Light-responsive fluids based on reversible wormlike micelle to rodlike micelle transitions

Abstract

A new class of light-responsive fluids based on reversible transitions between wormlike micelles and rodlike micelles is developed. The system is composed of a synthesized light-responsive surfactant, 4-decylazobenzene-4-(oxyethyl)-dihydroxyethylmethylammonium bromide (C₁₀AZODEMAB), and a salicylic acid derivative, 5-methyl salicylic acid (5 mS). 5 mS is highly efficient in inducing the micellar growth of C₁₀AZODEMAB at a constant concentration, i.e., [C₁₀AZODEMAB] = 30 mmol L⁻¹. Viscoelastic fluids are formed in a wide concentration region of 5 mS, and show UV light-induced shining behaviors. More importantly, they can return to their original states after visible light irradiation. Cryogen transmission electronic microscopy (cryo-TEM) and rheology measurements confirm that the light-induced rheological responses are attributed to the reversible transitions between wormlike micelles and rodlike micelles. UV-Vis and ¹H NMR spectra are employed to study the molecular interactions between C₁₀AZODEMAB and 5 mS before and after light irradiation systematically, which evidence the critical role of light-induced isomerization between trans-C₁₀AZODEMAB and cis-C₁₀AZODEMAB during the transition process well.

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1. Introduction

In recent years, stimuli-responsive fluids, i.e., temperature-,¹ pH-,^2–4 CO₂-,^5,6 and light-^7,8induced thinning or thickening fluids, have attracted much attention from scientists because of their potential applications in various fields such as drug-controlled release,⁹ nanomaterial fabrication,¹⁰ body protection,¹¹ and so on. Stimuli-responsive fluids based on surfactants have become one of the most interesting topics in the past decade, attributed to the development of colloid chemistry. It is well-known that surfactants form self-organized assemblies above the critical micelle concentration (cmc) with different morphologies such as spherical micelles, rodlike micelles, wormlike micelles, and vesicles, which follow the molecular packing parameter P (P = v/(a₀l_c)) well,¹² in which a₀ represents the effective headgroup area, and l_c and v represent the hydrophobic chain length and volume, respectively. The stimuli-induced transitions between aggregates, especially those between wormlike micelles and other assemblies, might result in significant rheological responses in the bulk phase,^1,3,5,8 endowing them with potential applications in industry. For example, light-responsive threadlike micelles were employed as drag-reducing fluids to enhance heat-transfer capabilities successfully.¹³

In general, there are mainly two routes in designing responsive fluids based on surfactants as mentioned previously.¹⁴ One is relying on the synthetic chemistry through introducing some special responsive moieties into molecules, and the other is through the formation of surfactant/hydrotrope binary systems. The latter route is more extensively studied for its simplicity by comparing with the former one, where stimuli-responsive transitions between aggregates with different morphologies and even phase transitions are realized by employing the binary systems.^1–8 Here, the stimuli-responsive component can be either additives or surfactants. However, hydrotropes with responsive groups are often employed because functional hydrotropes with desired structures are commercially available. For example, the azobenzene dyes and coumaric acid derivatives are often employed to develop light-responsive fluids.^15–17

Raghavan et al. reported that 5-methyl salicylic acid (5 mS) is highly efficient in inducing spherical micelle to vesicle transition via wormlike micelle in the cetyltrimethylammonium bromide (CTAB) aqueous solutions.¹⁸ More importantly, there present temperature-induced vesicle to wormlike micelle transitions in the CTAB/5 mS binary systems owing to the solubility variation of 5 mS at different temperatures. Similar transitions were also observed in the dodecyltrimethylammonium bromide (DTAB) and polymer amphiphile Pluronic P85 systems in the presence of 5 mS.^19,20 According to the same principle, we have recently studied a new category of responsive fluids based on 1-[2-(4-decylphenylazo-phenoxy)-ethyl]-3-methylimidazolium bromide (C₁₀AZOC₂IMB) and a salicylic acid derivative, 4-(trifluoromethyl) salicylic acid (4FS), which shows pH-, temperature-, and light-induced thickening behaviors.²¹ However, long and entangled wormlike micelles can be formed only in narrow molar ratio regions of [hydrotrope]/[surfactant] in those systems,^18–21 resulting in difficulties in preparation and the potential applications as well. Thereby, how to develop stimuli-responsive systems based on surfactants and salicylic acid derivatives with a wide wormlike micelle formation region is of particular interest and is also the major motivation of this work.

Recent works show that surfactants with hydroxyethyl moieties in the headgroups favor wormlike micelles formation since the presence of hydroxyl moiety endows possibility to form hydrogen bonds between molecules.^22–24 Without doubt, the hydroxyethyl containing surfactants provide an alternative way to enlarge the wormlike micelle formation region in the surfactant/hydrotrope binary systems. In this work, a new light-responsive surfactant, 4-decylazobenzene-4-(oxyethyl)-dihydroxyethylmethylammonium bromide (C₁₀AZODEMAB), is synthesized. Based on C₁₀AZODEMAB and 5 mS binary mixtures, a new family of light-rheological responsive fluids is developed that has a wide wormlike micelle formation region. The light-induced rheological responses are illustrated by the rheology and cryogen transmission electron microscopy (cryo-TEM) results. Furthermore, the interaction between C₁₀AZODEMAB and 5 mS is studied systematically at the molecular level by the spectra methods including both UV-Vis and ¹H NMR techniques. The present work not only enriches the diversity of light-responsive fluids but also provides important information about molecular interactions between surfactants and hydrotropes, which is fundamental importance in colloids and interfaces.

2. Material and methods

2.1. Material

4-Decylazobenzene-4-(oxyethyl)-dihydroxyethylmethylammonium bromide (C₁₀AZODEMAB) was synthesized according to the similar procedure as reported previously²¹ (Scheme S1, ESI†), 5-methyl salicylic acid (5 mS, 98%) was purchased from Acros and used as received. All samples were prepared directly by dissolving weighed amounts of C₁₀AZODEMAB and 5 mS in ultrapure deionized water (Millipore) under continuous stirring until the solution became homogeneous at 80 °C, and then left in the dark incubator at 30 °C at least 48 h before any measurements.

2.2. Light irradiation

For the light-triggered isomerization between trans-C₁₀AZODEMAB and cis-C₁₀AZODEMAB, OPTIMAX™ 365 (wavelength 365 nm, Spectronice, USA) and OPTIMAX™ 450 (wavelength 450 nm, Spectronice, USA) were employed as the UV light source and visible light source, respectively. In general, the irradiation sample volume was about 3 mL each, and the experimental temperature was 30 °C using a thermostatic water bath, and the distance between the sample and light source was fixed at 15 cm.

2.3. UV-Vis spectra measurements

UV-Vis spectra measurements were carried out on a UV-Vis Tu-1901 spectrophotometer (Pgeneral, China) using ultrapure deionized water (Millipore) as a blank at 30 °C.

2.4. Rheological measurements

Steady-shear and dynamic rheological measurements were performed on a RS 600 stress-controlled rheometer (TA Instruments) using a cone plate geometry (diameter 35 mm and cone angle 1°) at 30 °C. The distance between the sensor and the cone plate was adjusted to 52 μm for all measurements. A Peltier-based temperature controller was used to control the temperature and a solvent trap was used to minimize sample evaporation. The frequency spectra were conducted in the linear viscoelastic regions of the samples as determined by the dynamic stress sweep measurements previously. All samples were equilibrated for at least 30 min before measurements.

2.5. Cryo-TEM measurements

For cryo-TEM, a small amount (3–5 μL) of sample solution was deposited on the surface of a TEM copper grid covered by a holey carbon film. After blotting away the excess solution to form a thin liquid film, the grid was immediately plunged into liquid ethane cooled by liquid nitrogen (−175 °C). The specimens were maintained at approximately −173 °C and imaged in a transmission electron microscope (JEOL 2010) at an accelerating voltage of 200 kV under low dose conditions.

2.6. ¹H NMR measurements

All samples for ¹H NMR were prepared in D₂O (99.8 atom% D, Acros), and the ¹H NMR spectra were recorded on a 400 MHZ Bruker-BioSpin spectrometer at 30 °C. All chemical shifts were measured from the internal residual proton of HOD in D₂O, and no diamagnetic bulk susceptibility correction was needed and the solvent effects on the shifts were negligible.

3. Results and discussion

3.1. Light-induced micellar structure reversible transitions

C₁₀AZODEMAB is a typical single-tailed cationic surfactant with its cmc about 0.039 mmol L⁻¹ and 0.22 mmol L⁻¹ before and after UV light irradiation (ESI Fig. S1†), respectively. The cmc values are smaller than that of surfactant containing trimethylammonium headgroup, indicating the hydrophilicity of dihydroxyethylmethylammonium headgroup is much weaker than that of trimethylammonium headgroup.^12,25 For example, the cmc values of CTAB¹² and C₁₆DHAB,²⁵ those having the same hydrophobic chain length, are 0.9 and 0.75 mmol L⁻¹ at 25 °C, respectively. Fig. 1 shows the dependency of the zero-shear viscosity of the C₁₀AZODEMAB/5 mS binary systems on the concentration of 5 mS ([5 mS]). It's clear that the solution viscosity is a function of [5 mS] at the constant concentration of C₁₀AZODEMAB ([C₁₀AZODEMAB]) i.e., 30 mmol L⁻¹, which is far larger than its cmc. The C₁₀AZODEMAB aqueous solution is transparent and behaves as a Newtonian fluid with a low viscosity (ESI Fig. S2†) in the absence of 5 mS, suggesting the formation of spherical micelles. However, the viscosity increases nearly linearly to about 10⁴ times of that of the pure C₁₀AZODEMAB aqueous solution upon increasing [5 mS] until precipitation appears when [5 mS] is above 35 mmol L⁻¹, indicating the 5 mS-induced micellar growth in C₁₀AZODEMAB aqueous solution or wormlike micelles formation.

Fig. 1 [5 mS] dependent zero-shear viscosity of C₁₀AZODEMAB/5 mS binary systems before and after UV light irradiation.

It was reported previously that 5 mS was highly efficient in inducing micellar growth and could even induce vesicles formation in both surfactant and polymer systems,^18–20 whereas only wormlike micelles are observed in this work. Since the hydrophilicity of dihydroxyethylmethylammonium headgroup is weaker in comparing with the trimethylammonium headgroup surfactants such as CTAB as suggested from the cmc values.^12,25 That's to say, the presence of hydroxyethyl moiety strengths the hydrophobicity of C₁₀AZODEMAB headgroup in some ways. Simultaneously, it should enlarge the headgroup area of C₁₀AZODEMAB due to the stronger steric hindrance of dihydroxyethylmethylammonium headgroup. From the viewpoint of the molecular packing parameter P,¹² C₁₀AZODEMAB should have a relatively smaller P value and reduce the efficiency of 5 mS in increasing P, resulting in the P values of C₁₀AZODEMAB/5 mS binary systems mainly locating in the region of 1/3–1/2. Thus, only wormlike micelles are formed. It is noticed that nearly all samples show significantly UV light-induced thinning behaviors as shown in Fig. 1. The zero-shear viscosities of the C₁₀AZODEMAB/5 mS binary systems drop about 100 times after UV light irradiation for 3 hours (ESI Fig. S3†) when [5 mS] is above 10 mmol L⁻¹, suggesting the formation of micelles with the shorter lengths, i.e., rodlike micelles. In other words, the UV light-induced wormlike micelle to rodlike micelle transitions might happen in the C₁₀AZODEMAB/5 mS binary systems.

In order to clarify the importance of light on the rheological responses of C₁₀AZODEMAB/5 mS binary systems, a typical composition of 30 mmol L⁻¹ C₁₀AZODEMAB and 34 mmol L⁻¹ 5 mS is employed, which shows the highest zero-shear viscosity about 10 Pa s in the wormlike micelle formation region. On a macro level, the sample is a very thick fluid with the strong tendency to trap bubbles, however, it becomes a thin one after UV light irradiation (the insert images in Fig. 2a). Fig. 2a shows the steady-shear rheological responses of the systems before and after light irradiation. It is clear that the sample shows the typical shear-thinning behavior regardless of UV light irradiation or not, whereas its zero-shear viscosity drops about 100 times after UV light irradiation. In addition, the system shows perfectly reversible rheological response that the viscosity returns to its original state after visible light irradiation for 3 hours. The dynamic rheological responses (Fig. 2b) show that the original sample behaves as an elastic fluid because the elastic module G′ is commonly larger than the viscous modulus G′′ in the whole measured frequency region.¹⁸ However, it transforms into a pure viscous one after UV light irradiation that the elastic module G′ is always below the viscous modulus G′′ instead.

Fig. 2 (a) Steady-shear rheological responses of 30 mmol L⁻¹ C₁₀AZODEMAB/34 mmol L⁻¹ 5 mS after UV and visible light irradiation as repeated three times, and the insert images represent the photographic images before and after light irradiation, respectively. (b) The corresponding dynamic rheological responses before and after UV light irradiation.

Generally speaking, the light-responsive variation is often attributed to the microstructural changes of the self-organized assemblies. In order to evaluate the morphology changes of aggregates, cryo-TEM is employed. It is clear that very long and entangled wormlike micelles are observed in the original sample (assigned by black arrows in Fig. 3a), whereas short rodlike micelles are observed instead after UV light irradiation (assigned by white arrows in Fig. 3b). The cryo-TEM results confirm the light-induced wormlike micelle to rodlike micelle transitions evidently, and the formation of micelles with the shorter lengths supports the UV light-induced thinning behaviors perfectly. Since the light-induced reversible trans–cis isomerization of azobenzene surfactant C₁₀AZODEMAB is essential during the transition, the isomerization is also monitored by the UV-Vis spectra (ESI Fig. S4†). It is observed that the absorbance spectra of 30 mmol L⁻¹ C₁₀AZODEMAB/34 mmol L⁻¹ 5 mS changes a lot after UV light irradiation for 3 hours, and then the spectrum returns to its original shape with a slight variation in intensity through a 3 hours visible light irradiation. Based on the maximum absorbance intensity at 336 nm, the recovered trans-content is estimated to about 89.4% after one UV light and visible light irradiation cycle,²⁶ indicating the existing of light-induced reversible transitions.

Fig. 3 Cryo-TEM images of 30 mmol L⁻¹ C₁₀AZODEMAB/34 mmol L⁻¹ 5 mS before (a) and after (b) UV light irradiation.

3.2. Interactions between C₁₀AZODEMAB and 5 mS

3.2.1 Light-induced reversible isomerization of C₁₀AZODEMAB. The reversible isomerization between trans-C₁₀AZODEMAB and cis-C₁₀AZODEMAB plays the critical role in the light-induced rheological responses of C₁₀AZODEMAB/5 mS binary systems, and the microstructural changes of aggregates as well. The reversible isomerization of 0.05 mmol L⁻¹ C₁₀AZODEMAB is monitored by the time-dependent UV-Vis spectra as shown in Fig. 4. Initially, the dominant characteristic absorbance peak of trans-C₁₀AZODEMAB at 346 nm is observed due to the π–π* absorption band of the trans-azobenzene moiety²⁷ (Fig. 4a). However, the peak drops significantly after UV light irradiation for 20 seconds that shows little effect upon further increasing irradiation time. Two peaks at 308 nm and 436 nm appear instead, respectively, owing to the π–π* and n–π* absorption band of the cis-azobenzene moiety.²⁷ The reverse process from cis-C₁₀AZODEMAB to trans-C₁₀AZODEMAB transition is also realized perfectly upon visible light irradiation as shown in Fig. 4b, confirming the light-induced reversible isomerization of C₁₀AZODEMAB. Such light-induced reversible transitions in the diluted C₁₀AZODEMAB aqueous solutions can be repeated many times.

Fig. 4 Time-dependent UV-Vis spectra of 0.05 mmol L⁻¹ trans-C₁₀AZODEMAB under UV light irradiation (a) and cis-C₁₀AZODEMAB under visible light irradiation (b), respectively. Upward and downward arrows represent the increase of irradiation time t (0 s ≤ t ≤ 22 s) with a sequential variation (Δt) of 2 s each. (c) ¹H NMR spectra of 10 mmol L⁻¹ C₁₀AZODEMAB before and after UV light irradiation for 3 h.

Fig. 4c shows the ¹H NMR spectra of 10 mmol L⁻¹ C₁₀AZODEMAB before and after UV light irradiation. The trans-azobenzene moiety shows two double peaks at 6.81 ppm and 7.54 ppm as assigned as H₁ and H′₁, H₂ and H′₂, respectively. There is about 91% trans-C₁₀AZODEMAB in the original bulk phase as calculated from the ¹H NMR spectrum. After UV light irradiation, two peaks, in terms of the singlet at 6.55 ppm assigned as H′₂ and the triplet at 6.70 ppm assigned as H₁, H′₁ and H₂, respectively, are observed. It's clear that the cis-C₁₀AZODEMAB becomes dominant after UV light irradiation, whereas there is still about 20% trans-isomers remained.

3.2.2 Effect of light on the molecular interactions between C₁₀AZODEMAB and 5 mS. The effect of light irradiation on the molecular interactions between C₁₀AZODEMAB and 5 mS is also studied by the UV-Vis spectra. Fig. 5 shows the UV-Vis spectra of 0.1 mmol L⁻¹ 5 mS, 0.025 mmol L⁻¹ C₁₀AZODEMAB, and the binary system composed of 0.1 mmol L⁻¹ 5 mS and 0.025 mmol L⁻¹ C₁₀AZODEMAB before and after UV light irradiation for 10 s, respectively. It is noticed that the characteristic absorption peaks of 5 mS at 303 nm and trans-azobenzene moiety of C₁₀AZODEMAB at 345 nm are combined into one peak with the maximum wavelength of 310 nm. From the viewpoint of 5 mS, a significant red-shift from 303 nm to 310 nm is occurred, indicating the strong interactions between 5 mS and trans-C₁₀AZODEMAB attributed to the cationic–π interaction between the aromatic ring of 5 mS and the polar C₁₀AZODEMAB headgroup.^21,27 After UV light irradiation, the characteristic absorption peaks of 5 mS at 303 nm and cis-azobenzene moiety at 307 nm are combined into one peak with the maximum wavelength of 304 nm for the binary system. The slightly red-shift from 303 nm to 304 nm of 5 mS suggests the weakness in the molecular interactions between 5 mS and cis-C₁₀AZODEMAB. That's to say, UV-light irradiation weakens the interaction between 5 mS and C₁₀AZODEMAB because of the isomerization of C₁₀AZODEMAB.

Fig. 5 UV-Vis spectra of 0.1 mmol L⁻¹ 5 mS, 0.025 mmol L⁻¹ C₁₀AZODEMAB and binary system containing 0.1 mmol L⁻¹ 5 mS and 0.025 mmol L⁻¹ C₁₀AZODEMAB before and after UV-light irradiation for 10 s, respectively.

¹H NMR is powerful in explaining the intermolecular interactions between surfactants and additives because the chemical shifts reflect the variation in micro-environment of molecules.^28–30 Fig. 6a and b show five typical ¹H NMR spectra of 5 mS/C₁₀AZODEMAB binary systems before and after UV light irradiation, respectively. Significant changes are observed upon increasing [5 mS]. First of all, significant broadening and compress of the proton signal resonance are observed from either 5 mS or C₁₀AZODEMAB, suggesting the occurrence of 5 mS-induced micellar growth,^26,31 i.e. the spherical micelle to wormlike micelle transition. Furthermore, the chemical shifts of protons located in the headgroup and azobenzene moiety of C₁₀AZODEMAB shift toward upfield evidently as well as H_a and H_b of 5 mS, indicating the micro-environmental changes of protons during the process. It should be mentioned that the signals from the protons H_c and H_d of 5 mS is invisible because of the overlap from the trans-azobenzene moiety. However, all the spin–spin splitting of the peaks of 5 mS can be distinguished well after UV-light irradiation when [5 mS] is below 7 mmol L⁻¹ (Fig. 6b). The viscosity (Fig. 2) and cryo-TEM (Fig. 3) results confirm that trans-C₁₀AZODEMAB favors very long and entangled wormlike micelles in the presence of 5 mS, whereas cis-C₁₀AZODEMAB/5 mS binary systems prefer to form rodlike micelles. Therefore, the prominently weakened broadening of the resonance shown in Fig. 6b could be attributed to the formation of micelles with the smaller size, i.e., rodlike micelles.

Fig. 6 [5 mS] dependent ¹H NMR spectra of 5 mS/C₁₀AZODEMAB binary systems with [C₁₀AZODEMAB] = 10 mmol L⁻¹ before (a) and after (b) UV-light irradiation for 3 h, respectively.

To further understand the role of 5 mS, the dependence of chemical shifts of 5 mS and C₁₀AZODEMAB protons on [5 mS] is shown in Fig. 7. Before UV irradiation, nearly all chemical shifts of the protons H_a and H_b of 5 mS (Fig. 7a) and the protons around the headgroup such as H₆, H₇, H₈ and H₁₀ of trans-C₁₀AZODEMAB (Fig. 7c) shift toward upfield, indicating the strong interactions between 5 mS and trans-C₁₀AZODEMAB. For trans-C₁₀AZODEMAB, the chemical shifts shifting toward upfield suggests that 5 mS is penetrated into palisade layer of trans-C₁₀AZODEMAB micelles.^26,31 The largest chemical shift changes (Δδ = δ_{binary mixture} − δ_pure) of H₆, H₇, H₈ and H₁₀ of C₀AZOC₂DEMAB are 0.26, 0.13, 0.11 and 0.12 ppm before UV irradiation, whereas those are 0.25, 0.22, 0.15 and 0.16 ppm after UV irradiation (Fig. 7d). For one thing, 5 mS affects the proton H₆ of C₁₀AZODEMAB more remarkable, indicating the interactions between 5 mS and H₆ is the strongest regardless of UV irradiation or not. On the other hand, the interactions between 5 mS and C₁₀AZODEMAB especially that between 5 mS and H₇ are strengthened after UV irradiation, indicating that the distance between 5 mS and H₇, H₈ or H₁₀ is shortened, and thereby more remarkable Δδ occurs. That's to say, 5 mS should become closer to the charged interface of C₁₀AZODEMAB headgroup after UV light irradiation.^26,31

Fig. 7 Dependence of the typical protons chemical shifts of 5 mS and C₁₀AZODEMAB on [5 mS] before (a) and (c), and after (b) and (d) UV light irradiation for 5 mS and C₁₀AZODEMAB, respectively.

For 5 mS, the chemical shifts shifting upfield upon increasing [5 mS] before UV light irradiation (Fig. 7a) imply that H_a and H_b of 5 mS shift to the nonpolar part of C₁₀AZODEMAB micelles.^26,31 In other word, the aromatic ring of 5 mS is penetrated into the nonpolar core of C₁₀AZODEMAB micelles. Similar tendency is also observed in the C₁₀AZODEMAB/5 mS binary systems after UV light irradiation (Fig. 7b) except the proton H_d of 5 mS that shifts toward downfield, indicating H_d should be very close to the charged interface.³⁰ Since the chemical shifts of H_c and H_d of 5 mS cannot be visible because of the overlap from the trans-azobenzene moiety, we compared the chemical shifts of H_a and H_b of 5 mS at [5 mS] = 1 and 9 mmol L⁻¹ before and after UV irradiation, respectively. It is observed that the values of chemical shift changes (Δδ = δ_{[5 mS]=1} − δ_{[5 mS]=9}) of H_a and H_b are 0.13 and 0.08 ppm before UV light irradiation (Fig. 7a), respectively. Those of H_a and H_b are 0.14 and 0.06 ppm after UV light irradiation (Fig. 7b), respectively. It is clear that the change in Δδ of H_a is far larger than that of H_b regardless of UV irradiation or not, indicating H_a should be located in a more nonpolar environment than that of H_b, and the tendency strengthened after UV light irradiation. However, no significant micro-environmental change happens on H_a and H_b upon light irradiation because light irradiation shows little effect on Δδ. Since the cationic–π interactions between aromatic chemical of 5 mS and the polar headgroup of C₁₀AZODEMAB is dominant (Fig. 5), therefore, the most reasonable molecular arrangement of 5 mS and C₁₀AZODEMAB in micelles might be represented by the following figure (Fig. 8).

Fig. 8 Possible molecular arrangement of 5 mS and C₁₀AZODEMAB in micelles before (a) and after (b) UV irradiation.

4. Conclusions

In summary, we have reported a new light-responsive surfactant 4-decylazobenzene-4-(oxyethyl)-dihydroxyethylmethylammonium bromide (C₁₀AZODEMAB). Based on the C₁₀AZODEMAB/5 mS binary systems, a new family of light-responsive fluids with significant rheological changes is developed due to the reversible transitions between wormlike micelles and rodlike micelles, in which the light-induced reversible trans–cis isomerization of azobenzene surfactant C₁₀AZODEMAB is essential. Furthermore, the molecular interactions between C₁₀AZODEMAB and 5 mS before and after light irradiation are studied systematically by the UV-Vis and ¹H NMR spectra. 5 mS is solubilized and inserted into the palisade layer of C₁₀AZODEMAB micelles that induces the formation of wormlike micelles in the binary systems. Another, UV light irradiation increases the hydrophilicity increase of C₁₀AZODEMAB and also alters the conformation of molecules, resulting in a larger cmc and thereby, weakening the molecular interactions. As a result, micelles with a shorter length, i.e., rodlike micelles, formed. In comparison with other 5 mS containing binary systems,^18–20 wormlike micelles are formed in a wider concentration region of 5 mS with controlled viscosities in the current systems, providing more fascinating perspectives in application fields such as drag-reducing fluids.³²

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC 21273165 and 21573164).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05529j

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