃OHCOOHSO₃H] and [C₆H₄COOHSO₃H], their pK_a1 values in aqueous solution are, respectively, 2.89, 2.62 and 0.58, and their pK_a2 values are 5.51, 11.95 and 3.90, respectively.^46–48 Under the influence of solution pH value, they exhibit different structural forms in water at different solution pH values, which will strongly influence the interactions between the IL and hydrotrope, and sequentially change the self-assembly structure of the ILs in water. Considering the fact that NMR is a powerful means for the investigation of the interactions among substances, ¹H NMR experiments were carried out at different pH values to better understand the molecular interaction between the ILs and hydrotrope. Fig. 9 presents the chemical structure and the H atom numbering of [C₆H₄COOHCOOH], which was selected as a representative hydrotrope for NMR studies. Fig. 10 shows the proton resonance of [C₆H₄COOHCOOH] at 10 mM of [C₁₄mim]Br and 30 mM of [C₆H₄COOHCOOH] in the pH range from 2.35 to 6.48. It was found that the H1 proton firstly shifts to upfield until pH value of 3.84, and then to downfield with the increase of pH value. However, the reverse was observed for the shift of H2 proton. The shift of ¹H NMR signals can be rationalized as follows: there are two carboxyl acid groups in phthalic acid, which can be ionized depending on solution pH value. When pH value is lower than pK_a1 (2.89), most of phthalic acid exists as neutral molecules.⁴⁶ With the increase of solution pH value from pK_a1 (2.89) to pK_a2 (5.51), a number of phthalic acid appear with one ionized carboxylic acid moiety. This special molecular structure makes phthalic acid to be bond to the head of [C₁₄mim]⁺ cation with its aromatic ring penetrating into hydrocarbon chains through hydrophobic effect and electrostatic attraction, resulting in an non-polar environment in which phthalic acid was located.⁴⁹ Further increase of pH value will result in the transition of phthalic acid from one ionized carboxylic acid moiety to two ionized carboxylic acid moiety. Due to the stronger electrostatic attraction, phthalic acid was bond to the head of [C₁₄mim]⁺ cation with its aromatic ring present in water, leading to the involvement of phthalic acid in an polar environment.⁵⁰ Therefore, H1 proton shifts to upfield firstly and then to downfield.⁴⁷ However, with the increase of solution pH value, the electrostatic interaction between phthalic acid with one ionized carboxylic acid moiety and imidazolium cation makes the electron density of H2 proton to decrease, leading to the proton shift to downfield. When phthalic acid exists in two ionized carboxylic acid moiety, H2 proton in phthalic acid was present in water environment because of the stronger interaction between phthalic acid and the cation as well as the enhanced hydrophilicity of phthalic acid, which leads to an increase in electron density of H₂ proton and thus a upfield shift.⁴⁶

Fig. 9 The chemical structure and H atom numbering of [C₆H₄COOHCOOH].

Fig. 10 The proton chemical shifts of [C₆H₄COOHCOOH] as a function of pH value.

It was known that the self-assembly formation of ILs in aqueous solution is dictated by a balance between the repulsive headgroup interactions and the attractive forces arising from a need to minimize the exposure of hydrophobic core to water.²⁹ Due to the electrostatic interaction between [C₁₄mim]⁺ and phthalic acid with one ionized carboxylic acid, the electrostatic repulsion between IL headgroups was screened and the hydrocarbon chain packing becomes tighter. According to the molecular packing parameter model,^50,51 the larger sized aggregates performed when a tight molecular packing was achieved. Compared with phthalic acid with one ionized carboxylic acid moiety, phthalic acid with two ionized carboxylic acid moiety has stronger hydrophilicity, and cannot effectively bind to the IL headgroups. As a result, the electrostatic repulsion between IL headgroups cannot be shield and a loose packing between hydrocarbon chains was adapted. Therefore, the molecular number of phthalic acid containing one ionized carboxylic acid increases with increasing pH value, resulting in the growth of ILs aggregates and transition of spherical micelles into vesicles at a certain pH value. As pH value further increases, the molecular number of phthalic acid containing two ionized carboxylic acid moiety increases as well. This particular molecular structure disfavors the formation of vesicle, leading to the transition from vesicle to spherical micelle. Thus it seems likely that pH triggered structural change of ionic liquids aggregate in aqueous hydrotrope solutions was resulted from the pH dependent hydrophilicity/hydrophobicity of the hydrotrope and its binding ability to ionic liquid headgroup.

Conclusions

In summary, a class of 6 novel pH-responsive fluids has been designed in the present work. Such pH-responsive fluids have three main advantages: (1) they can be easily prepared by the use of conventional ILs and cheap and commercially available hydrotrope; (2) they can self-assemble into micelle and vesicle in aqueous solutions, and the structural transition between micelle and vesicle can be reversibly switched by solution pH; (3) the pH range can be tuned by using different hydrotropes. It is shown that the transition between micelle and vesicle induced by solution pH is ascribed to the change in the chemical structure and hydrophilicity/hydrophobicity of the hydrotropes under different pH conditions. These findings can shed light on the simple and promising strategy for fabricating pH-responsive self-assembly structures based on the commonly used ionic liquids and commercial hydrotropes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21273062, 21133009 and 21203057).

References

1. I. K. Voets, A. de Keizer and M. A. C. Stuart, Adv. Colloid Interface Sci., 2009, 147−148, 300–318.
2. D. Langevin, Adv. Colloid Interface Sci., 2009, 147−148, 170–177.
3. A. H. Gröschel, A. Walther, T. I. Löbling, J. Schmelz, A. Hanisch, H. Schmalz and A. H. E. Müller, J. Am. Chem. Soc., 2012, 134, 13850–13860.
4. C. de las Heras Alarcón, S. Pennadam and C. Alexander, Chem. Soc. Rev., 2005, 34, 279–285.
5. S. L. Huang and R. C. MacDonald, Biochim. Biophys. Acta, 2004, 1665, 134–141.
6. C. Park, K. Lee and C. Kim, Angew. Chem., Int. Ed., 2009, 48, 1275–1278.
7. Y. Zhao, Chem. Rec., 2007, 7, 286–294.
8. H. Kim, T. Kim and M. Lee, Acc. Chem. Res., 2011, 44, 72–82.
9. J. F. Lutz, Adv. Mater., 2011, 23, 2237–2243.
10. Z. Zhou, S. Zhu and D. Zhang, J. Mater. Chem., 2007, 17, 2428–2433.
11. B. S. Tian and C. Yang, J. Phys. Chem. C, 2009, 113, 4925–4931.
12. Y. Z. You, K. K. Kalebaila, S. L. Brock and D. Oupicky, Chem. Mater., 2008, 20, 3354–3359.
13. C. Park, K. Oh, S. C. Lee and C. Kim, Angew. Chem., Int. Ed., 2007, 46, 1455–1457.
14. C. H. Lee, L. W. Lo, C. Y. Mou and C. S. Yang, Adv. Funct. Mater., 2008, 18, 3283–3292.
15. B. Wang, C. Xu, J. Xie, Z. Yang and S. Sun, J. Am. Chem. Soc., 2008, 130, 14436–14437.
16. R. Casasús, E. Climent, M. D. Marcos, R. Martínez-Mánez, F. Sancenón, J. Soto, P. Amorós, J. Cano and E. Ruiz, J. Am. Chem. Soc., 2008, 130, 1903–1917.
17. Z. Chu and Y. Feng, Chem. Commun., 2010, 9028–9030.
18. E. R. Gillies, T. B. Jonsson and J. M. J. Fréchet, J. Am. Chem. Soc., 2004, 126, 11936–11943.
19. M. Antonietti and S. Förster, Adv. Mater., 2003, 15, 1323–1333.
20. I. W. Hamley, Soft Matter, 2005, 1, 36–43.
21. Q. Gao, Y. Xu, D. Wu, W. L. Shen and F. Deng, Langmuir, 2010, 26, 17133–17138.
22. A. Ghosh, M. Haverick, K. Stump, X. Yang, M. F. Tweedle and J. E. Goldberger, J. Am. Chem. Soc., 2012, 134, 3647–3650.
23. C. R. López-Barrón, D. Li, L. Derita, M. G. Basavaraj and N. Wagner, J. Am. Chem. Soc., 2012, 134, 20728–20732.
24. H. Kawasaki, M. Souda, S. Tanaka, N. Nemoto, G. Karlsson, M. Almgren and H. Maeda, J. Phys. Chem. B, 2002, 106, 1524–1527.
25. J. M. Gebicki and M. Hicks, Nature, 1973, 243, 232–237.
26. K. Edwards, M. Silvander and G. Karlsson, Langmuir, 1995, 11, 2429–2434.
27. F. Blochiner, M. Blocher, P. Walde and P. L. Luisi, J. Phys. Chem. B, 1998, 102, 10284–10287.
28. H. Niedermeyer, J. P. Hallett, I. J. Villar-Garcia, P. A. Hunt and T. Welton, Chem. Soc. Rev., 2012, 41, 7780–7802.
29. H. Y. Wang, J. J. Wang, S. B. Zhang and X. P. Xuan, J. Phys. Chem. B, 2008, 112, 16682–16689.
30. J. Wang, H. Wang, S. Zhang, H. Zhang and Y. Zhao, J. Phys. Chem. B, 2007, 111, 6181–6188.
31. J. Bowers, C. P. Butts, P. J. Martin, M. C. Vergara-Gutierrez and R. K. Heenan, Langmuir, 2004, 20, 2191–2198.
32. H. Wang, Q. Feng, J. Wang and H. Zhang, J. Phys. Chem. B, 2010, 114, 1380–1387.
33. B. L. Bhargava and M. L. Klein, Soft Matter, 2009, 5, 3475–3480.
34. X. Q. Fan and K. S. Zhao, Soft Matter, 2014, 10, 3259–3270.
35. N. A. Smirnova, A. A. Vanin, E. A. Safonova, I. B. Pukinsky, Y. A. Anufrikov and A. L. Makarov, J. Colloid Interface Sci., 2009, 336, 793–802.
36. J. Yuan, X. Bai, M. Zhao and L. Zheng, Langmuir, 2010, 26, 11726–11731.
37. K. Singh, D. G. Marangoni, J. G. Quinn and R. D. Singer, J. Colloid Interface Sci., 2009, 335, 105–111.
38. S. Ghosh, C. Ghatak, C. Banerjee, S. Mandal, J. Kuchlyan and N. Sarkar, Langmuir, 2013, 29, 10066–10076.
39. S. Mandal, J. Kuchlyan, D. Banik, S. Ghosh, C. Banerjee, V. Khorwal and N. Sarkar, ChemPhysChem, 2014, 15, 3544–3553.
40. K. S. Rao, T. Singh and A. Kumar, Langmuir, 2011, 27, 9261–9269.
41. Y. Gu, L. Shi, X. Cheng, F. Lu and L. Zheng, Langmuir, 2013, 29, 6213–6220.
42. K. S. Rao, P. S. Gehlot, H. Gupta, M. Drechsler and A. Kumar, J. Phys. Chem. B, 2015, 119, 4263–4274.
43. H. Yan, Y. Long, K. Song, C. Tung and L. Zheng, Soft Matter, 2014, 10, 115–121.
44. J. D. Holbrey and K. R. Seddon, J. Chem. Soc., Dalton Trans., 1999, 2133–2140.
45. C. C. Villa, F. Moyano, M. Ceolin, J. J. Silber, R. D. Falcone and N. M. Correa, Chem.–Eur. J., 2012, 18, 15598–15601.
46. Y. Lin, X. Han, J. Huang, H. Fu and C. Yu, J. Colloid Interface Sci., 2009, 330, 449–455.
47. Y. Yang, J. Dong and X. Li, J. Colloid Interface Sci., 2012, 380, 83–89.
48. CRC Handbook of Chemistry and Physics, ed. M. H. William, CRC Press, 92nd edn, 2011.
49. T. Shikata, H. Hirata and T. Kotaka, Langmuir, 1988, 4, 354–359.
50. S. J. Bachofer, U. Simonis and T. A. Nowicki, J. Phys. Chem., 1991, 95, 480–488.
51. Y. Yan, W. Xiong, X. Li, T. Lu, J. Huang, Z. Li and H. Fu, J. Phys. Chem. B, 2007, 111, 2225–2230.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12010a

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