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C2-symmetric benzene-based organogels: A rationally designed LMOG and its application in marine oil spill
Journal of Molecular Liquids 190 (2014) 94–98
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
C2-symmetric benzene-based organogels: A rationally designed LMOG and its application in marine oil spill Shi-Lin Yu a,1, Xiao-Qiu Dou b,1, Da-Hui Qu a,⁎, Chuan-Liang Feng b,⁎⁎ a b
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 9 July 2013 Received in revised form 19 October 2013 Accepted 31 October 2013 Available online 14 November 2013 Keywords: Organogelator PEB Hydrogen bonding π–π stacking Selective gelation Marine oil spill
a b s t r a c t With the efforts to exploit the marine and marine transport development, marine oil spill often happens in the process of marine transportation, which has a great negative effect on the environment. 1,4-Bi(phenylalanineethylene glycol monohexyl ether)-benzene (PEB) may be useful to solve this problem, which shows selective gelation of oil from aromatic solvents in the presence of water. A set of techniques including Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and circular dichroism (CD) have been employed to confirm a β-turn arrangement and a fibrous structure of PEB gelators, which is formed through hydrogen bonding, π–π stacking, and van der Waals interactions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, the synthesis and preparation of low molecular weight organic gels (LMOGs) have attracted great interest because of their potential applications in sensors, templated materials, drug delivery agents, enzyme-immobilization matrices, cosmetics, water purification, as well as in phase selective gelation [1–8]. These organogelators are a family of small molecules (typically with a molecular weight 1000 Da) that can immobilize organic solvents and self-assemble into fibrous, tubular, or helical structures through various non-covalent interactions, including hydrogen bonding, π–π stacking, and van der Waals interactions [9–13]. Although the gelation mechanism [14,15] of LMOGs has been extensively studied, it is still difficult to predict the formation of gels from low molecular compound. Thus, a major challenge in this field is the rational design and synthesis of gelator molecules [16–19]. So far, most of LMOGs have been derived from the amphiphilic [20–22] species, with good gelation abilities. However, these amphiphilic gelators exist as highly hydrophobic derivatives that can probably lead to the formation of kinetically trapped aggregates before their selfassembly, which directly influences the following assembly efficiency and controllability in turn. Recently, our group reported a new family
⁎ Corresponding author. Fax: +86 2164252288. ⁎⁎ Corresponding author. Fax: +86 2154747651. E-mail addresses: [email protected] (D.-H. Qu), [email protected] (C.-L. Feng). 1 These authors contributed equally to this work and should be considered co-first authors. 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.10.031
of effective low-molecular-weight hydrogelators with a modular architecture based on a C2-1,4-diamide benzene core. The highly symmetrical chemical structures of the gelators render their initial distribution in water and no trapped aggregates are formed before the assembly is triggered [23]. The subsequent self-assembly process, which results in the formation of versatile hydrogels, is highly efficient and can be performed in a controlled manner [24]. Although a series of C2 benzene based hydrogelators have been developed, gelators that can gel in organic solvent in preference to water from a given mixture have been not reported. Herein, we found that incorporation of an alkyl chain at the C terminus of the C2-gelator precursor leads to the development of efficient gelator for organic solvents. Typically, this C2 based organic gelator showed selective gelation of organic solvents in the presence of water, which has tremendous implications for resolving problems such as marine oil spill. The details of the findings are presented in this report. 2. Experimental 2.1. Materials and synthesis 1-Ethyl-3-(3-dimethyllaminopropyl)carbodiimide hydrochloride was purchased from Darui. All other chemicals were obtained from Aldrich and used without further purification. All solvents were reagent grade, which were dried and distilled prior to use according to standard procedures. The molecular structures of unknown compounds were confirmed using 1H nuclear magnetic resonance (1H NMR) and high resolution ESI mass spectrometry.
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
The organogelator (PEB) (Scheme 1) was synthesized according to the procedure described as follows:1,4-benzenedicarbonyl dichloride (2.3 g, 11.1 mmol) in 30 mL dry dichloromethane (DCM) was added dropwise into a solution of L-phenylalanine methyl ester hydrochloride (5.0 g, 20.0 mmol) and Et3N (8.3 mL, 59.3 mmol) in 80 mL dry DCM at 0 °C. The mixture was stirred at room temperature for 12 h. Water (80 mL) was added, then extracted with CH2Cl2 (50 × 3 mL). The organic layer was washed with brine (100 × 3 mL), dried over Na2SO4 and evaporated in vacuo to give compound 1 (5.1 g, 93.0%). 1H NMR (CDCl3, 400 MHz, 298 K): δ = 7.75(s, 4H), 6.62–7.30(m, 10H), 5.04– 5.09(m, 2H), 3.77(s, 6H), 3.19–3.31(m, 4H). For the hydrolysis, aqueous NaOH (20.0 mL, 2.0 M) was added to a suspension of compound 1 (5.0 g, 10.2 mmol) in MeOH (50.0 mL). The mixture was stirred at room temperature for 5 h and a clear solution was obtained. The solution was then acidified with concentrated HCl to pH b 3, and a gel-like precipitate formed. The gel phase was filtered, washed with ultrapure water 3 times, and finally dried in the vacuum oven to give pure compound 2 (4.6 g, 98%). 1H NMR (DMSO-d6, 400 MHz, 298 K): δ = 12.77(s, 2H), 8.80(d, J = 8 Hz, 2H), 7.81(s, 4H),7.14–7.30(m, 10H), 4.60(m, 2H), 3.07(d, J = 3.07, 4H). A mixture of compound 2(1 g, 2.2 mmol), ethylene glycol monohexyl ether(1.6 g, 10.9 mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) (1.7 g, 8.7 mmol), and 4-dimethylamiopryidine (DMAP) (0.5 g, 4.3 mmol) in CH2Cl2(10.0 mL) was stirred at room temperature overnight under nitrogen atmosphere. The solution was diluted with 20.0 mL CH2Cl2, and washed by water (50 × 2 mL), dried over Na2SO4, and evaporated under reduced pressure to get the crude product, which was purified by column chromatography (SiO2, CH2Cl2: MeOH = 100:1) to afford the compound 3 (1.0 g, 62.0%) as a oily solid. 1H NMR (DMSO-d6, 400 MHz, 298 K): δ = 8.97(d, J = 8 Hz, 2H), 7.83(s, 4H), 7.15–7.30(m, 10 H), 4.60–4.70(m, 2H), 4.16(t, J = 12.32Hz, 4H), 3.50(t, J = 14.12Hz, 4H), 3.32(t, J = 64Hz, 4H), 3.12(d, J = 192Hz, 4H), 1.36–1.43(m, 4H), 1.12–1.20(m, 12H), 0.789(t,
O
O
Cl
+
Cl
2.2. 1H nuclear magnetic resonance (1H NMR) experiments 1 H NMR studies were carried out on a Brüker Advance III 400 Instrument operating at 400 MHz. All spectra were recorded in DMSO or CDCl3.
2.3. Mass experiments Mass spectra were recorded on a Waters Q-Tof Mass Instrument by positive mode electrospray ionization. Methanol was used as the solvent. 2.4. Scanning electron microscopic (SEM) study Images were obtained using a FEI QUANTA 250 Microscope. Samples were prepared by depositing dilute solutions of gel materials on silicon slices and drying under vacuum, and coated with gold on a sputtering coater. 2.5. Atomic force microscopy (AFM) Images were obtained using a Vecco NanoScope IIIa Atomic force microscope and Mikro Masch NSC11 cantilevers/tips (radius of curvature less than 10 nm). Fiber diameters were measured by Nanoscope 5.30r3sr3 software. Samples were prepared by depositing dilute solutions of gel materials on mica plate and drying under vacuum. 2.6. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra of compound 3 xerogels were taken using Bruck EQUINOX55 Instrument. The KBr disk technique was used for the
O O
O NH
O NH
H
O O
O O OH
NH H
2. HCl H
2
HN H
O O
HO
OH NH H
1
HO
1. NaOH,MeOH
HN
1
HN
O O
O O
O
O O
O
H NH O 2
O O
H
J = 68Hz, 6H). HRMS (ESI) (m/z): calcd for C42H56N208, 716.4; found 715.4 [M].
Et3N DCM
O
95
HN
2
EDCI,DMAP
O
+
OH
H
O
O O O
O O
O
O NH
HN
H
H
3 Scheme 1. The synthesis of compound 3.
DCM
96
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
3. Results and discussion
Table 1 Organogelation properties of 3 in organic solvents (G, turbid gel; TG, transparent gel; I, insoluble; VS, viscous solution; S, solution). Solvent
Status of compound 3
% oil recovered
1-Hexane 1-Heptane CCl4 Toluene p-Xylene Benzene Isobutyl alcohol CH2Cl2 Methoxybenzene DMF CHCl3 Acetone DMSO THF Aniline
I G TG G TG TG VS S TG S S S S S S
– 99.5 76.3 99.6 89.2 95 – – 97 – – – – – –
The organogels, with fibrous structures, were prepared and characterized by SEM, AFM, FT-IR, and CD spectroscopy, respectively. Selective organo-gelation from oil/water of compound 3 is further studied. 3.1. Organogel formation We chose 14 kinds of common solvents (such as 1-heptane, CCl4, toluene, and so on) with different polarities for gelation test. 10 mg of compound 3 was added into solvent (1 mL), the suspension was then heated to 80 °C to form clear solution. The gelation can be observed in some organic solvents after cooling back to room temperature within 3 min. The particular result is presented in Table 1 and Fig. 1. The polarity of solvent(Table S-1)[26] has an important effect on whether the solvent gelates. We found the solvent may be gelated when its polarity is about 0.2–3.5. 3.2. Characterization
solid-state measurement. The samples were then scanned between the wavelengths of 4000 and 400 cm−1 at an interval of 1.9285 cm−1.
It has been demonstrated that C2-benzene based gelators have great self-assembly abilities to form fibrous network through the combination of hydrogen bonds, hydrophobic, and π–π interactions [27]. To get a visual insight into the gelation morphology of compound 3, scanning electron microscopy (SEM) was used to characterize. The xerogel of 3 prepared from dimethylbenzene exhibited an interconnected
2.7. Circular dichroism spectroscopy (CD) studies M A JASCO J-815 CD spectrometer was used to collect CD data. Spectra were measured in a 0.5 mm path length cell.
aniline
S
—
Fig. 1. 1 mL a) benzene with 10 mg of gelator; b) anisole with 10 mg of gelator; c) dimethylbenzene with 10 mg of gelator; d) methylbenzene with 20 mg of gelator; e) phenixin with 10 mg of gelator; f) heptane with 10 mg of gelator 3.
Fig. 2. a) SEM image of self-assembled compound 3 in dimethylbenzene. b)AFM image of self-assembled compound 3 in dimethylbenzene.
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
20
and a trough near 252 nm (nπ* transition), suggesting the backbones of compound 3 adopt similar β-sheet-like configuration [25]. The FT-IR of compound 3 is showed in Fig. 4. The spectrum of compound 3 exhibits strong and broad band at around 3437 cm−1 and 3303 cm−1, which correspond to the N\H stretching vibrations of functional groups engaged in hydrogen bonds (H-bonds). The spectrum of compound 3 shows distinctive absorption bands at 1736 cm−1 (C_O in ester group vibration), 1650 cm−1 (C_O stretching in amide group), and 1543 cm−1 (N\H bonding in amide group).
15 10
CD [mdeg]
97
5 0 -5
3.3. Selective organogelation from oil/water -10 -15 210
225
240
255
270
285
300
λ [nm] Fig. 3. CD spectrum of compound 3 at the concentration of 0.01 mM in dimethylbenzene.
100
Transmittance [%]
95
Purification of water contaminated with toxic organic solvent by selective gelation is always a daunting task that becomes challenging. To this end, only very few phase-selective gelation of oil from oil/water mixtures were reported [28]. Interestingly, we found that organogelator 3 is suitable for the selective gelation of an oil from an oil/water mixture due to their insolubility in water and nice organogelation ability in some organic solvents. In a fundamental procedure, 2 mL of water and 0.5 mL of dimethylbenzene were mixed, and 10 mg of compound 3 was added. The gelator 3 was then solubilized in this two-phase solution by heating and also shaken vigorously to ensure homogeneous dispersion of oil in water (Fig. 5). After cooling the mixed solution to room temperature, the dimethylbenzene layer was gelated, and the water layer remained intact in liquid state. Then, the organogel was separated from the water simply by filtration.
90
4. Conclusion 85
3303
3437
1542
1743 80
1635 75
3500
3000
2500
2000
1500
1000
500
Wavenumbers [cm-1] Fig. 4. FT-IR of xerogel of compound 3.
network of thin fibrils with a thickness of a few tens of nanometers (Fig. 2a). The morphology of the xerogel was further confirmed from atomic force microscopy (AFM). Magnification of the fibers showed that each fiber was formed by the folding orientation of several small fibers (Fig. 2b). The AFM image clearly reveals the fibrous network structure of the gel, which is in consistent with the SEM result. As a helpful tool to determine the secondary structures of amino acid compounds, circular dichroism (CD) is performed to study the superstructures of compound 3 in the phase of dimethylbenzene. Compound 3 shares the common feature of β-sheet structure according to the CD spectra shown in Fig. 3. It exhibits a peak near 242.5 nm (ππ* transition)
In summary, we synthesized a new kind of organogel with high productivity and good gelation ability. This organogel enriches the scaffold of C2-symmetric benzene-based gelators, realizing organo- and hydrogelation using the same scaffold in the simple synthesis method, which is not common. Furthermore, the ability of the selective gelation for organic solvents of organogels makes it possible to find tremendous applications in the field of marine oil spill. Acknowledgments We acknowledge the support from the National Science Foundation of China (51173105), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Foundation for the Author of National Excellent Doctoral Dissertation of China (200957), the Fok Ying Tong Education Foundation (121069), Research Fund for the Doctoral Program of Higher Education of China, SRF for ROCS, SEM, and the Project of Young Teachers Funding (20100073120006). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2013.10.031.
Fig. 5. Selective organogelation from oil/water of compound 3.
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References [1] E.D. Sone, E.R. Zubarev, S.I. Stupp, Angew. Chem. 114 (2002) 1781–1785. [2] S. Bhattacharya, Y.K. Ghosh, Chem. Commun. (2001) 185–186. [3] Y. Ono, K. Nakashima, M. Sano, Y. Kanekiyo, K. Inoue, J. Hojo, S. Shinkai, Chem. Commun. (1998) 1477–1478. [4] S. Kobayashi, K. Hanabusa, N. Hamasaki, M. Kimura, H. Shirai, S. Shinkai, Chem. Mater. 12 (2000) 1523–1525. [5] C. Zhan, J. Wang, J. Yuan, H. Gong, Y. Liu, M. Liu, Langmuir 19 (2003) 9440–9445. [6] V. Bekiari, P. Lianos, Chem. Mater. 18 (2006) 4142–4146. [7] S. Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H. Shinkai, K. Hanabusa, J. Am. Chem. Soc. 124 (2002) 6550–6551. [8] T. Polubesova, S. Nir, D. Zakada, O. Rabinovitz, C. Serban, L. Groisman, B. Rubin, Environ. Sci. Technol. 39 (2005) 2343–2348. [9] N. Mohmeyer, H.W. Schmidt, Chem. Eur. J. 11 (2005) 863–872. [10] N. Mohmeyer, H.W. Schmidt, Chem. Eur. J. 13 (2007) 4499–4509. [11] M. Suzuki, Y. Nakajima, M. Yumoto, M. Kimura, H. Shirai, K. Hanabusa, Org. Biomol. Chem. 2 (2004) 1155–1159. [12] M. Suzuki, M. Nanbu, M. Yumoto, H. Shiraib, K. Hanabusa, New J. Chem. 29 (2005) 1439–1444. [13] H. Yang, T. Yi, Z. Zhou, Y. Zhou, J. Wu, M. Xu, F. Li, C. Huang, Langmuir 23 (2007) 8224–8230.
[14] D. Das, A. Dasgupta, S. Roy, R.N. Mitra, S. Debnath, P.K. Das, Chem. Eur. J. 12 (2006) 5068–5074. [15] J.C. Tiller, Angew. Chem. 115 (2003) 3180–3183; Angew. Chem. Int. Ed. 42 (2003) 3072–3075. [16] X. Yan, D. Xu, X. Chi, J. Chen, S.X. Ding, Y. Yu, F. Huang, Adv. Mater. 24 (2012) 362–369. [17] Dave J. Adams, Macromol. Biosci. 11 (2011) 160–173. [18] X. Li, J. Li, Y. Gao, Y. Kuang, J. Shi, B. Xu, J. Am. Chem. Soc. 132 (2010) 17707–17709. [19] Y. Zhang, Y. Kuang, Y. Gao, B. Xu, Langmuir 27 (2) (2011) 529–537. [20] S. Koutsopoulos, S. Zhang, J. Control. Release 160 (2012) 451–458. [21] F. Zhao, Y. Gao, J. Shi, Hayley M. Browdy, B. Xu, Langmuir 27 (4) (2011) 1510–1512. [22] M.T. McClendon, S.I. Stupp, Biomaterials 33 (2012) 5713–5722. [23] C. Feng, X. Dou, D. Zhang, Holger Schönherr, Macromol. Rapid Commun. 33 (2012) 1535–1541. [24] X. Dou, P. Li, D. Zhang, C. Feng, Soft Matter 8 (2012) 3231–3238. [25] M. Ma, Y. Kuang, Y. Gao, Y. Zhang, P. Gao, B. Xu, J. Am. Chem. Soc. 132 (2010) 2719–2728. [26] J. Catalan, V. Lopez, P. Pkrez, R. Martin-Villamil, J.-G. Rodriguez, Liebigs Ann. (1995) 241–252. [27] X. Dou, X. Yang, P. Li, Z. Zhang, Holger Schönherr, D. Zhang, C. Feng, Soft Matter 8 (2012) 9539–9544. [28] S. Debnath, et al., Chem. Eur. J. 14 (2008) 6870–6881.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
C2-symmetric benzene-based organogels: A rationally designed LMOG and its application in marine oil spill Shi-Lin Yu a,1, Xiao-Qiu Dou b,1, Da-Hui Qu a,⁎, Chuan-Liang Feng b,⁎⁎ a b
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 9 July 2013 Received in revised form 19 October 2013 Accepted 31 October 2013 Available online 14 November 2013 Keywords: Organogelator PEB Hydrogen bonding π–π stacking Selective gelation Marine oil spill
a b s t r a c t With the efforts to exploit the marine and marine transport development, marine oil spill often happens in the process of marine transportation, which has a great negative effect on the environment. 1,4-Bi(phenylalanineethylene glycol monohexyl ether)-benzene (PEB) may be useful to solve this problem, which shows selective gelation of oil from aromatic solvents in the presence of water. A set of techniques including Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and circular dichroism (CD) have been employed to confirm a β-turn arrangement and a fibrous structure of PEB gelators, which is formed through hydrogen bonding, π–π stacking, and van der Waals interactions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, the synthesis and preparation of low molecular weight organic gels (LMOGs) have attracted great interest because of their potential applications in sensors, templated materials, drug delivery agents, enzyme-immobilization matrices, cosmetics, water purification, as well as in phase selective gelation [1–8]. These organogelators are a family of small molecules (typically with a molecular weight 1000 Da) that can immobilize organic solvents and self-assemble into fibrous, tubular, or helical structures through various non-covalent interactions, including hydrogen bonding, π–π stacking, and van der Waals interactions [9–13]. Although the gelation mechanism [14,15] of LMOGs has been extensively studied, it is still difficult to predict the formation of gels from low molecular compound. Thus, a major challenge in this field is the rational design and synthesis of gelator molecules [16–19]. So far, most of LMOGs have been derived from the amphiphilic [20–22] species, with good gelation abilities. However, these amphiphilic gelators exist as highly hydrophobic derivatives that can probably lead to the formation of kinetically trapped aggregates before their selfassembly, which directly influences the following assembly efficiency and controllability in turn. Recently, our group reported a new family
⁎ Corresponding author. Fax: +86 2164252288. ⁎⁎ Corresponding author. Fax: +86 2154747651. E-mail addresses: [email protected] (D.-H. Qu), [email protected] (C.-L. Feng). 1 These authors contributed equally to this work and should be considered co-first authors. 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.10.031
of effective low-molecular-weight hydrogelators with a modular architecture based on a C2-1,4-diamide benzene core. The highly symmetrical chemical structures of the gelators render their initial distribution in water and no trapped aggregates are formed before the assembly is triggered [23]. The subsequent self-assembly process, which results in the formation of versatile hydrogels, is highly efficient and can be performed in a controlled manner [24]. Although a series of C2 benzene based hydrogelators have been developed, gelators that can gel in organic solvent in preference to water from a given mixture have been not reported. Herein, we found that incorporation of an alkyl chain at the C terminus of the C2-gelator precursor leads to the development of efficient gelator for organic solvents. Typically, this C2 based organic gelator showed selective gelation of organic solvents in the presence of water, which has tremendous implications for resolving problems such as marine oil spill. The details of the findings are presented in this report. 2. Experimental 2.1. Materials and synthesis 1-Ethyl-3-(3-dimethyllaminopropyl)carbodiimide hydrochloride was purchased from Darui. All other chemicals were obtained from Aldrich and used without further purification. All solvents were reagent grade, which were dried and distilled prior to use according to standard procedures. The molecular structures of unknown compounds were confirmed using 1H nuclear magnetic resonance (1H NMR) and high resolution ESI mass spectrometry.
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
The organogelator (PEB) (Scheme 1) was synthesized according to the procedure described as follows:1,4-benzenedicarbonyl dichloride (2.3 g, 11.1 mmol) in 30 mL dry dichloromethane (DCM) was added dropwise into a solution of L-phenylalanine methyl ester hydrochloride (5.0 g, 20.0 mmol) and Et3N (8.3 mL, 59.3 mmol) in 80 mL dry DCM at 0 °C. The mixture was stirred at room temperature for 12 h. Water (80 mL) was added, then extracted with CH2Cl2 (50 × 3 mL). The organic layer was washed with brine (100 × 3 mL), dried over Na2SO4 and evaporated in vacuo to give compound 1 (5.1 g, 93.0%). 1H NMR (CDCl3, 400 MHz, 298 K): δ = 7.75(s, 4H), 6.62–7.30(m, 10H), 5.04– 5.09(m, 2H), 3.77(s, 6H), 3.19–3.31(m, 4H). For the hydrolysis, aqueous NaOH (20.0 mL, 2.0 M) was added to a suspension of compound 1 (5.0 g, 10.2 mmol) in MeOH (50.0 mL). The mixture was stirred at room temperature for 5 h and a clear solution was obtained. The solution was then acidified with concentrated HCl to pH b 3, and a gel-like precipitate formed. The gel phase was filtered, washed with ultrapure water 3 times, and finally dried in the vacuum oven to give pure compound 2 (4.6 g, 98%). 1H NMR (DMSO-d6, 400 MHz, 298 K): δ = 12.77(s, 2H), 8.80(d, J = 8 Hz, 2H), 7.81(s, 4H),7.14–7.30(m, 10H), 4.60(m, 2H), 3.07(d, J = 3.07, 4H). A mixture of compound 2(1 g, 2.2 mmol), ethylene glycol monohexyl ether(1.6 g, 10.9 mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) (1.7 g, 8.7 mmol), and 4-dimethylamiopryidine (DMAP) (0.5 g, 4.3 mmol) in CH2Cl2(10.0 mL) was stirred at room temperature overnight under nitrogen atmosphere. The solution was diluted with 20.0 mL CH2Cl2, and washed by water (50 × 2 mL), dried over Na2SO4, and evaporated under reduced pressure to get the crude product, which was purified by column chromatography (SiO2, CH2Cl2: MeOH = 100:1) to afford the compound 3 (1.0 g, 62.0%) as a oily solid. 1H NMR (DMSO-d6, 400 MHz, 298 K): δ = 8.97(d, J = 8 Hz, 2H), 7.83(s, 4H), 7.15–7.30(m, 10 H), 4.60–4.70(m, 2H), 4.16(t, J = 12.32Hz, 4H), 3.50(t, J = 14.12Hz, 4H), 3.32(t, J = 64Hz, 4H), 3.12(d, J = 192Hz, 4H), 1.36–1.43(m, 4H), 1.12–1.20(m, 12H), 0.789(t,
O
O
Cl
+
Cl
2.2. 1H nuclear magnetic resonance (1H NMR) experiments 1 H NMR studies were carried out on a Brüker Advance III 400 Instrument operating at 400 MHz. All spectra were recorded in DMSO or CDCl3.
2.3. Mass experiments Mass spectra were recorded on a Waters Q-Tof Mass Instrument by positive mode electrospray ionization. Methanol was used as the solvent. 2.4. Scanning electron microscopic (SEM) study Images were obtained using a FEI QUANTA 250 Microscope. Samples were prepared by depositing dilute solutions of gel materials on silicon slices and drying under vacuum, and coated with gold on a sputtering coater. 2.5. Atomic force microscopy (AFM) Images were obtained using a Vecco NanoScope IIIa Atomic force microscope and Mikro Masch NSC11 cantilevers/tips (radius of curvature less than 10 nm). Fiber diameters were measured by Nanoscope 5.30r3sr3 software. Samples were prepared by depositing dilute solutions of gel materials on mica plate and drying under vacuum. 2.6. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra of compound 3 xerogels were taken using Bruck EQUINOX55 Instrument. The KBr disk technique was used for the
O O
O NH
O NH
H
O O
O O OH
NH H
2. HCl H
2
HN H
O O
HO
OH NH H
1
HO
1. NaOH,MeOH
HN
1
HN
O O
O O
O
O O
O
H NH O 2
O O
H
J = 68Hz, 6H). HRMS (ESI) (m/z): calcd for C42H56N208, 716.4; found 715.4 [M].
Et3N DCM
O
95
HN
2
EDCI,DMAP
O
+
OH
H
O
O O O
O O
O
O NH
HN
H
H
3 Scheme 1. The synthesis of compound 3.
DCM
96
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
3. Results and discussion
Table 1 Organogelation properties of 3 in organic solvents (G, turbid gel; TG, transparent gel; I, insoluble; VS, viscous solution; S, solution). Solvent
Status of compound 3
% oil recovered
1-Hexane 1-Heptane CCl4 Toluene p-Xylene Benzene Isobutyl alcohol CH2Cl2 Methoxybenzene DMF CHCl3 Acetone DMSO THF Aniline
I G TG G TG TG VS S TG S S S S S S
– 99.5 76.3 99.6 89.2 95 – – 97 – – – – – –
The organogels, with fibrous structures, were prepared and characterized by SEM, AFM, FT-IR, and CD spectroscopy, respectively. Selective organo-gelation from oil/water of compound 3 is further studied. 3.1. Organogel formation We chose 14 kinds of common solvents (such as 1-heptane, CCl4, toluene, and so on) with different polarities for gelation test. 10 mg of compound 3 was added into solvent (1 mL), the suspension was then heated to 80 °C to form clear solution. The gelation can be observed in some organic solvents after cooling back to room temperature within 3 min. The particular result is presented in Table 1 and Fig. 1. The polarity of solvent(Table S-1)[26] has an important effect on whether the solvent gelates. We found the solvent may be gelated when its polarity is about 0.2–3.5. 3.2. Characterization
solid-state measurement. The samples were then scanned between the wavelengths of 4000 and 400 cm−1 at an interval of 1.9285 cm−1.
It has been demonstrated that C2-benzene based gelators have great self-assembly abilities to form fibrous network through the combination of hydrogen bonds, hydrophobic, and π–π interactions [27]. To get a visual insight into the gelation morphology of compound 3, scanning electron microscopy (SEM) was used to characterize. The xerogel of 3 prepared from dimethylbenzene exhibited an interconnected
2.7. Circular dichroism spectroscopy (CD) studies M A JASCO J-815 CD spectrometer was used to collect CD data. Spectra were measured in a 0.5 mm path length cell.
aniline
S
—
Fig. 1. 1 mL a) benzene with 10 mg of gelator; b) anisole with 10 mg of gelator; c) dimethylbenzene with 10 mg of gelator; d) methylbenzene with 20 mg of gelator; e) phenixin with 10 mg of gelator; f) heptane with 10 mg of gelator 3.
Fig. 2. a) SEM image of self-assembled compound 3 in dimethylbenzene. b)AFM image of self-assembled compound 3 in dimethylbenzene.
S.-L. Yu et al. / Journal of Molecular Liquids 190 (2014) 94–98
20
and a trough near 252 nm (nπ* transition), suggesting the backbones of compound 3 adopt similar β-sheet-like configuration [25]. The FT-IR of compound 3 is showed in Fig. 4. The spectrum of compound 3 exhibits strong and broad band at around 3437 cm−1 and 3303 cm−1, which correspond to the N\H stretching vibrations of functional groups engaged in hydrogen bonds (H-bonds). The spectrum of compound 3 shows distinctive absorption bands at 1736 cm−1 (C_O in ester group vibration), 1650 cm−1 (C_O stretching in amide group), and 1543 cm−1 (N\H bonding in amide group).
15 10
CD [mdeg]
97
5 0 -5
3.3. Selective organogelation from oil/water -10 -15 210
225
240
255
270
285
300
λ [nm] Fig. 3. CD spectrum of compound 3 at the concentration of 0.01 mM in dimethylbenzene.
100
Transmittance [%]
95
Purification of water contaminated with toxic organic solvent by selective gelation is always a daunting task that becomes challenging. To this end, only very few phase-selective gelation of oil from oil/water mixtures were reported [28]. Interestingly, we found that organogelator 3 is suitable for the selective gelation of an oil from an oil/water mixture due to their insolubility in water and nice organogelation ability in some organic solvents. In a fundamental procedure, 2 mL of water and 0.5 mL of dimethylbenzene were mixed, and 10 mg of compound 3 was added. The gelator 3 was then solubilized in this two-phase solution by heating and also shaken vigorously to ensure homogeneous dispersion of oil in water (Fig. 5). After cooling the mixed solution to room temperature, the dimethylbenzene layer was gelated, and the water layer remained intact in liquid state. Then, the organogel was separated from the water simply by filtration.
90
4. Conclusion 85
3303
3437
1542
1743 80
1635 75
3500
3000
2500
2000
1500
1000
500
Wavenumbers [cm-1] Fig. 4. FT-IR of xerogel of compound 3.
network of thin fibrils with a thickness of a few tens of nanometers (Fig. 2a). The morphology of the xerogel was further confirmed from atomic force microscopy (AFM). Magnification of the fibers showed that each fiber was formed by the folding orientation of several small fibers (Fig. 2b). The AFM image clearly reveals the fibrous network structure of the gel, which is in consistent with the SEM result. As a helpful tool to determine the secondary structures of amino acid compounds, circular dichroism (CD) is performed to study the superstructures of compound 3 in the phase of dimethylbenzene. Compound 3 shares the common feature of β-sheet structure according to the CD spectra shown in Fig. 3. It exhibits a peak near 242.5 nm (ππ* transition)
In summary, we synthesized a new kind of organogel with high productivity and good gelation ability. This organogel enriches the scaffold of C2-symmetric benzene-based gelators, realizing organo- and hydrogelation using the same scaffold in the simple synthesis method, which is not common. Furthermore, the ability of the selective gelation for organic solvents of organogels makes it possible to find tremendous applications in the field of marine oil spill. Acknowledgments We acknowledge the support from the National Science Foundation of China (51173105), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Foundation for the Author of National Excellent Doctoral Dissertation of China (200957), the Fok Ying Tong Education Foundation (121069), Research Fund for the Doctoral Program of Higher Education of China, SRF for ROCS, SEM, and the Project of Young Teachers Funding (20100073120006). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2013.10.031.
Fig. 5. Selective organogelation from oil/water of compound 3.
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