Aggregation Behavior of Imidazolium-Based Surface-Active Ionic

Article pubs.acs.org/Langmuir

Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Photoresponsive Cinnamate Counterions in the Aqueous Solution Yanhui Bi,† Liuchen Zhao,‡ Qiongzheng Hu,§ Yan’an Gao,∥ and Li Yu*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, P.R. China China Research Institute of Daily Chemical Industry, Taiyuan 030001, P.R. China § Department of Chemistry, University of Houston, Houston, Texas 77204, United States ∥ China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China ‡

S Supporting Information *

ABSTRACT: Two imidazolium-based surface active ionic liquids (SAILs) with photoresponsive cinnamate aromatic counterions, viz. 1-dodecyl-3methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]), were newly synthesized, and their self-assembly behaviors in aqueous solutions were systematically explored. Results of surface tension and conductivity measurements show that both [C12mim][CA] and [C12mim][PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C12mim][CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C12mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/ H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. Investigation on the temperature-dependent self-assembly nanostructures demonstrates that the higher temperature leads to a looser arrangement. Under UV irradiation, trans−cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases. This work is expected to enrich the investigations of phase behaviors formed in SAILs systems and receive particular attention due to their unique properties and potential applications in drug delivery, biochemistry, materials science, etc.

1. INTRODUCTION In recent years, with the rising challenges of environmentally benign chemical processing, ionic liquids (ILs) have attracted increasing attention due to their specific physicochemical properties, e.g., high ionic conductivity, negligible vapor pressure, and convenient designability. The good properties drive that the ILs have been shown to be excellent candidates in the fields of catalysis, nanostructure materials, organic synthesis, electrochemistry, and liquid/liquid extraction.1−5 The great superiority of ILs is that their chemical and physical properties can be effectively and easily designed through choosing the cation, anion, and substituent. Ionic liquids with long alkyl side chains are regarded as amphiphiles that are named surface active ionic liquids (SAILs), which is a kind of functional ILs with combined properties of ILs and surfactants. They can form aggregates with specific structures, shapes, and properties. The aggregation behavior of SAILs in aqueous solutions has been a focus of recent investigations.6,7 © 2015 American Chemical Society

Driven by reduction of surface energy, surfactant molecules with different structures can self-assembly into multifarious aggregates depending on their particular natures. A variety of aggregates including micelles, vesicles, lyotropic liquid crystals (LLCs), and gels can be formed.8−12 Among these aggregate structures, LLCs, representing an intermediate phase between solids and liquids, have become a significant class of aggregate and attracted widespread interest in fundamental research and practical applications over the past few decades.13−16 The phase behavior of LLCs mainly relies on both the concentration of the aqueous solution and the temperature, which is different from that of the more well-known single component thermotropic liquid crystal system.17 The LLC phase is closely related to biology because of its prevalence of organized lipid Received: August 27, 2015 Revised: October 19, 2015 Published: October 28, 2015 12597

DOI: 10.1021/acs.langmuir.5b03216 Langmuir 2015, 31, 12597−12608

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Langmuir structures in living systems.14 Based on these unique properties, LLCs have been widely applied in chemical reaction, material science,18−20 protein crystallization,21 drug delivery, etc.22,23 LLCs formed by SAILs have drawn particular attention recently. Zhao et al. investigated LLCs formed by N-alkyl-Nmethylpiperidinium bromides (CnPDB, n = 12, 14, 16), demonstrating that different hydrophobic interactions resulted in different types of phase diagrams in water.24 LLCs aggregated by N-alkyl-N-methylpyrrolidium bromides in water and ethylammonium nitrate (EAN) were also reported, which confirmed that the solvent effect played an important role in the phase behavior.25 In addition, our group found that 1-butyl3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]) could form LLCs with multimesophases, namely, normal hexagonal (H1), lamellar liquid crystal (Lα), and reverse bicontinuous cubic phase (V2), in the water-rich environment of H2O-EAN mixed solvents.26 We also investigated LLCs formed by imidazolium-based SAILs, 1-dodecyl-3-methylimidazolium salicylate (C12mimsal) and 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate (C12mimHNC), illustrating that the π−π interactions between imidazolium cations and the aromatic counterions resulted in the specific LLCs phase behavior.27 Self-assembly system with stimuli responsive groups can be rationally designed to obtain the desired structures, morphologies, and properties under external stimuli. They have been emerging as potential candidates in practical applications such as catalysis, diagnostics, drug delivery, and biosensors.28 The stimuli can be largely categorized into redox,29 light,30 temperature,31 pH,32 CO2,33 ultrasound,34 and hydrocarbon.35 In particular, light is generally considered as an ideal external stimulus, as it enables readily control of materials without directly introducing external species.36 Abe et al. applied an azobenzene-based cationic surfactant, which shows reversible trans/cis photoisomerization, as a “photoswitchable” agent to control viscosity variations.37 Huang’s group constructed a photomodulated multistate and multiscale molecular selfassembly system by utilizing a binary-state molecular switch in the CTAB aqueous solution mixed with sodium (4phenylazo-phenoxy)-acetate (AzoNa). Depending on the UV light irradiation time, the molecular assemblies including wormlike micelles, vesicles, lamellar structures, and small micelles were obtained, resulting in a distinct change in solution properties at the macroscopic scale.38 Zhao and his coworkers observed that azobenzene-containing amphiphilic diblock copolymers with poly(acrylic acid) as the hydrophilic block self-assembled into micelles in water and displayed reversible morphological transformation by alternative exposure to UV and visible light.39 Phenylalkenes is another classic photoresponsive unit that can also be applied to the photoresponsive system. Wang et al. synthesized cinnamatebased light-responsive ionic liquids and found that UV light irradiation could modulate the conductivity of the ILs.40 Raghavan et al. reported a notable type of photorheological fluids that underwent a transformation from long wormlike micelles into shorter micelles by employing CTAB and a phenylalkene derivative, trans-ortho-methoxycinnamic acid (OMCA).41 Wang et al. constructed a photoresponsive aqueous system of tetradecyldimethylamine oxide (C14DMAO) mixed with para-coumaric acid (PCA), and found the aggregation transformation from bilayer vesicles into wormlike micelles.30

To date, most of the literature has been mainly focused on the transformation between simple colloidal aggregates such as micelles and vesicles in the photoresponsive system. However, there are only a few investigations on photoresponsive LLC phases. Eastoe’s group employed a photolyzable anionic surfactant, 4-hexylphenylazosulfonate (C6PAS), to manipulate the ordering behavior of lyotropic lamellar (Lα) phases by UV light.42 Hughes et al. described novel LLC materials based on azobenzene photoresponsive amphiphiles that actuated the transition between different LLC forms depending on illumination conditions. The UV irradiation led to the disruption of the ordered LLC phases while the visible light reversed the transition.43 They also investigated effect of the azobenzene group positions within the comparable photoresponsive amphiphiles on the ability to form LLCs and their corresponding photoresponsive behavior.17 However, LLC phases formed by photoresponsive SAILs have been rarely reported. In this work, we applied a facile method to prepare SAILs with photoresponsive cinnamate counterions, namely, 1dodecyl-3-methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]) (chemical structures are shown in Scheme 1 in the Supporting Information) and evaluated their ability to form LLC phases. Interestingly, both hexagonal liquidcrystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. Under UV irradiation, trans−cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs with higher cmc values. In addition, after UV light irradiation, the self-assembled LLC phases stayed the same but their structural parameters obviously changed. It is the first time to observe the photoresponsive LLC phases constructed by SAILs based on phenylalkene moieties. This work is expected to expand their applications in the fields of drug delivery, biochemistry, etc.

2. EXPERIMENTAL SECTION 2.1. Materials. 1-Methylimidazole (99%) was purchased from Acros Organics and distilled prior to use. 1-Bromododecane (98%), trans-cinnamic acid (99%), and trans-4-hydroxycinnamic acid (98%) were the products of J&K Scientific, Ltd., and used as received. Ethyl alcohol was obtained from Sinopharm Chemical Reagent Co. The deionized water was distilled three times and was used throughout all the experiments. 2.2. Synthesis of SAILs. 1-Dodecyl-3-methylimidazolium bromide (C12mimBr) was synthesized and purified according to the methods reported in the literature.44 [C12mim][CA] was freshly prepared as follows according to the neutralization method. An aqueous solution of [C12mim]Br was available to pass through a column filled with anion exchange resin to obtain [C12mim][OH]. Trans-cinnamic acid was dissolved in alcohol, which was then neutralized with aqueous solution of equimolar [C12mim][OH] and stirred about 10 h to obtain [C12mim][CA]. After removed the solvent by vacuum rotary evaporation and then dried under vacuum for 2 days, the final product was obtained. [C12mim][PCA] was obtained following the similar procedure as above. [C12mim][CA] and [C12mim][PCA] molecules were confirmed by 1H NMR spectroscopy with a Bruker Avance 300 spectrometer in CDCl3, and the 1H NMR peak of CDCl3 (7.26 ppm) was used as the reference in determining the proton chemical shifts (details presented in the Supporting Information). For the as-prepared [C12mim][CA] and [C12mim][PCA], Thermogravimetric/Differential Thermal Analyzer (Diamond TG/DTA, Perki12598


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Langmuir nElmer, American) was employed to determine the melting temperatures, 15 and 39 °C, and the thermal decomposition temperatures, 220 and 225 °C, respectively. As the previous reported, the impurity in ILs may have a significant effect on their physicochemical properties.45 Similar to the synthesis process of [Cnmim][OMCA] reported by Wang’s group, the main impurity in the [C12mim][CA] and [C12mim][PCA] was bromide.40 Therefore, we confirmed the bromide content in these ILs by means of a Br− selective electrode.40 And the Br− content of [C12mim][CA] and [C12mim][PCA] was less than 0.00004 mol·L−1 and 0.00002 mol·L−1, respectively. The quality of the ILs prepared in this work is available to study the aggregation behavior in aqueous solution. 2.3. Micellization of [C12mim][CA] and [C12mim][PCA]. Surface tension measurements were carried out on a model JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., accuracy ±0.1 mN/m) using the ring method. All measurements were repeated for at least three times until the values were reproducible. Electrical conductivity measurements were performed on a low frequency conductivity analyzer (model DDSJ-308A, Shanghai Precision & Science Instrument Co., Ltd. of China). Each electrical conductivity data point was recorded when its accuracy was better than 1% within 2 min. The temperature of surface tension and electrical conductivity measurements was controlled by a HAAKE DC30-K20 thermostatic bath (Karlsruhe, Germany) with an uncertainty of within ±0.1 °C. 2.4. Sample Preparation of LLC Phase. The samples were prepared by weighing the synthesized SAILs and water as designed compositions in weight percent (wt %) into stoppered glass vials sealed with Parafilm. The samples were mixed and homogenized by repeated vortex mixing and centrifugation. Then, they were put in a thermostat at 25 °C for at least 2 weeks to reach equilibrium before further investigation. It should be noted that the LLC phases are sensitive to water evaporation loss, so each step of samples preparation should keep them sealed as long as possible. 2.5. UV-Light Irradiation. SAILs samples were irradiated with a CHF-XM35−500W ultrahigh pressure short arc mercury lamp with optical filter (365 nm) at room temperature. As for the SAILs aqueous solution, samples (20 mL) were placed in a quartz tumbler with a cover, and irradiation was applied for 30 min under stirring. As for the LLC phase, the light irradiation sample was prepared by weighting appropriate amounts of SAILs and water into a quartz vial according to the results of POM and SAXS. After the LLC phase equilibrated, the sample was irradiated with the lamp for 30 min at room temperature. To avoid overheating, experimental temperature was kept at 25 °C by putting the lamp in a well with a water circulating jacket. The distance between the sample and light source was fixed at 10 cm. 2.6. Characterizations. Polarized Optical Microscopy Observations. Liquid crystal textures were observed with a polarized optical microscope (XPF-800C, Tianxing, Shanghai, China) at room temperature. Images were captured with a digital camera (TK-9301EC, JVC, Japan). Small-Angle X-ray Scattering Measurements. Small-angle X-ray scattering (SAXS) measurements were performed on the Anton Paar SAXSess mc2 X-ray scattering system with Ni-filtered Cu Kα radiation (0.154 nm) operating at 2 kW (50 kV and 40 mA). The samples were placed in a stainless steel tank and sealed with transparent film. The distance between the sample and the detector was 264.5 mm, and the wavelength of X-rays is 1.542 Å. The exposure time was 600 s for each sample. All samples were held in a vacuum steel holder to provide thermal contact with the computer controlled Peltier heating system (Hecus MBraun, Graz, Austria). The determination of the LLC phases was based on the characteristic diffraction pattern of the SAXS ratio of Bragg peaks. In particular, the relative peak positions are 1:√3:√4:√7 for hexagonal phase and √3:√4 for the cubic phase. Rheological Measurements. Rheological properties were determined on a HAAKE RS6000 Rheometer with a cone−plate sensor system (Ti, diameter = 35 mm, cone angle = 1°, distance = 52 μm). The experimental temperature was controlled by a cyclic oil bath (Phoenix) within an error of ±0.1 °C. In oscillatory measurements, dynamic frequency spectra were conducted in the linear viscoelastic

regimes of each sample as determined from dynamic stress-sweep measurements.

3. RESULTS AND DISCUSSION 3.1. Surface Properties and Micellization Parameters. Surface tension and electrical conductivity measurements were performed to characterize the surface activities and micellization thermodynamic properties of synthesized SAILs in the aqueous solution. The plots of surface tension (γ) at 25 °C as a function of concentration (C) for [C12mim][CA] and [C12mim][PCA] are depicted in Figure 1. It is clear that, for

Figure 1. Surface tension plots as a function of concentration for [C12mimCA] and [C12mim][PCA ]in the aqueous solution at 25 °C, respectively.

each SAIL, the surface tension of aqueous solutions decreases initially with the increasing SAILs concentration, suggesting that the SAILs molecules are adsorbed at the air−liquid interface.46 Then, a plateau region appears in the (γ-C) plots, indicating that the micelles have been formed, and above which a nearly constant value of surface tension (γcmc) is obtained. The breakpoint in the (γ-C) plots is assigned to the critical micelle concentration (cmc), which originates from the micellization of the synthesized amphiphilic compounds. Noteworthy is that the absence of a minimum around the breakpoint demonstrates the high purity of the prepared SAILs. The estimated cmc and γcmc values for [C12mim][CA] and [C12mim][PCA] are listed in Table S1 (Supporting Information), together with the corresponding data of C12mimBr reported previously.47 As shown in Table S1, the cmc values of [C12mim][CA] and [C12mim][PCA] in the aqueous solution are 2.5 and 2.7 mmol·L−1, respectively, which is notably smaller than that of C12mimBr, suggesting that the SAILs studied here have higher surface activity to form micelles. As is well-known that electrostatic interactions and hydrophobic interactions are considered as two key roles played in the formation of micelles.48 The difference associated with the surface activity of these SAILs can be attributed to the following two reasons. On one hand, electrostatic repulsions between hydrophilic headgroups are screened more effectively, which originates from the weaker hydration of the cinnamate aromatic counterion compared to the inorganic anion (e.g., Br−), thereby enhancing surfactant molecules adsorption at the air−liquid interface and facilitating the formation of micelles. On the other hand, the enhanced hydrophobicity of SAILs molecules derived from the incorporation of cinnamate aromatic group can also be responsible for the variation of cmc values. It has a weak 12599


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Figure 2. Plots of electrical conductivity as a function of concentration for (a) [C12mim][CA] and (b) [C12mim][PCA] in the aqueous solution at different temperatures.

S1, the values of Γmax increase in the order C12mimBr < [C12mim][CA] < [C12mim][PCA], while the Amin values reduce in the same order. Compared with C12mimBr, the higher Γmax values and lower Amin values of [C12mim][CA] and [C12mim][PCA] imply a denser arrangement of the prepared SAILs molecules at the air−liquid interface.52 The explanation for this phenomenon is mainly attributed to the π−π interaction offered by the aromatic counterions. As for [C12mim][PCA] and [C12mim][CA], the hydrophilic effect induced by the additional hydroxyl group of [C12mim][PCA] leads to a higher Γmax value and lower Amin value. The concentration dependencies of the electrical conductivity at different temperature for [C12mim][CA] and [C12mim][PCA] aqueous solutions are depicted in Figure 2. The change of line slopes originates from the micellization of amphiphiles, and the concentration of the break point is designated to be the cmc value. Table S2 (Supporting Information) shows the cmc values at different temperatures and the relevant thermodynamic parameters calculated according to the equations shown in the Supporting Information. Obviously, for the SAILs investigated, the cmc values obtained by this method at 25 °C are in good accordance with those of surface tension measurements (Table S1). It can be also found that, with increasing temperature, the cmc values of [C12mim][CA] and [C12mim][PCA] increase slightly, which is attributed to following two opposite processes.9 First, the hydration degree of hydrophilic headgroups decreases with the increase of temperature, which facilitates micelle formation and decreases cmc. Second, the orderly water structures surrounding the hydrophobic groups are destroyed as the temperature increases, which is disadvantageous to micelle formation and increases the cmc value. The increase of cmc values dependent on temperature (Table S2) indicates that the second process plays a dominant role in the micellization process of the SAILs studied in this work. Based on the mixed electrolyte model of micellar solution,52 the degree of counterion binding (β) can be estimated from the electrical conductivity measurements by using equation β = 1 − α. Here α can be obtained by ratio between the slopes of conductivity curves above and below the cmc. The values of β for [C12mim][CA] and [C12mim][PCA] at different temperatures are summarized in Table S2. For [C12mim][CA] and [C12mim][PCA], β values decrease with the increasing temperature. There are two major opposite aspects as for aromatic ring counterion to bind to micelles while the temperature rises. First, the hydration of cinnamate weakens with the increase of temperature, which is in favor of counterions binding to micellar surface. However, the

tendency to be moved away from the micellar interface because of the hydrophobic effect, which plays a crucial role in promoting micelle formation. Thus, based on the synergistic effect of the above two aspects, the cmc values of the investigated SAILs are lower than that of the traditional imidazolium-based SAIL, C12mimBr. It is noted that the cmc value of [C12mim][PCA] is slightly higher, compared with [C12mim][CA]. This is probably due to the introduction of an additional hydroxyl group for [C12mim][PCA], which can form hydrogen bonding with water molecules49 and then weaken the hydrophobicity of the counterion and reduce the surface activity of [C12mim][PCA] in aqueous solution. Based on the surface tension plots, two additional micellization parameters, i.e., adsorption efficiency (pC20) and effectiveness of surface tension reduction (Πcmc) of the SAILs at the air−liquid interface can be obtained according to the equation reported earlier.50 Πcmc demonstrates the maximum reduction of surface tension caused by the dissolution of surfactant molecules. It is generally employed to evaluate effectiveness of surfactants to lower the surface tension of the solvent.50 In Table S1, the values of Πcmc increase in the order of [C12mim][PCA] < C12mimBr < [C12mim][CA], which reveals the effectiveness of decreasing surface tension gradually increases. [C12mim][CA] molecules induce a higher Πcmc value compared to C12mimBr due to increase of the hydrophobic interactions of [C12mim][CA] in the presence of the aromatic counterion. However, as for [C12mim][PCA], the hydrophilicity of hydroxyl plays a dominated role in decreasing the surface tension. Therefore, the Πcmc value of [C12mim][PCA] is lower than C12mimBr. pC20 is used to evaluate the ability of decreasing surface tension for surfactants and a higher pC20 value demonstrates a greater adsorption efficiency of surfactant molecules. As listed in Table S1, pC20 values for [C12mim][CA] and [C12mim][PCA] are higher than that of C12mimBr, which reveals the superior efficiency of the SAILs investigated here in decreasing surface tension of water. This difference is probably due to the bulky size of cinnamate anions, which can be less hydrated by water and more strongly attached to the imidazolium cations, thereby reducing the electrostatic repulsions between the cationic headgroups. In addition, the hydrophobic effect of cinnamate aromatic groups should also be considered. For the adsorption of SAILs at the air−liquid interface, the maximum surface excess concentration (Γmax) and the area occupied by a single surfactant molecule (Amin) at the air− liquid interface reflect the arrangement of surfactant molecules at the interface. They can be estimated by applying the Gibbs adsorption isotherm to the surface tension data.51 From Table 12600


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formed by the SAIL/water systems were further confirmed by SAXS. SAXS is an efficient technique to evaluate the details of LLC phases. The SAXS characterization on different types of LLC phases is dependent on the long-range order in the LLC states, which can be represented by the characteristic positions of Bragg reflections. Figure 4 exhibits the SAXS patterns of [C12mim][CA]/water mixture with the [C12mim][CA] concentration of 50, 55, 60 wt % at 25 °C, respectively. The SAXS spectra show three distinct Bragg scattering peaks with relative q values in the ratio of q1/q2/q3 = 1:√3:2, which can be assigned to (100), (110), and (200), respectively, indicating the presence of hexagonal phase (H1). The SAXS data is consistent with the POM observation (shown in Figure 3(a−c)). In Figure 4a, it is also clear that the first Bragg peak shifts to the right with increasing the concentration of [C12mim][CA], demonstrating a smaller distance between the adjacent cylinders.27 The formation of a cubic phase (V2) in 65−70 wt % [C12mim][CA] was confirmed by the observation of three representative Bragg peaks with a relative ratio of √3:√4:√11 (Figure 4b). They can be designated as (111), (220), and (311), respectively, suggesting a structure of V2 phase. The variety of LLCs obtained in the [C12mim][CA]/water binary system is analogous to that of another SAIL, N-hexadecyl-N′methylpyrrolidinium bromide (C16MPB), in water or EAN25 as well as C12mimSal/water binary system.27 In comparison to the LLC phase formed by C12mimBr/water binary system, [C12mim][CA] shows peculiar phase behavior at higher concentrations. According to a previous report,54 only H1 phase can be constructed using C12mimBr. However, in this work, diversified LLC phases (viz. H1 and V2 phases) were observed in [C12mim][CA] aqueous solution. The existence of π−π interaction between imidazolium cations and aromatic anions is primarily responsible for decrease of the Amin value when the concentration of SAILs with aromatic counterions increases,27 resulting in formation of cubic phase (V2) that has a higher P value. Compared to [C12mim][CA], [C12mim][PCA] exhibits a quite different self-assembly behavior in water at 25 °C. As shown in Figure 5, when the concentrations of [C12mim][PCA] change from 65 to 95 wt %, three scattering peaks with the ratio of 1:√3:2 in the appropriate q-range were observed, implying the existence of long-range ordered hexagonal structure (H1). This is probably because the presence of the hydroxyl group improves hydration of cinnamate anions, which is unfavorable to π−π interactions between imidazolium cations and aromatic anions. When the concentration of [C12mim][PCA] increases, the Amin value at the air−liquid interface does not significantly decrease to induce change of the P value. Therefore, the [C12mim][PCA]/H2O binary system can form a more unitary LLC phase compared to the [C12mim][CA]/H2O system. The structure parameters of LLC phases can be calculated from the SAXS patterns based on the periodic order of LLC states. The H1 phase contains infinitely long cylinder-like aggregates packed in a hexagonal array and separated by a continuous region,55 where the hydrophobic tails are located in the interior of the cylinder-like aggregates and the hydrophilic headgroups are surrounded by H 2 O molecules. 25 For [C12mim][CA]/H2O and [C12mim][PCA]/H2O binary systems, a variety of structural parameters of the H1 phase, including the repeat lattice parameter (α0), the radius of a cylindrical aggregate (dH), the thickness of the solvent layer

intensification of molecular thermal motion at higher temperature is infaust to the counterion binding. For [C12mim][CA] and [C12mim][PCA], β values decrease with the increasing temperature, possibly due to the fact that the stronger thermal movement of cinnamate counterions plays a predominant role over the weaker hydration. As a result, at higher temperature the attraction between the headgroup and counterion around the stern layer is decreased. In addition, it is observed that [C12mim][PCA] has much smaller β values than [C12mim][CA] at the same temperature. This discrepancy is primarily caused by the weaker hydrophobic interactions of [C12mim][PCA] due to the additional hydroxyl, which results in less counterions existing at the micellar surface. As is shown in Table S2, the negative values of ΔGθm and ΔHθm indicate that micellization process of the prepared SAILs in the aqueous solution is a spontaneous and exothermic process in the investigated temperature range. In addition, the negative values of ΔHθm are much lower than those of −TΔSθm, indicating that the micellization process for the SAILs is enthalpy-driven. This observation is consistent with the reported results for other SAILs, e.g., C14mimBF4 and C16mimBF4.53 Compared to [C12mim][PCA], [C12mim][CA] has a more negative ΔGθm value, indicating that the insertion of a hydroxyl group weakens hydrophobic interactions between aromatic counterions and hinders the formation of micelles. 3.2. LLC Phase Behavior and Characterizations. To further investigate the aggregation behavior of [C12mim][CA] and [C12mim][PCA] in the aqueous solution, we varied the concentrations of SAILs in the binary systems. It was observed that gradually increasing the concentrations of SAILs resulted in the construction of anisotropic LLC phases in water, which were determined using POM (Figure 3). As shown in Figure

Figure 3. POM images for (a−c) [C12mim][CA]/water and (d−f) [C12mim][PCA]/water binary systems with different SAIL concentrations at 25 °C: (a) 50, (b) 55, (c) 60, (d) 65, (e) 85, and (f) 95 wt %.

3a−c, the smoke-like textures were observed with concentrations ranging from 50 to 60 wt %, respectively, representing a hexagonal phase (H1) of the [C12mim][CA]/water mixtures. When the [C12mim][CA] concentration was further increased to 65 or 75 wt %, no brightness was observed for the binary system, and only isotropic cubic liquid crystal phase region was obtained (data not shown). However, in terms of the [C12mim][PCA]/water mixture system, when the concentrations of [C12mim][PCA] fell into the range of 65−95 wt %, the smoke-like optical textures, indicative of the hexagonal phase (H1), were unambiguously observed. These LLC phases 12601


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Figure 4. SAXS patterns of the (a) H1 and (b) V2 phases formed at various concentrations of [C12mim][CA] aqueous solutions at 25 °C.

hexagonal array. Such similar phenomenon was also observed in the H1 phase formed by the N-dodecyl-N-methylpiperidinium bromide (C12PDB)/ H2O binary system,25 as well as the C12mimBr/1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)/H2O ternary system.56 As for hexagonal phase of the [C12mim][CA]/H2O system in this study, when the concentration of [C12mim][CA] increases from 50 to 60 wt %, the number of H2O molecules involved in the cylinder palisade layer decreases due to the stronger hydrophobic interactions between [C12mim][CA] molecules than those between [C12mim][CA] and H2O molecules.57 Such a change can also be reflected by the diminishing αs values with the increasing the concentration of [C12mim][CA], demonstrating a denser alignment of the SAILs inside the cylinders. It is obvious that [C12mim][CA] has a smaller area at the hydrophilic/hydrophobic interface compared to C12PDB at the same concentration.24 The strong electrostatic screening effect and π−π interactions might account for that. As shown in Table S3, for the H1 phase constructed by the [C12mim][PCA]/H2O system, the α0 value becomes smaller when the concentration of [C12mim][PCA] increases and the variation of structural parameters with increasing the concentration of [C12mim][PCA] is analogous to that of the H1 phase formed in the [C12mim][CA]/H2O system. 3.3. Effect of Temperature on Phase Behavior. The temperate-dependent self-assembly behavior of [C12mim][CA] and [C12mim][PCA] in aqueous solutions was also studied. Figure S1a (Supporting Information) presents the SAXS spectra for the binary system with the concentration of [C12mim][CA] at 55 wt % (representative of the H1 phase) at the temperature ranging from 25 to 55 °C. The SAXS peaks at higher temperature shift to a higher q position, indicative of a smaller distance between adjacent cylinders. Quantitative analysis of the SAXS patterns with structural parameters (α0, dH, dw, and αs) is presented in Table S4 (Supporting Information), which illustrates the effect of temperature on the H1 phase formed in the [C12mim][CA]/H2O binary system. With increasing temperature, a smaller repetitive lattice spacing appears. This is probably due to the temperatureinducing conformational fluctuation of the surfactant molecules with soft hydrophobic chains, which may shorten the average length of alkyl chains and lead to [C12mim][CA] molecules extruding each other more strongly. It can be reflected by the shrinkage of dH values. The gradually reduced dw values can be attributed to the hydrogen bond interactions among water molecules, which are weakened with the increase of temperature. A similar phenomenon was also observed in the H1 phase formed by other binary systems.24,27

Figure 5. SAXS curves of the H1 phase formed by different concentrations of [C12mim][PCA] in the aqueous solution at 25 °C.

(dW) between cylinders, and the area per molecule at the hydrophilic/hydrophobic interface (αs), were calculated according to the equations shown in the Supporting Information, and they are summarized in Table S3 (Supporting Information). From Table S3, the α0 values are observed to decrease from 46.19 to 43.03 Å when the [C12mim][CA] concentration increases from 50 to 60 wt %, which indicates the distance between two centers of the adjacent columns shows a declining tendency with increasing the concentration of [C12mim][CA]. In comparison to the structural parameters of hexagonal phase formed by the [C4mim][C12H25SO4]/H2O system at the same concentration,26 the H1 phase of [C12mim][CA]/H2O binary system has a lower α0 value. A possible reason for this phenomenon is that the electrostatic screening effect provided by cinnamate anions as well as π−π interactions between imidazolium headgroups and cinnamate anions, play vital roles in decreasing electrostatic repulsions between polar headgroups. As a result, denser cylindrical aggregates with lower α0 values are obtained in the [C12mim][CA]/H2O binary system. Moreover, the dH values increase from 16.62 to 16.91 Å, respectively, while the dW values decrease from 12.95 to 9.22 Å, correspondingly, when the [C12mim][CA] concentration varies from 50 wt % to 60 wt %. These results combined with the tendency of α0 values suggest that the column micellar radius (dH) becomes larger and the solvent layer turns to be thinner with increasing the [C12mim][CA] concentration. That is to say, higher concentration of [C12mim][CA] in the H1 phase gives rise to a denser arrangement of SAIL molecules in a 12602


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Figure 6. Dynamic frequency sweep plots for (a) the H1 phase of 55 wt % [C12mim][CA], (b) the V2 phase of 70 wt % [C12mim][CA], and (c) the H1 phase of 65 wt % [C12mim][PCA] at 25 °C.

weaker hydrogen-bond interactions between water molecules at higher temperature result in decline of the dw value. 3.4. Rheological Properties. In order to investigate the macroscopic physicochemical properties of the LLCs formed in the [C12mim][CA]/H2O and [C12mim][PCA]/H2O binary systems, the rheological measurements were carried out. The data are depicted as plots of the elastic modulus (G′) and viscous modulus (G″) as functions of the oscillatory shear frequency (ω) at a fixed shear stress (Figure 6). Figure 6a exhibits dynamic frequency sweep curves for the H1 phase aggregated in the [C 12 mim][CA]/H 2 O binary system. Throughout the whole investigated frequency region, both elastic modulus (G′) and viscous modulus (G″) are almost frequency-independent. In addition, G′ value is higher than G″, demonstrating that the H1 phase in [C12mim][CA]/H2O system is more elastic than viscous. Such a rheological character shows apparent elastic gel-like behavior,25 incompatible with the classical model of H1 phase. Although it is usual for lamellar phase to display such a rheogram,58 analogous phenomena have also been reported for the H1 phase in C14PDB/H2O, C16PDB/ H2O24 and C16MPB/H2O systems.25 As for the V2 phase formed by [C12mim][CA] in the aqueous solution (shown in Figure 6b), G′ tends to level off and maintains at an apparent plateau value throughout the whole frequency range, while G″ shows a tendency of decline and its value is smaller than that of G′. Therefore, the V2 phase is more elastic than viscous. The predominance of elastic properties is increasingly obvious in the binary system. The rheological curves of the H1 phase formed in the [C12mim][PCA]/H2O binary system are shown in Figure 6c, which are representative for the hexagonal crystal phase and follow the typical Maxwell’s model. At the lower frequency region, G″ is higher than G′, indicating that the H1 phase shows a viscous behavior. With rise of frequency, both G′ and G″ increase and then intersect at ωco. After that, G′ continues to increase and exceeds G″ as the frequency further increases,

To illustrate the effect of temperature on the V2 phase in the [C12mim][CA]/H2O system, the sample with 70 wt % [C12mim][CA] was taken as an example. Figure S1b presents the SAXS spectra for 70% [C12mim][CA] system at different temperature, and it is obvious that at 25 or 45 °C there exists cubic phase (V2) in this binary system on account of the presence of two SAXS scattering peaks with the ratio of √3:√4. With further increasing temperature to 55 °C, the SAXS peaks assigned to cubic phase gradually disappear and characteristic peaks of lamellar phase with relative q position ratio at 1:2 occur, indicating the V2 phase turns into Lα phase. The disorder-to-order transition further suggests the temperature sensitivity of LLCs formed by [C12mim][CA] molecules. In terms of the [C12mim][PCA]/H2O binary system, we chose the sample with 65% [C12mim][PCA] as a representative to investigate the impact of temperature on the H1 phase. Figure S2 exhibits its SAXS patterns at different temperatures. With the temperature increasing from 25 to 45 °C, the SAXS patterns with relative peak positions of 1:√3:2 are unchanged in this binary system and the sample presents a hexagonal columnar structure, implying the thermal stability of the topological structures during this temperature range. In addition, the Bragg scattering peaks move toward higher q values with increasing the temperature, which is similar to the tendency of the H1 phase formed by [C12mim][CA]/H2O system. However, when the temperature was increased to 55 °C, the first Bragg peak decreases and the second peak disappears, indicating the topological structure of LLC basic unit is destroyed. Structural parameters estimated from SAXS curves at 25−45 °C are summarized in Table S4. Similar to the trends of [C12mim][CA]/H2O system, dH and dW values decrease from 16.62 and 7.29 Å to 14.01 and 6.15 Å, respectively. The higher temperature causes the hydrophobic chains of [C12mim][PCA] to soften and interdigitate more vigorously, leading to decrease of the dH value. In addition, the 12603


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can be attributed to the transformation from trans-cinnamate to cis-formation. Upon irradiation of UV light, transformation of trans-cinnamate to cis-formation induces the variation of net dipole moment, which affects the hydrophobic/hydrophilic balance of cinnamate isomers. As previously reported,59 the trans-isomers are always more hydrophobic while the cisisomers are more hydrophilic for varieties of cinnamate derivatives. Therefore, compared to the corresponding transSAILs, the hydrophobicity of cis-[C12mim][CA] and cis[C12mim][PCA] molecules is weakened, which is disadvantaged for the formation of micelles. In addition, under UV light irradiation, the rodlike trans conformation of phenylalkenes in the surfactant molecules transformed to their cis “bent” conformation, which accompanied by the increase of steric hindrance.30 Therefore, the rodlike trans-state of [CA]− and [PCA]− can strongly combined with [C12mim]+ and effectively decrease the electrostatic repulsions between [C12mim]+ headgroups. While the cis “bent” conformation goes against the association with [C12mim]+ due to the bigger steric hinder, and the electrostatic repulsions between hydrophilic headgroups are screened inefficiently. Based on the above two points, the cmc values of [C12mim][CA] and [C12mim][PCA] after UV irradiation are higher than those of SAILs without UV irradiation. The adsorption efficiency (pC20) and effectiveness of surface tension reduction (Πcmc) of the SAILs at the air− liquid interface after UV irradiation are also described in Table S5. After UV irradiation, the Πcmc value of [C12mim][CA] is lower than the sample without UV irradiation, which reveals the effectiveness of decreasing surface tension decreases after UV irradiation. While, the [C12mim][PCA] aqueous solution shows the opposite trend. The pC20 values of [C12mim][CA] and [C12mim][PCA] after UV irradiation both increase, which implies the superior efficiency in decreasing surface tension of water after exposing UV irradiation. After UV irradiation, we also calculated the maximum surface excess concentration (Γmax) and the area occupied by a single surfactant molecule (Amin) at the air−liquid interface. The lower Γmax values and higher Amin values of [C12mim][CA] and [C12mim][PCA] after UV irradiation demonstrate a looser arrangement of the cisisomer molecules at the air−liquid interface. This can be attributed to bigger steric hinder of the cis “bent” conformation hindering close arrangement of surfactant molecules. To examine the effect of UV light irradiation on the LLC phase, SAXS measurements were conducted and the obtained SAXS plots are given in Figure 8. As shown in Figure 8a, before UV irradiation, it exhibits periodic SAXS peaks with relative positions 1:√3:2 for an ordered hexagonal phase. Upon UV light irradiation, the SAXS patterns still show characteristic peaks of H1 phase, which implies the UV light has not changed the aggregate type of the LLC. However, the SAXS peaks after UV light irradiation shift to a higher q position, indicating a shorter distance between adjacent cylinders. For the H1 phase formed by the [C12mim][CA]/H2O binary system, the lattice spacing (α0) values are observed to decrease from 46.19 to 43.47 Å before and after UV light irradiation. Similar decrease trend of structural parameters was also obtained in the [C12mim][PCA]/H2O system (Figure 8b). These results can be attributed to the photoisomerization of phenylalkenes in [C12mim][CA] and [C12mim][PCA] molecules. After UV light irradiation, the trans-cinnamate transforms into cis-formation accompanied by remarkable influence on the aggregation of SAIL molecules. The variation in the structural parameters of hexagonal phase formed by [C 12 mim][CA]/H 2 O and

implying that the sample is dominated by the elastic property. In contrast, G″ decreases with rise of frequency. At last, G′ tends to stabilization and reaches a limiting constant value at the high frequency, while G″ drops to a minimum, and then increases slightly again. These results imply that H1 phase formed in the [C12mim][PCA]/H2O system exhibits a viscoelastic behavior, which is analogous to the previous reports on H1 phase formed in C12PDB/H2O,24 C16MPB/ EAN25 as well as C12mimSal/H2O mixtures.27 The different rheological properties of the H1 phase formed in [C12mim][CA]/H2O and [C12mim][PCA]/H2O binary systems, respectively, can be attributed to the existence of hydroxyl in [C 12 mim][PCA] molecule. Compared to [C12mim][PCA], [C12mim][CA] has weaker steric volume, which easily leads to more attraction among different cylinders in the H1 phase. Such a cylinder conglutination behaves like the layers in the lamellar phase and therefore the H1 phase formed by [C12mim][CA]/H2O system exhibits homologous rheological properties similar to that of the lamellae. 3.5. Effect of UV Light Irradiation on Phase Behavior. According to the previous reports,59 phenylalkenes is a class of typical photoisomerization compounds, such as orthomethoxycinnamic acid (OMCA), para-coumaric acid (PCA), cinnamic acid (CA), etc. Since both [C12mim][CA] and [C12mim][PCA] molecules have a photosensitive phenylalkene unit, it is anticipated that both of the [C12mim][CA]/H2O and [C12mim][PCA]/H2O systems may form photoresponsive aggregates. To examine the effect of UV light irradiation on the aggregation behavior of [C12mim][CA] and [C12mim][PCA] in aqueous solution, surface tension measurements were performed to characterize their surface activities and micellization properties in the aqueous solution after UV irradiation. The plots of surface tension (γ) at 25 °C as a function of concentration (C) for [C12mim][CA] and [C12mim][PCA] after UV irradiation are depicted in Figure 7. The micellar and interfacial parameters for [C12mim][CA] and [C12mim][PCA] are listed in Table S5 (Supporting Information).

Figure 7. Surface tension plots as a function of concentration for [C12mim][CA] and [C12mim][PCA] in the aqueous solution upon UV irradiation at 25 °C, respectively.

As shown in Table S5, the cmc values of [C12mim][CA] and [C12mim][PCA] in the aqueous solution after UV irradiation are 2.7 and 3.2 mmol·L−1, respectively, which is apparently higher than those of the values without UV irradiation. This suggests that after UV irradiation the SAILs studied here have inferior surface activity to form micelles. The obvious change 12604


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Figure 8. SAXS patterns for the LLCs before and after UV irradiation at 25 °C: (a) 50 wt % [C12mim][CA] /H2O and (b) 65 wt % [C12mim][PCA]/H2O binary systems.

For the [C12mim][CA]/H2O and [C12mim][PCA]/H2O systems studied in this work, the hydrophilic headgroups are identical. The only discrepancy lies in the alteration of the counterions. The electrostatic potentials at the 0.001 e/bohr3 isodensity surfaces of [CA]− and [PCA]− were calculated (Figure 10). It is apparent that they are both electronegative.

[C12mim][PCA]/H2O systems upon UV light irradiation may be due to the molecular-geometrical changes or the difference in hydrophobicity of the two isomers of the SAILs. 3.6. DFT Calculations. In order to better understand the electrostatic interactions between the imidazolium cation and the cinnamate counterion, we also performed density functional theory (DFT) calculations via Gaussian 09 package using a hybrid functional B3LYP with the basis 6-31G (d,p).60 The optimized structural models for the [C12mim][CA] and [C12mim][PCA] are shown in Figure 9. Through DFT

Figure 10. B3LYP/6-31G (d, p) electrostatic potentials, in Hartrees, at the 0.001 e/bohr3 isodensity surfaces of (a) [CA]− and (b) [PCA]−.

On one hand, the more electronegativity of [PCA]− indicates that the electrostatic attractions between it and [C12mim]+ is higher than that of [CA]− and [C12mim]+, which is more favorable to the combination [PCA]− and [C12mim]+. On the other hand, compared to [CA]−, the bigger steric volume of [PCA]− impedes the bind between it and [C12mim]+. The lower β value of [C12mim][PCA] (Table S2) implies the later factors plays a dominant role. The electrostatic repulsions between the cation of SAILS are screened less effectively by [PCA]−, which is unfavorable to the formation of micelles and results in a higher cmc value compared to [C12mim][CA]. 3.7. Mechanisms of the Formation of the LLC Phase. It is well-known that the theory of molecular packing parameter (P) (shown in eq 2),62 introduced by Israelachvili et al., is usually employed to explain and predict the aggregate geometry of surfactant solutions. v P= (2) la

Figure 9. Geometries of (a) [C12mim][CA] and (b) [C12mim][PCA] molecules optimized using the polarizable continuum model at the B3LYP/6-31G(d,p) level.

calculation, the length of hydrophobic chain is calculated to be 15 Å, which is approximately equal to the theoretical value (16.68 Å) according to the Tanford equation below.61 L = 1.5 + 1.265n

(1)

where n is the number of carbon atoms in the hydrocarbon chain. For the [C12mim][CA]/H2O and [C12mim][PCA]/H2O systems, the radius of a cylindrical aggregate (dH) is about 17 Å (Table S4), which implies that the hydrophobic alkyl chain of SAILs is freely stretching in the hexagonal phase. To quantify the difference in the interactions between [C12mim][CA]/H2O and [C12mim][PCA]/H2O, the interaction energies (Eint) for the two binary systems were also calculated by DFT calculations, which are −24.67 and −20.20 kJ/mol, respectively. The less negative interaction energy between [C12mim][PCA] and H2O molecules indicates weaker stability for the [C12mim][PCA]/H2O complex, which is much easier to be destroyed. This is favorable to the formation of micelles. However, compared to [C12mim][CA], the steric volume of the additional hydroxyl group of [C12mim][PCA] is unfavorable to the formation of aggregates. The higher cmc value of [C12mim][PCA] indicates the dominant role played by the later factor.

Here ν, l, and a are the volume and length of hydrophobic chain, and the effective headgroup area of surfactant, respectively. For the surfactant solution, the value of P can be used to speculate the type of the formed aggregates, including spherical micelles (P < 1/3), rodlike or wormlike micelles (1/3 < P < 1/2), bilayers, vesicles, and bicontinuous cubic LLC phases (1/2 < P < 1), planar bilayers (P ≈1), inverted micelles (P > 1). As for the [C12mim][CA] and [C12mim][PCA] molecules, the volume of hydrophobic chain V can be estimated from the Tanford equation61 12605


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where n is the number of carbon atoms in the hydrocarbon chain. Therefore, the V value of the SAILs prepared in this work is calculated to be 350.2 Å. Through DFT calculations, the length of hydrophobic chain is 15 Å (Figure 9). According to the results of surface tension measurements, the values of effective headgroup area (a) for [C12mim][CA] and [C12mim][PCA] are 76 and 41 Å2, respectively. Based on these parameters, the corresponding packing parameter P calculated is 0.31 and 0.57, respectively. Hence, we can predict that [C 12mim][CA] prefers to form spherical micelles and [C12mim][PCA] tends to construct bilayers, vesicles, or bicontinuous cubic LLC phases. However, compared to the traditional SAILs (e.g., C12mimBr), for [C12mim][CA] and [C12mim][PCA], there exist π−π interactions between the cinnamate anion and imidazolium cation. The electrostatic, hydrophobic, and π−π interactions synergistically affect the self-assembly behavior of [C12mim][CA] molecules in the aqueous solution, which induces that the electrostatic repulsions between imidazolium headgroups are effectively screened. Therefore, the P value increases significantly, resulting in the transition from spherical micelles to the LLC phase with rodlike hexagonal arrays. Although [C12mim][PCA] molecules also have these cooperative interactions, the steric volume of hydroxyl plays an important role in weakening the electrostatic repulsions between polar headgroups. Thus, the P value decreases but the aggregates still exhibit the hexagonal phase.

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AUTHOR INFORMATION

Corresponding Author

*Phone number: +86-531-88364807. Fax number: +86-53188564750. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2014GSF117001), and the Natural Science Foundation of Shandong Province of China (No. ZR2011BM017).

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4. CONCLUSIONS In summary, two structurally similar photoresponsive SAILs, [C12mim][CA] and [C12mim][PCA], were synthesized, and their phase behavior was systematically investigated by various techniques, e.g., surface tension, electrical conductivity, POM, SAXS, and rheology measurements. The results show both of the newly synthesized SAILs exhibit higher surface activity than the traditional SAILs such as C12mimBr. [C12mim][CA] can form hexagonal liquid crystalline phase (H1) as well as cubic liquid crystalline phase (V2) in the aqueous solution. However, the [C12mim][PCA]/H2O binary system only displays a single hexagonal liquid-crystalline phase (H1) in a broad concentration range. Quantitative analysis of SAXS patterns reveals that the increase of concentration of SAILs leads to a denser molecular arrangement, while the increase of temperature results in the opposite tendency. Upon UV irradiation that induced the trans−cis photoisomerization of phenylalkenes, the prepared SAILs formed micelle with higher cmc values, which indicates the inferior surface activity. In addition, self-assembled LLC phases stayed the same but their structural parameters obviously changed after UV light irradiation. The SAILs with light-responsive behavior investigated in this work are expected to have potential applications in drug delivery, biochemistry, and material science.

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the aqueous solution at 25 °C, theory for the calculation of thermodynamic parameters of micellization, structural parameters of liquid-crystalline phases, table of thermodynamic parameters of micelle formation for [C12mim][CA] and [C12mim][PCA] in the aqueous solution at different temperature, table of structural parameters of the H1 phase formed by SAILs in the aqueous solution with different concentrations at 25 °C, SAXS curves of samples at different temperature, table of structural parameters of the H1 phase formed by the synthesized SAILs in aqueous solutions at different temperature, and table of surface property parameters of [C12mim][CA] and [C12mim][PCA] in the aqueous solution after UV light irradiation at 25 °C. (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03216. More details about 1H NMR spectroscopy, chemical structures of prepared SAILs, table of surface property parameters of [C12mim][CA] and [C12mim][PCA] in 12606


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Aggregation Behavior of Imidazolium-Based Surface-Active Ionic

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