Subscriber access provided by University of Otago Library
Article
A low-cost phase-selective organogelator for rapid gelation of crude oil at room temperature Changliang Ren, Feng Chen, Feng Zhou, Jie Shen, Haibin Su, and Huaqiang Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04027 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
A low-cost phase-selective organogelator for rapid gelation of crude oil at room temperature Changliang Ren,† Feng Chen,† Feng Zhou,‡ Jie Shen,† Haibin Su,‡ and Huaqiang Zeng*† †
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669.
‡
Division of Materials Science, 50 Nanyang Avenue, Nanyang Technological University, Singapore, 639798
ABSTRACT: Frequent marine oil spills pose a significant threat to the environment and marine’s ecosystem. We recently reported a highly tunable molecular gelling scaffold, which enables us to identify a few first examples of phase-selective organogelators (PSOGs) able to instantly gel crude oil of various types with room-temperature operation. In this study, we demonstrate here the high robustness and reliability of this modular gelling scaffold in consistently and combinatorially producing high capacity PSOGs. Such a unique feature has allowed us to carry out a systematic study of 48 gelators via a two-step screening process and discover another powerful carboxybenzyl-based gelator with comparable gelling properties but with a cost lowered by more than 300%, pointing to a good commercial potential for rapid cleanup of oil spills while effectively eliminating environmental pollutions caused by spilled oil.
Crude oil gelation at room temperature
1. INTRODUCTION Marine oil spills constantly cause disastrous impact on the environment and ocean life, and result in enormous socioeconomic burdens.1,2 For instance, the BP oil spill in 2010 alone released 4.9 million barrels of crude oil into the Gulf of Mexico over a wide area of more than 175,000 km,2 producing long-term damages to the marine’s ecosystem and causing $54 billion for BP in terms of clean-up, environmental and economic damages and penalties. The high frequency of oil spills urgently calls for an effective solution. However, the prevailing approaches for treating oil spills, including use of booms and skimmers,3 sorbents,4-7 bioremediation,8 dispersant,9,10 solidifiers11 or in situ burning, are of low efficiency, particularly for large-scale oil spill or in rough water.11-14 In particular, for widely used dispersant,13 the treated oil still stay in water and oil-caused damages and pollutions thus largely remain for long periods of time. Since the pioneering work by Bhattacharya and his co-workers in 2001,15 phase-selective organogelators (PSOGs) have emerged as promising oil-scavenging materials.15-29 Prior to our recent report,29 the hitherto developed gelators unfortunately suffer from either having a need to use toxic or hot or a large amount of carrier solvent for dissolution of gelator before being applied to gel organic liquids19-27 or an extremely slow action in oil gelation in the powder form.28,29 To date, there are only four known types of PSOGs able to gel crude oil, including the reports via the use of hot gelator-containing petrol22 or toluene27 solutions, sugar- or monopeptide-based PSOGs for extremely slow oil gelation at room temperature in the powder form by Sureshan28 and us,29 respectively, and our use of gelator-containing environmentally friendly solvents (e.g., EtOH and ethyl acetate) for achieving instant room-temperature gelation of both weathered and unweathered crude oils of wide densities and viscosities.29
Fig. 1 Chemical structures of monopeptide-based gelators with R1 = amino acids, R2 = n-alkyl chains and R3 shown in (b). All the gelators were named as R3-AA-R2; for instance, Nap-G-Leu-C4 refers to the gelator consisting of Nap-G, Leu and n-butyl groups.
The key to our recent success in finding the first examples of excellent PSOGs capable of instantly turning crude oil from liquid into floating solid in the presence of water at room temperature lies in our unique design of an unusually robust yet modular molecular gelling scaffold containing fluorenylmethyloxycarbonyl (Fmoc) group at R3 position (Figure 1a).29 This modularly tunable
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
monopeptide-based gelling scaffold not only consistently enables us to produce Fmoc-containing high capacity gelators that differ from each other by two types of side chains (e.g., R1 and R2) but also allows combinatorial fine-tuning of the gelator’s solubility and aquatic toxicity. These two features combine to produce two highly soluble non-toxic PSOGs that concurrently overcome most of the technical barriers for practical uses. Nevertheless, these gelators are still quite costly, ranging from $2.0 to $5.3 for treating one liter of crude oil. A cost breakdown analysis shows that the Fmoc group, which is contained in the structure as R3 (Figure 1) and provides critically important aromatic - stacking forces to help drive the formation of H-bonded 1D columnar packing of the gelator molecules, accounts for more than 80% cost of the gelling material. Naturally, we therefore decided to replace the aromatic Fmoc group with other low-cost aromatic motifs (Figure 1b), which, similar to Fmoc, could also provide aromatic - stacking to assist the self-assembly of gelator molecules. In our current work, we demonstrated a high tolerance in aromatic R3 group and high reliability of the molecular gelling network in producing high capacity PSOGs in a combinatorial format and in a way mostly independent of the choice of side chains R1 and R2. These features are not often seen in all previously reported molecular gelling scaffolds. By using this scaffold, we have systematically looked into a total of 48 monopeptide-based structures that differ from each other only by groups of R1, R2 and R3 (Figure 1). Among these 48 possible gelators, the best two carboxybenzyl-based gelators were found to possess low BMGC values for gelling four types of crude oil (12 80 g of gelator for treating one liter of crude oil) with costs ($0.2 $1.1 per liter of crude oil) at least three times lower than our previously reported Fmoc-based gelators.29
2. EXPERIMENTAL PROCEDURES 2.1. Reagents and compound characterizations. All the reagents were obtained from commercial suppliers and used as received unless otherwise noted. 1H and 13C NMR spectra were recorded on a Bruker ACF-400 spectrometer with 13C spectra proton-decoupled. The solvent signal of CDCl3 was referenced at δ= 7.26 ppm and 77 ppm for 1H and 13C NMR, respectively. Coupling constants (J values) were reported in Hertz (Hz). MALDI-TOF mass spectra were acquired with Bruker Ultraflextreme (Bruker Daltonik GmbH, Germany) equipped with Bruker Smartbeam II 355-nm nitrogen laser with an accelerating voltage of 25 kV in the linear configuration. Mass spectra were measured by using the positive mode of mass spectroscopy. The matrix used in the experiment was 1, 8, 9-trihydroxyanthracene purchased from Alfa Aesar and used directly without further purification. The solid matrix was dissolved at 20 mg/mL in chloroform. A volume of 10 L matrix solution was then mixed with 20 L of the sample solution in chloroform. An aliquot of 1 L of the resulting solution was spotted onto the MALDI sample plate and air-dried at room temperature. The dried plate was then inserted into MALDI instrument. Selection of the laser used for ionization was performed directly through the software and required no adjustments to the individual lasers. The optimum laser power for the nitrogen laser was between 10 and 25 microjoules. 2.2. Typical experimental procedure for synthesis of Cbzbased gelator molecules. Z-Ile-OH (265 mg, 1.0 mmol) and BOP
Page 2 of 7
(486 mg, 1.1 mmol) were dissolved in DCM/DMF (10 mL/50 mL) to which DIEA (0.39 ml, 2.2 mmol) was added and the reaction mixture was allowed to stirred continuously for 12 hours at room temperature. Solvent was removed in vacuo and the crude product was dissolved in DCM (50 mL), washed with water (2 x 50 mL) and dried over Na2SO4 to give the crude product, which was subjected to column purification (MeOH/CH2Cl2 = 1/25) to yield the pure product Z-Ile-C4 as a white solid. Yield: 288 mg, 90%. 2.3. Stable to inversion method for MGC determination. The oil-gelling abilities of designed amino acid-based gelators were examined in a variety of oils, including four types of crude oils by the “stable to inversion” method. Briefly, the gelator and oil were mixed in a sealed sample vial, and heated on the hotplate until all the gelator molecules were dissolved. The solution was then cooled to room temperature under ambient conditions. The sample was regarded as a gel if no flow was observed within 30 sec after inverting the sample vial. 2.4. Scanning electron microscopy (SEM). SEM images were obtained using a field-emission scanning electron microscope (JEOL JSM-6700F, Japan). A small amount of petrol gel was placed on copper tape attached aluminum stub, and allowed to dry overnight under ambient conditions. Sample was then sputtercoated with a thin layer of Pt, and subjected to SEM observation at an accelerating voltage of 20 kV. 2.5. Rheological Study. Rheological studies of gels at biphasic MGCs (BMGCs) were performed using an ARES-G2 rheometer (TA Instruments, U.S.A.) equipped with a plate (8 mm diameter). The gels were equilibrated at 25ºC between the plates that were adjusted to a gap of 2.0 mm. The storage modulus (G’) and loss modulus (G”) of gels were first measured in strain sweep (0.01– 100%) modes at a constant frequency of 1 Hz, followed by a frequency scan of 1.0 to 100 rad/s under the controlled strain of 0.1%. Experiments were repeated twice to ensure the reproducibility. 2.6. Computational method. The COMPASS force field (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) - developed by H. Sun31 was used to optimize the geometry and calculate the energy of all molecules. COMPASS force field is based on state-of-the-art ab initio and empirical parametrization techniques. For instance, the valence parameters and atomic partial charges were supported by ab initio data, and the van der Waals (vdW) parameters were derived by fitting the experimental data of cohesive energies and equilibrium densities. The convergence tolerance is 2x10-5 kcal/mol for the energy, 0.001 kcal/mol/Å for the force, 0.001 GPa for the stress and 10-5 Å for the displacement. The Ewald method is used for calculating the electrostatic and the van der Waals terms. The accuracy is 10-5 kcal/mol. The repulsive cutoff is 6 Å for the van der Waals term. For the periodical structure, the box vector along the stacking direction is also optimized together with the molecules.
3. RESULTS AND DISCUSSION 3.1. Molecular design of modularly tunable gelator library. In our early design of the gelling scaffold shown in Figure 1a for combinatorial screening of effective gelators, ease in synthesis, tunability in structure diversity and reliability in forming fibrous structure for forming 3D net to trap oil molecules were top three
ACS Paragon Plus Environment
Page 3 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
criteria that were concurrently considered by us. In this regard, we designed a monopeptide-based gelling scaffold that is easily accessible via one-step synthesis from Fmoc-protected amino acid and that however remains largely ignored by other researchers. The biggest advantage of such a gelling scaffold lies in its great tunability in structure via three groups, i.e., R1-R3, which play distinctively different roles. More specifically, while the main backbone contains a defined chirality for reducing entropic penalties and two secondary amide bonds for forming a onedimensionally aligned H-bonded columnar structure via intermolecular H-bonds, a rigid planar conjugated aromatic group such as Fmoc at R3 position could provide important aromatic π-π stacking forces to further stabilize the H-bonded columnar stacking. The additionally available combinatorial variations at R1 and R2 positions might lead to finely tunable gelling ability and particularly solubility and aquatic toxicity of the resultant gelators for oil gelation with room-temperature operation in a safe manner. These design considerations indeed help us to identify two practically non-toxic Fmoc-based gelators able to instantly gel crude oils at room temperature. A high performance PSOG capable of rapidly gelling crude oil at room temperature is characterized by having both outstanding gelling ability toward crude oil and high solubility in environmentally benign solvents such as ethanol and ethyl acetate. During our early screening of Fmoc-based PSOGs that differ only by R1 group, we found that gelators derived from isoleucine (Ile) and leucine (Leu) with the same R2 group generally possess the best gelling ability and solubility, respectively. For R2 group, we found that butyl (C4) and hexyl (C6) side chains as a group differ insignificantly from octyl (C8) and decyl (C10) as a group. Accordingly, during our primary screening to identify low-cost aromatic motifs suitable for replacing Fmoc group, we focus on diversifying the library at R1 using Ile and Leu, and at R2 using C4 and C6 side chains. Following identification of suitable aromatic motifs at R3, a secondary screening centered on both R1 and R2 will be carried out. With respect to a material cost of ~ $1.5 for 10 gram of Fmoc-based gelators, 10 gram for the other six aromatic motifs listed in Figure 1b was estimated to range from $0.15 - $0.28 with Ph and Nap-A being the cheapest and most expensive, respectively. 3.2. High and broad spectrum oil-gelling abilities. It was thought that diesel, having intermediate boiling points of 200 320 °C, should resemble the crude oil better than petrol, and molecules unable to gel diesel highly likely won’t gel crude oil either. Therefore, in the primary screening, diesel was chosen as the representative oil to assess the gelling ability of gelators. The gelators’ minimum gelation concentrations (MGCs in % w/v, mg/100 μL) were determined by using the “stable to inversion” method. Comparison among all 24 gelators derived from six aromatic motifs, two R1 (e.g., Ile and Leu) and two R2 (e.g., C4 and C6) demonstrates that gelators derived from aromatic motifs Nap and Z perform better than those made from Gal and Ph do (Table 1). Although Nap-based gelators exhibit superior gelling ability with MGCs ranging from 0.32 to 0.37 % w/v, they unfortunately suffer from extremely bad solubility in all commonly used organic solvents and won’t dissolve even at high temperature. Due to this solubility problem, rather than Nap, the Z group was used in the subsequent secondary screening. If gelators derived from the Z group could display gelling ability
Table 1. Gelling abilities (MGC values in % w/v, mg/100 μL) of gelators toward diesel. a,b Nap-G-Leu-C4
I
C8-Gal-Leu-C4
S
Ph-Leu-C4
Nap-G-Leu-C6
0.37
C8-Gal-Leu-C6
S
Ph-Leu-C6
S
2.15
Ph-Ile-C4
2.75
Nap-G-Ile-C4
I
C8-Gal-Ile-C4
Nap-G-Ile-C6
I
C8-Gal-Ile-C4
S
Ph-Ile-C6
2.78
Nap-A-Leu-C4
0.35
C16-Gal-Leu-C4
2.27
Z-Leu-C4
0.66
Nap-A-Leu-C6
0.32
C16-Gal-Leu-C6
1.80
Z-Leu-C6
1.03
Nap-A-Ile-C4
I
C16-Gal-Ile-C4
1.10
Z-Ile-C4
0.12
I
C16-Gal-Ile-C6
0.61
Z-Ile-C6
0.73
Nap-A-Ile-C6 a
S
S = soluble, I = insoluble.
b
All the MGC values were determined by
using a heating-cooling (stable-to-inversion) method. Table 2. Gelation abilities (MGC values in % w/v, mg/100 μL) of gelators in oils of varying types at room temperature.a,b,c Petrol
Diesel
Grissik
Arab Light
Arab Heavy
Ratawi
Z-Ala-C4 Z-Ala-C6
1.96
0.63
2.07
7.80
4.58
5.77
5.50
1.72
4.06
2.80
1.83
1.23
Z-Ala-C8
4.17
1.09
3.71
3.25
2.87
2.98
Z-Ala-C10
5.95
0.78
1.97
1.97
1.98
1.53
Z-Val-C4 Z-Val-C6
0.50
0.19
0.95
1.94
2.91
2.10
1.95
0.67
1.69
4.43
3.23
2.60
Z-Val-C8
2.24
0.34
2.70
1.87
3.26
3.22
Z-Val-C10
1.29
0.71
1.14
4.33
3.26
2.88
Z-Ile-C4 Z-Ile-C6
0.51
0.12
0.42
0.94
1.40
0.98
2.95
0.73
3.90
6.25
3.28
2.58
Z-Ile-C8
2.24
0.29
0.98
0.77
1.23
0.89
Z-Ile-C10
2.50
0.85
1.16
0.86
1.23
0.71
Z-Leu-C4 Z-Leu-C6
S
0.66
3.04
6.64
5.68
4.20
S
1.03
4.02
4.90
5.08
2.80
Z-Leu-C8
S
0.47
2.34
5.48
7.40
5.12
Z-Leu-C10
1.97
0.86
1.03
0.76
0.90
0.58
Z-Asn-C4 Z-Asn-C6
1.05
0.31
2.75
2.50
S
10.10
0.17
0.10
0.71
0.86
1.78
1.03
Z-Asn-C8
0.95
0.11
0.90
0.87
1.83
1.41
Z-Asn-C10
0.57
0.14
0.90
1.73
2.37
5.01
Z-Gln-C4 Z-Gln-C6
I
1.16
6.05
S
3.57
3.63
I
0.86
6.55
S
2.36
2.20
Z-Gln-C8
I
0.69
1.29
S
3.40
3.97
Z-Gln-C10
I
1.72
1.02
3.83
1.73
1.29
a
S = soluble, I = insoluble. b Crude oils (Grissik, Arab Light, Arab Heave and Ratawi), which make a good representation of the majority of the world’s actively traded oils, were obtained from Singapore Refining Co. (SRC) Pte. Ltd. c All the MGC values were determined by using a heatingcooling (stable-to-inversion) method.
comparable to Fmoc-based gelators, the corresponding material cost could be reduced by more than 70%. We then expanded Z-based gelator library by incorporating four more types of amino acids (e.g., alanine = Ala, valine = Val, asparagine = Asn and glutamine = Gln) and two more alkyl chains of different lengths (e.g., C8 and C10). The oil-
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
gelling ability of the whole library consisting of 24 gelators was systematically tested against six types of oils with four being crude oil (e.g., Grissik, Arab Light, Arab Heavy and Ratawi). In terms of both density and viscosity, these four types of crude oil cover wider ranges than those of the world’s most actively traded benchmark oils (Tables S1 and S2, Supporting Information). Except for 11 cases where the gelators are either too soluble or insoluble in the tested oil, all the other 133 combinatorial testings produced gelled oil. Among the 24 gelaotors, Z-Ile-C4, Z-Ile-C8, Z-Ile-C10, ZLeu-C10, Z-Val-C4, Z-Asn-C6, Z-Asn-C8 and Z-Asn-C10 as a group exhibit the best oil-gelling ability with MGCs ranging from 0.17 to 0.57% w/v for petrol, 0.10 to 0.29% w/v for diesel, 0.42 to 1.03% w/v for Grissik, 0.76 to 0.94% w/v for Arab Light, 0.90 to 1.23% w/v for Arab Heavy, and 0.58 to 1.23% w/v for Ratawi. The as-formed thermoreversible gels are highly stable for months under ambient conditions, suggesting their potential applications in treating real oil spill accidents. SEM images of selected gelators illustrate extensive formation of the fibrous structures (Figure 2a), which subsequently should assemble in oil into a 3D gelling network capable of efficeintly trapping oil molecules via surface tension and capillary forces and eventually turning them from liquid to solid state. 3.3. Rapid gelation of crude oil at room temperature. In the marine oil spill treatment, there exists a clear need to separate oil from water. This real scenario was mimicked in the lab by using a biphasic phase separation experiment where oil was placed on seawater in a vial and gelators were added to test their ability to selectively gel oil from oily water into water-floating solid for easy separation. Undoubtedly, gelators that require heating or maintenance in hot solution to achieve gelation are not suitable for such applications. Instead, a room-temperature protocol would be more desirable and practical. This alternative protocol nevertheless requires the gelators to have a sufficiently high solubility in environmentally friendly carrier solvents. As recently reported by us,29 we believe EtOH and ethyl acetate, which are thousands of times less toxic than crude oil, should constitute good solvents for use as carrier solvents. After screening the solubility of eight gelators with the best gelling ability toward crude oil in these two solvents, Z-Ile-C4 and Z-Ile-C8 were found to be highly soluble in a solvent mixture containing ethanol and ethyl acetate (3:2, v:v), and a gelator concentration of as high as 150 mg/mL can be maintained at room temperature. In a typical biphasic gelation experiment, 40 μL of gelatorcontaining solution was added to a biphasic system comprising 0.5 mL of oil on top and 2 mL of seawater at the bottom. While leaving aqueous phase intact, selective gelation of oil generally took place immediately when gentle shaking was applied to simulate the choppy wave motion. For instance, Grissik became fully gelled within 50 seconds and floated on water (Figure 2b). Following this protocol, The BMGC value for Z-Ile-C4 against Grissik is determined to be 1.2% w/v (mg/100 μL). This corresponds to a capability to immobilize Grissik at 65 times its own weight. The oil-gelling abilities of Z-Ile-C4 and Z-Ile-C8 were examined against all six oils and summarized in Table 3. Under the biphasic conditions, all six oils can be instantly gelled within 6 minutes under gentle shaking with good BMGC values ranging from 1.2% w/v to 3.9% w/v and 1.6% w/v to 4.5% w/v
Page 4 of 7
Table 3. Biphasic minimum gelation concentrations (BMGC values in % w/v, mg/100 μL) for room-temperature oil gelation in the presence of seawater by Z-Ile-C4 and Z-Ile-C8.a,b
Oil
BMGC (% w/v) Density Viscosity (kg/L) (mPas, 25 oC) Z-Ile-C4 Z-Ile-C8
Petrol
0.73
0.4
1.6
2.3
Diesel
0.83
5.2
1.4
1.8
Grissik
0.75
0.7
1.2
1.6
Arab Light
0.83
2.7
2.2
3.9
Weathered Arab Light
0.85
11.1
3.3
4.9
Arab Heavy
0.89
42.5
3.7
4.2
Weathered Arab Heavy
0.94
294.4
7.2
8.0
Ratawi
0.91
81.9
3.9
4.5
a
Crude oils (Grissik, Arab Light, Arab Heave and Ratawi), which make a good representation of the majority of the world’s actively traded oils, were obtained from Singapore Refining Co. (SRC) Pte. Ltd. All the BMGC values were determined by adding gelator-containing solution in EtOH:ethyl acetate (v:v, 3:2) into a biphasic system containing oil on top and water at the bottom. Table 4. Temperature effect on BMGC values for oil gelation in the presence of seawater by Z-Ile-C4.a Oil
BMGC (% w/v) 0 C
15 C
25 C
Grissik
0.9
1.0
1.2
Arab Light
1.5
2.0
2.2
Arab Heavy
6.5
5.8
3.7
Ratawi
13.2
7.2
3.9
a
All the BMGC values were determined by adding gelator-containing solution in EtOH:ethyl acetate (v:v, 3:2) into a biphasic system containing oil on top and water at the bottom.
for Z-Ile-C4 and Z-Ile-C8, respectively. Application of more vigorous shaking further shortened the gelling time significantly. In general, denser and more viscous crude oils such as Arab Heavy and Ratawi require longer time and more gelator for a full gelation. Moreover, the pHs (2-12) and water compositions (e.g. seawater, groundwater and plain water) exert negligible influence on the phase-selective gelation process and BMGC values, suggesting the robustness of the phenomenon. In terms of gelling ability and gelling time, Z-Ile-C4 consistently outperforms Z-IleC8. To test their efficiency on gelling weathered oils, Arab Light (600 mL) and Arab Heavy (600 mL) each was placed in a 1L glass bottle containing 200 mL of seawater, and exposed to sunlight in Singapore at 25-32oC for one week. This weathering process increases both density and viscosity from 0.83 kg/L and 2.7 mPas at 25 oC to 0.85 kg/L and 11.1 mPas at 25 oC for Arab Light, and from 0.89 kg/L and 42.5 mPas at 25 oC to 0.94 kg/L and 294.4 mPas at 25 oC for Arab Heavy (Table 3). Even with these hugely increased viscosities after weathering, Z-Ile-C4 still displayed near-excellent gelling ability toward both weathered Arab Light and Arab Heavy with respective BMGC values of 3.3 and 7.2%. For oil spill treatment using PSOGs, the solidified oil needs to
ACS Paragon Plus Environment
Page 5 of 7
b)
c) G', G'' (Pa)
a)
Z-Ile-C4
104
100 G' G''
105 104
Hb
e)
1 10 (rad/s)
106
0.1
d)
G' G''
105
103 0.1
G', G'' (Pa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1 10 (rad/s)
100
Ha
100 mM 50 mM 12.5 mM 3.2 mM 0.8 mM
Z-Ile-C4 f)
Fig. 2 (a) SEM micrographs of the as-formed gels of Z-Ile-C4, Z-Ile-C8, Z-Val-C4 and Z-Asn-C8 in petrol (From left to right and from top to bottom, (b) phase-selective gelation of six oils (from left to right: petrol, diesel, Grissik, Arab Light, Arab Heavy and Ratawi) by Z-Ile-C4 in the presence of seawater at room temperature, (c) dynamic rheological studies of diesel and Arab Heavy phase-selectively gelled by Z-Ile-C4 at 1.4% w/v and 3.7% w/v, respectively; G’ = storage modulus, G” = loss modulus, frequency sweep = ω, and stress amplitude = 0. (d) phase-selective gelation of an oil slick of Arab Light with a thickness of 150 µm on seawater without agitation and removal from water via filtration, (e) concentration-dependent shifts of 0.15 ppm and 0.30 ppm for Ha and Hb from Z-Ile-C4, respectively, upon diluting from 100 to 0.8 mM in CDCl3 at 25ºC and (f) computationally optimized H-bonded 1D columnar structure formed by Z-Ile-C4 under periodic conditions. The small balls in (f) refer to O- and H-atoms involved in forming intermolecular H-bonds.
be stiff enough to withstand influences from choppy wave action for oil collection via filtration. For this purpose, mechanical strengths of the organogels produced from Z-Ile-C4 at BMGC values in different oils were evaluated by rheological study at 25 o C. As shown in Figure 2c and S2), the storage modulus G’ not only is essentially independent of frequency but also remain much larger than loss modulus G’’ in all four frequency sweep experiments, suggesting excellent elasticity of formed gels. The remarkable stiffness and strength of the gels can be inferred from high values of the storage modulus G’, an indicator of gel strength, with the corresponding values of 7.8×104, 3.6×104, 1.9×105 and 3.2×105 Pa for diesel, Arab light, Arab heavy and Ratawi, respectively. As spraying is the only method to apply the gelators on large scale treatment of spilled oil, a spray experiment was conducted to
assess the efficiency of gelator-containing solution in gelling crude oil. In this experiment, 5 mL of Arab Light was added to seawater to form an oil slick of 160 µm in thickness. 15 minutes later after evenly spraying 1.2 mL of solution containing Z-Ile-C4 at a concentration of 150 mg/mL onto the oil, the top oil slick became full gelled without any agitation. The gelled oil was stiff enough and can be easily scooped out using a strainer with fine pores, leaving clean seawater behind (Figure 2d). Increasing the thickness of oil slick from 150 µm to 1 mm increases the gelling time from 15 mins to 40 mins. Lastly, since some parts of sea surface temperature can go as low as 0 C as in Atlantic ocean, we have also investigated the temperature effect on the gelling ability of Z-Ile-C4 (Table 4). We found that for light crude oils (Gissik and Arab Light), BMGC
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
values decrease with decreasing temperatures from 25 to 0 degree, and this trend is reversed for heavy crude oils (Arab Heavy and Ratawi). 3.4. Structure of H-bonded one-dimensional gelling network. As recently demonstrated by us via a combined use of X-ray crystallography and 1H NMR dilution methods,29 Fmoc-based gelator molecules such as F-Phe-C4 undergo one-dimensionally aligned self-assembly primarily driven by the highly directional H-bonding forces among amide bonds and strong aromatic π-π stacking forces among Fmoc groups. For Z-Ile-C4, we also observed concentration-dependent upshifts of 0.15 ppm and 0.30 ppm for the two amide H-atoms (Ha and Hb) in Z-Ile-C4 upon dilution from 100 to 0.8 mM (Figure 2e). These changes, which are consistent with those seen for F-Phe-C4, suggest the involvement of amide H-atoms in forming intermolecular Hbonds that are largely responsible for the formation of 1D stacked structure. Since our repetitive attempts to grow the crystals failed, we have instead used the computation as a tool to shed some insights into the self-assembled 1D structure formed from Z-IleC4 (Figure 2f). From the computationally optimized structure, formation of intermolecular H-bonds, aromatic π-π stacking and Van der Waals interactions among side chains are clearly present, which work collectively to drive the formation of 1D H-bonded structure. These 1D structures then associate with each other to produce 1D fibers and fiber bundles, which further assemble into a 3D net for “freezing” oil into solid state.
4. CONCLUSIONS In conclusion, through a two-step combinatorial screening in our effort to find a suitable low-cost replacement of costly Fmoc group, we have examined the gelling abilities of 48 gelator molecules sharing the same type of monopeptide-based gelling scaffold, and identified a highly effective Cbz-based gelator, ZIle-C4. This gelator molecule possesses both high solubility in environmentally friendly solvents and outstanding biphasic gelling ability. In the presence of seawater, both un-weathered crude oils of widely ranging densities and viscosities and weathered crude oils with hugely increased viscosities can be gelled rapidly and efficiently at room temperature with low BMGC values of 12 - 72 gram per liter of crude oil. More importantly, compared to our recently elaborated Fmoc-based gelators that cost from $2.0 to $5.3 for treating one liter of crude oil,29 treatment using Z-Ile-C4 comes with much lower costs ($0.2 - $0.6 per liter of un-weathered crude oil and up to $1.1 for highly weathered heavy crude oil). Given the average cost of $100 per liter of spilled crude oil in the United States,30 Z-Ile-C4 might offer an efficient and commercially viable solution to alleviate the severe environmental damage caused by oil spills in the future.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Synthetic procedures and characterizations (1H NMR, 13C NMR, HRMS). gelling data, SEM and rheology data for 48 gelator molecules.
Page 6 of 7
AUTHOR INFORMATION Corresponding Author *Email:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore).
REFERENCES (1) Schmidt-Etkin, D. In Oil Spill Sci. Technol.; Fingas, M., Ed.; Gulf Professional Publishing: Boston, 2011, p 7-48. (2) Schrope, M. Oil spill: Deep wounds. Nature 2011, 472, 152-154. (3) Broje, V.; Keller, A. A. Improved mechanical oil spill recovery using an optimized geometry for the skimmer surface. Env. Sci. Technol. 2006, 40, 7914-7918. (4) Yuan, J. K.; Liu, X. G.; Akbulut, O.; Hu, J. Q.; Suib, S. L.; Kong, J.; Stellacci, F. Superwetting nanowire membranes for selective absorption. Nat. Nanotechnol. 2008, 3, 332-336. (5) Bi, H.; Yin, Z.; Cao, X.; Xie, X.; Tan, C.; Huang, X.; Chen, B.; Chen, F.; Yang, Q.; Bu, X.; Lu, X.; Sun, L.; Zhang, H. Carbon Fiber Aerogel Made from Raw Cotton: A Novel, Efficient and Recyclable Sorbent for Oils and Organic Solvents. Adv. Mater. 2013, 25, 59165921. (6) Ruan, C.; Ai, K.; Li, X.; Lu, L. A Superhydrophobic sponge with excellent absorbency and flame retardancy. Angew. Chem., Int. Ed. 2014, 53, 5556-5560. (7) Sabir, S. Approach of cost-effective adsorbents for oil removal from oily water. Crit. Rev. Env. Sci. Tech. 2015, 45, 1916-1945. (8) Atlas, R. M. Petroleum biodegradation and oil spill bioremediation. Mar. Pollut. Bull. 1995, 31, 178-182. (9) Steen, A.; Findlay, A. Frequency of dispersant use worldwide. Int. Oil Spill Con. Proc. 2008, 2008, 645-649. (10) Prince, R. C. Oil spill dispersants: Boon or bane? Env. Sci. Technol. 2015, 49, 6376-6384. (11) Fingas, M.; Fieldhouse, B. In Oil Spill Sci. Technol.; Fingas, M., Ed.; Gulf Professional Publishing: Boston, 2011, p 713-733. (12) Swannell, R. P.; Lee, K.; McDonagh, M. Field evaluations of marine oil spill bioremediation. Microbiol. Rev. 1996, 60, 342-365. (13) Kleindienst, S.; Paul, J. H.; Joye, S. B. Using dispersants after oil spills: Impacts on the composition and activity of microbial communities. Nat. Rev. Microbiol. 2015, 13, 388-396. (14) Fingas, M. In Oil Spill Sci. Technol.; Fingas, M., Ed.; Gulf Professional Publishing: Boston, 2011, p 429-433. (15) Bhattacharya, S.; Krishnan-Ghosh, Y. First report of phase selective gelation of oil from oil/water mixtures. Possible implications toward containing oil spills. Chem. Commun. 2001, 185-186. (16) Trivedi, D. R.; Ballabh, A.; Dastidar, P. An easy to prepare organic salt as a low molecular mass organic gelator capable of selective gelation of oil from oil/water mixtures. Chem. Mater. 2003, 15, 3971-3973. (17) Ballabh, A.; Trivedi, D. R.; Dastidar, P. New series of organogelators derived from a combinatorial library of primary ammonium monocarboxylate salts. Chem. Mater. 2006, 18, 37953800. (18) Xue, M.; Gao, D.; Liu, K.; Peng, J.; Fang, Y. Cholesteryl derivatives as phase-selective gelators at room temperature. Tetrahedron 2009, 65, 3369-3377. (19) Yu, H.; Liu, B.; Wang, Y.; Wang, J.; Hao, Q. Gallic ester-based phase-selective gelators. Soft Matter 2011, 7, 5113-5115.
ACS Paragon Plus Environment
Page 7 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
(20) Prathap, A.; Sureshan, K. M. A mannitol based phase selective supergelator offers a simple, viable and greener method to combat marine oil spills. Chem. Commun. 2012, 48, 5250-5252. (21) Basak, S.; Nanda, J.; Banerjee, A. A new aromatic amino acid based organogel for oil spill recovery. J. Mater. Chem. 2012, 22, 11658-11664. (22) Rajkamal; Chatterjee, D.; Paul, A.; Banerjee, S.; Yadav, S. Enantiomeric organogelators from D-/L-arabinose for phase selective gelation of crude oil and their gel as a photochemical micro -reactor. Chem. Commun. 2014, 50, 12131-12134. (23) Jadhav, S. R.; Vemula, P. K.; Kumar, R.; Raghavan, S. R.; John, G. Sugar-derived phase-selective molecular gelators as model solidifiers for oil spills. Angew. Chem., Int. Ed. 2010, 49, 7695-7698. (24) Tsai, C.-C.; Cheng, Y.-T.; Shen, L.-C.; Chang, K.-C.; Ho, I. T.; Chu, J.-H.; Chung, W.-S. Biscalix[4]-arene derivative as a very efficient phase selective gelator for oil spill recovery. Org. Lett. 2013, 15, 5830-5833. (25) Mukherjee, S.; Shang, C.; Chen, X.; Chang, X.; Liu, K.; Yu, C.; Fang, Y. N-Acetylglucosamine-based efficient, phases-selective
organogelators for oil spill remediation. Chem. Commun. 2014, 50, 13940-13943. (26) Wang, D.; Niu, J.; Wang, Z.; Jin, J. Monoglyceride-Based Organogelator for Broad-Range Oil Uptake with High Capacity. Langmuir 2015, 31, 1670-1674. (27) Kesava Raju, C. S.; Pramanik, B.; Kar, T.; Rao, P. V. C.; Choudary, N. V.; Ravishankar, R. Low molecular weight gels: potential in remediation of crude oil spillage and recovery. RSC Advances 2016, 6, 53415-53420. (28) Vibhute, A. M.; Muvvala, V.; Sureshan, K. M. A Sugar-Based Gelator for Marine Oil-Spill Recovery. Angew Chem, Int Ed 2016, 55, 7782–7785. (29) Ren, C.; Ng, G. H. B.; Wu, H.; Chan, K.-H.; Shen, J.; Teh, C.; Ying, J. Y.; Zeng, H. Instant Room-Temperature Gelation of Crude Oil by Chiral Organogelators. Chem. Mater. 2016, 28, 4001-4008. (30) Fingas, M. In Oil Spill Sci. Technol.; Fingas, M., Ed.; Gulf Professional Publishing: Boston, 2011, p 3-5. (31) Sun, H. COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds. J. Phys. Chem. 1998, B102, 7338-7364.
ACS Paragon Plus Environment
