Light induced micelle to vesicle transition in an aqueous solution of a surface active ionic liquid

Abstract

A new surface active ionic liquid 4-butylazobenzene-4′-hexyloxy-trimethyl-ammoniumtrifluoro-acetate (BHATfO) was synthesized to achieve light-responsive and reversible micelle–vesicle transformation, which was verified by means of UV-spectroscopy, surface tension, conductivity, dynamic light scattering and transmission electron microscopy.

---

Self-assemblies formed by amphiphiles which are in response to external stimuli such as pH, temperature, light, redox, magnetic filed, have attracted much attention,¹ and been applied in many fields like drug delivery, nano-material synthesis, switchable catalysis, surface coating and so forth.² The light-responsive amphiphiles are of special importance since light is a mild and non-invasive energy source and convenient to obtain and be controlled within a precise spatial place.³ One of the most common light-responsive elements used to construct amphiphiles is the azobenzene group.⁴ The reversible switching between trans and cis isomers of azobenzene was achieved by using alternative UV/vis-light irradiation, thus the structure of amphiphile aggregate containing azobenzene unit may be changed.^5–7 Moreover, it was found that the combination of azobenzene unit containing molecule with another surfactant can also result in abundant changes in the size and/or morphology of corresponding assemblies.^3,7a–c,8 However, in the absence of any other additives, only one investigation involved the vesicle–micelle transition in aqueous solution of amphiphilic copolymers containing azobenzene unit under UV irradiation has been reported,^7d yet no investigations concerned the micelle–vesicle transition by small amiphiphiles containing azobenzene unit without additives have been presented.

In recent decades, ionic liquids (ILs) have been paid much attention due to their unique properties such as the wide liquid state range, the negligible vapor pressure, the favorable solvation behavior, high thermal stability, and the high reactivity and selectivity.⁹ Some types of ILs with long alkyl chains can self-assemble to form micelles,¹⁰ which was denoted as surface active ionic liquid (SAIL). SAIL possesses some unique properties due to its combination of ionic liquid and surfactant, such as stronger self-aggregation tendency in aqueous solution and non-aqueous solvent like hydrophobic ionic liquids, novel surface properties, good ability in formulation of high-temperature stable microemulsion, etc.^10b,11 Recently, a few studies on the formation of vesicles from SAIL alone without additives were reported,¹² among which one report¹²ⁱ involved the transition from micelle to vesicle driven by change of pH was investigated. However, to the best of the authors' knowledge, no report involved the micelle–vesicle transition by light-responsive SAIL alone has been presented.

In the present work, we synthesized a new simple light-responsive surface active ionic liquid 4-butylazobenzene-4′-hexyloxy-trimethylammonium trifluoroacetate (BHATfO) to achieve the light-responsive and reversible micelle–vesicle transformation by this SAIL alone without additives, which was investigated by UV-spectroscopy, surface tension, conductivity, dynamic light scattering (DLS) and transmission electron microscopy (TEM).

The synthetic route of 4-butylazobenzene-4′-hexyloxy-trimethylammonium trifluoroacetate (BHATfO) is shown in Scheme S1 in ESI.† The orange product was obtained with total yield being about 10%. The melting point of BHATfO determined by Micro-DSC III (Setaram, France) was 77 °C (Fig. S1 in ESI†), characterizing the feature of the ionic liquid. The chemical structure of BHATfO is shown in Scheme 1.

Scheme 1 Chemical structure of BHATfO.

It's well-known that the azobenzene derivatives generally presented in trans-isomer because it is more thermodynamically stable than that in cis-isomer, however, it can be transformed into cis-isomer by UV irradiation.^6,13 Fig. 1a shows the UV-vis spectrum of 0.05 mM aqueous solution of trans-BHATfO by the solid line, where the typical maximum absorption wavelength at 350 nm is resulted from the π → π* transition. The UV-vis spectrum of trans/cis mixture after UV irradiation at 365 nm for 1 hour is also presented in Fig. 1a as dashed line, where the absorption peak at 350 nm disappeared while two new peaks at 310 nm and 436 nm appear due to the n → π* transition of cis-isomer. The absorption peak at 350 nm was regenerated by further vis-light irradiation (natural light) for 1 hour as indicated by dotted line in Fig. 1a. However, the absorption band of trans-isomer at 350 nm was not fully re-converted since some residual cis-form may remain in the system. With the assumptions that the absorption of the isomer in the cis-isomer at 373 nm (the middle wavelength between the peaks 310 nm and 436 nm) is negligible and BHATfO in the initial state before irradiation exists only in the pure trans-isomer,¹⁴ the compositions of the isomer mixtures in the different irradiation stages can be calculated and are shown in Fig. 1a. About 67.2% trans-isomer exists in the mixture after further vis-light irradiation, which can be used to explain that the absorption peak at 350 nm doesn't fully regenerate. Changes of the absorbance value at 350 nm after several vis and UV irradiation cycles (up to seven cycles) are shown in Fig. 1b, indicating a good reversibility in the cis–trans transformation.

Fig. 1 (a) UV-vis spectra of aqueous solution of 0.05 mM BHATfO at different states: initial state ((1) solid line), after UV irradiation ((2) dashed line), and after vis-light irradiation ((3) dotted line); (b) changes of absorbance values at 350 nm for seven cycles.

The critical aggregation concentrations of BHATfO before and after UV irradiation were determined by measurements of surface tension. The breakpoints of the plots of surface tension against ln C (C refers to the surfactant molar concentration) displayed in Fig. 2 are taken as critical aggregation concentration (CAC),¹⁵ which are summarized in Table 1. The amount of surfactant adsorbed on per unit area of the liquid/air surface can be deduced by the Gibbs adsorption equation,¹⁶ which gives the express of maximum surface excess Γ_max as:

Γ_max = −((∂γ/∂ ln C)_T)/2RT (1)

where T is the absolute temperature; R is the universal ideal-gas constant; (∂γ/∂ ln C)_T can be obtained from the slope of linear plot of the surface tension against ln C near CAC. The average minimum surface area A_min then can be calculated from Γ_max:

A_min = 10¹⁴/N_AΓ_max (2)

with N_A referring to the Avogadro's constant. It can be seen from Table 1 that the minimum surface area per molecule before UV irradiation is smaller than that after UV irradiation, which may be attributed to that the planar trans-isomer is dominated before UV irradiation and stacks closer than the bent cis-isomer at the interface.^6b,17 The CAC values were also checked by the measurements of conductivity (see Fig. S2 in ESI†), which are listed in Table 1 and show well agreement with those determined by surface tension method.

Fig. 2 Plots of surface tension against the ln C (C refers to the concentration of BHATfO). The filled circles and open circles refer to the experimental data collected before UV irradiation and after UV irradiation, respectively. The lines are linear fitting results.

Table 1 Critical aggregation concentrations (CAC), the maximum surface excess Γ_max, and the average minimum surface area A_min for BHATfO in aqueous solution before and after UV irradiation at T = 298.15 K

	CAC/mmol L^−1		10^6 Γmax/mol m^−2	Amin/nm^2
	Surface tension	Conductivity
Before UV irradiation	0.41 ± 0.02	0.37 ± 0.02	4.21 ± 0.05	0.39 ± 0.02
After UV irradiation	0.54 ± 0.02	0.52 ± 0.01	3.67 ± 0.04	0.45 ± 0.01

---

Dynamic light scattering (DLS) measurements were carried out to show the size of the aggregates formed in BHATfO aqueous solution before and after UV irradiation. The size distributions of 2.5 mM BHATfO aqueous solution (above the CAC) for different irradiation stages are shown in Fig. 3a. It may be clearly seen that the small size aggregate of 10 nm constructed from trans-isomer (see line (i)) grows up into large size aggregate of one order of magnitude higher (see line (ii)) formed mainly by cis-isomer after UV irradiation; it returns to the small size aggregate (see line (iii)) after further irradiation by visible light, however, the average size is slightly different from that before UV irradiation, which was attributed to that the aggregates are made up of trans/cis mixture after further irradiation by vis-light while the aggregates formed before UV irradiation are constructed by almost pure trans-isomer. The size distributions of the aggregate by alternative UV/vis light irradiation for five circles are shown in Fig. 3b and c, suggesting good reversibility.

Fig. 3 (a) Size distributions of 2.5 mM BFATfO aqueous solution (i) before UV irradiation, (ii) after UV irradiation, and (iii) further irradiation by visible light. (b) and (c) Sizes distributions upon five cycles with alternative irradiation by UV/vis light.

Transmission electronic microscopy (TEM) measurements were performed to further check the size change of the aggregates by alternative UV/vis light irradiation. The TEM images of the 2.5 mM BHATfO aqueous solution at different irradiation stages are presented in Fig. 4a–c, where small size aggregates at initial state and after further vis-light irradiation are observed while large size aggregates after UV irradiation appear. The sizes of the aggregates coincide reasonably well with those determined by DLS measurements. In order to further visualize the morphology of the large size aggregate after irradiation by UV light, negative stained TEM and cryo-TEM images for 2.5 mM BHATfO aqueous solution were obtained and displayed in Fig. 4d and e, where vesicle structure is clearly displayed.

Fig. 4 TEM images of 2.5 mM BFATfO aqueous solution (a) before UV irradiation, (b) after UV irradiation, and (c) further irradiation by visible light. (d) and (e) Negative stained TEM and cryo-TEM images of 2.5 mM BHATfO aqueous solution after UV irradiation. The scale bars shown in the (a–d) are 200 nm.

The micelle–vesicle–micelle transition process could be described as shown in Scheme 2. The initially synthesized nearly pure trans-BHATfO aggregates into small micelles after CAC firstly; after UV irradiation, the planar trans-isomer almost completely turns into bent cis-isomer accompanied by the transition of micelle into vesicle; further irradiation by visible light results in a mixture of trans- and cis-isomers with trans-isomer being dominated and regeneration of small size micelles.

Scheme 2 Illustration of micelle–vesicle–micelle transition.

To sum up, the synthesized surface active ionic liquid BHATfO showed reversible transition between trans- and cis-form by UV/vis light irradiation. Its aggregation behaviors in aqueous solutions before and after UV irradiation were investigated by means of UV-spectroscopy, surface tension, conductivity, DLS, and TEM. For the first time, the micelle to vesicle to micelle transition triggered by UV/vis irradiation was observed in a single amphiphilic ionic liquid, which shows potential applications in fields like drug delivery, material synthesis, etc.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Projects 21173080, 21373085 and 21303055) and the Fundamental Research Funds for the Central Universities (WJ1516001).

Notes and references

1. (a) P. Brown, C. P. Butts and J. Eastoe, Soft Matter, 2013, 9, 2365; (b) Z. L. Zhu, C. A. Dreiss and Y. J. Feng, Chem. Soc. Rev., 2013, 42, 7174.
2. (a) P. Broz, S. Driamov, J. Ziegler, B. H. Nadav, S. Marsch, W. Meier and P. Hunziker, Nano Lett., 2006, 6, 2349; (b) K. Sakai, E. G. Smith, G. B. Webber, M. Baker, E. J. Wanless, V. Butun, S. P. Armes and S. Biggs, Langmuir, 2006, 22, 8435; (c) Y. C. Liu, A. L. M. Le Ny, J. Schmidt, Y. Talmon, B. F. Chmelka and C. T. Lee Jr, Langmuir, 2009, 25, 5713; (d) J. Su, F. Chen, V. L. Cryns and P. B. Messersmith, J. Am. Chem. Soc., 2011, 133, 11850; (e) Y. Yang, L. Yue, H. L. Li, E. Maher, Y. G. Li, Y. Z. Wang, L. X. Wu and V. W. Yam, Small, 2012, 8, 3105; (f) J. Wei, Y. Y. Liu, J. Chen, Y. H. Li, Q. Yue, C. X. Pan, Y. L. Yu, Y. H. Deng and D. Y. Zhao, Adv. Mater., 2014, 26, 1782.
3. (a) Y. Y. Lin, X. H. Cheng, Y. Qiao, C. L. Yu, Z. B. Li, Y. Yan and J. B. Huang, Soft Matter, 2010, 6, 902; (b) H. Yan, Y. Long, K. Song, C. H. Tung and L. Q. Zheng, Soft Matter, 2014, 10, 115.
4. J. Eastoe and A. Vesperinas, Soft Matter, 2005, 1, 338.
5. S. Shinkai, K. Mastsuo, A. Harada and O. Manabe, J. Chem. Soc., Perkin Trans. 2, 1982, 2, 1261.
6. (a) T. Hayashita, T. Kurosawa, T. Miyata, K. Tanaka and M. Igawa, Colloid Polym. Sci., 1994, 272, 1611; (b) K. Ho-Cheol, B. M. Lee, J. Yoon and M. Yoon, J. Colloid Interface Sci., 2000, 231, 255; (c) J. Yang, H. Y. Wang, J. J. Wang, Y. Zhang and Z. J. Guo, Chem. Commun., 2014, 50, 14979.
7. (a) Q. Jin, C. Luy, J. Ji and S. Agarwal, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 451; (b) M. H. Li and P. Keller, Soft Matter, 2009, 5, 927; (c) E. Blasco, J. del Barrio, C. Sanchez-Somolinos, M. Pinol and L. Oriol, Polym. Chem., 2013, 4, 2246; (d) S. Shrivastava and H. Matsuoka, Langmuir, 2014, 30, 3957.
8. (a) J. Eastoe, M. S. Dominguez, P. Wyatt and A. J. Orr-Ewing, Langmuir, 2004, 20, 6120; (b) B. L. Song, Y. F. Hu and J. X. Zhao, J. Colloid Interface Sci., 2009, 333, 820; (c) L. Li, Y. Yang, J. F. Dong and X. F. Li, J. Colloid Interface Sci., 2010, 343, 504; (d) A. Matsumura, K. Tsuchiya, K. Torigoe, K. Sakai, H. Sakai and M. Abe, Langmuir, 2011, 27, 1610; (e) Y. Yang, J. F. Dong and X. F. Li, J. Dispersion Sci. Technol., 2013, 34, 47; (f) K. Jia, Y. M. Cheng, X. Liu, X. F. Li and J. F. Dong, RSC Adv., 2015, 5, 640.
9. (a) J. Dupont, Acc. Chem. Res., 2011, 44, 1223; (b) Ionic Liquids-New Aspects for the Future, ed. J. Kadokawa, InTech, Rijeka, Croatia, 2013.
10. (a) N. A. Smirnova and E. A. Safonova, Russ. J. Phys. Chem. A, 2010, 84, 1695; (b) Z. G. Asadov, G. A. Akhmedova, A. D. Aga-Zadeh, S. M. Narbalieva, A. M. Bagirova and R. A. Ragimov, Russ. J. Gen. Chem., 2012, 82, 1916; (c) W. Kunz, Z. Thomas and H. Agnes, Curr. Opin. Colloid Interface Sci., 2012, 17, 205.
11. (a) P. D. Galgano and O. A. El Seoud, J. Colloid Interface Sci., 2010, 345, 1; (b) K. S. Rao, P. S. Gehlot, T. J. Trivedi and A. Kumar, J. Colloid Interface Sci., 2014, 428, 267; (c) V. G. Rao, C. Banerjee, S. Ghosh, S. Mandal, J. Kuchlyan and N. Sarkar, J. Phys. Chem. B, 2013, 117, 7472; (d) M. Q. Ao, G. Y. Xu, J. Y. Pang and T. T. Zhao, Langmuir, 2009, 25, 9721; (e) T. M. Garcia, I. Ribosa, L. Perez, A. Manresa and F. Comelles, Langmuir, 2013, 29, 2536.
12. (a) C. C. Villa, F. Moyano, M. Ceolin, J. J. Silber, R. D. Falcone and N. M. Correa, Chem.–Eur. J., 2012, 18, 15598; (b) C. Banerjee, S. Mandal, S. Ghosh, J. Kuchlyan, N. Kundu and N. Sarkar, J. Phys. Chem. B, 2013, 117, 3927; (c) K. S. Rao, T. J. Trivedi and A. Kumar, J. Phys. Chem. B, 2012, 116, 14363; (d) L. L. Zhang, J. Liu, T. T. Tian, Y. Guo, X. Q. Ji, Z. H. Li and Y. X. Luan, ChemPhysChem, 2013, 14, 3454; (e) K. S. Rao, S. So and A. Kumar, Chem. Commun., 2013, 49, 8111; (f) L. J. Shi, Y. Wei, N. Sun and L. Q. Zheng, Chem. Commun., 2013, 49, 11388; (g) H. Y. Wang, L. M. Zhang, J. J. Wang, Z. Y. Li and S. J. Zhang, Chem. Commun., 2013, 49, 5222; (h) C. Tourne-Peteilh, B. Coasne, M. In, D. Brevet, J. M. Devoisselle, A. Vioux and L. Viau, Langmuir, 2014, 30, 1229; (i) H. Y. Wang, B. Tan, J. J. Wang, Z. Y. Li and S. J. Zhang, Langmuir, 2014, 30, 3971; (j) P. Li, Z. P. Du, G. Y. Wang, L. F. Zhi and H. Z. Sun, J. Dispersion Sci. Technol., 2014, 35, 364; (k) P. Li, W. X. Wang, Z. P. Du, G. Y. Wang, E. Z. Li and X. Li, Colloids Surf., A, 2014, 450, 52.
13. Smart Light-responsive Materials: Azobenzene-Containing Polymers and Liquid Crystals, ed. Y. Zhao and T. Ikeda, Wiley-Interscience, New Jersey, 2009.
14. Y. Zakreskyy, M. Richter, S. Zakreska, N. Lomadze, R. von Klitzing and S. Santer, Adv. Funct. Mater., 2012, 22, 5000.
15. L. D. Song and M. J. Rosen, Langmuir, 1996, 12, 1149.
16. M. J. Rosen, Surfactants and Interfacial Phenomena, 3rd edn, Wiley, New York, 2004.
17. T. Shang, K. A. Smith and T. A. Hatton, Langmuir, 2003, 19, 10764.

---

Footnote

† Electronic supplementary information (ESI) available: Synthetic process and characterizations of products; the experimental methods. See DOI: 10.1039/c5ra12047k

---