Wormlike Micelles with Photoresponsive Viscoelastic Behavior

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Wormlike Micelles with Photoresponsive Viscoelastic Behavior Formed by Surface Active Ionic Liquid/Azobenzene Derivative Mixed Solution Yanhui Bi,† Hongtu Wei,‡ Qiongzheng Hu,§ Wenwen Xu,† Yanjun Gong,† and Li Yu*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China ‡ China Research Institute of Daily Chemical Industry, Taiyuan 030001, People’s Republic of China § Department of Chemistry, University of Houston, Houston, Texas 77204, United States S Supporting Information *

ABSTRACT: The UV-light-stimulated self-assembly behavior of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), with an azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa), was investigated in aqueous solution. The properties and structures of the aggregates, formed at a concentration ratio equal to 2:1 ([C16mimBr]:[AzoCOONa]), were comprehensively characterized by rheometer and cryogenic transmission electron microscopy. Initially, viscoelastic wormlike micelles with a viscosity of 0.65 Pa·s were constructed in the C16mimBr/AzoCOONa system. Upon irradiation by UV light (365 nm), particularly fascinating is that the wormlike micelles become much longer and more entangled, exhibiting a high viscosity of 6.9 Pa·s. This can be attributed to photoisomerization of the AzoCOONa molecule from trans to cis form. It is the first time that, with exposure to UV or visible light, the aggregate type of the photoresponsive system has remained unchanged, with only a change of internal property parameters. The cation−π interaction prevailing over the hydrophobic interaction and electrostatic interaction between C16mimBr and AzoCOONa molecules is supposed to be responsible for this peculiar phase behavior. The wormlike micelles constructed with the SAIL and photosensitive additive exhibit controllable viscoelastic behavior in the photoresponsive process. In addition, the average contour length of wormlike micelles was found to slightly decrease with the increase of temperature. We expect this system will receive particular attention due to its unique properties and potential applications in drug delivery, biochemistry, and materials science, etc.

1. INTRODUCTION It is well-known that surfactants can self-assemble into various aggregates in aqueous solution, such as spherical micelles, wormlike micelles, vesicles, and liquid crystals.1−3 They are widely used as detergents in daily application, carriers for drug delivery,4 a sieving matrix for separation of DNA fragments,5 or even templates for the synthesis of materials.6 Among these aggregate structures, wormlike micelles have attracted great interest in fundamental research and practical application over the past few decades.7−10 Under specific environmental conditions, such as appropriate temperature, concentration, and salinity, small spherical micelles could grow along the direction of one dimension into long, thread-like, and flexible entangled wormlike micelles. The entanglement of such aggregates resulting in remarkable viscoelastic properties is analogous to that of flexible polymers in aqueous solution. However, wormlike micelles exist in a dynamic equilibrium, and thus the network constantly breaks and recombines, which is different from the polymer.11 Due to these unique properties, wormlike micelles have been widely applied in numerous fields such as tertiary oil recovery, rheology control,12 drag-reducing agents, and daily care products.13 © 2015 American Chemical Society

Recently, much attention has been paid to smart wormlike micelles,14 whose macroscopic physicochemical properties undergo an instantaneous and radical change in response to external stimuli. According to the review by Chu et al.,14 the external stimuli generally includes redox reaction,15 UV/vis light,16 temperature,17 pH,18 CO2,19 hydrocarbon,20 and combined stimuli.21 Among these external stimuli, UV/vis light presents superior advantages and has been widely applied. First, compared with the other stimuli (e.g., redox reagent, pH change, salinity, or stress), light stimulus has no additives to the samples, which avoids changes in the composition or thermodynamic conditions. Besides, light is readily available and has been extensively applied both in fundamental and practical applications. Last, but not least, superior to electric and magnetic fields, light can be directed at a precise spatial location, which is particularly worthy in nanoscience and nanotechnology applications. Received: January 11, 2015 Revised: February 23, 2015 Published: March 12, 2015 3789

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photocontrollable viscoelasticity, which could lead to expansion of their application in the fields of drug delivery, oilfields, and biochemistry, etc.

Therefore, light considered as an ideal external stimulus has driven many scientists to investigate the light-induced selfassembly behavior in aqueous solutions over the past decades. Sakai et al. used a mixed aqueous solution of an azobenzenemodified cationic surfactant and an anionic surfactant to control the formation and disruption of vesicles by light.22 Raghavan et al. obtained a simple type of photorheological fluid which underwent a transformation from long wormlike micelles into shorter micelles.23 They also observed the transition from vesicles to wormlike micelles by using a mixture of 4azobenzene carboxylic acid (ACA) and erucylbis(2hydroxyethyl)methylammonium chloride (EHAC) that led to nearly a million-fold increase in viscosity upon exposure to UV light.24 Eastoe’s group utilized a photolyzable anionic surfactant, 4-hexylphenylazosulfonate (C6PAS), to manipulate the ordering behavior of lyotropic lamellar (Lα) phases by UV light.25 Huang et al. created photomodulated multistate and multiscale molecular self-assemblies by utilizing a binary-state molecular switch in cetyltrimethylammonium bromide (CTAB) aqueous solution mixed with sodium (4-(phenylazo)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.26 Wang et al. investigated a photoresponsive aqueous system of tetradecyldimethylamine oxide (C14DMAO) mixed with p-coumaric acid (PCA), and found the aggregation transition from bilayer vesicles into wormlike micelles.16 In recent years, ionic liquids (ILs) have attracted increasing attention due to their specific physicochemical properties, e.g., high ionic conductivity, negligible vapor pressure, and designability. Ionic liquids with long alkyl side chains are generally regarded as amphiphilic compounds and named surface active ionic liquids (SAILs). The self-organization of imidazolium-type SAILs, particularly 1-alkyl-3-methylimidazolium salts, has significant effects on a variety of processes such as the extraction of compounds from IL-containing systems, the synthesis and purification of bulk ILs, the formation of dispersed or phase separated systems, and the aqueous biphase or multiphase liquid−liquid equilibrium systems.27−29 Many researchers,30−33 including our group,34−36 have extensively studied the self-assembled aggregates (e.g., micelles, vesicles, microemulsions, and lyotropic liquid crystals) that are formed by imidazolium-based SAILs in aqueous solution. For example, Huang’s group reported wormlike micelles constructed by 1hexadecyl-3-methylimidazolium bromide (C16mimBr) in the presence of salicylate sodium (NaSal) and found an impressive thermoresponsive transition behavior between micelles and elastic hydrogels.32 Zheng et al. reported viscoelastic wormlike micelles in a photomodulated molecular self-assembled system of C16mimBr/trans-cinnamic acid (trans-CA).33 In this work, viscoelastic wormlike micelles were constructed by an imidazolium-based SAIL, C16mimBr, and a photosensitive azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa). Interestingly, upon UV-light irradiation, the molecular self-assemblies are still wormlike micelles, only accompanied by a surprising increase in viscoelasticity, suggesting the formation of a more elongated and entangled architecture. The UV-light-induced trans-/cis-AzoCOONa molecules play a crucial role in the molecular self-assembly of C16mimBr in aqueous solution. Although varieties of lightresponsive systems have been reported previously, it is the first time for observation of the formation of wormlike micelles with

2. EXPERIMENTAL SECTION 2.1. Materials. The SAIL, C16mimBr, was synthesized and purified according to the methods reported in the literature.37 The product was purified by recrystallization from ethyl acetate five times and then dried under vacuum for 2 days. The structure of C16mimBr was ascertained by 1H NMR spectroscopy with a Bruker Avance 300 spectrometer. Azobenzene-4-carboxylic acid (greater than 98% in purity) purchased from TCI was dissolved in hot alcohol, which was then added to aqueous solution of equimolar NaOH (Sinopharm Chemical Reagent Co.) and stirred about 4 h to obtain sodium azobenzene 4-carboxylate (AzoCOONa). After removing the solvent by reduced pressure distillation and then drying under vacuum for 2 days, the final product was obtained. The deionized water was distilled three times. 2.2. Sample Preparation. C16mimBr solutions were obtained by dissolving different amounts of solid SAIL product directly in deionized water, followed by the addition of desired amounts of AzoCOONa. Since AzoCOONa has poor solubility in water and is difficult to dissolve at room temperature, the samples were heating to 45−50 °C, accompanied by gentle agitation until AzoCOONa was dissolved completely. Homogeneous solutions with orange color were prepared after continuous heating and stirring. The samples were put in a thermostat at 25 °C for at least 2 weeks to reach equilibrium. 2.3. UV-Light Irradiation. For light-triggered trans/cis transition, solution samples were irradiated with a CHF-XM35-500W ultrahigh pressure short arc mercury lamp with optical filter (365 nm). Samples were placed in a quartz tumbler, and irradiation was applied for a specific duration under stirring. To avoid overheating, experimental temperature was kept at 25 °C using a thermostat water bath, and the distance between the sample and light source was fixed at 10 cm. 2.4. 1H NMR Measurements. 1H NMR spectra were recorded on a Bruker Avance 300 nuclear magnetic resonance spectrometer (300 MHz). The samples were placed in 5 mm BBO probe with D2O as solvent, and the 1H NMR measurements were conducted at 25.0 ± 0.1 °C. 2.5. UV−Vis Spectroscopy. The UV−vis spectroscopy measurements of solutions before and after UV irradiation were carried out using a UV-4100 spectrophotometer at room temperature. The deionized water was utilized as a blank in the experiments. 2.6. Rheological Measurements. Rheological properties were determined on a HAAKE RS6000 rheometer equipped with a coaxial cylinder sensor system (Z41Ti). The experimental temperature was controlled by a cyclic oil bath (Phoenix) within an error of ±0.1 °C. The sample thickness in the middle of the cylinder sensor was 3.0 mm. In oscillatory measurements, dynamic frequency spectra were conducted in the linear viscoelastic regimes of each sample as determined from dynamic stress-sweep measurements. For the steadyshear experiments, the range of shear rate is from 0.01 to 100 s−1. The zero-shear viscosity for samples was obtained by extrapolation of the viscosity curve to the vertical coordinates. All of the samples were equilibrated at the experimental temperature for at least 15 min prior to measurement. The light-illuminated samples were wrapped with aluminum foil and loaded in near-dark conditions to prevent exposure to visible light. 2.7. Cryo-Transmission Electron Microscopy. A cryo-transmission electron microscopy (cryo-TEM) observation of C16mimBr/ AzoCOONa solutions was carried out in a controlled-environment vitrification system (CEVS). A micropipette was utilized to load a 5 μL solution onto a TEM copper grid, which was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on the mesh holes. After waiting for about 5 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by nitrogen) at −165 °C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 TEM (120 kV) at about −174 °C. 3790

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Figure 1. Molecular structures of trans-AzoCOONa (a) and cis-AzoCOONa (b); 1H NMR spectra of AzoCOONa in D2O before (c) and after (d) UV-light irradiation. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with Digital Micrograph.

3. RESULTS AND DISCUSSION 3.1. Photoisomerization of AzoCOONa. It has been wellcharacterized that azobenzene group can change its conformation in response to UV-/vis-light irradiation. When irradiated by UV light, the molecular structure of AzoCOONa, with each aromatic proton numbered, can undergo trans/cis transition, as shown in Figure 1a,b. The photoisomerization behavior of AzoCOONa was evaluated by 1H NMR. Parts c and d of Figure 1 represent the chemical shifts of aromatic protons in azobenzene group before and after UV-light irradiation, respectively. The 1H NMR spectra of AzoCOONa show three groups of peaks for each trans and cis form before (Figure 1c) and after (Figure 1d) UV-light irradiation. Before the UV-light irradiation, the integral area of peaks corresponding to the trans-AzoCOONa is apparently higher than that of the cis form. As calculated from 1H NMR spectra of the relevant aromatic protons peaks in anionic azobenzene group, there are approximately 85.2% trans-AzoCOONa and 14.8% cisAzoCOONa. When the sample was exposed to UV light for 20 min, the intensity of peaks corresponding to the trans conformation notably decreased whereas that of the cis isomer drastically increased, as shown in Figure 1d. The UV-lightirradiated sample thus corresponds to a photostationary equilibrium of the most cis (ca. 82.3%) molecules. Besides, compared to trans-AzoCOONa, the chemical shifts for aromatic protons of the cis form are located upfield, which can be interpreted by the magnetically anisotropic effect.26 To further elucidate the trans/cis transition, UV−vis spectroscopy was performed to study the light-responsive process. Figure 2 shows the absorption spectra of AzoCOONa aqueous solution (0.1 mM) after different UV-light-irradiation times. Before UV illumination, AzoCOONa solution displayed dominate absorption at about 325 nm which is ascribed to the π−π* transition of the trans-azobenzene moiety.26 As expected, UV stimulus gave rise to a remarkable decrease in intensity and obvious blue shift of the absorption peak as well as concomitant with a slight increase of band at about 430 nm. This can be attributed to the n−π* transition of the cis-azobenzene. It can

Figure 2. UV−vis absorption spectra of AzoCOONa solution (0.1 mM) at different UV-light-irradiation times.

also be found that the absorption spectra were unvaried by the UV-light irradiation after 20 min, showing that a photostationary state was attained on this time scale. The UV−vis results strongly demonstrate the photoisomerization of azobenzene from the trans to the cis state. 3.2. Photoresponsive Wormlike Micelles Based on C16mimBr and AzoCOONa. As proved by early reports, spherical micelles formed by C16mimBr in aqueous solutions could be converted into viscoelastic wormlike micelles in the presence of certain additives such as aromatic counterion salts.32,38 Besides, AzoCOONa could be considered as a binarystate molecular switch due to its special trans/cis isomerization of the azobenzene unit upon UV-light irradiation. Based on the preceding two points, it is anticipated that the C16mimBr/ AzoCOONa mixed solution can form photoresponsive aggregates with certain state and scale. We found that when a certain amount of AzoCOONa was added to C16mimBr aqueous solution, the binary mixtures appeared to be transparent and gel-like viscoelastic fluids initially. However, the sample gradually became more viscous as the UV-lightillumination time was extended, and the viscosity was unchanged after irradiation for 2.5 h. The increase in viscosity may result from a change of the aggregation state and the scale of molecules in the binary system. Since azobenzene can 3791

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Figure 3. Rheograms for aqueous solutions of C16mimBr (60 mM) and AzoCOONa (30 mM) before the UV-light irradiation at 25 °C: (a) steadyshear rheology showing the shear viscosity (η) as a function of the shear rate (γ̇); (b) dynamic frequency spectra exhibiting elastic (G′) and viscous (G″) modulus as a function of the oscillatory shear frequency (ω).

with the increase of frequency. At last, G′ tends to level off and reaches an apparent plateau value at the high frequency which is generally regarded as G0, while G″ drops to a minimum and then increases slightly again. This minimum point of G″ is also determined as Gmin ″ . Such analogous rheological behavior has also been observed at the molar ratio of C16mimBr and AzoCOONa at 2:1, with different concentrations of C16mimBr and AzoCOONa (Supporting Information Figure S1). This trend follows the typical Maxwell’s model, indicative of formation of viscoelastic wormlike micelles.41,42 For Maxwell fluid, G′ and G″ can be expressed by the following formulas:

undergo photoinduced isomerism coupled with apparent structural change, which directly affects the packing behavior of AzoCOONa and C16mimBr in aqueous solution while the concentration remains constant. Shorter wormlike micelles were obtained when AzoCOONa was in its trans form. In contrast, more and longer entangled wormlike micelles were constructed after the UV-light irradiation when transAzoCOONa was light-isomerized to cis form. Although the type of the aggregates remained unaltered, the scale and number density might vary. Rheological measurement was conducted to confirm the preceding hypothesis. The steady-shear measurements of the sample containing C16mimBr (60 mM) and AzoCOONa (30 mM) were first carried out before the UV-light irradiation. Figure 3a illustrates the steady-shear rate curve showing shear viscosity (η) as a function of shear rate (γ̇). Overall, the solution viscosity is independent of the shear rate at a low shear rate, while the viscosity remarkably reduces with the increase of the shear rate at a high shear rate above a threshold. At the low shear rate, the viscosity remains constant and this plateau value is considered as zero-shear viscosity (η0) which is 0.65 Pa·s in the binary mixture system before the UV-light irradiation. When the shear rate becomes high, the viscosity of the C16mimBr and AzoCOONa mixture decreases, showing a notable shearthinning phenomenon, which is also observed in the wormlike micelles formed by traditional cationic surfactants and salts.10,39 The typical shear-thinning behavior has been taken as evidence for the formation of long wormlike micelles,40 which is induced by the different flow states of wormlike micelles at different shear rates.17 At the low shear rate, wormlike micelles behave like Newtonian fluid, exhibiting a constant viscosity. However, at the high shear rate, the decline of viscosity can be attributed to the alignment of wormlike chains under the direction of flow.17 In addition to steady-shear rheology, oscillatory shear measurements were also performed to investigate the rheological response of the samples. Figure 3b exhibits dynamic frequency spectra of the C16mimBr (60 mM)/AzoCOONa (30 mM) mixture before the UV-light irradiation. 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 (σ = 1 Pa). As shown in Figure 3b, at the lower frequency region, G″ is larger than G′, indicating that the wormlike micelles show a viscous behavior. With the increase of the 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, implying that the solution is dominated by the elastic properties. In contrast, G″ decreases

G′(ω) = G0

ω 2τR 2 1 + ω 2τR 2

G″(ω) = G0

(1)

ωτR 1 + ω 2τR 2

(2)

Here, ω is the angular frequency. The relaxation time τR is estimated as eq 3, and G0 is the plateau modulus of G′ (ω) at the high frequency. In some systems investigated, G0 does not reach a constant value at high frequency. In these cases, the plateau modulus (G0) can be calculated from eq 4. ωco and G* represent the angular frequency and modulus of the intersection point when G′ is equal to G″, respectively. τR =

1 ωco

G0 = 2G*

(3) (4)

The consistence of these rheological data with the Maxwell model can also be illustrated by a semicircular shape of the Cole−Cole plot. The Cole−Cole plots of G″ against G′ can be obtained by the following equation: ⎛ G ⎞2 ⎛ G ⎞2 G ″ + ⎜G − 0 ⎟ = ⎜ 0 ⎟ ⎝ ⎝ 2 ⎠ 2 ⎠

(5)

Figure 4 shows the Cole−Cole plots for the C16mimBr/ AzoCOONa (molar ratio = 2:1) aqueous solutions with different concentrations at 25 °C. The concentrations of C16mimBr are 40, 50, 60, and 80 mM, and the corresponding concentrations of AzoCOONa are 20, 25, 30, and 40 mM, respectively. In Figure 4, original experimental results are shown in points. Then, solid lines are calculated and fitted according to eq 5. Obviously, at low frequencies, all of the dynamic data fit perfectly with the semicircles, indicating the presence of viscoelastic wormlike micelles in the solutions before the UV-light irradiation.43 However, the high frequency 3792

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viscosity represents that the length of wormlike micelles becomes longer in the solution. Oscillatory frequency sweep measurements were performed in order to further explore the viscoelastic properties of these irradiated viscous samples. The plots of G′ and G″ as a function of ω for aqueous solution containing C16mimBr (60 mM) and AzoCOONa (30 mM) after different UV-light-irradiation times are presented in Figure 6. The sample irradiated for different

Figure 4. Cole−Cole plots for the C16mimBr/AzoCOONa (2:1 molar ratio) aqueous solutions with different concentrations at 25 °C.

data slightly deviates from the semicircle Cole−Cole plots, such as the upturn of G″ at higher frequency shown in Figure 3b. This phenomenon is generally interpreted as the existence of Rouse modes or “breathe modes”,44 which are usually observed in other viscoelastic systems.39 Next, the evolution of sample rheology as a function of irradiation time was investigated. The viscosity versus steady rate profiles for the mixture of C16mimBr (60 mM) and AzoCOONa (30 mM) after various periods of UV-light irradiation (365 nm) are presented in Figure 5. According to

Figure 6. Dynamic frequency sweep of the C16mimBr (60 mM)/ AzoCOONa (30 mM) mixture upon exposure to UV light (365 nm) at different irradiation times: before (a) and after the UV-light irradiation for 0.5 (b), 1 (c), 1.5 (d), 2 (e), and 2.5 h (f), respectively.

times displays the typical characteristic of the viscoelastic behavior of wormlike micelles that follows Maxwell’s model, which is analogous to the sample without the UV-light irradiation. The properties of wormlike micelles rely on their structures, especially the average contour length of wormlike micelles generally representing Maxwell fluid behavior, and it can be calculated according to the following equation:46

Figure 5. Viscosity versus steady rate for the C16mimBr (60 mM)/ AzoCOONa (30 mM) mixture exposed to 365 nm UV light at different irradiation times at 25 °C.

G0′ L ≈ ″ Gmin le

(6)

where L is the average contour length and le is the average length between two entanglement points. Gmin ″ is the minimum of loss modulus in the high frequency region and G0′ is the plateau modulus of G′(ω). Although, le is not currently available, as a comparison, a typical value of 80−150 nm for wormlike micelles can be adopted47 to estimate L. Based on the preceding equations, the rheological parameters of C16mimBr/AzoCOONa wormlike micellar systems with different UV-light-irradiation times are listed in Table 1. It can be found that the value of G0′ rises with the increase of UVlight-irradiation time, which may be coupled to the micellar growth, resulting in longer micelles that are more entangled,48 because G′0 relies on the number of aggregates. The gradual increase of the average contour length (L) also demonstrates that the growth of wormlike micelles is enhanced by the UV-

the preceding rheological results, it can be inferred that wormlike micelles were formed in the C16mimBr/AzoCOONa aqueous solution before irradiation. When the sample was irradiated with UV light for different times, similar to the sample without the irradiation, the shear-thinning behavior was also observed with the increasing shear rate, implying the formation of wormlike micelles.45 Additionally, it was observed that η0 of the sample gradually increased with the extension of UV-light irradiation time. The η0 value reached the maximum after irradiation for 2.5 h and remained an almost constant value with the further increase of UV-light-irradiation time. Consequently, the zero-shear viscosity rose about 1 order of magnitude after the UV-light irradiation. Since longer micelles have the higher zero-shear viscosity, the drastic increase of 3793

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Table 1. Rheological Parameters of Wormlike Micelles at Different UV-Light Irradiation Time for Aqueous Solution with C16mimBr (60 mM) and AzoCOONa (30 mM) at 25 °C irradiation time (h)

η0 (Pa·s)

G0′ (Pa)

Gmin ″ (Pa)

ωco (rad·s−1)

τR (s)

L (nm)

0 0.5 1.0 1.5 2.0 2.5

0.651 0.893 2.48 3.61 4.21 6.91

3.85 13.1 16.9 17.5 18.1 18.2

2.21 7.42 5.90 4.71 3.22 2.55

2.15 4.64 1.00 0.77 0.46 0.28

0.47 0.22 1.00 1.30 2.17 3.57

139−261 141−265 229−430 297−557 450−843 571−1071

Figure 7. Cryo-TEM images of the C16mimBr (60 mM) and AzoCOONa (30 mM) mixture before (a) and after (b) the UV-light irradiation for 2.5 h.

Table 2. Rheological Parameters of Wormlike Micelles Formed in the Mixture of C16mimBr (60 mM) and AzoCOONa (30 mM) at Different Temperatures temp (°C)

η0 (Pa·s)

G0′ (Pa)

Gmin ″ (Pa)

ωco (rad·s−1)

τR (s)

L (nm)

15 20 25 30

2.06 1.40 0.713 0.425

2.28 3.19 3.85 4.12

1.10 1.66 2.21 2.55

0.215 0.599 2.15 3.59

4.65 1.67 0.465 0.278

166−311 154−288 139−261 129−242

On the basis of the rheological and cryo-TEM experiments, it is demonstrated that the long and flexible wormlike micelles exhibiting viscoelastic properties are presented in C16mimBr and AzoCOONa binary mixtures without the UV-light irradiation. Longer and more entangled wormlike micelles are obtained upon irradiation with UV light. To the best of our knowledge, the photoresponsive systems reported to date generally undergo the transformation of different self-assembly aggregates upon UV-light irradiation.16,20,22−26 It is quite interesting that our system can remain the same type of aggregates upon exposure to UV light, only accompanied by an increase in viscoelasticity. 3.3. Temperature Effect. The effect of temperature on the viscoelasticity of the wormlike micelles formed in the mixture of C16mimBr (60 mM) and AzoCOONa (30 mM) was also investigated in the range from 15 to 30 °C. The rheological curves are presented in Supporting Information Figure S2 and Figure S3. The corresponding calculated parameters of wormlike micelles are listed in Table 2. Similar to the previous reports,42,45 the average contour length (L) value of wormlike micelles formed in C16mimBr/AzoCOONa mixed solution reduces with the increase of temperature because the transformation of surfactant molecules between the cylindrical body and the hemispherical end of wormlike micelles becomes more rapid at high temperature.39,50 In addition, both η0 and τR values decrease with the increasing temperature. The alteration

light irradiation, which is consistent with the results of zero viscosity derived from the steady-state shearing rheological measurements. As shown in Table 1, in the range of irradiation time investigated, the highest L value of the C16mimBr/ AzoCOONa wormlike micellar system can reach 571−1071 nm. This is almost close to the longest average contour length of wormlike micelles formed in the N-methyl-N-cetylpyrrolidinium bromide (C16(MP)Br)/sodium laurate (SL) system (991−1304 nm),49 whereas shorter than that of wormlike micelles formed in the (3-(hexadecyloxy)-2-hydroxypropyl)trimethylammonium bromide (R16HTAB)/NaSal system (several thousand nanometers).10 In order to investigate the microstructural features of the aggregates formed in the C16mimBr/AzoCOONa system before and after UV-light irradiation, the cryo-TEM technique was used. Figure 7 shows the cryo-TEM images of aqueous solution with C16mimBr (60 mM) and AzoCOONa (30 mM) before and after UV-light irradiation for 2.5 h at room temperature, respectively. Figure 7a clearly demonstrates the formation of elongated flexible wormlike micelles in this viscoelastic solution before the UV-light irradiation. In contrast, the length of aggregates increases significantly after irradiation of UV light (Figure 7b). A three-dimensional network is constructed by the intertwining of longer and more entangled wormlike micelles. This result testifies to the speculation derived from the rheological measurements. 3794

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P=

(8)

Here ν, l, and a are the volume and length of the hydrophobic chain and the effective headgroup area of surfactant, respectively. In 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), vesicles (1/2 < P < 1), planar bilayers (P ≈ 1), and inverted micelles (P > 1). As reported,55 spherical micelles can form in the C16mimBr aqueous solution. Upon addition of AzoCOONa, its azobenzene group is vertically embedded in the hydrophobic interior of micelles.24 At the micelle/water interface, the association of the carboxylate anion of AzoCOONa molecule and the imidazolium cation of C16mimBr molecule through electrostatic interaction will reduce the micellar surface charge, which leads to a decrease of a value. This facilitates the growth of long, cylindrical micelles and makes the value of P increase and fall into the range of 1/3 < P < 1/2. Additionally, it has been reported that the key point of micellar growth is the degree of tight association of the aromatic counterion of the additive with the headgroup of surfactant at the micelle/water interface.20,23 After the UV-light irradiation, trans-AzoCOONa transforms into the cis formation accompanied by a large structural change, which remarkably influences the association of C16mimBr and AzoCOONa. The interactions between the azobenzene group of AzoCOONa and imidazolium cation of C16mimBr are discussed in detail later. Upon irradiation of UV light, transformation of transAzoCOONa to cis formation induces the variation of net dipole moment, which affects the hydrophobic/hydrophilic balance of azobenzene isomers.26 As previously reported,16,26,56 the trans isomers are always more hydrophobic while the cis isomers are more hydrophilic for varieties of azobenzene derivatives. In addition, it is reported that cis-azobenzene derivative has a higher steric hindrance than the trans transformation.26 As a result, the trans-AzoCOONa can strongly penetrate into the surfactant aggregates and the cisAzoCOONa is not available to intercalate into the aggregates and fails to accelerate the close packing of surfactant micelles. Therefore, in contrast to trans-AzoCOONa, both the hydrophobic/hydrophilic balance and steric hindrance caused by cisAzoCOONa are against the growth of wormlike micelles. For the C16mimBr/AzoCOONa system before and after UVlight irradiation studied in this work, the main discrepancy lies in the alteration of the AzoCOO− configuration. In order to

Figure 8. Arrhenius dependence of τR with the reciprocal of absolute temperature for the aqueous solution containing C16mimBr (60 mM) and AzoCOONa (30 mM).

rheological results accord to a linear relationship, implying that the relaxation time follows the Arrhenius equation45 ⎛E ⎞ τR = A exp⎜ a ⎟ ⎝ RT ⎠

ν la

(7) 51

where Ea is the activation energy, R is the gas constant, and A is a constant. Ea calculated from the slope is 144 kJ·mol−1, which falls into the Ea range of 70−300 kJ·mol−1 reported for the micellar system involving surfactants.44,50−52 The activation energy for the C16mimBr/AzoCOONa system is close to that of the wormlike micelles formed in NaOA/Et3NHCl systems, which is 157.0 kJ·mol−1.53 3.4. Mechanism of Self-Assembly Process Induced by UV Irradiation. In this section, we attempt to understand why wormlike micelles form in the C16mimBr/AzoCOONa mixture solution and why their viscosity increases when the samples are exposed to UV light. It is well-known that the theory of molecular packing parameter,54 shown in eq 8, was introduced by Israelachvili et al. in 1976 and is usually employed to study the aggregate geometry of surfactant solution.

Figure 9. B3LYP/6-31G(d,p) electrostatic potentials, in hartrees, at the 0.001 e/bohr3 isodensity surfaces of trans-AzoCOO− (a) and cis-AzoCOO− (b) counterions. 3795

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4. CONCLUSION The phase behavior of a light-responsive system based on the C16mimBr/AzoCOONa mixed system was investigated in this work. Interestingly, wormlike micelles can be constructed both before and after the UV-light irritation. Longer and more entangled wormlike micelles, with increased viscosity of about an order of magnitude, can be formed by self-assembly of C16mimBr/AzoCOONa in aqueous solution. This can be ascribed to the trans to cis photoisomerization of AzoCOONa molecule upon exposure to the UV light. Besides, temperature can cause a slight change of the average contour length of wormlike micelles. Compared to the hydrophobic/hydrophilic balance and electrostatic interaction, the cation−π interaction between the imidazolium ring of C16mimBr molecule and azobenzene group of AzoCOONa molecule plays a predominant role in the formation of longer and more entangled wormlike micelles after the UV-light irradiation. The wormlike micelles with light-responsive viscoelastic behavior investigated in this work are expected to have potential applications in some fields, e.g., drug delivery, oil recovery, and sensing.

better understand the electrostatic interaction between the imidazolium cation of C16mimBr and the carboxylate anion of AzoCOONa, we performed density functional theory (DFT) calculations via the Gaussian 09 package using a hybrid functional B3LYP with the basis 6-31G(d,p).57 The electrostatic potentials at the 0.001 e/bohr3 isodensity surfaces of trans-AzoCOO− and cis-AzoCOO− counterions were calculated (Figure 9). It is apparent that they are both electronegative. The less electronegativity of the cis-formation counterion means that the electrostatic attraction between it and C16mim+ is weaker than that of trans-AzoCOO− and C16mim+. This also affects the growth of wormlike micelles formed by C16mimBr/ AzoCOONa mixed solution. At the same time, the first phenyl ring of trans-AzoCOONa molecule combines with the imidazolium cation of C16mimBr molecule (Figure 10). However, when trans-AzoCOONa is

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ASSOCIATED CONTENT

S Supporting Information *

Text detailing more details about 1H NMR spectroscopy for C16mimBr and figures showing dynamic frequency spectra of C16mimBr/AzoCOONa (2:1 molar ratio) aqueous solution with different concentrations, steady-shear rheology and dynamic frequency sweep of the aqueous solution containing C16mimBr (60 mM) and AzoCOONa (30 mM) at different temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-531-88364807. Fax: +86-531-88564750. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Grant No. 21373128), Scientific and Technological Projects of Shandong Province of China (Grant No. 2014GSF117001), the Natural Science Foundation of Shandong Province of China (Grant No. ZR2011BM017), and the Project of Sinopec (Grant No. P13045).

Figure 10. Schematic illustration of the aggregates formed by the C16mimBr/AzoCOONa system in aqueous solution before and after the UV-light irradiation.

photoisomerized to cis-AzoCOONa, the second phenyl ring of AzoCOONa molecule is moved toward the headgroup of the adjacent C16mimBr molecule, which results in more overlap between the azobenzene group and imidazolium cation by cation−π interaction. The association of AzoCOONa with C16mimBr molecules is more tight due to the additional cation−π interaction, which facilitates the growth of wormlike micelles.58 Therefore, we propose the probable mechanism of the C16mimBr/AzoCOONa self-assembly process induced by UV irradiation (Figure 10). Upon UV-light irritation, the cation−π interaction between C16mimBr and AzoCOONa molecules is speculated to be dominant over the hydrophobic/ hydrophilic balance and electrostatic interaction, leading to the formation of longer and more entangled wormlike micelles.

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