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Journal of Spectroscopy
Volume 2014 (2014), Article ID 758765, 7 pages
http://dx.doi.org/10.1155/2014/758765
Research Article

Preparation and Characterization of Binary Organogels via Some Azobenzene Amino Derivatives and Different Fatty Acids: Self-Assembly and Nanostructures

1Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China
2Department of Chemistry, Hebei Normal University of Science and Technology, Qinhuangdao 066004, China

Received 9 March 2014; Accepted 7 April 2014; Published 11 June 2014

Academic Editor: Xinqing Chen

Copyright © 2014 Haiying Guo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In present work the gelation behaviors of binary organogels composed of azobenzene amino derivatives and fatty acids with different alkyl chains in various organic solvents were designed and investigated. Their gelation behaviors in 20 solvents were tested as new binary organic gelators. It showed that the length of alkyl substituent chains and azobenzene segment have played a crucial role in the gelation behavior of all gelator mixtures in various organic solvents. Longer alkyl chains in molecular skeletons in present gelators are favorable for the gelation of organic solvents. Morphological studies revealed that the gelator molecules self-assemble into different aggregates from lamella, wrinkle, to belt with change of solvents. Spectral studies indicated that there existed different H-bond formation and hydrophobic force, depending on different substituent chains in molecular skeletons. The present work may also give new perspectives for designing new binary organogelators and soft materials.

1. Introduction

In recent years, organogels have been attracting more attention as one class of important soft materials, in which organic solvents are immobilized by gelators [14]. Although gels are widely found in polymer systems, there has recently been an increasing interest in low-molecular-mass organic gelators (LMOGs) [58]. In recent years, physical gelation of organic solvents by LMOGs has become one of the hot areas in the soft matter research due to their scientific values and many potential applications in the biomedical field, including tissue engineering, controlled drug release, and medical implants [912]. The gels based on LMOGs are usually considered as supramolecular gels, in which the gelator molecules self-assemble into three-dimensional networks in which the solvent is trapped via various noncovalent interactions, such as hydrogen bonding, π-π stacking, van der Waals interaction, dipole-dipole interaction, coordination, solvophobic interaction, and host-guest interaction [1316]. Such organogels have some advantages over polymer gels: the molecular structure of the gelator is defined and the gel process is usually reversible. Such properties make it possible to design various functional gel systems and produce more complicated and defined, as well as controllable, nanostructures [1720].

In our reported work, the gelation properties of some cholesterol imide derivatives consisting of cholesteryl units and photoresponsive azobenzene substituent groups have been investigated [21]. We found that a subtle change in the headgroup of azobenzene segment can produce a dramatic change in the gelation behavior of both compounds. In addition, the gelation properties of bolaform and trigonal cholesteryl derivatives with different aromatic spacers have been characterized [22]. Therein, we have investigated the spacer effect on the microstructures of such organogels and found that various kinds of hydrogen bond interactions among the molecules play an important role in the formation of gels. Furthermore, in another relative research work, the gelation behaviors of some new azobenzene imide derivatives with different alkyl substituent chains and headgroups of azobenzene residues were investigated [23]. The experimental results indicated that more alkyl chains in molecular skeletons in synthesized imide gelators were favorable for the gelation of organic solvents.

As a continuous research work, herein, we have designed and prepared new binary organogels composed of aminoazobenzene derivatives and fatty acids with different alkyl chains. We have found that some of present mixtures of acid/amine compounds could form different organogels in various organic solvents. Morphological characterization of the organogels revealed different structures of the aggregates in the gels. We have investigated the effect of alkyl substituent chains in gelators on the microstructures of such organogels in detail and found different kinds of hydrogen bond interactions.

2. Experiments

2.1. Materials

The starting materials, 4-aminoazobenzene, 2-aminoazotoluene, stearic acid, palmitic acid, tetradecanoic acid, and dodecanoic acid, were purchased from Alfa Aesar Tianjin Chemicals, Aldrich Chemicals, and TCI Shanghai Chemicals, respectively. Other used reagents were all for analysis purity from Beijing Chemicals and were distilled before use.

2.2. Gelation Test

All mixed organogels were prepared according to a simple procedure. Firstly, these fatty acid and amine derivatives were mixed with 1 : 1 molar ratio according to the number matching of intermolecular carboxylic acid and amine group, respectively. Then, a weighted amount of binary mixtures and a measured volume of selected pure organic solvent were placed into a sealed glass bottle and the solution was heated in a water bath until the solid was dissolved. Then, the solution was cooled to room temperature in air and the test bottle was inversed to see if a gel was formed. When the binary mixtures formed a gel by immobilizing the solvent at this stage, it was denoted as “G.” For the systems in which only solution remained until the end of the tests, they were referred to as solution (S). When the binary mixtures formed into a few precipitate in some solvent, it was denoted as a “PS.” Critical gelation concentration refers to the minimum concentration of the gelator for gel formation.

2.3. Measurements

Firstly, the xerogel was prepared by a vacuum pump for 12–24 h. The dried sample thus obtained was attached to copper foil, glass, and CaF2 slice for morphological and spectral investigation, respectively. Before SEM measurement, the samples were coated on copper foil fixed by conductive adhesive tape and shielded by gold. SEM pictures of the xerogel were taken on a Hitachi S-4800 field emission scanning electron microscopy with the accelerating voltage of 5–15 kV. Transmission FT-IR spectra of the xerogel were obtained by Nicolet is/10 FT-IR spectrophotometer from Thermo Fisher Scientific Inc. by average 32 scans and at a resolution of 4 cm−1. The XRD measurement was conducted using a Rigaku D/max 2550PC diffractometer (Rigaku Inc., Tokyo, Japan). The XRD pattern was obtained using CuKα radiation with an incident wavelength of 0.1542 nm under a voltage of 40 kV and a current of 200 mA. The scan rate was 0.5°/min.

3. Results and Discussions

3.1. Gelation Behaviors of These Binary Mixtures

The gelation performances of all binary mixtures in 20 solvents are tested. The experimental data showed that the binary mixtures of fatty acids with different alkyl chains and 4-aminoazobenzene/2-aminoazotoluene could form organogels in special organic solvents, as listed in Table 1. The binary mixtures of fatty acids with different carbon numbers (18, 16, 14, and 12) and 4-aminoazobenzene are denoted as C18-Azo, C16-Azo, C14-Azo, and C12-Azo, respectively. Similarly, the binary mixtures of these acids and 2-aminoazotoluene are denoted as C18-Azo-Me, C16-Azo-Me, C14-Azo-Me, and C12-Azo-Me, respectively. Firstly, for the mixtures containing 4-aminoazobenzene, C18-Azo, C16-Azo, and C14-Azo can form organogel in ethanolamine, while C12-Azo do no form any organogel in present solvents. In addition, it is interesting to note that C16-Azo can form another organogel in nitrobenzene. However, for the mixtures containing 2-aminoazotoluene, only C18-Azo-Me and C16-Azo-Me can form gel in ethanolamine, respectively. Their photographs of all as-made organogels in different solvents were shown in Figure 1. The present research results indicated that length change of alkyl chains can have a profound effect upon the gelation abilities of these studied mixtures. It seemed that longer alkyl chains in molecular skeletons in present mixture gelators are more favorable for the present mixtures, which was similar to the recent reports [21, 22].

tab1
Table 1: Gelation behaviors of these binary organogels.
fig1
Figure 1: Photographs of as-made organogels: (a) C18-Azo; (b) C16-Azo; (c) C14-Azo; (d) C18-Azo-Me; (e) C16-Azo-Me, respectively.
3.2. Morphological Investigation of Organogels

Many researchers have reported that a gelator molecule constructs nanoscale superstructures such as fibers, ribbons, and sheets in a supramolecular gel [23, 24]. To obtain a visual insight into the gel nanostructures, the typical organized structures of these organogels were studied by SEM technique, as shown in Figure 2. From the present diverse images, it can be obviously observed that the nanostructures of all xerogels from ethanolamine and nitrobenzene are significantly different from each other, and the morphologies of the aggregates change from lamella, wrinkle, to belt with change of solvents and mixtures. In addition, more rod-like or belt-like aggregates with different sizes were prepared in gels of C16-Azo in nitrobenzene. In addition, it is interesting to note that these belt aggregates showed a tendency to aggregate together due to highly directional intermolecular interactions and/or solvent evaporation. The difference of morphologies can be mainly due to the different strengths of the intermolecular hydrophobic force between alkyl chains of fatty acids, which have played an important role in regulating the intermolecular orderly staking and formation of special aggregates.

fig2
Figure 2: SEM images of xerogels: (a) and ((c)–(f)): C18-Azo, C16-Azo, C14-Azo, C18-Azo-Me, and C16-Azo-Me in ethanolamine, respectively; (b) C16-Azo in nitrobenzene.
3.3. Spectral Investigation of Organogels

In addition, in order to further investigate the orderly stacking of xerogels nanostructures, XRD of all xerogels from gels were measured, as shown in Figure 3. Firstly, the curves of C16-Azo xerogels from ethanolamine and nitrobenzene show similar strong peaks in the angle region (2θ values, 3.56, 7.18, 10.90, 18.28, and 21.94°) corresponding to values of 2.48, 1.23, 0.81, 0.49, and 0.41 nm, respectively. In addition, for the curves of C14-Azo and C18-Azo xerogels from ethanolamine, weaker peaks appeared, suggesting more disordered structures in the gels. However, as for the curves of C18-Azo-Me and C16-Azo-Me from ethanolamine, the minimum 2θ values are 3.20 and 3.32°, corresponding to values of 2.76 and 2.66 nm, respectively. The difference of values between C16-Azo and C16-Azo-Me can be mainly assigned to the change of substituent groups linked to azobenzene segment in the molecular skeleton, which affected the assembly modes in the 3D stacking of organogels [22]. The XRD results described above demonstrated again that the many factors, such as chain length and substituent group, had great effect on the assembly modes of these gelator mixtures.

758765.fig.003
Figure 3: X-ray diffraction patterns of xerogels: (a) and ((c)–(f)): C18-Azo, C16-Azo, C14-Azo, C18-Azo-Me, and C16-Azo-Me in ethanolamine, respectively; (b) C16-Azo in nitrobenzene.

It is well-known that hydrogen bonding plays an important role in the self-assembly process of organogels [2527]. At present, in order to further clarify this and investigate the effect of many factors on assembly, we have measured the FT-IR spectra of all xerogels, as shown in Figure 4. Firstly, C18-Azo xerogel was taken as examples, as shown in Figure 4(a). Some main peaks were observed at 3354, 3224, 2918, 2850, 1724, 1633, and 1470 cm−1, respectively, which can be assigned to the N–H and O–H stretching, methylene stretching, carbonyl group band, amide I band, and methylene scissoring, respectively [2831]. These bands indicated the formation of hydrogen bonding interactions between intermolecular amino and carboxylic acid groups in the gel state, which can regulate the stacking of the gelator molecules to self-assemble into ordered structures. Similar spectra were observed for other xerogels.

758765.fig.004
Figure 4: FT-IR spectra of xerogels: (a) and ((c)–(f)): C18-Azo, C16-Azo, C14-Azo, C18-Azo-Me, and C16-Azo-Me in ethanolamine, respectively; (b) C16-Azo in nitrobenzene.

Considering the XRD results described above and the hydrogen bonding nature of these binary mixtures as confirmed by FT-IR measurements, a possible assembly mode for present xerogels was proposed and schematically shown in Figure 5. As for C18-Azo xerogel containing longer alkyl chain, due to the hydrophobic force of methylene chains, after the intermolecular hydrogen bonding and orderly stacking, the nanostructures of supramolecular assembly were obtained. For the binary mixture C12-Azo with shorter alkyl chain, the intermolecular interaction is not enough to connect the repeating units with each other, so no gels were formed.

758765.fig.005
Figure 5: Assembly modes for C18-Azo in organogel and C12-Azo in solution.

4. Conclusion

In summary, the gelation behaviors of binary organogels composed of azobenzene amino derivatives and fatty acids with different alkyl chains in various organic solvents were investigated. The experimental results indicated that their gelation behaviors solvents can be regulated by changing length of alkyl substituent chains and azobenzene segment. Longer alkyl chains in molecular skeletons in present gelators are favorable for the gelation of organic solvents. For the mixtures containing 4-aminoazobenzene, only C12-Azo cannot form any organogel in present solvents. While for the mixtures containing 2-aminoazotoluene, only C18-Azo-Me and C16-Azo-Me can form gel in ethanolamine, respectively. Morphological studies revealed that the gelator molecules self-assemble into different aggregates from lamella, wrinkle, to belt with change of solvents. Spectral studies indicated that there existed different H-bond formation and hydrophobic force, depending on different substituent chains in molecular skeletons. The prepared nanostructured materials have wide perspectives and many potential applications in nanoscience and material fields due to their scientific values. The present work may also give some insight to design and character new organogelators and soft materials.

Conflict of Interests

The authors declare that they have no direct financial relation with the commercial identities mentioned in this paper that might lead to a conflict of interests for any of the authors.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (no. 21207112), the Natural Science Foundation of Hebei Province (nos. B2012203060 and B2013203108), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (nos. Y2011113 and YQ2013026), and the Support Program for the Top Young Talents of Hebei Province.

References

  1. H. Sawalha, R. den Adel, P. Venema, A. Bot, E. Flöter, and E. van der Linden, “Organogel-emulsions with mixtures of β-sitosterol and γ-oryzanol: influence of water activity and type of oil phase on gelling capability,” Journal of Agricultural and Food Chemistry, vol. 60, no. 13, pp. 3462–3470, 2012. View at Publisher · View at Google Scholar · View at Scopus
  2. P. Rajamalli and E. Prasad, “Low molecular weight fluorescent organogel for fluoride ion detection,” Organic Letters, vol. 13, no. 14, pp. 3714–3717, 2011. View at Publisher · View at Google Scholar · View at Scopus
  3. S. Bouguet-Bonnet, M. Yemloul, and D. Canet, “New application of proton nuclear spin relaxation unraveling the intermolecular structural features of low-molecular-weight organogel fibers,” Journal of the American Chemical Society, vol. 134, no. 25, pp. 10621–10627, 2012. View at Publisher · View at Google Scholar · View at Scopus
  4. H. Yu and Q. Huang, “Improving the oral bioavailability of curcumin using novel organogel-based nanoemulsions,” Journal of Agricultural and Food Chemistry, vol. 60, no. 21, pp. 5373–5379, 2012. View at Publisher · View at Google Scholar · View at Scopus
  5. G. M. Newbloom, K. M. Weigandt, and D. C. Pozzo, “Electrical, mechanical, and structural characterization of self-assembly in poly(3-hexylthiophene) organogel networks,” Macromolecules, vol. 45, no. 8, pp. 3452–3462, 2012. View at Publisher · View at Google Scholar · View at Scopus
  6. D. Liu, D. Wang, M. Wang et al., “Supramolecular organogel based on crown ether and secondary ammoniumion functionalized glycidyl triazole polymers,” Macromolecules, vol. 46, no. 11, pp. 4617–4625, 2013. View at Publisher · View at Google Scholar · View at Scopus
  7. S. O'Sullivan and D. W. M. Arrigan, “Impact of a surfactant on the electroactivity of proteins at an aqueous-organogel microinterface array,” Analytical Chemistry, vol. 85, no. 3, pp. 1389–1394, 2013. View at Publisher · View at Google Scholar · View at Scopus
  8. H. Takeno, A. Maehara, D. Yamaguchi, and S. Koizumi, “A structural study of an organogel investigated by small-angle neutron scattering and synchrotron small-angle X-ray scattering,” The Journal of Physical Chemistry B, vol. 116, no. 26, pp. 7739–7745, 2012. View at Publisher · View at Google Scholar · View at Scopus
  9. J. Lu, J. Hu, Y. Song, and Y. Ju, “A new dual-responsive organogel based on uracil-appended glycyrrhetinic acid,” Organic Letters, vol. 13, no. 13, pp. 3372–3375, 2011. View at Publisher · View at Google Scholar · View at Scopus
  10. M. Yemloul, E. Steiner, A. Robert et al., “Solvent dynamical behavior in an organogel phase as studied by NMR relaxation and diffusion experiments,” The Journal of Physical Chemistry B, vol. 115, no. 11, pp. 2511–2517, 2011. View at Publisher · View at Google Scholar · View at Scopus
  11. P. D. Wadhavane, R. E. Galian, M. A. Izquierdo et al., “Photoluminescence enhancement of CdSe quantum dots: a case of organogel-nanoparticle symbiosis,” Journal of the American Chemical Society, vol. 134, no. 50, pp. 20554–20563, 2012. View at Publisher · View at Google Scholar · View at Scopus
  12. W. Liu, P. Xing, F. Xin et al., “Novel double phase transforming organogel based on β-cyclodextrin in 1,2-propylene glycol,” The Journal of Physical Chemistry B, vol. 116, no. 43, pp. 13106–13113, 2012. View at Publisher · View at Google Scholar · View at Scopus
  13. M. Bielejewski and J. Tritt-Goc, “Evidence of solvent-gelator interaction in sugar-based organogel studied by field-cycling NMR relaxometry,” Langmuir, vol. 26, no. 22, pp. 17459–17464, 2010. View at Publisher · View at Google Scholar · View at Scopus
  14. Y. Li, J. Liu, G. Du et al., “Reversible heat-set organogel based on supramolecular interactions of β-Cyclodextrin in N, N-dimethylformamide,” The Journal of Physical Chemistry B, vol. 114, no. 32, pp. 10321–10326, 2010. View at Publisher · View at Google Scholar · View at Scopus
  15. F. Xin, H. Zhang, B. Hao et al., “Controllable transformation from sensitive and reversible heat-set organogel to stable gel induced by sodium acetate,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 410, pp. 18–22, 2012. View at Publisher · View at Google Scholar · View at Scopus
  16. F. S. Schoonbeek, J. H. van Esch, R. Hulst, R. M. Kellogg, and B. L. Feringa, “Geminal bis-ureas as gelators for organic solvents: gelation properties and structural studies in solution and in the gel state,” Chemistry, vol. 6, no. 14, pp. 2633–2643, 2000. View at Google Scholar · View at Scopus
  17. P. Anilkumar and M. Jayakannan, “A novel supramolecular organogel nanotubular template approach for conducting nanomaterials,” The Journal of Physical Chemistry B, vol. 114, no. 2, pp. 728–736, 2010. View at Publisher · View at Google Scholar · View at Scopus
  18. Y. Marui, A. Kikuzawa, T. Kida, and M. Akashi, “Unique organogel formation with macroporous materials constructed by the freeze-drying of aqueous cyclodextrin solutions,” Langmuir, vol. 26, no. 13, pp. 11441–11445, 2010. View at Publisher · View at Google Scholar · View at Scopus
  19. A. R. Hirst, D. K. Smith, and J. P. Harrington, “Unique nanoscale morphologies underpinning organic gel-phase materials,” Chemistry, vol. 11, no. 22, pp. 6552–6559, 2005. View at Publisher · View at Google Scholar · View at Scopus
  20. M. Moniruzzaman and P. R. Sundararajan, “Low molecular weight organogels based on long-chain carbamates,” Langmuir, vol. 21, no. 9, pp. 3802–3807, 2005. View at Publisher · View at Google Scholar · View at Scopus
  21. T. Jiao, Y. Wang, F. Gao, J. Zhou, and F. Gao, “Photoresponsive organogel and organized nanostructures of cholesterol imide derivatives with azobenzene substituent groups,” Progress in Natural Science: Materials International, vol. 22, no. 1, pp. 64–70, 2012. View at Publisher · View at Google Scholar · View at Scopus
  22. T. Jiao, F. Gao, Y. Wang, J. Zhou, F. Gao, and X. Luo, “Supramolecular gel and nanostructures of bolaform and trigonal cholesteryl derivatives with different aromatic spacers,” Current Nanoscience, vol. 8, no. 1, pp. 111–116, 2012. View at Google Scholar · View at Scopus
  23. T. Jiao, Y. Wang, Q. Zhang, J. Zhou, and F. Gao, “Regulation of substituent groups on morphologies and self-assembly of organogels based on some azobenzene imide derivatives,” Nanoscale Research Letters, vol. 8, no. 1, article 160, pp. 1–8, 2013. View at Publisher · View at Google Scholar · View at Scopus
  24. P. Mukhopadhyay, Y. Iwashita, M. Shirakawa, S. Kawano, N. Fujita, and S. Shinkai, “Spontaneous colorimetric sensing of the positional isomers of dihydroxynaphthalene in a 1D organogel matrix,” Angewandte Chemie, vol. 45, no. 10, pp. 1592–1595, 2006. View at Publisher · View at Google Scholar · View at Scopus
  25. J. L. Gurav, I.-K. Jung, H.-H. Park, E. S. Kang, and D. Y. Nadargi, “Silica aerogel: synthesis and applications,” Journal of Nanomaterials, vol. 2010, Article ID 409310, 11 pages, 2010. View at Publisher · View at Google Scholar · View at Scopus
  26. A. S. Al Dwayyan, M. N. Khan, and M. S. Al Salhi, “Optical characterization of chemically etched nanoporous silicon embedded in sol-gel matrix,” Journal of Nanomaterials, vol. 2012, Article ID 713203, 7 pages, 2012. View at Publisher · View at Google Scholar · View at Scopus
  27. H. Guo, T. Jiao, X. Shen et al., “Binary organogels based on glutamic acid derivatives and different acids: solvent effect and molecular skeletons on self-assembly and nanostructures,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 447, pp. 88–96, 2014. View at Publisher · View at Google Scholar
  28. M. Zinic, F. Vögtle, and F. Fages, “Cholesterol-based gelators,” Topics in Current Chemistry, vol. 256, pp. 39–76, 2005. View at Publisher · View at Google Scholar · View at Scopus
  29. T. Y. Wang, Y. G. Li, and M. H. Liu, “Gelation and self-assembly of glutamate bolaamphiphiles with hybrid linkers: effect of the aromatic ring and alkyl spacers,” Soft Matter, vol. 5, no. 5, pp. 1066–1073, 2009. View at Publisher · View at Google Scholar · View at Scopus
  30. T. F. Jiao, Q. Q. Huang, Q. R. Zhang, D. B. Xiao, J. X. Zhou, and F. M. Gao, “Self-assembly of organogels via new luminol imide derivatives: diverse nanostructures and substituent chain effect,” Nanoscale Research Letters, vol. 8, no. 1, article 278, pp. 1–8, 2013. View at Publisher · View at Google Scholar · View at Scopus
  31. Y. G. Li, T. Y. Wang, and M. H. Liu, “Ultrasound induced formation of organogel from a glutamic dendron,” Tetrahedron, vol. 63, no. 31, pp. 7468–7473, 2007. View at Publisher · View at Google Scholar · View at Scopus