Article pubs.acs.org/Langmuir

Self-Assembly of Azobenzene Bilayer Membranes in Binary Ionic Liquid−Water Nanostructured Media Tejwant Singh Kang,†,∥ Keita Ishiba,† Masa-aki Morikawa,†,‡,§ and Nobuo Kimizuka*,†,‡,§ †

Department of Chemistry and Biochemistry, Graduate School of Engineering, and ‡Center for Molecular Systems (CMS), Kyushu University, and §JST CREST, 744-Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Department of Chemistry, Guru Nanak Dev University (GNDU), Amritsar 143005, Punjab, India S Supporting Information *

ABSTRACT: Anionic azobenzene-containing amphiphile 1 (sodium 4-[4-(N-methyl-N-dodecylamino)phenylazo]benzenesulfonate) forms ordered bilayer membranes in binary ionic liquid (1-ethyl-3-methylimidazolium ethyl sulfate, [C2mim][C2OSO3])−water mixtures. The binary [C2mim][C2OSO3]− water mixture is macroscopically homogeneous at any mixing ratio; however, it possesses fluctuating nanodomains of [C2mim][C2OSO3] molecules as observed by dynamic light scattering (DLS). These nanodomains show reversible heat-induced mixing behavior with water. Although the amphiphile 1 is substantially insoluble in pure water, it is dispersible in the [C2mim][C2OSO3]−water mixtures. The concentration of [C2mim][C2OSO3] and temperature exert significant influences on the self-assembling characteristics of 1 in the binary media, as shown by DLS, transmission electron microscopy (TEM), UV−vis spectroscopy, and zeta-potential measurements. Bilayer membranes with rodor dotlike nanostructures were formed at a lower content of [C2mim][C2OSO3] (2−30 v/v %), in which azobenzene chromophores adopt parallel molecular orientation regardless of temperature. In contrast, when the content of [C2mim][C2OSO3] is increased above 60 v/v %, azobenzene bilayers showed thermally reversible gel-to-liquid crystalline phase transition. The self-assembly of azobenzene amphiphiles is tunable depending on the volume fraction of [C2mim][C2OSO3] and temperature, which are associated with the solvation by nanoclusters in the binary [C2mim][C2OSO3]−water media. These observations clearly indicate that mixtures of water-soluble ionic liquids and water provide unique and valiant environments for ordered molecular self-assembly.

1. INTRODUCTION Ionic liquids (ILs) are attracting much interest in many disciplines of chemistry due to their unique properties as exemplified by very low vapor pressure, excellent thermal stability, and favorable solvating properties for a range of polar and nonpolar materials.1 Application of ILs have been focused on their use as green alternative solvents for chemical synthesis and separation and also as electrolytes in electrochemical devices.1,2 Meanwhile, self-assembly in ILs and their application to materials chemistry have been emerging as a promising area of research. Although ILs exhibit smaller cohesive energy compared to water,3,4 formation of ordered molecular assemblies has been demonstrated for bilayer membranes5−8 and low-molecular weight ionogels.5,8−10 These reports gave rise to the studies of IL-based amphiphiles self-assembled in water11−14 and ILs.15,16 The next confronting issues obviously involve uncovering self-assembling phenomena which are specific to IL−water binary mixtures. The mixtures of watersoluble IL and water have been recognized to display unique physicochemical properties which are different from the pure counterparts.17,18 They would be related to formation of solvent clusters19 and bicontinuous superstructures20,21 in binary aqueous IL mixtures, which vary depending on the © 2014 American Chemical Society

component chemical structures of IL, composition of the binary mixtures, and physical conditions such as temperature. However, understanding of ordered self-assembling phenomena in such binary IL−water systems has been largely lagging behind. The formation of clusters and bicontinuous structures of IL molecules in aqueous systems and their influence on molecular self-assembly are of pivotal importance not only as fundamental science but also for their wide range of technological applications. To get insight into the self-assembly phenomena in binary IL−water media, an anionic azobenzene-containing amphiphile 1 (sodium 4-[4-(N-methyl-N-dodecylamino)phenylazo]benzenesulfonate, Scheme 1) was employed in this study. As a hydrophilic IL, [C2mim][C2OSO3] was selected because of its chemical stability and miscibility with water over the whole concentration range.21,22 A sulfonate headgroup is introduced in the azobenzene amphiphile 1, in expectation that electrostatic binding of the [C2mim]+ ion to the sulfonate group influences molecular orientation of the azobenzene amphiReceived: January 4, 2014 Revised: February 14, 2014 Published: February 15, 2014 2376

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[C2mim][C2OSO3] in the binary mixture, above 2 v/v %), [C2mim]+ ion is present in large excess compared to 1 or 2 (Supporting Information, Table S1 and the discussion on page S9). 2.2. Methods. Dynamic light scattering (DLS) and zeta(ζ)potential measurements were performed on a Zeta-Sizer Nanoseries (nano-ZS) instrument (Malvern Instruments) equipped with built-in temperature controller having an accuracy of ±0.1 K. The samples were contained in small volume quartz cuvettes, and an average of 10 measurements has been adopted as DLS data. ζ-Potential data were obtained by repeating 10 sets of measurements whose results were statistically averaged. Dark field optical microscopy (DFOM) was performed using a Nikon optical microscope (Nikon Eclipse 80i) equipped with a dark field condenser. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010 electron microscope at a working voltage of 120 kV. Samples was prepared by putting a drop of various dispersions of amphiphile 1 (500 μM) on the carbon-coated copper grid. Residual liquid was blotted, and the samples were poststained with uranyl acetate. The UV−vis absorption spectra were measured using a JASCO 670 spectrophotometer, equipped with an built-in temperature controller having an accuracy of ±0.1 K, using a quartz cuvette (path length, 1 mm). Samples were equilibrated for 10 min to reach the thermal equilibrium before each measurement. 1H NMR spectra were recorded using a Brüker 300 MHz spectrometer. The proton chemical shifts were referenced with respect to an external standard D2O.

Scheme 1. Molecular Structures of Azobenzene Amphiphile 1, Hydrophilic Ionic Liquid [C2mim][C2OSO3], and Methyl Orange 2a

a

Methyl orange was employed as a monomeric standard.

philes. We have previously reported that molecular orientation of chromophore-containing synthetic bilayer membranes showed changes in binary water−ethanol systems specifically at an ethanol content of ∼40 vol%.23−25 The chromophorecontaining amphiphiles provide us means to investigate influences of solvent molecules on the molecular self-assembly, because their interactions can be effectively reflected in the spectral characteristics. Self-assembly of azobenzene amphiphiles has been widely studied for a wide range of molecular systems such as monolayers at air−water interfaces, Langmuir− Blodgett films, aqueous micelles, and bilayer membranes.23,26−30 This is because azobenzene chromophores possess π−π* absorption bands in the ultraviolet (UV) and visible (vis) region, which are associated with transition dipole moments oriented parallel to the short and long axes of the molecule. When azobenzene chromophores regularly aligned in molecular assemblies are photoexcited, the resultant excited state delocalizes among strongly interacting chromophores, and these exciton interactions cause pronounced spectral blue or red shifts depending on the molecular orientation (i.e., parallel orientation, H-aggregates or head-to-tail orientation, J-aggregates).30−32 In this study, UV−vis spectroscopy of azobenzene amphiphiles was combined with dynamic light scattering (DLS), dark-field optical microscopy, transmission electron microscopy, 1H NMR spectroscopy, and zeta-potential measurements to comprehensively investigate their molecular assembling characteristics in binary [C2mim][C2OSO3]−water systems.

3. RESULTS AND DISCUSSION 3.1. Dynamic Light Scattering (DLS) Measurements. Scheme 1 shows molecular structures of azobenzene-containing amphiphile 1 (sodium 4-[4-(N-methyl-N-dodecylamino)phenylazo]benzenesulfonate) and IL (1-ethyl-3-methylimidazolium ethyl sulfate, [C2mim][C2OSO3]) used in this study. Methyl orange 2 was employed as a monomeric standard because its chromophore structure is identical to that of 1. Although [C2mim][C2OSO3] is totally miscible with water, formation of clusters and bicontinuous network structures has been indicated for [C2mim][C2OSO3]−water mixtures based on the molecular dynamic simulation.21 To verify the presence of nanostructures in the bulk aqueous [C2mim][C2OSO3], dynamic light scattering (DLS) was measured. As shown in Figure S1 (Supporting Information), nanosized domains were observed for the [C2mim][C2OSO3]−water mixtures at ambient temperature (without 1, [C2mim][C2OSO3] = 30 and 60 v/v %). Scattering intensity with hydrodynamic diameters Dh of ca. 160 and 300 nm were observed for aqueous solutions containing 30 and 60 v/v % of [C2mim][C2OSO3], respectively (Figure S1A,C, Supporting Information). These nanostructures would be populated by association of alkyl side chains of imidazolium cations in water, because formation of temporally fluctuating nanoaggregates has been also indicated for the aqueous solution of more hydrophilic 1ethyl-3-methylimidazolium methyl sulfate [C2mim][C1SO3].19 Interestingly, the scattering intensity disappeared upon heating to 75 °C, indicating temperature-driven mixing of these nanostructures with water. The correlation constant shown in Figure S1B (Supporting Information), which is a measure of intensity fluctuations in number of photons arriving at a detector at a set time interval, similarly decreased upon elevating the temperature. It supports the decrease in number of nanoaggregates, which may show structural fluctuation especially at higher temperatures. Although some macroscopic liquid−liquid binary phase of IL−water has been reported to exert temperature-driven mixing (upper critical solution temperature-type phase change),18 the present data clearly demonstrate that even the macroscopically homogeneous,

2. MATERIALS AND METHODS 2.1. Materials. High purity ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate, [C2mim][C2OSO3], was purchased from Kanto Chemical Company, Inc., and used after drying at 60−70 °C for 2 days. Karl Fischer titration indicated water content of ca. 500 ppm. The azobenzene amphiphile 1 (Scheme 1) was synthesized according to the procedure described in the literature.28 1 was purified by recrystallization from ethanol and analyzed through 1H NMR and mass spectroscopy. 1 was dissolved in methanol to give stock solutions, and the weighted amounts of methanol solutions were evaporated to dryness in a sample cuvette under flow of nitrogen. Dispersions of 1 in [C2mim][C2OSO3]−water binary solvent mixtures (content of [C2mim][C2OSO3], 2, 10, 30, 60, and 90 v/v %) were prepared by adding the required amounts of binary [C2mim][C2OSO3]−water mixtures to make a specified final concentration of 1. The 10, 30, 60, and 90 v/v % of [C2mim][C2OSO3] correspond to 1.0, 3.9, 12.4, and 46.0 mol %, respectively, at 20 °C (Table S1 in Supporting Information, the density of [C2mim][C2OSO3]; 1.24 g· cm−3).33 An equimolar ratio (50 mol % of [C2mim][C2OSO3]) is attained at ca. 91.3 v/v % [C2mim][C2OSO3] in water. The dispersions were then ultrasonicated for 5 min to ensure macroscopic homogeneity. For all the measurements, the concentration of 1 was kept at 500 μM (above critical aggregate concentrations, cac), if not stated otherwise. Under the experimental conditions (content of 2377

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[C2mim][C2OSO3]), respectively, which are ascribed to cac (critical aggregate concentration, Figure S2 in Supporting Information). These cac values are considerably lower than those reported for micellar aggregates,34 indicating formation of ordered molecular assemblies, i.e., bilayer membranes.28−30 The increase in cac at higher [C2mim][C2OSO3] composition reflects altered molecular assembly characteristics, which is shown by distinct thermally induced UV−vis absorption spectral changes as discussed later. In Figure 1, dependence of hydrodynamic diameter (Dh) of 1 on the concentration of [C2mim][C2OSO3] (A) and temperature (B) are shown, respectively. The Dh value observed below the [C2mim][C2OSO3] content of 30 v/v % is about 7−9 nm, while it became larger by more than 10 orders of magnitude at increased [C2mim][C2OSO3] compositions of 60 v/v % (Dh ∼ 42 nm)−90 v/v % (Dh ∼ 110−130 nm, Figure 1A). These Dh values observed at ambient temperature are smaller than those observed for the bulk aqueous [C2mim][C2OSO3] without 1 (Dh of 160−300 nm, Figure S1A,C in Supporting Information), indicating that the nanodomains of [C2mim]C2OSO3] existing in the pure aqueous mixtures underwent changes through interactions with the molecular assemblies. It is likely that introduction of amphiphilic molecular assemblies induces an aggregate structure different from that in the pure [C2mim][C2OSO3]−water mixtures to accommodate headgroup solvation of the assemblies. Meanwhile, a pronounced increase in hydrodynamic diameter was also observed upon heating the dispersion of 1 in 60 vol% [C2mim][C2OSO3]: the Dh value showed significant increase above 55 °C and reached a value of ca. 600 nm at 75 °C (Figure 1B). The observed changes in DLS diameter were reversible with respect to the temperature changes, as shown by filled squares in Figure 1A,B. As nanosized domains of [C2mim][C2OSO3] in bulk [C2mim][C2OSO3]−water mixtures became miscible with water and disappeared at elevated temperatures (Figures S1, Supporting Information), the notable increase in Dh at higher temperatures should also occur as a consequence of interactions between bilayer 1 and [C2mim][C2OSO3] molecules. We would postulate the possibility of nanosegregated [C2mim][C2OSO3] domains or bicontinuous structures20,21 formed at these elevated temperatures in which bilayer 1 is involved as a necessary component. The increase in hydrodynamic diameter of aqueous micelles upon addition of imidazolium ionic liquids has been reported for sodium dodecyl sulfate34 and sodium dodecyl benzenesulfonate.35 However, in these aqueous micellar systems, the observed increase in hydrodynamic diameter remains within ca. 5−6 fold. The thermally induced dynamic structural changes as observed in Figure 1B seem to be a unique behavior for the ordered azobenzene bilayer assemblies in the binary [C2mim][C2OSO3]−water mixtures, as will be discussed below. 3.2. Microscopic Observations of 1 in Binary [C2mim][C2OSO3]−Water Mixtures. To observe aggregate morphology in the [C2mim][C2OSO3]−water binary media, dark field optical microscopy (DFOM) was conducted. As shown in Figure S5 (Supporting Information), scattering dots were observable at volume fractions of 2−60 v/v % [C2mim][C2OSO3] (Figure S5A−D), indicating the presence of molecular aggregates. Consistent with the thermally reversible DLS-data, they gave similar images after the heat treatment (Figure S5E−H). Figure 2 shows the corresponding TEM micrographs. At lower contents of [C2mim][C2OSO3] (2 and 10 v/v %), amphiphile 1 gave rodlike nanostructures of

binary IL−water system shows thermally induced mixing behavior at the nanolevel. It is considered that these nanoaggregates (or nanodomains) of ionic liquid molecules dispersed in aqueous mixtures possess hydrated interfaces with water, and consequently their physicochemical properties cannot be equated with those of bulk (pure) ILs. From this viewpoint, it would be more appropriate to distinguish nanodomains of IL-forming molecules in homogeneous IL− water mixtures from the bulk ILs. We therefore describe them as [C2mim][C2OSO3]−water mixtures hereafter. The azobenzene amphiphile 1 is substantially insoluble in pure water at room temperature presumably due to overwhelming hydrophobicity of the ionic azobenzene sulfonate group. To our surprise, however, it is readily dispersible in [C2mim][C2OSO3]−water mixtures. As the amphiphile 1 is soluble neither in aqueous NaCl nor in NaBr, the enhanced solubility of 1 in binary [C2mim][C2OSO3]−water mixtures is reasonably ascribed to the electrostatic binding of [C2mim]+ ions to the azobenzene sulfonate headgroups. 34,35 To investigate critical aggregation concentration (cac) of 1 in the binary [C2mim][C2OSO3]−water mixtures, dynamic light scattering (DLS) measurements were conducted at varied solvent compositions (Figure 1, Figures S2−S4 in Supporting Information). The DLS count rates showed discontinuous increases at concentration ranges of 10−22 μM (2−30 vol% [C2mim][C2OSO3]) and 140−193 μM (60−90 v/v %

Figure 1. (A) Dependence of hydrodynamic diameter (Dh) observed for amphiphile 1 on volume fraction of [C2mim][C2OSO3] in the binary [C2mim][C2OSO3]−water mixture ([1] = 500 μM, above cac; temperature, 25 °C). (B) Temperature dependence of Dh. IL; 60 v/v %. Data plotted here are taken from the number percent analysis of DLS. 2378

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indicate that [C2mim][C2OSO3]−water mixtures serve as unique media to disperse ionic amphiphiles even they are poorly soluble in pure water. When the content of [C2mim][C2OSO3] is increased to 30 v/v %, dots with average diameter of 10−20 nm are abundantly observed as indicated by arrows (Figure 2C). At the [C2mim][C2OSO3] content of 60 v/v % (Figure 2D), dark domains with a larger diameter of ∼50 nm are seen, which are consistent with the DLS data (Supporting Information, Figure S3C,D). It is noticeable that blurred images are coexisting in Figure 2C, which are considered as nanodomains of [C2mim][C2OSO3] poststained by uranyl acetate. This is consistent with the DLS data shown in Figure S1 (Supporting Information). We found that the specimens at higher content of [C2mim][C2OSO3] became viscous, and a thick layer was inevitably formed upon dropping them on the carbon-coated TEM grid. It allows coexistence of [C2mim][C2OSO3] nanodomains on the carbon-coated TEM grid which often appeared as blurred images upon poststaining. 3.3. UV−vis Absorption Spectra of 2 in the Binary [C2mim][C2OSO3]−Water Media. In methanol, the azobenzene amphiphile 1 is molecularly dissolved and gave absorption maxima (λmax) at 274 and 429 nm (Figure S6, Supporting Information). These peaks are associated with the π−π* transition moment along the short and long axis of the azobenzene chromophore, respectively. The latter visible absorption maximum is consistent with that reported for sodium 4-[4-(N-methyl- N-oct ylamin o)ph en ylaz o]benzenesulfonate36 and methyl orange 2. Methyl orange has been widely employed as a spectroscopic probe for microenvironments,37 because of the sensitivity of absorption spectra against solvent polarity. In pure water, 2 gives a red-shifted λmax around 464 nm (Figure S7a, Supporting Information). This considerable red shift from methanol solution occurs as a consequence of the lowered LUMO energy level of monomeric dye 2 in water. When 2 is dissolved in [C2mim][C2OSO3]− water mixtures (temperature, 25 °C), the absorption peak showed a slight red shift from 464 nm (in pure water) to 470

Figure 2. Transmission electron microscopy images of 1 dispersed in binary [C2mim][C2OSO3]−water mixtures. Content of [C2mim][C2OSO3]: (A) 2 v/v %, (B) 10 v/v %, (C) 30 v/v %, (D) 60 v/v %. Arrows in Figure 2C indicate the aggregates. The blurred dark spots in panel C are considered to be nanodomains of [C2mim][C2OSO3] stained by uranyl acetate. All samples were poststained with uranyl acetate. [1] = 500 μM.

typically 20−30 nm in length and 6−7 nm in width (Figure 2A,B). As the molecular length of 1 as determined by CPK molecular model is 2.65 nm, the observed TEM width corresponds to a bilayer structure as schematically shown in Scheme 2. Although the formation of bilayer membranes from single-chained azobenzene-containing ammonium amphiphiles has been extensively studied in water,28,30 the present results

Scheme 2. Schematic Molecular Orientation Models of Amphiphile 1 Self-Assembled in Binary [C2mim][C2OSO3]−Water Mixtures: (A) Parallel Orientation of Azobenzene Chromophores at 2−60 v/v % [C2mim][C2OSO3]. (B) Tilt Chromophore Orientation (θ = 54.7°)

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Figure 3. Temperature dependence of UV−vis absorption spectra in binary [C2mim][C2OSO3]−water mixtures. Content of [C2mim][C2OSO3]: (A) 2 v/v %,(B) 30 v/v %, (C) 60 v/v %, (D) 90 v/v %. [1] = 500 μM.

Figure 4. Temperature dependence of absorption λmax observed for 1 and 2 in binary [C2mim][C2OSO3]−water mixtures. Content of [C2mim][C2OSO3]: (A) 2 v/v %, (B) 30 v/v %, (C) 60 v/v %, (D) 90 v/v %. [1] = [2] = 500 μM.

nm (in 30 v/v % [C2mim][C2OSO3], Figure S7a−d in Supporting Information). On the other hand, further increase in the content of [C2mim][C2OSO3] to 60 and 90 v/v % caused a reversely directed spectral blue shift to 463 and 441

nm, respectively (Figure S7d−f, Supporting Information), reflecting a decrease in the net polarity of binary media as probed by 2. The spectral blue shift observed beyond the [C2mim][C2OSO3] content of 30 v/v % is consistent with the 2380

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compared to 1 under the present conditions. The thermally intact parallel orientation of azobenzene chromophores is reasonably explicable by quantitative electrostatic adsorption of [C2mim]+ ion to the bilayer surface. When the content of [C2mim][C2OSO3] is increased to 60 v/v %, λmax of 1 is observed around 429 nm (temperature, 25 °C, Figure 3C). This λmax is also considerably blue-shifted compared to that observed for 2 dissolved in the corresponding binary water−[C2mim][C2OSO3] (60 v/v %) mixture (λmax, 463 nm, Figure S7e in Supporting Information), also indicating the formation of bilayer membrane with parallel chromophore orientation. The formation of ordered molecular assembly is further supported by a large, full spectral width at halfmaximum intensity (fwhm) of 116 nm observed for 1 (Figure 3c; Figure S9 and Table S1 in Supporting Information), which value exceeds those observed for the other investigated [C2mim][C2OSO3]−water mixtures (Table S1). Interestingly, in contrast to those observed for the binary mixtures with lower contents of [C2mim][C2OSO3], heating the dispersion from 25 °C to 75 °C caused red shift of the λmax from 429 to 460 nm with an isosbestic point at 432 nm. The spectral changes in heating and cooling cycles were thermally reversible, which is consistent with the reversible morphology changes in DFOM before and after the heating (Figure S5D,H in Supporting Information). The 460 nm band observed at 55−75 °C is slightly red-shifted compared to that observed for 2 (452 nm, Figure 4; Figure S7e in Supporting Information) and is apparently distinguishable from a broad absorption around 420−460 nm measured for 1 below cac (25 μM under 60 v/v % [C2mim][C2OSO3], Figure S10 in Supporting Information). By considering together the formation of molecular assemblies even at these elevated temperatures (Figure 1B), the 460 nm species of 1 (above cac) at 75 °C is assignable to azobenzene chromophores in the liquid crystalline state. The presence of an isosbestic point (at 432 nm) indicates that thermal equilibrium exists between the gel state (crystalline bilayer) and the liquid crystalline state. The UV−vis spectra obtained for 1 below cac is comparable to those obtained for 2 dispersed in aqueous [C2mim][C2OSO3] (60 v/v %): a λmax of 462 nm observed at 25 °C shifted to a broad absorption around 452 nm at 75 °C (Figure S7e in Supporting Information). Note that 2, which is monomerically dissolved in the aqueous [C2mim][C2OSO3] (60 v/v %) media, showed such a temperature dependence. As described previously, the absorption peak of 2 reflects the polarity of medium microenvironment;37 that is, its spectral change reflects local solvation of the chromophore. The temperature-dependent absorption spectra in binary [C2mim][C2OSO3]−water media ([C2mim][C2OSO3], 60 v/v %, Figure S7e) indicate that micropolarity of [C2mim][C2OSO3] (60 v/v %)−water mixtures experienced by 2 decreased at elevated temperatures. The observed thermally induced changes in the solvation of 2 in binary [C2mim][C2OSO3]− water mixture are explainable by dissolution of molecular clusters19,21 to water at higher temperatures. It consequently lowered the polarity of the binary media as reflected in the absorption spectral change of 2 (λmax, 452 nm at 75 °C). This is consistent with the decrease in DLS-size and intensity of nanodomains (Figure S1C, Supporting Information). The temperature dependence of absorption spectra observed for 1 below cac (Figure S10, Supporting Information) is interpretable in this light. It is interesting that the thermal gel-to-liquid crystalline phase transition is induced at 60 v/v % [C2mim]-

increase in the DLS domain size (Figure S1A,C, Supporting Information), indicating that 2 is electrostatically adsorbed on the hydrophobic nanodomains of [C2mim][C2OSO3] existing above that specific content. Temperature dependence of UV− vis spectra in each solution is also included in Figure S7a−f (Supporting Information). As shown in Figure S1 (Supporting Information), scattering intensities observed for nanodomains of [C2mim][C2OSO3] formed at higher volume fractions decrease as a result of mixing with water. At 75 °C, binary aqueous [C 2mim][C2 OSO3 ] solutions of 2 below the [C2mim][C2OSO3] content of 30 v/v % gave λmax at 462 nm, whereas they were observed at 452 and 435 nm at contents of 60 and 90 v/v % [C2mim][C2OSO3], respectively (Figure S7e,f, Supporting Information). The λmax observed for these high-[C2mim][C2OSO3] content solutions at elevated temperatures are still red-shifted compared to that observed in methanol (λmax = 429 nm, Figure S6, Supporting Information), indicating that these aqueous [C2mim][C2OSO3] mixtures possess higher polarity that is advantageous for self-assembly via solvophobic interactions.8 3.4. UV−vis Spectra of 1 and Its Self-Assembly in the Binary [C2mim][C2OSO3]−Water Media. Formation of ordered self-assemblies from 1 in the binary [C2mim][C2OSO3]−water media was then investigated by UV−vis absorption spectroscopy. The influence of [C2mim][C2OSO3] molecules on the molecular orientation of 1 is investigated above cac. Figure 3A−D depicts temperature dependence of UV−vis absorption spectra in various binary [C2mim][C2OSO3]−water mixtures ([1] = 500 μM). At room temperature, 1 in binary [C2mim][C2OSO3]−water mixtures (2−30 v/v % of [C2mim][C2OSO3]) showed λmax around 418−420 nm, which is considerably blue-shifted compared to 2 in the corresponding binary media (λmax at 462 nm, Figures S7b−d and S8). As developed nanostructures were observed in TEM (Figure 2), the observed spectral blue shifts would be reasonably ascribed to excitonic interactions occurring among oriented azobenzene chromophores (Scheme 2A), similarly to those observed for aqueous azobenzene bilayer membranes.28,30 According to the molecular exciton model,31,32 the observed spectral blue shifts observed for 1 indicate parallel orientation of azobenzene chromophores in binary [C2mim][C2OSO3]− water mixtures (2−30 v/v % of [C2mim][C2OSO3]) (Scheme 2A). As described previously, enhanced solvation of 1 in the binary media occurred as a consequence of the adsorption of [C2mim]+ ions. The binding of [C2mim]+ ions to azobenzene sulfonate groups not only enhances solvation of the ionic groups to the binary media but would also screen the electrostatic repulsions operating between the sulfonate headgroups, which militates for the parallel orientation of azobenzene chromophores. The azobenzene bilayers formed in 2−30 v/v % [C2mim][C2OSO3] are thermally intact, as can be seen from absorption spectra which showed slight or negligible changes upon heating. (Figure 3A,B, 2−30 v/v % of [C2mim][C2OSO3]). The absorption λmax observed for 1 and 2 in binary [C2mim][C2OSO3]−water mixtures and their temperature dependence are summarized in Figure 4. The absorption λmax of 418−422 nm observed at 25−75 °C (solid lines in Figure 4A,B) are considerably blue-shifted compared to that of 2 (dashed lines, 462 nm at 75 °C, 2−30 v/v % [C2mim][C2OSO3]), indicating that parallel orientation of the azobenzene chromophore is well preserved in bilayer 1 even at elevated temperatures. As shown in Table S1 (Supporting Information), the [C2mim]+ ion is present in large excess 2381

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S12 (Supporting Information). The ζ-potential reflects the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed aggregates, which reveals the degree of counterion binding to molecular assemblies.42−44 In aqueous mixtures with 2−10 v/v % [C2mim][C2OSO3], the ζ-potential remained almost constant irrespective of the temperature changes. At 30 v/v % [C2mim][C2OSO3], the ζ-potential showed a slight increase with increasing temperature. This change was not reflected in the UV−vis spectra, where λmax did not change within the temperature range investigated (Figure 3B). On the other hand, at the [C2mim][C2OSO3] content of 60 v/v %, a considerably large negative ζ-potential (∼−46 mV) was specifically observed at 25 °C. This negative ζ-potential provides an account for the observed stability of bilayer assemblies in this binary media which were stably dispersed without precipitation even after a period of 6 months. Upon heating, the ζ-potential showed a decline, and it was observed reproducibly, as is consistent with the temperature-dependent changes in λmax (Figure 3C, Figure 4C). As the ζ-potential obtained for the bulk binary mixture without amphiphile 1 ([C2mim][C2OSO3]: 60 v/v %) shows no such temperature dependence (Figure S12), the notable ζpotential of −46 mV observed for 1 at 25 °C indicates the presence of specific interactions between bilayer 1 and the binary [C2mim][C2OSO3] (60 v/v %)−water media. As shown in Figures S2 and S3 (Supporting Information), the dispersion structure of 1 in binary [C2mim][C2OSO3]−water mixtures varied specifically at composition regimes between 30 and 60 v/ v % [C2mim][C2OSO3], which caused different thermal characteristics for the bilayer as observed by UV−vis spectroscopy (Figure 3B,C). This would be ascribed to aqueous [C2mim][C2OSO3] which reveals multifaceted solution properties depending on the experimental conditions and techniques. It is reported that [C2mim][C2OSO3] is completely dissociated in water in a dilute concentration regime,45 and the SO3− group in the ethyl sulfate anion is hydrogen bonded with water.46 Meanwhile, the ethyl sulfate anion of [C2mim][C2OSO3] binds to zwitterionic micelles of N-hexadecyl-N,N-dimethylaminio-1propanesulfonate, and consequently a negative ζ-potential (−30 mV) was observed.47 It is also reported that a related IL with a more hydrophilic methyl sulfate anion, [C2mim][C1OSO3] behaves as a surface-active compound and it lowers the surface tension of water to ∼37 mN m−1.48 In the present [C2mim][C2OSO3]−water binary mixtures, nanostructured nonpolar domains of [C2mim][C2OSO3] are formed in the bulk, where the size of nanodomains varies dependent on both the composition and temperature. These temporally fluctuating nanodomains interact with azobenzene bilayers, and the negative surface of the azobenzene bilayer 1 would be compensated by the adsorbed C2mim+ cations aligned near the surface (Scheme 2). It is natural to assume that fluctuating nanodomains of [C 2 mim][C 2 OSO 3 ] interact with the azobenzene bilayers to form solvating fluid layers. Especially at the higher [C2mim][C2OSO3] content of 60%, the adsorption layer next to the C2mim−bound bilayer surface would be self-assembled from ethyl sulfate anions, so that alternate layering of [C2mim]+ and [C2OSO3]− ions could occur at the interface. It has been reported that the cationic layer−anionic layer interfaces of [C2mim][C2OSO3] show extremely low shear stress as determined by surface force measurements.49 Therefore, the ζ-potential of self-assembled structures of 1 in [C2mim][C2OSO3]−water binary mixtures will reflect net charges of the stationary [C2mim][C2OSO3]

[C2OSO3]. As described previously, when 1 is dispersed in the [C2mim][C2OSO3] (60 v/v %)−water mixture, the binary media adopt a liquid structure different from that of the bulk mixture owing to the solvation of azobenzene bilayers. Yet the thermally enhanced mixing of [C2mim][C2OSO3] and water in the bulk dispersion system seemingly occurred, as the consequently enhanced effective concentration of [C2mim][C2OSO3] promoted solvation of the azobenzene bilayers which facilitated the gel-to-liquid crystal phase transition. At 90 v/v % [C2mim][C2OSO3], on the other hand, λmax observed for 1 at 452 nm at 25 °C showed a blue shift to 444 nm upon heating to 75 °C (Figure 3D; Table S1, Supporting Information). These peaks are red-shifted from those of 2 (Figure 4D; Figure S7f in Supporting Information). As described previously, 1 in this binary mixture gives a large Dh value that indicates formation of a molecular assembly (Figure 1a), and accordingly, we consider that the observed 452 nm band observed at ambient temperature is ascribed to azobenzene chromophores in the oriented bilayer with the angle of inclination slightly larger than cos−1(1/(3)1/2) = 54.7° (Scheme 2B). The molecular exciton model31,32 shows that exciton splitting in dimers or linear or lamellar aggregates becomes zero at the angle of inclination of cos−1(1/(3)1/2) = 54.7°. It indicates that the excitonic interactions among chromophores will not be significantly reflected in spectral shifts when they are orientated around this particular inclination angle. Upon an increase in temperature, the absorption spectra of 1 showed hypochromism and λmax was observed at 444 nm, which is still red-shifted compared to that of 2 (435 nm at 75 °C, Figure 4D). This spectral change was thermally reversible, and we presume that 1 underwent phase transition from the slightly tilt bilayer to the liquid crystalline state. Thus, the bilayer 1 shows a thermally responsive nature even in the [C2mim][C2OSO3]-rich environment. Although the absorbance spectra of amphiphiles 1 and 2 are susceptible to change depending on medium pH,38 it was confirmed to be constant throughout the measurement, and thus the binary [C2mim][C2OSO3]−water media is chemically stable (Figure S11, Supporting Information). The observed wide variety of absorption spectra are characteristic of bilayer membranes, which exert excitonic interactions and thermal gel-to-liquid crystal phase transitions.28−30 These features are not available for micellar aggregates, which only reveal weak intermolecular interactions and are devoid of salient spectral shifts.27,39 The presence of bilayer membranes in binary [C2mim][C2OSO3]−water mixtures was further supported by 1H NMR measurements, where no NMR signals were observed for 1 when dispersed in binary [C2mim][C2OSO3]−D2O mixtures. The line width of 1 H NMR signals is determined by spin−spin relaxation (T2, transverse relaxation), and the broader lines indicate shortened T2, which is caused by the slowing of molecular reorientation rates.40 It has been demonstrated that bilayers in the ordered gel state show considerably broadened (or weakened) peaks,41 and it is reasonable that the rotational and translational motions of amphiphile 1 dispersed in [C2mim][C2OSO3]−water mixtures are significantly suppressed in the tightly packed crystalline bilayer structures on the NMR time scale. 3.5. Temperature Dependence of the ζ-Potential in the Binary [C2mim][C2OSO3]−Water Media. To get more insight into the properties of [C2mim][C2OSO3]−water mixtures that influence self-assembly of 1, the temperature dependence of ζ- potential was measured as shown in Figure 2382

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(5) Kimizuka, N.; Nakashima, T. Spontaneous Self-Assembly of Glycolipid Bilayer Membranes in Sugar-philic Ionic Liquids and Formation of Ionogels. Langmuir 2001, 17, 6759−6761. (6) Nakashima, T.; Kimizuka, N. Vesicles in Salt: Formation of Bilayer Membranes from Dialkyldimethylammonium Bromides in Ether-Containing Ionic Liquids. Chem. Lett. 2002, 1018−1019. (7) Gayet, F.; Marty, J.-D.; Brulet, A.; Viguerie, N. L. Vesicles in Ionic Liquids. Langmuir 2011, 27, 9706−9710. (8) Nakashima, T.; Kimizuka, N. Controlled Self-assembly of Amphiphiles in Ionic Liquids and the Formation of Ionogels by Molecular Tuning of Cohesive Energies. Polym. J. 2012, 44, 665−671. (9) Ikeda, A.; Sonoda, K.; Ayabe, M.; Tamaru, S.; Nakashima, T.; Kimizuka, N.; Shinkai, S. Gelation of Ionic Liquids with a Low Molecular-Weight Gelator Showing Tgel above 100 °C. Chem. Lett. 2001, 1154−1155. (10) Bideau, J. L.; Viau, L.; Vioux, A. Ionogels, Ionic Liquid Based Hybrid Materials. Chem. Soc. Rev. 2011, 40, 907−925. (11) Singh, T.; Kumar, A. Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions. J. Phys. Chem. B 2007, 111, 7843−7851. (12) Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. Conductivities, Volumes, Fluorescence, and Aggregation Behavior of Ionic Liquids [C4mim][BF4] and [Cnmim]Br (n = 4, 6, 8, 10, 12) in Aqueous Solutions. J. Phys. Chem. B 2007, 111, 6181−6188. (13) Rao, K. S.; Singh, T.; Trivedi, T. J.; Kumar, A. Aggregation Behavior of Amino Acid Ionic Liquid Surfactants in Aqueous Media. J. Phys. Chem. B 2011, 115, 13847−13853. (14) Zhao, Y.; Gao, S.; Wang, J.; Tang, J. Aggregation of Ionic Liquids [Cnmim]Br (n = 4, 6, 8, 10, 12) in D2O: A NMR Study. J. Phys. Chem. B 2008, 112, 2031−2039. (15) Li, N.; Zhang, S.; Zheng, L.; Dong, B.; Li, X.; Yu, L. Aggregation Behavior of Long-Chain Ionic Liquids in an Ionic Liquid. Phys. Chem. Chem. Phys. 2008, 10, 4375−4377. (16) Shi, L.; Li, N.; Zheng, L. Aggregation Behavior of Long-Chain N-Aryl Imidazolium Bromide in a Room Temperature Ionic Liquid. J. Phys. Chem. C 2011, 115, 18295−18301. (17) Widegren, J. A.; Laesecke, A.; Magee, J. W. The Effect of Dissolved Water on the Viscosities of Hydrophobic Room-Temperature Ionic Liquids. Chem. Commun. 2005, 1610−1612. (18) Kohno, Y.; Ohno, H. Ionic Liquid/Water Mixtures: From Hostility to Conciliation. Chem. Commun. 2012, 48, 7119−7130. (19) Stark, A.; Zidell, A. W.; Hoffman, M. M. Is the Ionic Liquid 1Ethyl-3-methylimidazolium Methanesulfonate [emim][MeSO3] Capable of Rigidly Binding Water? J. Mol. Liq. 2011, 160, 166−179. (20) Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R. How Water Dissolves in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2012, 51, 7468−7471. (21) Bernardes, C. E. S.; Minas da Piedade, M. E.; Canongia Lopes, J. N. The Structure of Aqueous Solutions of a Hydrophilic Ionic Liquid: The Full Concentration Range of 1-Ethyl-3-methylimidazolium Ethylsulfate and Water. J. Phys. Chem. B 2011, 115, 2067−3074. (22) Köddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. The Association of Water in Ionic Liquids: A Reliable Measure of Polarity. Angew. Chem., Int. Ed. 2006, 45, 3697−3702. (23) Kawasaki, T.; Tokuhiro, M.; Kimizuka, N.; Kunitake, T. Hierarchical Self-Assembly of Chiral Complementary Hydrogen Bond Networks in Water. Reconstitution of Supramolecular Membranes. J. Am. Chem. Soc. 2001, 123, 6792−6800. (24) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Tokuhiro, M.; Kunitake, T. Formation of Stable Bilayer Membranes in Binary Aqueous−Organic Media from a Dialkyl Amphiphile with a Highly Dipolar Head Group. J. Am. Chem. Soc. 1996, 118, 5808−5809. (25) Kimizuka, N.; Tokuhiro, M.; Miyauchi, H.; Wakiyama, T.; Kunitake, T. Bilayer Formation in Ethanol from Dialkylammonium Amphiphile Appended with Nitroaniline Moiety. Chem. Lett. 1997, 1049−1050. (26) Heesemann, J. Studies on monolayers. 1. Surface Tension and Absorption Spectroscopic Measurements of Monolayers of Surface-

layer self-assembled on the surface of assemblies 1, which could vary depending on conditions such as the content of [C2mim][C2OSO3] and temperature. The observed large negative ζ-potential at ambient temperature is intriguing, but it indicates that the formation of the electrical double layer in binary [C2mim][C2OSO3]−water systems is considerably different from that of the well investigated dilute aqueous electrolytes.50

4. CONCLUSION We demonstrated that azobenzene amphiphile 1 forms ordered bilayer membranes in binary [C2mim][C2OSO3]−water mixtures. Azobenzene amphiphiles 1 assume regular parallel molecular orientation in the bilayer, depending on the content of [C2mim][C2OSO3] in the binary mixture and temperature. These bilayer assemblies are thermally intact below the [C2mim][C2OSO3] content of ∼30 v/v %. Above 60 v/v % [C2mim][C2OSO3], on the other hand, thermally reversible phase transition was observed between the parallel-stacked or tilt crystalline azobenzene bilayer (gel state) and the liquid crystalline state, depending on the content of [C2mim][C2OSO3]. The observed changes in the aqueous [C2mim][C2OSO3] (60 v/v %) mixture are related to the selfassembling nature of [C2mim][C2OSO3] molecules to form nanoclusters in the bulk binary aqueous mixture as supported by ζ-potential and DLS data. As far as we are aware, this work provides the first observations that molecular orientation of ordered molecular assemblies in IL−water mixtures show significant dependence on the solvent composition, solution nanostructures, and temperature. Together with the water−IL biphasic systems,51 the present findings offer a new perspective in understanding and designing self-assembly phenomena and their functions in binary IL−water systems.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S12 and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-92-802-2832; fax: +81-92-802-2838; e-mail: n-kimi@ mail.cstm.kyushu-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grant-in-Aid for Scientific Research (S) 25220805 and JST-CREST. T.S.K is highly thankful to JSPS for providing Postdoctoral Fellowship for Foreign Researchers.

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REFERENCES

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