Investigation on the aggregation behavior of photo-responsive system composed of 1-hexadecyl-3-methylimidazolium bromide and 2-methoxycinnamic acid

Abstract

A novel fluid system composed of 2-methoxycinnamic acid (trans-OMCA) and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr) in an aqueous solution was investigated. The compounds trans-OMCA and C₁₆mimBr in an aqueous solution can self-assemble and form viscoelastic worm-like micelles. The concentrations of trans-OMCA and C₁₆mimBr have a significant influence on the rheological properties of the system. The samples were characterized by rheological measurements. The structural isomerization of trans-to-cis for trans-OMCA occurred after UV light irradiation. The transformation of the system after UV light irradiation was determined by UV-vis absorption spectroscopy, rheological measurement and cryo-TEM observation. Surface tension measurements were carried out to investigate the role of trans-OMCA and UV light in C₁₆mimBr aqueous solution. Critical aggregation concentration (cac), effectiveness of surface tension reduction (Π_cac), maximum excess surface concentration (Γ_max) and minimum area occupied per surfactant molecule (a_s) were investigated. Critical packing parameter was introduced to express the mechanism of aggregation behavior transition.

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1. Introduction

Surfactants can aggregate into highly organized structures, for example, micelles, fibers, vesicles, and lamellar phases.^1–4 As one of the most important structures, worm-like micelles have received considerable attention because of their particular micellar structure and unique rheological response.^5–9 These micelles with flexible and elongated characters can easily entangle into a dynamic reversible network structure. The structures can break and reform; in this way, they are usually called “living” or “equilibrium” polymers.^10–12 Worm-like micelles can be applied in wide-spread fields and offer great potential in numerous aspects because of their fascinating properties, for example, rheology control, drag reduction agent, heat transfer, drug delivery, and enhanced oil recovery.^13–15

Scientists have already focused on craft adaptable materials inspired by nature.^16–18 The structure or functionality of these materials can be dynamically altered when stimulated by the factors, such as light, temperature, electricity, and pH.^19–27 Among these systems formed by the environmentally stimulated materials, smart worm-like micelles are a particular area of highly promising development.²⁸ The smart worm-like systems usually rely on small amphiphilic molecules and their self-assembly. The structural change may lead to macroscopic characteristic variations such as viscosity and elasticity. Smart worm-like micelles have attracted considerable interest due to their promising characteristics, and have many technological applications.^27,29–31 Compared with other factors, such as pH, salinity, redox reagent, and temperature, light plays a vital and important role and has a set of significant advantages.^32–34 Systems using light can be operated in a clean environment without any other additional chemicals, avoiding any changes in thermodynamic conditions. Besides, light is a reliable and cheap energy resource, and can be adjusted at an accurate distance, which is valuable in research fields such as nanoscience, nanotechnology, and medicine.^35,36

Therefore, light has always been used as a stimulus by exploiting trans-to-cis isomerization in worm-like micelles.^34,37–40 The investigation of the light-responsive worm-like micelles was first reported in 1980s.^41,42 There are mainly two ways of obtaining light-responsive worm-like micelles: adding certain light-sensitive additives into surface active ionic liquid solutions or introducing functional groups into the molecules.^27,43–45 Light-responsive worm-like micelles always take the advantage of some light-induced trans-to-cis isomerization of additives containing certain functional groups or surfactants.^46,47 Obviously, these changes could affect the aggregation behavior of the molecules inside the aggregates; thus, the properties of worm-like micelles would alter as well.

In this study, we investigated the aggregation behavior of 2-methoxycinnamic acid (trans-OMCA) and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr) in an aqueous solution, and the transition of micelles induced by UV irradiation was observed. Structures of C₁₆mimBr and trans-OMCA are shown in Scheme 1. We just want to construct a photo-responsive system with the help of the surfactant and the photo-isomerized molecule. In the present study, the system is composed of C₁₆mimBr and trans-OMCA, which contain aromatic rings in their molecules. The complex interactions, such as electrostatic, hydrophobic and π–π stacking interactions, exist in the system, which may result in the rich phase behaviors. This inspired us to carry out this investigation. We expect that this study will help to better understand the photo-responsive system and expand its application.

Scheme 1 Chemical structure of C₁₆mimBr (a) and OMCA (b).

2. Experimental section

2.1 Materials

The compounds 1-methylimidazole (99%), 1-bromohexadecane (97%), dichloromethane (99.5%), tetrahydrofuran (THF) (99%), and 2-methoxycinnamic acid (trans-OMCA) (98%) were purchased from Aladdin Chemical Reagent Co., Ltd (Shanghai, China). Ultrapure deionized water was obtained from Erie water purification systems. Solutions containing trans-OMCA were prepared with a slight excess of base (NaOH).

2.1.1 Synthesis of 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr). C₁₆mimBr was synthesized according to a previously reported study.⁴⁸ The solution of 1-bromohexadecane in dichloromethane was added to 1-methylimidazole dissolved in dichloromethane. The mixture was stirred under a nitrogen atmosphere for 48 hours at 75–80 °C. Then, the product was cooled to room temperature and dichloromethane was evaporated. The product was recrystallized from THF at least three times and dried under vacuum for 48 hours at 50 °C. The product was characterized by ¹H-NMR spectroscopy (400 MHz) using D₂O as the solvent. The ¹H-NMR result was recorded as follows:

C₁₆mimBr δ_H: 9.03 (s, 1H), 7.37 (t, 2H), 4.00 (t, 2H), 3.72 (s, 3H), 1.68 (s, 2H), 1.13 (m, 26H), 0.61 (t, 3H).

2.2 Characterization

2.2.1 UV irradiation. Samples were irradiated by a CHF-XM500-500W ultrahigh pressure short arc mercury lamp with a 365 nm filter for two hours. The experiment was carried out in a water bath at 25 °C with constant stirring.

2.2.2 UV-vis spectroscopy measurement. The UV-vis spectroscopy measurements of the samples before and after UV irradiation were carried out on a UNIC-2802 UV-vis spectrophotometer at room temperature. For this measurement, water was used as the solution.

2.2.3 Rheology measurements. An AR2000 rheometer (TA Instruments, New Castle, DE) was employed to investigate rheological properties of the samples. Samples were run on a cone and plate geometry (diameter, 40 mm; cone angle, 2°). Dynamic frequency sweep measurement was conducted in the linear viscoelastic region. The region was determined from the dynamic strain sweep measurement. The shearing rate ranged from 0.001 to 1000 s⁻¹. The zero-shear viscosity (η₀) was obtained from viscosity curve to the vertical coordinates. All the experiments were conducted at 25 °C.

2.2.4 Cryo-TEM. 5 µL of the solution was loaded onto a TEM carbon grid using a micropipette. TEM carbon grid was blotted with two pieces of filter paper to obtain a thin film suspended on the mesh hole. About 5 seconds later, samples were plunged into a reservoir of liquid ethane at −165 °C. The samples were stored in liquid nitrogen till they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 TEM (120 kV) at −174 °C. The phase contrast was enhanced by under-focus. Images were obtained on a Gatan Multiscan CCD and processed with a Digital Micrograph. The cryo-TEM observations were performed at the China University of Petroleum (Eastern China).

2.2.5 Surface tension measurements. Surface tension measurements were carried out using a model JYW-200B surface tensiometer (Chengde Dahua Testing Instrument Co., Ltd, Hebei, China). The temperature was maintained at 25 °C with the help of a thermostatic bath. Surface tension was measured through a ring method, and all the tests were repeated at least twice until the results were repeatable.

3. Results and discussion

3.1 Phase behavior of C₁₆mimBr/trans-OMCA system

To investigate the phase behavior of C₁₆mimBr/trans-OMCA system, different amounts of trans-OMCA were added into a series of C₁₆mimBr solutions with a constant concentration. Fig. 1 shows the phase diagram of the C₁₆mimBr/trans-OMCA system at 25 °C. Phase I and phase II are divided by a solid line; these regions have different aggregation behaviors. As shown in this figure, samples in phase I are clear and transparent with no viscoelasticity. In particular, when C₁₆mimBr concentration is lower than 80 mM, the liquid is not viscoelastic even when the concentration of trans-OMCA is up to 100 mM. However, with the addition of trans-OMCA in certain C₁₆mimBr aqueous solutions, changes can be observed, and the samples turn into highly viscoelastic solutions. As observed in the figure, the fluid in the bottle cannot flow even when the bottle is inverted. It is predicted that worm-like micelles are formed in this region, which will be proved in the following sections.

Fig. 1 Phase diagram of C₁₆mimBr and trans-OMCA system at 25 °C.

3.2 Rheological properties of trans-OMCA and C₁₆mimBr system

3.2.1 Effect of trans-OMCA concentration on rheological properties. To investigate the effect of trans-OMCA concentration on the micelle formation of C₁₆mimBr/trans-OMCA system at 25 °C, samples were prepared by adding different amounts of trans-OMCA to C₁₆mimBr solutions with a fixed concentration (150 mM). Fig. 2a shows the steady-shear rate viscosity curves of these systems. The viscosity of the system is very low and displays a Newtonian behavior^49,50 under low trans-OMCA concentrations. However, changes appear when a certain amount of trans-OMCA is added. The fluid exhibits high viscosity at low shear rates, and the solution displays a Newtonian property. Nevertheless, viscosity decreases while the shear rate increases, and shear-thinning behavior is observed. High viscosity in the Newtonian plateau can be attributed to the presence of entangled networks of worm-like micelles, and perhaps this is an indication of long worm-like micelles, which will be studied in the following section. Many reports have shown that the entangled network is aligned as the shear rate increases.⁵¹ However, the fluid cannot reach an equilibrium condition above a critical shear rate, and viscosity falls at this instance.

Fig. 2 Viscosity measurement as a function of trans-OMCA concentration (20–100 mM) at a fixed C₁₆mimBr solution (150 mM) at 25 °C: (a) viscosity versus shear rate curves; (b) zero-shear viscosity versus trans-OMCA concentration.

Zero-shear viscosity (η₀) can be obtained according to the Carreau model,⁵⁰ and it is the value of viscosity at the plateau in Fig. 2a. Fig. 2b presents the curve of η₀ against trans-OMCA concentration for this system. In this figure, we can observe that η₀ increases with trans-OMCA concentration and reaches a peak, and then decreases. This phenomenon is similar to the behavior of some ionic surfactants and aqueous systems of salts.⁵² The increase in η₀ may be an indication of growth in micellar length. The addition of trans-OMCA can stimulate the electrostatic repulsions among the headgroups of the surfactant molecules. Consequently, the micellar length increases, micelles begin to overlap, and an entangled network is formed. It has been considered that the decrease in η₀ is a result of branched worm-like micelles.⁵³ The branched structures tend to slide along the micelles, and thus the viscosity will decrease.

Viscoelastic properties of the wormlike micellar samples were studied by frequency sweep measurement. The dynamic rheological behavior for the C₁₆mimBr/trans-OMCA system was studied as a function of trans-OMCA concentration at a certain C₁₆mimBr concentration, which is shown in Fig. 3a. The lines show the relationship of elastic modulus (G′) and viscous modulus (G″) versus oscillation frequency (ω). At low ω, G′ is smaller than G″, and the sample shows viscous behavior. Elastic behavior is presented at high ω, when G′ is higher than G″ and G′ shows a plateau value at that time. The samples show viscoelastic property, which indicates the formation of a rigid network of entangled worm-like micelles. A crossover point exists according to the lines; the crossover frequency is called ω_co, and the crossover modulus corresponds to G_co. As shown in Fig. 5a, ω_co moves backward and then forward as trans-OMCA concentration increases from 40 mM to 60 mM. A higher ω_co value means poorer viscoelastic behavior. The values of G′ and G″ are fitted as eqn (1) and (2) below.⁵⁴

where G₀ stands for the plateau value of shear modulus and τ stands for the stress relaxation time. G₀ and τ can be calculated from eqn (3) and (4).

G₀ = 2G_co (3)

Fig. 3 (a) Dynamic frequency sweep for different trans-OMCA concentration (40 mM, 50 mM, and 60 mM) in a 150 mM C₁₆mimBr solution at 25 °C; (b) Cole–Cole plots of this system.

To better understand whether the Maxwell model fits the data, Cole–Cole plots are commonly investigated. The Cole–Cole plot is a curve of G″ as a function of G′. It can be obtained from eqn (5).

The Cole–Cole plots are shown in Fig. 3b. It can be clearly seen that at low frequency, all the experimental points follow the Maxwell fluid, which means that the solution behaves as an ideal Maxwell material within the lower shear frequency range. This is a proof that worm-like micelles are formed in this situation. However, when trans-OMCA concentration is lower than 30 mM, the system has no viscoelastic response.

3.2.2 Effect of surfactant concentration on rheological properties. It can be seen clearly that the concentration of trans-OMCA has great influence on the viscosity of the C₁₆mimBr/trans-OMCA system. The impact of surfactant (C₁₆mimBr) concentration on this system was studied. The steady state curves for different surfactant concentration solutions are shown in Fig. 4a. It can be seen from the figure that samples are Newtonian fluids at low concentrations; the viscosities of these samples are low and are not affected by the shear rate. However, when C₁₆mimBr concentration is higher than 80 mM, these samples are non-Newtonian fluids exhibiting a high constant viscosity at low shear rates and shear thinning behavior at high shear rates, which is a typical indication of worm-like micelles.⁵⁵Fig. 4b shows the curve of η₀ obtained from Fig. 4a. According to this figure, the value of η₀ increases with the addition of C₁₆mimBr. Low viscosity at low C₁₆mimBr concentrations represents spherical or short rod-like micelles, and the addition of C₁₆mimBr may favor the growth of micelles.⁵⁶

Fig. 4 (a) Viscosity versus shear rate curves measurement as a function of C₁₆mimBr (40–200 mM) concentration at a fixed trans-OMCA solution (50 mM) at 25 °C; (b) zero-shear viscosity versus trans-OMCA concentration.

3.3 Effect of UV-vis light irradiation on C₁₆mimBr/trans-OMCA system

3.3.1 UV-vis absorption spectra of C₁₆mimBr/trans-OMCA system subjected to UV irradiation. Molecules of trans-OMCA and cis-OMCA exhibit different wavelength absorptions in the ultraviolet region; the structural transition can be detected by UV-vis spectra. In this way, the absorbance spectra of trans-OMCA before and after UV irradiation were obtained to prove the trans-to-cis transition in Fig. 5. Fig. 5a shows that, before irradiation, trans-OMCA has absorbance peaks at 269 and 312 nm. However, UV light irradiation results in blue shifts to 255 and 292 nm. Significant reduction in the intensity is also presented. According to the literature, cis-isomer has higher steric hindrance and lower conjugation degree, indicating that the maximum absorption wavelength and the value of intensity are lower than that of the trans-isomer.²⁷ This blue shift is a proof of trans-to-cis transition. Besides, the UV irradiated sample was irradiated by visible light for 24 hours, and there was no change in the absorbance spectra, as shown by the black line in Fig. 5a. This phenomenon is in accordance with a previously published study.⁴²

Fig. 5 UV-vis spectra before and after UV irradiation for 2 hours of (a) trans-OMCA (50 mM) and (b) trans-OMCA (50 mM) + C₁₆mimBr (50 mM). All the samples were diluted 100 times before investigation.

3.3.2 Rheological properties of C₁₆mimBr/trans-OMCA system after UV irradiation. Then, we studied the effect of UV light irradiation on the system. After being irradiated for 2 hours, the sample changed into a flowing fluid with low viscosity from the viscoelastic fluid. Fig. 6a shows the steady shear rheological curve of 150 mM C₁₆mimBr and 50 mM trans-OMCA solution after UV irradiation. It can be found that the solution is a Newtonian fluid. The value of η₀ decreases sharply compared with the value of 150 mM C₁₆mimBr and 50 mM trans-OMCA solution before irradiation, as shown in Fig. 2a. This interesting phenomenon indicates the length of worm-like micelles decreases, and they even turn into spherical micelles. Fig. 6b shows the dynamic rheology of this sample. It can be clearly seen that G″ > G′ in all frequency regions without a crossover, and this is a characteristic of a viscous fluid. We can conclude that UV irradiation could convert the viscoelastic fluid to a thin, viscous one, and the length of wormlike micelles may decrease, which could be ascribed to the phase transition from worm-like micelles to rod-like or spherical micelles.

Fig. 6 Typical rheology results of a solution containing 150 mM C₁₆mimBr and 50 mM trans-OMCA after UV irradiation for 2 hours at 25 °C: (a) viscosity versus shear rate; (b) dynamic frequency sweep.

3.3.3 Cryo-TEM investigation on C₁₆mimBr/trans-OMCA system. Cryo-TEM technique was further employed to confirm the effect of UV irradiation for this system. Cryo-TEM images before and after 120 min of UV irradiation are shown in Fig. 7. As shown in Fig. 7a, a typical worm-like micelle network could be observed, and the network is formed by numerous worm-like micelles. After UV irradiation, as shown in Fig. 7b, worm-like micellar structures are damaged and cannot be detected anymore. The change in worm-like micelles structure could be attributed to the trans-to-cis isomerization induced by UV irradiation. This phenomenon is in accordance with the rheological results of the trans-OMCA (50 mM) + C₁₆mimBr (150 mM) before and after UV irradiation.

Fig. 7 Cryo-TEM images of 150 mM C₁₆mimBr + 50 mM trans-OMCA (a) before UV irradiation and (b) after UV irradiation.

3.3.4 Surface tension measurements. Surface tension measurements were carried out to evaluate the aggregation behavior of the aqueous solutions. The aqueous solution of trans-OMCA was irradiated by UV light for 120 min before the experiment, and it turned into the cis form. Fig. 8 shows the surface tensions (γ) of the aqueous solutions of C₁₆mimBr, C₁₆mimBr/trans-OMCA and C₁₆mimBr/cis-OMCA. All the experiments were carried out at 25 °C. It can be suggested that surface tensions of all the three solutions decrease with the addition of surfactant before reaching a plateau at certain C₁₆mimBr concentrations. The critical aggregation concentrations (cac) obtained from this figure are listed in Table 1. The C₁₆mimBr/trans-OMCA solution and C₁₆mimBr/cis-OMCA solution have lower cac values. Both trans-OMCA and cis-OMCA molecules have opposite charges against surfactant headgroups. They can compress the electric double layer surrounding the micelles and finally reduce the electrostatic repulsion effect among the headgroups. Finally, cac values decrease. The effectiveness of surface tension reduction (Π_cac)⁵ can be calculated from the following equation:

Π_cac = γ₀ − γ_cac (6)

where γ₀ is the surface tension of pure solvent and γ_cac is the surface tension of the sample at the cac. Π_cac shows the equilibrium surface tension reduction caused by the introduction of these molecules. The values in Table 1 shows that Π_cac increases in the order C₁₆mimBr < cis-C₁₆mimBr/trans-OMCA < C₁₆mimBr/trans-OMCA.

Fig. 8 Surface tension (γ) of C₁₆mimBr aqueous solution, C₁₆mimBr/trans-OMCA (50 mM) aqueous solution and the C₁₆mimBr/cis-OMCA (50 mM) aqueous solution at 25 °C.

Table 1 Surface properties of C₁₆mimBr, C₁₆mimBr/trans-OMCA and C₁₆mimBr/cis-OMCA systems at 25 °C (trans-OMCA and cis-OMCA concentrations are 50 mM)

	cac (mM)	Γ cac (mN m^−1)s	Π cmc (mN m^−1)	Γ max (µmol m^−2)	a s (Å^2)	P
C16mimBr	0.566	38.98	33.05	2.06	80.7	0.261
C16mimBr/trans-OMCA	0.010	37.37	34.63	3.31	50.2	0.420
C16mimBr/cis-OMCA	0.019	38.60	33.39	2.99	55.6	0.379

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The maximum excess surface tension (Γ_max) and the minimum area occupied per surfactant molecule (a_s) are introduced to clearly describe the aggregation behavior in an aqueous solution.⁵ They are obtained from the Gibbs adsorption isotherm:

where R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature, N_A is Avogadro's number (6.022 × 10²³ mol⁻¹). As indicated in Table 1, the C₁₆mimBr/trans-OMCA system obtains the highest Γ_max value and the lowest a_s value. The C₁₆mimBr headgroups can adsorb the anionic counterion in trans-OMCA, which makes the surfactant molecules pack more closely. However, cis-OMCA is more hydrophilic than trans-OMCA and fewer molecules are absorbed.²⁷ This is the reason why the C₁₆mimBr/cis-OMCA system is different from the C₁₆mimBr/trans-OMCA system.

3.4 Mechanism of micelle transition induced by UV irradiation

The mechanism of micelle transition can be explained by the theory of molecular packing parameter.⁵⁷where υ₀ is the hydrocarbon core volume of a surfactant, a_s is the equilibrium area per molecule at the surfactant–water interface, and l₀ is the length of hydrocarbon chain. The value of P means the critical packing parameter, which can be used to determine the shape of micelles. The critical condition for the formation of different micelles are P = 1/3, P = 1/2, and P = 1, respectively. When P < 1/3, spherical micelles can be observed; when 1/3 < P < 1/2, rod-like or worm-like micelles are obtained; when 1/2 < P < 1, vesicles exists; when P ≈ 1, planar bilayers are observed; and when P > 1, inverted micelles are obtained.

In our system, the value of υ₀ and l₀ can be calculated by the experimental formula introduced by Tanford.⁵⁸

υ₀ = 27.4 + 26.9N_c (10)

l₀ = 1.5 + 1.265N_c (11)

where N_c is the carbon atom number of the alkyl chain.

According to formulas (10) and (11), the value of υ₀ and l₀ for C₁₆mimBr are 457.8 ų and 21.7 Å, respectively. The value of a_s has been calculated and shown in Table 1. As a result, the critical packing parameter can be calculated, and the value is 0.261. It is apparent that spherical micelles can be observed in the aqueous solution of C₁₆mimBr and no worm-like micelles would appear theoretically, which is in correspondence with the experimental results. We can calculate the value of P for the other two systems, and the results are shown in Table 1. They are 0.420 and 0.379, respectively, which are in the range of 1/3–1/2 where rod-like micelles and worm-like micelles can be detected. Once introduced, the compensation ion with opposite charge in trans-OMCA tends to insert into the spherical micelles, and the surface charges can be shielded because of electrostatic repulsion. This will lead to the reduction of a_s, and the value of P increases at the same time. As a result, worm-like micelles appear with the addition of a certain amount of trans-OMCA. After UV irradiation, trans-OMCA would transform to cis-OMCA. As reported before, cis-OMCA is more hydrophobic than trans-OMCA. The different hydrophobicity is attributed to the net dipole moment. The dipoles tend to add up in the cis-OMCA; however, they tend to cancel each other in the trans-OMCA.²⁷ Less cis-OMCA could combine with C₁₆mimBr because of the hydrophobic interaction and steric hindrance. Thus, the value of a_s increases and P decreases, and therefore spherical or rod-like micelles will appear after UV irradiation. This process can be clearly seen in Fig. 9.

Fig. 9 Mechanism of the aggregation behavior for the photo-responsive system.

4. Conclusions

In this study, we have investigated the rheological behavior and UV light influence of the C₁₆mimBr/trans-OMCA system in an aqueous solution. Rheological measurements were used to observe the influence of trans-OMCA and C₁₆mimBr concentrations. UV-vis absorption spectroscopy, rheological measurement and cryo-TEM observations were carried out to investigate the influence of UV irradiation on the structural change of trans-OMCA and the micelle transition from worm-like micelles to spherical or rod-like micelles. Surface tension measurements were employed to investigate the effect of trans-OMCA and UV light irradiation on the micelle formation process. Critical packing parameter theory was used to discover the mechanism of micelle transition.

Acknowledgements

The study was supported by the National Science Fund for Distinguished Young Scholars (51425406), National Natural Science Foundation of China (51174221).

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