UV-Responsive Behavior of Multistate and ... - ACS Publications

Oct 5, 2018 - The steady- state shear rheology property was measured by changing the shear rates from 0.01 to 1000 s. −1 . The dynamic shear rheolog...
0 downloads 0 Views 7MB Size
Download PDF
Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

UV-Responsive Behavior of Multistate and Multiscale SelfAssemblies Constructed by Gemini Surfactant 12-3-12·2Br− and trans-o‑Methoxy-cinnamate Yan Tu,† Zhicheng Ye,† Cheng Lian,† Yazhuo Shang,*,† Hongni Teng,‡ and Honglai Liu† †

Downloaded via KAOHSIUNG MEDICAL UNIV on October 19, 2018 at 14:28:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Key Laboratory for Advanced Materials, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Department of Applied Chemistry, College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China S Supporting Information *

ABSTRACT: Photoresponsive systems with adjustable selfassembly morphologies and tunable rheological properties have aroused widespread concern of researchers in recent years because of their prospect applications in controlled release, microfluidics, sensors, and so forth. In this paper, we combine a cationic Gemini surfactant 12-3-12·2Br− and trans-2-methoxycinnamate (trans-OMCA) together to create a representative UV-responsive self-assembly system. The system displays abundant self-assembly behaviors, and the self-assemblies with different states and different scales including wormlike micelles, vesicles, and lyotropic liquid crystals (LCs) as well as an aqueous two-phase system (ATPS) are observed even at lower surfactant concentration. The UV-responsive behavior of the formed self-assemblies is investigated systematically. The results have shown that the photoisomerization of OMCA from trans form to cis form under UV light irradiation alters the hydrophobicity and steric hindrance effect of OMCA and thus affects the molecular packing at the micellar interface and further leads to the transformation of assembly morphologies. The long wormlike micelles can gradually transform into much shorter rodlike micelles under UV irradiation and companied by the decrease of solution viscosity by 2 orders of magnitude. In addition, the vesicles can evolve into multistate self-assembly structures including the ATPS, wormlike micelles, rod-like micelles, and small spherical micelles depending on the UV irradiation time. The ATPS and its adjacent anisotropic LC phase can respectively combine into a single phase and separate into ATPS under UV irradiation. The morphologies of assemblies in the 12-3-12·2Br−/trans-OMCA mixed system can be tailored by adjusting the system composition and duration of UV light irradiation on purpose. The photoresponsive system with abundant self-assembly behaviors and tunable rheological properties has wide application prospect in numerous fields such as drug delivery, materials science, smart fluids, and so forth, and the macroscopic phase separation and combination provide novel strategies for effective separation and purification of certain substances.

■

INTRODUCTION

a result, the photoresponsive self-assembly system has received extensive attention from researchers. It is well known that surfactants can form a variety of self-assemblies such as spherical micelles,19 wormlike micelles,20,21 vesicles22,23 or gels,24,25 and even liquid crystals (LCs)26 because of their amphiphilic properties. Correspondingly, photoresponsive surfactants become the hot target of research. Until now, a number of UV surfactants have been synthesized and used to construct photoresponsive self-assembly systems.27−29 However, the time-consuming, energy-consuming, and complicated

Smart self-assembly materials that significantly respond to external stimuli1 including pH,2,3 temperature,4,5 CO2,6,7 redox,8,9 light,10,11 and magnetic field12,13 have been one of the most important scientific concerns over the past decades. Stimuli-responsive self-assemblies with tunable aggregate morphologies and rheological properties have aroused considerable interest because of their extensive application in rheology control, drag reduction agent, heat transfer, drug delivery, and so on.14−16 Compared with other responsive methods, light stimuli methods have significant advantages.17,18 The light is easily accessible, clean, and pollution-free. Furthermore, it does not directly interfere with the composition of the system and can be precisely regulated. As © XXXX American Chemical Society

Received: August 28, 2018 Revised: September 30, 2018 Published: October 5, 2018 A

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir synthesis of photoresponsive surfactants limit the development of photoresponsive self-assembly systems to some degree. Therefore, finding a simple and feasible way for constructing photoresponsive self-assemblies is still the goal pursued by researchers. Introducing photosensitive molecules to surfactant solutions directly has become an alternative way to create a photoresponsive self-assembly system during the past decades, and great efforts have been devoted to explore the photoresponsive behavior of a mixed system containing surfactants and photosensitive molecules.30−33 Huang et al. utilized cetyltrimethylammonium bromide (CTAB) mixed with sodium(4(phenylazo) phenoxy)-acetate (AzoNa) to establish the photomodulated multistate molecular self-assemblies.34 Yu et al. constructed viscoelastic wormlike micelles by mixing imidazolium-based SAIL and sodium azobenzene 4-carboxylate (AzoCOONa) together.30 It is worth mentioning that cinnamic acid and its derivatives are typical photoresponsive compounds and have attracted wide attention because of their applications in medicine, cosmetics, food, daily chemistry, and so on.35 Raghavan et al. developed a simple photorheological fluid using trans-ortho-methoxycinnamic acid (OMCA) and CTAB and realized the tunable viscosity by the photoisomerization of trans-OMCA to cis-OMCA under UV irradiation.36 Hao et al. investigated photoinduced transition from vesicles to wormlike micelles constructed by tetradecyldimethylamine oxide (C14DMAO) and para-coumaric acid.37 Dai’s group and Dong’s group also have done a lot of related work and achieved great achievements in this area.32,38,39 However, almost all of those studies focus on a certain state photoresponsive micellar system in microscale, such as a wormlike micelle system and vesicle system. By far, few studies have been conducted on the photoresponsive behavior of self-assemblies with different morphologies in different scales formed in the same system, which not only limits the comprehensive understanding to the mechanism of the photoresponsive behavior but also restricts the application scope of photoresponsive systems. Both the nature of photoresponsive molecules and the properties of surfactants play essential roles in determining the photoresponsive behavior of the mixed systems.40 The properties of surfactants including the length and the structure of the surfactant alkyl chains and head groups as well as charges carried by the surfactant head groups affect the selfassembly behavior of the system directly. Gemini surfactants are a kind of novel surfactants composed of two amphiphilic chains connected by a spacer at or near the hydrophilic head groups. Gemini surfactants are well known not only for their structural diversity by varying the alkyl chains, the head groups, the spacer, and the counter-ions but also for their unique physicochemical properties such as much lower critical micelle concentration and more abundant self-assembly behavior compared with their conventional counterparts bearing a single alkyl chain and single head group.41−44 The abundant self-assembly behavior of Gemini surfactants provides a premise for studying the photoresponsive behavior systematically. However, to the best of our knowledge, the study on photoresponsive systems containing Gemini surfactants is rather few; comprehensive study on the photoresponsive behavior of different self-assemblies formed in the mixed system containing Gemini surfactants is definitely worthwhile.41,45

In this paper, we combine a cationic Gemini surfactant propanediyl-α,ω-bis(N-dodecyl-N,N-dimethylammonium bromide) (12-3-12·2Br−) and a photosensitive molecule trans-2methoxy-cinnamate (trans-OMCA) together to create a representative photoresponsive self-assembly system. The unique properties of Gemini surfactants itself and the interaction between Gemini surfactants and OMCA endow the system abundant self-assembly behaviors, and the selfassemblies with different states and in different scales including wormlike micelles, vesicles, and lyotropic LCs as well as the aqueous two-phase system (ATPS) are observed even at lower surfactant concentration. The UV-responsive behavior of these assemblies is investigated systematically by rheology, transmission electron microscopy (TEM), and 1H NMR, as well as dynamic light scattering (DLS). The possible photoresponsive mechanism of the self-assemblies with different states and in different scales for the 12-3-12·2Br−/trans-OMCA system is proposed. We expect that the present study on the photoresponsive behavior of self-assemblies with different morphologies and in different scales formed in the same system not only deepens the comprehensive understanding to the mechanism of the photoresponsive behavior but also broadens the application scope of photoresponsive systems.

■

EXPERIMENTAL SECTION

Materials. 12-3-12·2Br− was synthesized according to our previous work reported in the literature.46 The structure of 12-3-12· 2Br− was ascertained by 1H NMR spectroscopy (Bruker Avance 400), and elemental analysis (Vario EL III) verified that the purity of surfactants was above 97%. Trans-o-Methoxycinnamic acid (99%) was purchased from TCI Chemical Industry Development Co., Ltd. and used without further purification. Moreover, trans-o-methoxycinnamate (trans-OMCA) used in this paper was prepared by mixing trans-o-methoxycinnamic acid with a little excess of sodium hydroxide (NaOH) in solution. The chemical structure of 12-3-12·2Br− and the isomerization illustration of trans-OMCA to cis-OMCA are shown in Scheme 1. Ultrapure water from Millipore system was used to prepare all samples.

Scheme 1. Chemical Structure of 12-3-12·2Br− and (a) the Isomerization Illustration of trans-OMCA to cis-OMCA (b)

Sample Preparation. A series of 12-3-12·2Br−/trans-OMCA mixed solutions with the desired compositions were prepared by quantitatively adding the trans-OMCA solution to the 12-3-12·2Br− solutions, and then, the solutions were let to stand in an incubator (25 ± 0.1 °C) for 48 h for equilibrium. UV-Light Irradiation. An ultra-high-pressure short arc mercury lamp (CHF-XM35-500W) with a 365 nm optical filter was used to illuminate the samples. The samples were placed in a 10 mL quartz crucible and maintained at 25 ± 0.1 °C. Rheological Measurements. The rheological properties were measured using a cone plate system (CP50-1) with a radius of 50 mm and a taper angle of 1° of the physical MCR 302 rheometer (Anton Paar; Gratz, Austria). A cover was used to reduce evaporation of water B

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir in samples. The sample was added to a Peltier plate and equilibrated at 25 ± 0.1 °C for about 5 min before the measurements. The steadystate shear rheology property was measured by changing the shear rates from 0.01 to 1000 s−1. The dynamic shear rheology measurement was carried out in the range of 0.05−100 Hz with a fixed stress value in the linear viscoelastic region. The experimental temperature was controlled at 25.0 ± 0.1 °C. Dynamic Light Scattering. The size measurements of aggregates were taken by a Malvern Nano ZS instrument at 25 ± 0.1 °C with a 173° backscattering detector and a He−Ne laser of 633 nm. All samples were filtered in a quartz cuvette about 1 mL by 0.45 μm microporous filters and maintained at 25 ± 0.1 °C for 120 s before measurement. The hydrodynamic radii of samples were obtained using the CONTIN method. Transmission Electron Microscopy. The microstructures of aggregates were observed by a transmission electron microscope (Jeol JEM-1400, Japan). First, an appropriate amount of the solution was dropped on the side of the copper mesh containing the carbon support film, and we waited several minutes for the sample to adsorb on the copper mesh. The excess solution was sucked with a filter paper. The samples were stained with phosphotungstate for 10−20 s, and then, the excess solution was dried with a filter paper for observation.

through the TEM observation and DLS measurement. Increasing the total concentration of the mixture, two aggregates with different morphologies coexist in the solution and the solution macroscopically separates into two phases spontaneously (III), namely, a typical ATPS form.44 The system exhibits more complicated phenomena when the total concentration of the system is higher (IV). The three-phase systems and four-phase systems as well as the long-range ordered aggregate on mesoscopic scale, namely, LC phase (b),44 appear as expected. It is worth noticing that the LC phase is adjacent to ATPS. The LC phase and ATPS will be further studied in detail in the following. Rheological Properties and UV-Responsive Behaviors of the Wormlike Micelle System. Rheological Behavior. Wormlike micelles have extensive application because of the special rheological properties of their aqueous solutions. Figure 2a provides the steady-shear viscosity for the mixed systems of 40 mM 12-3-12·2Br− with different concentrations of transOMCA. The viscosity of the pure 12-3-12·2Br− solution (Figure 2a) is low and is almost about 0.001 Pa·s. It is manifested as a typical Newtonian fluid.42 However, with the addition of trans-OMCA, the viscosity of samples increases significantly, unambiguously implying the growth of the length of micelles. The micelles will further entangle with each other and tend to form a three-dimensional network structure. The formed network structure is difficult to be destroyed under relatively lower shear rate and therefore the platform appears (zero-shear viscosity, η0). When the shear rate is higher, the network structure is destroyed and the shearing-thinning behavior occurs. The shear-thinning phenomenon and η0 are the typical signs of wormlike micelles.47 Obviously, the wormlike micelles begin to form when the trans-OMCA concentration is higher than 12 mM. Figure 2b shows the variation of η0 of the wormlike micelle systems with the concentration of trans-OMCA. η0 increases sharply at 12−22 mM, and it reaches the maximum of 2.58 Pa·s at approximately 22 mM. With the further addition of trans-OMCA, η0 decreases. After the addition of trans-OMCA molecules, the counter ions of trans-OMCA can bind to the positively charged head group of 12-3-12·2Br− and induce the growth of micelles because of the effect of electrostatic shielding. When micelles grow further, they become flexible and can entangle into a network structure, which results in the rapid increase in viscosity. Finally, the wormlike micelles transform into a branched structure or break into several pieces when the concentration of trans-OMCA is higher and hence the viscosity decreases. To further study the viscoelastic properties of wormlike micelles, plots of the viscoelastic modulus (G′: elastic modulus; G″: viscous modulus) versus the frequencies are shown in Figure 3a. When the concentration of trans-OMCA is higher than 16 mM, the system shows a viscous behavior of G″ > G′ at lower frequency regions; then, both G′ and G″ increase and intersect at a specific frequency. Further, G′ and G″ continue to grow and G′ surpasses G″ (G″ < G′), and finally, G′ tends to be gentle. This is consistent with the typical Maxwell model,48 indicating the formation of typical wormlike micelles. The Cole−Cole plots (Figure 3b) can be also used to intuitively determine whether these rheological data fit with the Maxwell model. Obviously, the data perfectly follow semicircle curves at lower frequency, proving the formation of viscoelastic wormlike micelles in solutions. Although the results slightly deviate from the semicircle Cole−Cole plots

■

RESULTS AND DISCUSSION Phase Behaviors of the 12-3-12·2Br− and trans-OMCA Binary System. A series of 12-3-12·2Br−/trans-OMCA mixed solutions were prepared, and the phase diagram was drawn based on the observed apparent phenomena. The phase diagram and the typical appearance phenomena corresponding to each area are provided in Figure 1. Figure 1 reveals that the

Figure 1. Phase diagram of the 12-3-12·2Br−/trans-OMCA system at 25 °C. I: L1 (colorless and transparent micelle phase); II: L2 (bluish vesicle area); III: ATPS; IV: complicated phase area (including the three-phase system); and V: precipitation area.

12-3-12·2Br−/trans-OMCA binary system has abundant phase behaviors. Region I (L1) is a colorless and transparent phase. The viscosity of samples in this area increases significantly with the addition of trans-OMCA, indicating that there may be formation of larger aggregates, which will be discussed in detail in the following study of rheological properties. Region II (L2) is a homogeneous bluish phase and turns out to be vesicles C

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. Steady-shear viscosity curves for a mixed system containing 40 mM 12-3-12·2Br− and trans-OMCA (0−32 mM) at 25 °C (a); the variation of zero-shear viscosity of samples with concentration of trans-OMCA (b).

Figure 3. Dynamic frequency sweep (a) and Cole−Cole plots (b) for 40 mM/L 12-3-12·2Br− with different concentrations of trans-OMCA (12, 16, 20, 24, and 28 mM) at 25 °C (G′: elastic modulus; G″: viscous modulus).

Figure 4. Effect of UV irradiation time on steady-shear rheology (a) and the change of zero-viscosity with the prolongation of UV irradiation time (b) for the 40 mM 12-3-12·2Br−/22 mM trans-OMCA system.

studied further by measuring the viscosity. The results have shown that trans-OMCA is indeed an effective photoresponsive molecule in the mixed system of Gemini surfactant/trans-OMCA. η0 of the mixed system decreases significantly after UV irradiation, as shown in Figure S2. In addition, the sample of 40 mM 12-3-12·2Br−/22 mM transOMCA has excellent light response; therefore, the 40 mM 123-12·2Br−/22 mM trans-OMCA system is specified as a typical sample and studied in detail in the following experiment. The sample of 40 mM 12-3-12·2Br−/22 mM trans-OMCA is typical wormlike micelles (Figure 4a) and shows a high viscosity (2.58 Pa·s) before UV irradiation. With the

at higher frequency, this phenomenon is usually explained by Rouse modes.49 UV-Responsive Behaviors. The photoisomerization of the photosensitive molecules trans-OMCA with UV irradiation time is studied by measuring the UV−vis spectrum. As shown in Figure S1 in Supporting Information, trans-OMCA is gradually isomerized to cis-OMCA under UV irradiation and trans-OMCA can photoisomerize to cis-OMCA completely within 15 min under the studied condition, indicating that trans-OMCA has a sensitive and an efficient light response. The UV-responsive behavior of the mixed solutions of 40 mM 12-3-12·2Br− and trans-OMCA with different concentrations is D

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 5. Variation of elastic modulus G′ and viscous modulus G″ with oscillation frequency for the mixed system of 40 mM 12-3-12·2Br−/22 mM trans-OMCA under different UV irradiation times (0, 10, 20, 40, 60, 80, 100, and 120 min).

Table 1. Variation of Rheological Parameters of the 40 mM 12-3-12·2Br−/22 mM trans-OMCA System with the Prolongation of UV Irradiation Time at 25 °C 40 mM 12-3-12·2Br−:22 mM OMCA

G* [Pa]

G0′ [Pa]

Gmin ′ [Pa]

ωCO [rad/s]

τ [s]

le [nm]

L [nm]

0 min 10 min 20 min 40 min 60 min 80 min 100 min

10.26 9.66 8.63 7.65 6.67 6.56 5.86

21.66 20.79 16.77 13.78 12.09 10.89 8.67

5.31 5.91 6.14 6.86 6.91 7.08 7.48

8.46 10.5 12.9 20.3 26.3 33.7 44.46

0.118 0.095 0.078 0.049 0.038 0.030 0.022

119−145 123−150 131−159 141−170 152−184 153−186 163−198

463−561 405−490 370−449 314−380 293−355 284−344 256−310

prolongation of UV irradiation time, the shear-thinning behavior of the system gradually disappears accompanied by a gradual decrease of the viscosity of the system. It suggests that the long wormlike micelles are gradually destroyed after a period of UV irradiation and can be transformed into rodlike micelles; these results have been confirmed by TEM observation. The zero-shear viscosity (Figure 4b) of the studied system drops rapidly within 40 min and gently decreases as the UV irradiation time increases further, and the viscosity of the system remains constant over 200 min. Within 200 min, the viscosity of the system decreases by about 2 orders of magnitude (from 2.58 to 0.03 Pa·s). The results indicate that the wormlike micelles formed by the Gemini surfactant 12-3-12·2Br− and trans-OMCA have excellent photoresponsiveness. After UV irradiation, the system changes from wormlike micelles to rodlike micelles. To further illustrate the change of the self-assembly behavior and rheological properties, we investigated the viscoelasticity of the system by the frequency sweep method. As shown in Figure 5, G″ is greater than G′ at lower frequencies and G′ is greater than G″ at higher frequencies for the studied system when the irradiation time is within 100 min. The results conform to the Maxwell model; it indicates that the sample of 40 mM 12-3-12·2Br−/22 mM trans-OMCA displays as typical wormlike micelles within 100 min UV irradiation time. With further prolongation of the UV irradiation time (120 min), both the values of G′ and G″ decrease remarkably and the viscosity modulus of the system is larger than the elastic modulus in the whole frequency range, indicating that the fluid is predominantly a low viscosity fluid. It is shown that the longer wormlike micelles are gradually destroyed under UV

irradiation, which is consistent with the results obtained by steady-shear rheology measurements (Figure 4a). The results of Cole−Cole curves in Figure S3 in Supporting Information also indicate that the longer wormlike micelles are gradually destroyed with the prolongation of UV irradiation time, which are in agreement with dynamic viscoelastic results. To further investigate the morphology changes of micelles with the prolongation of UV irradiation time in detail, more rheological parameters are calculated from the viscoelastic data of the system. According to the literature,50 the average length of the wormlike micelles and the equivalent lengths of the two entanglement point of the wormlike micelles have the following relationship G0′ L = ″ Gmin le le =

(1)

ξ 5/3 lp2/3

ij k T yz ξ = jjj B zzz j G0′ z k {

(2)

1/3

(3)

Among them, L represents the average extension length; le is the equivalent length between two entanglement points of the wormlike micelle. Gmin ′ is the lowest value of G′ in the graph; G′0 = 2G*, and G* is the loss modulus at the G′ and G″ intersections. lp is the persistent length in the present work, lP takes 15−20 nm. kB is the Boltzmann constant (kB = 1.3806 × 10−23 J/K). T is the Kelvin temperature; ξ is the hydraulic correlation length (representing the size of the mesh in the E

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 6. Effect of the UV irradiation time on the steady-shear rheology for the 5 mM 12-3-12·2Br−/12 mM trans-OMCA system (a); particle size distribution of aggregates in samples before and after UV irradiation (30 min) (b).

Figure 7. TEM images for the sample of 5 mM 12-3-12·2Br−/12 mM trans-OMCA with the prolongation of UV irradiation time [(a) 0; (b) 2; (c) 15; (d) 20; (e) 30; and (f) 40 min].

micelle network). The calculated parameters from Figure 5 are listed in Table 1. It can be seen from the Table 1 that the rheological parameters change regularly with the prolongation of the UV irradiation time. G0 is usually proportional to the number density of the micellar aggregates and therefore reflects the degree of entanglements of micelles.51 The decrease of G0 and τ signifies the decrease in the length of wormlike micelles. The increase of le indicates that the structure of the system changes from a tight grid structure to a loose structure. The decrease of L indicates that UV irradiation results in the shortening of the wormlike micelles. These quantitative changes indicate that the long wormlike micelles with a tightly wound three-dimensional network are gradually transformed into short rigid rod micelles with the increase of the UV irradiation time. It can be seen that the length and the morphology of aggregates can be regulated by controlling the UV irradiation time. Multistate UV-Responsive Behaviors of the Vesicle System. The UV-responsive behavior of a typical sample in the vesicle region (5 mM 12-3-12·2Br−/12 mM trans-OMCA) is studied. It can be seen that the sample appears bluish before UV irradiation (Figure 6a, inserted picture). In addition, the aggregate morphology of the sample is characterized by DLS and TEM. The DLS measurements indicate that the size of the aggregates is about 60 nm (Figure 6b), and the TEM

observation then provides the vivid structure of typical vesicles with aqueous cores (Figure 7a). In addition, the viscosity of the sample is lower and is manifested as the typical Newtonian fluid before UV irradiation. With the prolongation of UV irradiation time, the homogeneous solution separates into two phases under UV irradiation (within 10 min), the ATPS formed. Then, the sample turns into a colorless and transparent homogeneous solution after 15 min UV irradiation time. At the same time, the shear-thinning phenomenon can be observed; this presents the formation of wormlike micelles. With further prolongation of the UV irradiation time, the solution appears to be a Newtonian fluid and the average particle size of the sample changes into 8 nm. The result reveals that the wormlike micelles are further transformed into rodlike or even spherical micelles. To further explore the changes of the aggregate structure in detail under UV irradiation, the microstructure of the aggregates was observed by TEM combined with a negative staining technique. TEM observation (Figure 7) shows that the 5 mM 12-3-12·2Br−/12 mM trans-OMCA system is almost vesicles with a diameter of about 60 nm (a) before UV irradiation. This is consistent with the DLS results. After 2 min UV irradiation, there appears the coexistence of a wormlike network structure and bigger spherical-like aggregates in the system (Figure 7b), corresponding to the ATPS. With F

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir continuous prolongation of the irradiation time to 15 min, a dense and intertwined three-dimensional network structure formed (Figure 7c), implying the formation of wormlike micelles. Then, the three-dimensional network structure gradually branched into shorter micelles with the prolongation of UV irradiation time (Figure 7d). With further prolongation of the UV light time to 30 min, the micelles are almost transformed into spherical micelles (Figure 7e). Finally, the system is completely transformed into small spherical micelles above 30 min (Figure 7f) UV light irradiation. Both the macroscopic and microscopic views of the light-responsive behavior in the vesicle region indicate that the self-assembled morphology of the aggregates has undergone a multistate transformation adjusted by the UV irradiation time. Obviously, the vesicles can be adjusted to ATPS, wormlike micelles, rodlike micelles, and spherical micelles by controlling the UV irradiation time, thus realizing the photoresponsive regulation of the aggregate structure macroscopically and microscopically. Macro UV-Responsive Behaviors of the 12-3-12·2Br−/ trans-OMCA Mixed System. The above study has shown that the single vesicle solution can separate into two phases (namely ATPS) after a certain time of UV irradiation. Then, how about the UV-responsive behaviors of the ATPS and whether the two coexisting phases can immerge with each other and reunion as a single phase or not? All of these are very significant to separation and purification of certain substances. Therefore, the photoresponsive behaviors of ATPS and its adjacent LC phase are explored systematically. During the experiments, we found that the ATPS can be transformed into a homogeneous and transparent micellar phase after UV irradiation (about 40 min). For the ATPS, no anisotropic phenomenon is observed under the polarizer. TEM observation suggests that the aggregates in the top phase of ATPS are nearly spherical micelles, and the steady-shear rheology results (Figure 8a) reveal that the bottom phase of ATPS is mainly wormlike micelles. After the UV irradiation, the two coexisting phases are combined into a single phase composed of short rodlike micelles just as seen in Figure 8a. For the single LC phase, the hexagonal sector LC can be observed through the polarizing optical microscope (Figure 8b). With the prolongation of UV irradiation time to 40 min, two kinds of aggregates with different morphologies (wormlike micelles and spherical micelles) formed. Those aggregates cannot immerge with each other, leading to the formation of ATPS, as shown in Figure 8b. It can be seen that the macroscopic phase separation and combination of surfactant systems containing trans-OMCA can be realized by UV irradiation. The macroscopic photoresponsive behavior of the studied system provides novel strategies for the separation and purification of substances. Mechanism of the UV-Responsive Behavior of the 123-12·2Br−/trans-OMCA Mixed System. To further understand the UV-responsive behavior of the studied system, the interaction between surfactant and OMCA was studied in detail. 1H NMR is one of the most powerful techniques to explore the molecular interaction between surfactants and additives. The chemical shifts can reflect variation in the microenvironment of the system.52,53 Figure 9 shows the 1H NMR spectra of 10 mM 12-3-12·2Br−/trans-OMCA (changes from 0 to 5 mM) binary mixtures before and after UV irradiation. Before the UV irradiation, it is observed that there are obvious shifts in proton signal peaks referring both to the surfactant 12-3-12·2Br− and to trans-OMCA, and obvious

Figure 8. Steady-shear rheology for different systems before and after UV irradiation: ATPS (30 mM 12-3-12·2Br−/40 mM trans-OMCA; a) and LC phase (30 mM 12-3-12·2Br−/60 mM trans-OMCA; b).

broadening and compression of peaks are also observed with the increase of OMCA concentration. It is well known that the broadening of the resonance signal peaks is often attributed to the formation of large aggregates because of the strong interaction between 12-3-12·2Br− and trans-OCMA.54 The introduction of trans-OMCA molecules leads to the chemical shift of surfactant signal peaks because of the shielding effect between the positively charged surfactants and negatively charged cinnamates. Correspondingly, the chemical shift of OMCA signal peaks toward the upfield indicates that part of trans-OMCA molecules are penetrated into the palisade layer of aggregates and participate in the micelle formation. The broadening and compression of the peaks should be attributed to the structural change of the micellar aggregates.55 Proton signal peaks indicate that trans-OMCA is isomerized to cisOMCA after UV irradiation. All of the spin-splitting peaks of cis-OMCA and 12-3-12·2Br− can be well distinguished. The proton signal peaks of in all mixed systems are basically the same as that of the pure 12-3-12·2Br− except for the weak chemical shift, whereas the signal peaks of cis-OMCA molecules only change the height of the signal peaks, that is, the height of peaks increases with the increase of cis-OMCA concentration. It indicates that the intermolecular interaction between 12-3-12·2Br− and cis-OMCA should be weakened after trans-OMCA is isomerized to cis-OMCA. The 12-3-12· 2Br−/cis-OMCA system is apt to form the smaller aggregates.56 Furthermore, we performed density functional theory calculation to obtain the binding energy via the Gaussian 09 package using a hybrid functional B3LYP with the basis 6311G(d,p) (Figure 10).30 The results confirm that the binding G

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

wormlike micelles and vesicles. Previous studies have shown that cis-OMCA has stronger hydrophily and steric resistance effect compared with trans-OMCA.36 The binding ability of cisOMCA to 12-3-12·2Br− is weaker than that of trans-OMCA (Figure 10). When trans-OMCA is isomerized to cis-OMCA, the cis-OMCA molecules tend to escape from the micellar layer and into the polar aqueous solution, resulting in partial charge recovery and effective cross-sectional area increase and the selfassembly conformation changes correspondingly. It is reasonable that when the coexisting micelles with different morphologies have the similar morphologies after UV irradiation, the phase combination appears macroscopically. On the contrary, the phase separation phenomenon will appear when the aggregates with different morphologies coexist in the system after the UV irradiation. Figure 11 depicts the possible

Figure 9. 1H NMR spectra of 12-3-12·2Br−(10 mM)/trans-OMCA (0−5 mM) systems before and after UV light irradiation for 1 h.

Figure 11. Mechanism of the UV-responsive behavior of the selfassemblies formed in the mixed system of 12-3-12·2Br−/trans-OMCA.

energy between cis-OMCA and 12-3-12·2Br− (−710.17 kJ· mol−1) is weaker than that between trans-OMCA and 12-3-12· 2Br− (−726.06 kJ·mol−1).

mechanism of the photoresponsive behavior of the mixed system of 12-3-12·2Br−/trans-OMCA. Obviously, the perfect combination of the cationic Gemini surfactant and photosensitive trans-OMCA creates multistate and multiscale selfassemblies and these assemblies display novel UV-responsive behaviors because of the difference of hydrophobicity and steric hindrance effect between trans-OMCA and cis-OMCA.

■

CONCLUSIONS The perfect combination of the cationic Gemini surfactant 123-12·2Br− and photosensitive molecule trans-OMCA creates abundant self-assemblies with different states and different scales including wormlike micelles, vesicles, and lyotropic LCs as well as ATPS even at lower surfactant concentration. These assemblies display significant UV-responsive behaviors because of the difference of hydrophobicity and steric hindrance effect between trans-OMCA and cis-OMCA. The twisting wormlike micelles can gradually transform into short rodlike micelles under UV irradiation. In addition, vesicles then can evolve into spherical micelles after undergoing a series of transformation including the ATPS, wormlike micelles, and rodlike micelles only by adjusting the UV irradiation time. Interestingly, the ATPS composed of different micelles (spherical micelles in the top phase and wormlike micelles in the bottom phase) will combine into a single phase by forming unique short rodlike micelles under UV irradiation. However, the adjacent anisotropic LC phase of ATPS will separate into two phases, namely, ATPS, after UV irradiation. Obviously, the morphologies of assemblies in the present system can be tailored by adjusting system composition and duration of UV light

Figure 10. B3LYP/6-311G(d,p) electrostatic potentials, in hartrees, at the 0.001 e/bohr3 isodensity surfaces of trans-OMCA/12-3-12·2Br− (a) and cis-OMCA/12-3-12·2Br− (b).

From the viewpoint of intermolecular interactions, the formation of abundant aggregates in the 12-3-12·2Br−/transOMCA binary system can be attributed to the unique structure of the Gemini surfactant and strong association between the head groups of cationic 12-3-12·2Br− and anionic transOMCA, as well as the hydrophobic interaction for the benzene ring parts of OMCA penetrating into the palisade layers of the micelles. It leads to the compression of the electric double layer and the reduction of the cross-sectional area. Correspondingly, the critical packing parameter (CPP) increases, leading to the spherical micelle (CPP ≤ 1/3) to transform into longer wormlike micelles (1/3 < CPP < 1/2).57 If the transOMCA concentration is further increased, the micellar surface double layer becomes more compressed and the linear wormlike micelles will self-curl into global vesicles with the CPP increase to 1/2−1 and then promotes the growth of H

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(8) Nakahata, M.; Takashima, Y.; Harada, A. Redox-Responsive Macroscopic Gel Assembly Based on Discrete Dual Interactions. Angew. Chem., Int. Ed. 2014, 53, 3617−3621. (9) Yang, Q.; Tan, L.; He, C.; Liu, B.; Xu, Y.; Zhu, Z.; Shao, Z.; Gong, B.; Shen, Y.-M. Redox-Responsive Micelles Self-Assembled from Dynamic Covalent Block Copolymers for Intracellular Drug Delivery. Acta Biomater. 2015, 17, 193−200. (10) Peng, S.; Guo, Q.; Hartley, P. G.; Hughes, T. C. Azobenzene Moiety Variation Directing Self-Assembly and Photoresponsive Behavior of Azo-Surfactants. J. Mater. Chem. C 2014, 2, 8303−8312. (11) Takahashi, Y.; Kishimoto, M.; Kondo, Y. Photoinduced Formation of Threadlike Micelles from Mixtures of a Cationic Surfactant and a Stilbene Amphiphile. J. Colloid Interface Sci. 2016, 470, 250−256. (12) Brown, P.; Alan Hatton, T.; Eastoe, J. Magnetic Surfactants. Curr. Opin. Colloid Interface Sci. 2015, 20, 140−150. (13) Brown, P.; Bromberg, L.; Rial-Hermida, M. I.; Wasbrough, M.; Hatton, T. A.; Alvarez-Lorenzo, C. Magnetic Surfactants and Polymers with Gadolinium Counterions for Protein Separations. Langmuir 2016, 32, 699−705. (14) Ezrahi, S.; Tuval, E.; Aserin, A. Properties, Main Applications and Perspectives of Worm Micelles. Adv. Colloid Interface Sci. 2006, 128, 77−102. (15) Rogers, S. A.; Calabrese, M. A.; Wagner, N. J. Rheology of Branched Wormlike Micelles. Curr. Opin. Colloid Interface Sci. 2014, 19, 530−535. (16) Jiang, Z.; Jia, K.; Liu, X.; Dong, J.; Li, X. Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle Transitions by SingleTailed Pyrrolidone Surfactants. Langmuir 2015, 31, 11760−11768. (17) Bansal, A.; Zhang, Y. Photocontrolled Nanoparticle Delivery Systems for Biomedical Applications. Acc. Chem. Res. 2014, 47, 3052− 3060. (18) Lee, H.-Y.; Diehn, K. K.; Sun, K.; Chen, T.; Raghavan, S. R. Reversible Photorheological Fluids Based on Spiropyran-Doped Reverse Micelles. J. Am. Chem. Soc. 2011, 133, 8461−8463. (19) Bernheim-Groswasser, A.; Zana, R.; Talmon, Y. Sphere-toCylinder Transition in Aqueous Micellar Solution of a Dimeric (Gemini) Surfactant. J. Phys. Chem. B 2000, 104, 4005−4009. (20) Zhao, M.; Yan, Z.; Dai, C.; Du, M.; Li, H.; Zhao, Y.; Wang, K.; Ding, Q. Formation and Rheological Properties of Wormlike Micelles by N-Hexadecyl-N-Methylpiperidinium Bromide and Sodium Salicylate. Colloid Polym. Sci. 2015, 293, 1073−1082. (21) Kamada, M.; Pierlot, C.; Molinier, V.; Aubry, J.-M.; Aramaki, K. Rheological Properties of Wormlike Micellar Gels Formed by Novel Bio-Based Isosorbide Surfactants. Colloids Surf., A 2018, 536, 82−87. (22) Abdelkader, H.; Alani, A. W. G.; Alany, R. G. Recent Advances in Non-Ionic Surfactant Vesicles (Niosomes): Self-Assembly, Fabrication, Characterization, Drug Delivery Applications and Limitations. Drug Delivery 2014, 21, 87−100. (23) Du, N.; Song, R.; Zhu, X.; Hou, W.; Li, H.; Zhang, R. Vesicles Composed of One Simple Single-Tailed Surfactant. Chem. Commun. 2014, 50, 10573−10576. (24) Jabeen, S.; Chat, O. A.; Maswal, M.; Ashraf, U.; Rather, G. M.; Dar, A. A. Hydrogels of Sodium Alginate in Cationic Surfactants: Surfactant Dependent Modulation of Encapsulation/Release toward Ibuprofen. Carbohydr. Polym. 2015, 133, 144−153. (25) Pei, X.; Zhao, J.; Ye, Y.; You, Y.; Wei, X. Wormlike Micelles and Gels Reinforced by Hydrogen Bonding in Aqueous Cationic Gemini Surfactant Systems. Soft Matter 2011, 7, 2953−2960. (26) Fong, C.; Le, T.; Drummond, C. J. Lyotropic liquid crystal engineering-ordered nanostructured small molecule amphiphileselfassembly materials by design. Chem. Soc. Rev. 2012, 41, 1297−1322. (27) Peng, S.; Guo, Q.; Hughes, T. C.; Hartley, P. G. Reversible Photorheological Lyotropic Liquid Crystals. Langmuir 2013, 30, 866− 872. (28) Martin, N.; Sharma, K. P.; Harniman, R. L.; Richardson, R. M.; Hutchings, R. J.; Alibhai, D.; Li, M.; Mann, S. Light-Induced Dynamic Shaping and Self-Division of Multipodal Polyelectrolyte-Surfactant

irradiation. Reasonably, the rheological behavior of the system can be tuned on purpose. It is worth mentioning that the macroscopic photoresponsive behaviors of the studied system including macroscopic phase separation and combination provide novel strategies for effective separation and purification of certain substances. The photoresponsive system with abundant self-assembly behaviors and tunable rheological properties has wide application prospect in numerous fields such as drug delivery, materials science, and smart fluids as well as separation and purification.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02914.

■

UV−vis spectroscopy of trans-OMCA system with and without 12-3-12·2Br− after different UV irradiation time; zero-shear viscosity of 40 mM 12-3-12·2Br−/transOMCA solutions after UV irradiation; and Cole−Cole plots of the 40 mM 12-3-12·2Br−/22 mM trans-OMCA system after different UV irradiation times (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone & Fax: 86 21 6425 2767. ORCID

Yazhuo Shang: 0000-0003-1598-4711 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (project no. 21476072) and the Fundamental Research Funds for the Central Universities.

■

REFERENCES

(1) Löwik, D. W. P. M.; Leunissen, E. H. P.; van den Heuvel, M.; Hansen, M. B.; van Hest, J. C. M. Stimulus Responsive Peptide Based Materials. Chem. Soc. Rev. 2010, 39, 3394−3412. (2) Li, Y.; Leng, M.; Cai, M.; Huang, L.; Chen, Y.; Luo, X. Ph Responsive Micelles Based on Copolymers Mpeg-Pcl-Pdea: The Relationship between Composition and Properties. Colloids Surf., B 2017, 154, 397−407. (3) Kang, W.; Wang, P.; Fan, H.; Yang, H.; Dai, C.; Yin, X.; Zhao, Y.; Guo, S. A Ph-Responsive Wormlike Micellar System of a Noncovalent Interaction-Based Surfactant with a Tunable Molecular Structure. Soft Matter 2017, 13, 1182−1189. (4) Zhang, Y.; Tang, H.; Wu, P. Multiple Interaction Regulated Phase Transition Behavior of Thermo-Responsive Copolymers Containing Cationic Poly(Ionic Liquid)S. Phys. Chem. Chem. Phys. 2017, 19, 30804−30813. (5) Wang, J.; Du, R.; Zhang, X. Thermoresponsive Polyrotaxane Aerogels: Converting Molecular Necklaces into Tough Porous Monoliths. ACS Appl. Mater. Interfaces 2018, 10, 1468−1473. (6) Liu, H.; Wang, W.; Yin, H.; Feng, Y. Solvent-Driven Formation of Worm-Like Micelles Assembled from a CO2-Responsive Triblock Copolymer. Langmuir 2015, 31, 8756−8763. (7) Zhang, Y.; Feng, Y.; Wang, J.; He, S.; Guo, Z.; Chu, Z.; Dreiss, C. A. CO2-switchable wormlike micelles. Chem. Commun. 2013, 49, 4902−4904. I

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Microarchitectures Via Azobenzene Photomechanics. Sci. Rep. 2017, 7, 41327. (29) Akamatsu, M.; FitzGerald, P. A.; Shiina, M.; Misono, T.; Tsuchiya, K.; Sakai, K.; Abe, M.; Warr, G. G.; Sakai, H. Micelle Structure in a Photoresponsive Surfactant with and without Solubilized Ethylbenzene from Small-Angle Neutron Scattering. J. Phys. Chem. B 2015, 119, 5904−5910. (30) Bi, Y.; Wei, H.; Hu, Q.; Xu, W.; Gong, Y.; Yu, L. Wormlike Micelles with Photoresponsive Viscoelastic Behavior Formed by Surface Active Ionic Liquid/Azobenzene Derivative Mixed Solution. Langmuir 2015, 31, 3789−3798. (31) Zhao, M.; Gao, M.; Dai, C.; Du, M.; Liu, Y.; Zou, C.; Tao, J.; Wang, X.; Wang, T. Investigation on a novel photo-responsive system formed by N -methyl- N -cetylpyrrolidinium bromide and ortho -methoxycinnamic. J. Mol. Liq. 2016, 223, 329−334. (32) Zhao, M.; Gao, M.; Dai, C.; Zou, C.; Yang, Z.; Wu, X.; Liu, Y.; Wu, Y.; Fang, S.; Lv, W. Investigation of Novel Triple-Responsive Wormlike Micelles. Langmuir 2017, 33, 4319−4327. (33) Lin, Y.; Cheng, X.; Qiao, Y.; Yu, C.; Li, Z.; Yan, Y.; Huang, J. Creation of Photo-Modulated Multi-State and Multi-Scale Molecular Assemblies Via Binary-State Molecular Switch. Soft Matter 2010, 6, 902−908. (34) Wu, Z.; Yan, Y.; Huang, J. Advanced Molecular Self-Assemblies Facilitated by Simple Molecules. Langmuir 2014, 30, 14375−14384. (35) Singh, T. S.; Mitra, S. Interaction of Cinnamic Acid Derivatives with Serum Albumins: A Fluorescence Spectroscopic Study. Spectrochim. Acta, Part A 2011, 78, 942−948. (36) Ketner, A. M.; Kumar, R.; Davies, T. S.; Elder, P. W.; Raghavan, S. R. A Simple Class of Photorheological Fluids: Surfactant Solutions with Viscosity Tunable by Light. J. Am. Chem. Soc. 2007, 129, 1553− 1559. (37) Wang, D.; Dong, R.; Long, P.; Hao, J. Photo-Induced Phase Transition from Multilamellar Vesicles to Wormlike Micelles. Soft Matter 2011, 7, 10713. (38) Du, M.; Dai, C.; Chen, A.; Wu, X.; Li, Y.; Liu, Y.; Li, W.; Zhao, M. Investigation on the Aggregation Behavior of Photo-Responsive System Composed of 1-Hexadecyl-3-Methylimidazolium Bromide and 2-Methoxycinnamic Acid. RSC Adv. 2015, 5, 68369−68377. (39) Jia, K.; Hu, J.; Dong, J.; Li, X. Light-Responsive Multillamellar Vesicles in Coumaric Acid/Alkyldimethylamine Oxide Binary Systems: Effects of Surfactant and Hydrotrope Structures. J. Colloid Interface Sci. 2016, 477, 156−165. (40) Sharma, R.; Kamal, A.; Abdinejad, M.; Mahajan, R. K.; Kraatz, H.-B. Advances in the Synthesis, Molecular Architectures and Potential Applications of Gemini Surfactants. Adv. Colloid Interface Sci. 2017, 248, 35−68. (41) Tu, Y.; Gao, M.; Teng, H.; Shang, Y.; Fang, B.; Liu, H. A Gemini Surfactant-Containing System with Abundant Self-Assembly Morphology and Rheological Behaviors Tunable by Photoinduction. RSC Adv. 2018, 8, 16004−16012. (42) Pletneva, V. A.; Molchanov, V. S.; Philippova, O. E. Viscoelasticity of Smart Fluids Based on Wormlike Surfactant Micelles and Oppositely Charged Magnetic Particles. Langmuir 2015, 31, 110−119. (43) Zou, M.; Dong, J.; Yang, G.; Li, X. A Comprehensive Study on Micellization of Dissymmetric Pyrrolidinium Headgroup-Based Gemini Surfactants. Phys. Chem. Chem. Phys. 2015, 17, 10265−10273. (44) Teng, H.; Liu, W.; Chen, Y.; Wang, X.; Zhang, H. Effect of Additives on the Phase Behavior of Sds/Ctab/H2o Systems. Tenside Surfactants Deterg. 2018, 55, 24−29. (45) Yu, L.; Fang, B.; Jin, H.; Chen, J.; Li, K.; Tian, M.; Yang, M.; Jin, L. Rheological Properties of a Novel Photosensitive Micelle Composed of Cationic Gemini Surfactant and 4-Phenylazo Benzoic Acid. J. Dispersion Sci. Technol. 2015, 36, 1770−1776. (46) Chen, L.; Shang, Y.; Liu, H.; Hu, Y. Effect of the Spacer Group of Cationic Gemini Surfactant on Microemulsion Phase Behavior. J. Colloid Interface Sci. 2006, 301, 644−650. (47) Ito, T. H.; Miranda, P. C. M. L.; Morgon, N. H.; Heerdt, G.; Dreiss, C. A.; Sabadini, E. Molecular Variations in Aromatic

Cosolutes: Critical Role in the Rheology of Cationic Wormlike Micelles. Langmuir 2014, 30, 11535−11542. (48) Granek, R.; Cates, M. E. Stress Relaxation in Living Polymers: Results from a Poisson Renewal Model. J. Chem. Phys. 1992, 96, 4758−4767. (49) Acharya, D. P.; Kunieda, H. Wormlike Micelles in Mixed Surfactant Solutions. Adv. Colloid Interface Sci. 2006, 123, 401−413. (50) Koehler, R. D.; Raghavan, S. R.; Kaler, E. W. Microstructure and Dynamics of Wormlike Micellar Solutions Formed by Mixing Cationic and Anionic Surfactants. J. Phys. Chem. B 2000, 104, 11035− 11044. (51) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Phase and Rheological Behavior of Salt-Free Alkyltrimethylammonium Bromide/Alkanoyl-N-Methylethanolamide/Water Systems. Langmuir 2003, 19, 9173−9178. (52) Yu, D.; Huang, X.; Deng, M.; Lin, Y.; Jiang, L.; Huang, J.; Wang, Y. Effects of Inorganic and Organic Salts on Aggregation Behavior of Cationic Gemini Surfactants. J. Phys. Chem. B 2010, 114, 14955−14964. (53) Zhang, Y.; Liu, L.; Liu, X.; Fang, Y. Reversibly Switching Wormlike Micelles Formed by a Selenium-Containing Surfactant and Benzyl Tertiary Amine Using Co2/N2 and Redox Reaction. Langmuir 2018, 34, 2302−2311. (54) Lu, Y.; Zhou, T.; Fan, Q.; Dong, J.; Li, X. Light-Responsive Viscoelastic Fluids Based on Anionic Wormlike Micelles. J. Colloid Interface Sci. 2013, 412, 107−111. (55) Baglioni, P.; Braccalenti, E.; Carretti, E.; Germani, R.; Goracci, L.; Savelli, G.; Tiecco, M. Surfactant-Based Photorheological Fluids: Effect of the Surfactant Structure. Langmuir 2009, 25, 5467−5475. (56) Chen, A.-J.; Hsu, I.-J.; Wu, W.-Y.; Su, Y.-T.; Tsai, F.-Y.; Mou, C.-Y. A Fluorescent Organic Nanotube Assembled from Novel PPhenylene Ethynylene-Based Dicationic Amphiphiles. Langmuir 2013, 29, 2580−2587. (57) Islam, M. S.; Shortall, S. M.; Mekhail, G. M.; Callender, S. P.; Madkhali, O.; Bharwani, Z.; Ayyash, D.; Kobernyk, K.; Wettig, S. D. Effect of Counterions on the Micellization and Monolayer Behaviour of Cationic Gemini Surfactants. Phys. Chem. Chem. Phys. 2017, 19, 10825−10834.

J

DOI: 10.1021/acs.langmuir.8b02914 Langmuir XXXX, XXX, XXX−XXX