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Polar Solvent-Induced Unprecedented Supergelation of (Un)Weathered Crude Oils at Room Temperature Juntong Li, Yanping Huo, and Huaqiang Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01643 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018
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Graphic for the Table of Contents 1.8% w/v A heating-cooling process
0.5% w/v 5% ethyl acetate as solvent
Oil Supergelation
3.2% w/v Ethyl acetate/EtOH as solvent
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Polar Solvent-Induced Unprecedented Supergelation of (Un)Weathered Crude Oils at Room Temperature Juntong Li,† Yanping Huo*† and Huaqiang Zeng*‡ †
Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, China 510006
‡
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669.
ABSTRACT: Use of carrier solvents to assist dissolution of phase-selective organogelators (PSOGs) before application in oil gelation is a common approach for solution-based gelators. Due to the competition in H-bonds by the polar carrier solvent, decreased gelling ability of PSOGs was often observed. That is, whereas data are available, the previously documented biphasic minimum gelling concentrations (BMGCs) are much larger than the MGCs determined using heating-cooling cycle for the same PSOG against the same oil. In this study, we show that, by minimizing amount of polar carrier solvent used, the gelling ability of PSOGs actually can be enhanced very substantially, rather than being weakened. More specifically, we demonstrate that use of a minute amount of polar carrier solvents of different types (e.g., ethyl acetate, acetone, acetonitrile and tetrahydrofuran) significantly enhances the gelling ability of seven structurally different organogelators in hydrophobic oil. In particular, with the use of 5 vol% essentially non-toxic ethyl acetate, application of this previously unexplored strategy onto four monopeptide-based PSOGs produces up to 11-fold improvement in biphasic gelling ability toward seven (un)weathered crude oils of widely ranging viscosities. While collectively overcoming many problematic issues (slow gelling action, low gelling ability or a need to use hot or toxic solvent for dissolution of gelator) associated with PSOGs, this surprisingly simple yet powerful and reliable method produces unprecedented rapid supergelation of crude oil at room temperature, with BMGCs of as low as 0.38% w/v (e.g., 3.8 g per liter of crude oil) and an averaged reduction in material cost of gelators by 85-97%.
INTRODUCTION Environmental cleanup and recovery from frequently occurring marine oil spills is difficult, often requiring a vital and enduring commitment for years.1 To alleviate disastrous consequences to the environment and ecosystems caused by spilled oils, various measures, such as in situ burning, beach raking and dredging as well as use of skimmers, booms,2 solidifiers,3 sorbents,4-10 oileating bacterial11 and dispersants,12,13 have been implemented. Nevertheless, these methods are either of low efficiency and/or difficult to operate on large-scale oil treatments, or incapable of removing oil from water body (e.g., use of dispersants), thereby with oil-caused environmental pollution and damages largely remaining for long periods of time. We14-16,43 and others17-42 believe that phase-selective organogelators (PSOGs), which can selectively congeal oil into floating gels in the presence of water for easy collection and reclamation of treated oil, might serve as a unique and efficient type of oil-controlling material for marine oil spill treatment on large scale and with room-temperature operations. Compared to solution-based PSOGs that require carrier solvent(s) for dissolution of gelators, the most advantageous feature of powder PSOGs is their ability to gel oil in the powder form at room temperature.14,16,41-43 With one recently reported exception,42 PSOGs of this category generally come with a set of limitations, including tremendous difficulty in their development, slow gelling action (e.g., complete gelation of crude oil often takes hours or even days) and low gelling ability.16,41,43 We recently discovered that, when wetted to contain gelator/acetonitrile a1:1 weight ratio, the “wetted” powder gelator Ac-Ile-C8 (Figure 1a) exhibits gelling speeds drastically boosted by one to two orders of
a)
b)
H-bonds Packing by F-Phe-C4 Figure 1. a) Structures of monopeptide-based phase-selective organogelators Ac-Ile-C8,14 Z-Ile-C4,15 F-Leu-C416 and F-Leu-C6.16 b) Crystal structure of F-Phe-C4,16 illustrating one-dimensionally aligned columnar packing of gelators mediated by intermolecular H-bonds.
magnitudes toward many types of (un)weathered crude oils at room temperature.14 This novel wetting strategy however offers little help in enhancing the low gelling capacity of Ac-Ile-C8 in the powder form. As such, a gelator loading of as much as 18% w/v for “wetted” powder gelator Ac-Ile-C8 is needed in order to completely gel Arab Heavy (a type of heavy crude oil) within 14 min in the presence of water without any mechanical agitation at room temperature,14 while the same gelator, when used in solution-based format, requires a much smaller loading of 3.5%
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w/v to instantly solidify the same type of crude oil within mins at room temperature.16 Aside from its ability to gel oil in the powder form, Ac-Ile-C8 could also function as a solution-based PSOG. This gelator, together with Z-Ile-C4, F-Leu-C4 and F-Leu-C6, was undoubtedly among the best solution-based PSOGs ever developed since 2001.25 These four PSOGs all possess (1) an excellent solubility of 150 mg/mL (0.53 M) in essentially nontoxic mixed carrier solvents, i.e., ethanol and ethyl acetate (3:2, v:v), (2) have similar minimum gelling capacities (see MGC44 and bMGC45 values in Table S1) in phase-selectively gelling crude oil in the presence of water with room-temperature operations, (3) are non-toxic toward marine lives16 and (4) are the very first type of PSOGs that enable instant phase-selective and room-temperature gelation of seven types of crude oils of widely ranging viscosities in the presence of water. In terms of cost, Ac-Ile-C8 carries a material cost of $0.2−$1.1 for treating per liter of (un)weathered crude oil, which is similar to Z-Ile-C4 but four times lower than our first-generation of solution-based gelators F-Leu-C4 or FLeu-C6. Still, all these solution-based PSOGs are disadvantageously characterized by its relatively low gelling capacity. For instance, as compiled in Table S1, MGC and bMGC values for Ac-Ile-C8 to gel light (e.g., Grissik and Arab Light) and heavy (e.g., Arab Heavy and Ratawi) crude oils range from 0.8 to 2.4% w/v and from 1.8 to 3.8% w/v, respectively. Since practically more meaningful values are bMGCs (or BMGCs) associated with room-temperature gelation without heating, not MGCs that require a heating for oil gelation, the low gelling capacities in terms of bMGC values obviously result in relatively high costs and limit its commercial applications in oil spill treatment in the practical setting. A low BMGC value generally requires the gelator to concurrently possess a low MGC value and a high solubility in essentially non-toxic organic solvents as the carrier solvent for practical uses. Given a lack of solid principles for reliable prediction of either gelling properties or solubilities from molecular structures, even very careful and extensive alterations in structure might not deliver desired outcomes. In fact, despite that all the above mentioned four PSOGs, possessing outstanding gelling properties with respect to all PSOGs developed by others, have been combinatorially optimized from a total of 84 PSOGs,1416 their lowest MGC and bMGC values for heavy crude oils (Arab Heavy and Ratawi) are still as high as 0.82 and 2.8% w/v (Table S1), respectively. Circumventing such a laborious and unproductive need via alterations in molecular structure, in this work we demonstrate a simple yet powerful polar solvent-assisted strategy as an alternative, reliable and general approach to dramatically enhance the biphasic gelling abilities of all four PSOGs in Figure 1a, without any structural, alteration, in seven types of (un)weathered crude oils in the presence of water. We anticipate that this new principle will expand the utility of low molecular weight organogelators-based soft materials with augmented gelling abilities for specific applications.
a)
b)
c)
Figure 2. a) Solvent effects at a fixed solvent/oil volume fraction of 10 vol% on BMGC values for room-temperature oil gelation of Petrol, Diesel, Arab Light and Arab Heavy by Ac-Ile-C8. A BMGC value of 3% refers to the cases where addition of gelator at 3% w/v produced a clear solution, not gelation. b) A summarized correlation between BMGC and percentage of EA vs oil (vol%) for all four gelators (Ac-Ile-C8, Z-Ile-C4, F-Leu-C4 and F-Leu-C6) against four crude oils. c) (B)MGCs determined by adding gelator-containing EA (5 vol%) into oil (solid lines, BMGCs) or gelator in the powder form into oil containing 5 vol% of EA, followed by a heatingcooling process (dotted lines, MGCs). In a) and b), MGCs44 and bMGCs45 were determined under different conditions. ACN = acetonitrile, EA = ethyl acetate, DMK = acetone, DCM = dichloromethane, WAL = weathered Arab Light and WAH = weathered Arab Heavy.
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RESULTS AND DISCUSSION Experimental effects of nine polar organic solvents. Our strategy employs a minute amount of polar solvent to substantially boost the gelling ability of gelators without re-synthesizing many other structurally related gelators in a difficult-to-predict and unproductive fashion. This strategy was largely inspired by our recent finding that powder gelator Ac-Ile-C8 wetted by polar acetonitrile of equal weight (molar ratio of acetonitrile:gelator = 7:1) displays remarkably increased gelling speeds in gelling crude oils.14 Although the exact mechanism underlying these pronounced improvements in gelling speed still remain elusive to us at the moment, we naturally hypothesized that certain polar solvent(s) might have similar effects, but, rather than to enhance the gelling speeds of powder gelators, might augment the gelling ability of solution-based gelators. Bearing the above conjecture in mind, we decided to test it out using nine types of polar solvents against four types of oils (e.g., Petrol, Diesel, Arab Light and Arab Heavy) in a biphasic setting comprising oil on top and water at the bottom. In our initial screening, the volumetric ratio of polar solvent to oil is fixed to 1:10, i.e., 10 vol% of oil. More specifically, certain amount of AcIle-C8 dissolved in 50 L of polar solvent was added to 500 L oil, followed by gently shaking the solution to check if gelation develops or not at room temperature within two minutes. The minimum amount of Ac-Ile-C8, which leads to rapid gelation, was used to calculate BMGC values (Figure 2a and Table S2). Presumably as a result of a significant disruption of the H-bonded structure responsible for the fiber formation, strongly H-bonding solvents (e.g., AcOH, DMSO, DMF and EtOH) result in BMGC values of 3% w/v, which are larger than either MGCs44 and bMGCs45 (except for bMGC of Arab Heavy) determined for the same gelator. Among the remaining five polar solvents, while acetonitrile, acetone and THF all can make BMGC values lower than the corresponding MGC and bMGC values for two crude oils (Arab Light and Arab Heavy), ethyl acetate is the only solvent that could produce the largest improvements in gelling ability of AcIle-C8 against all four oils, and obtained BMGCs are much lower than the corresponding bMGCs and even MGCs. In particular, the use of 10 vol% ethyl acetate improves the gelling capacity of AcIle-C8 toward crude oils of Arab Light and Arab Heavy by close to five-fold (BMGCs of 0.38 and 0.61% w/v vs bMGCs of 2.2 and 3.5%, respectively). It might be worth pointing out that ethyl acetate is the only solvent that could enhance the gelling ability of Ac-Ile-C8 in both Petrol and Diesel, with BMGCs lower than the corresponding MGCs, not to mention bMGCs. The above study on solvent effects arising from strongly Hbonding solvents such as EtOH explains well why bMGC values previously determined using a mixed solvent (EtOH:ethyl acetate = 3:2)45 are quite large (1.8 to 3.8% w/v, Table S1). Nevertheless, as detailed further below, very large enhancements in gelling ability via the use of a single non-strongly H-bonding polar solvent (ethyl acetate, acetone, etc) is considered very surprising and unprecedented in the field of organogelators. Room-temperature supergelation of four crude oils by all four PSOGs. Our subsequent screening looks into the ranges in percentage of ethyl acetate required to achieve effective
enhancements of gelling ability for all four gelators Ac-Ile-C8, ZIle-C4, F-Leu-C4 and F-Leu-C6 in four crude oils (Grissik, Arab Ligth, Arab Heavy and Ratawi) with viscosities ranging from 0.7 to 82 mPas at 25 oC. In a typical experiment, 5 mg of gelator was dissolved in 25 L ethyl acetate and added into 500 l of oil on top of 2 mL water, resulting in a starting ethyl acetate/oil volume fraction of 5 vol%. The resultant solution mixture was shaked for up to two minutes at room temperature. If gelation occurs, more oil was added until solution just becomes clear. If no gelation is observed, less oil was used until gelation just takes place. The minimum amount of gelator or oil, which leads to rapid gelation, was used to calculate BMGC values. The same screening procedure was repeated for the other seven starting volume fractions of 2 to 20 vol%, which correspond to additions of 10 to 100 L of ethyl acetate (Table S3). The obtained 128 BMGC values summarized in Table S3 were then plotted against vol% of ethyl acetate without referring to the identity of gelators (Figure 2b). In this way, two notable trends readily emerge, which are independent of structure of gelators. First, addition of 5 mg gelator of any type, which was dissolved in 10 L of ethyl acetate, into 500 l of oil of any type always leads to gelator precipitating out of oil, indicating that 10 L ethyl acetate is insufficient to bring 5 mg gelator into any tested oil at room temperature. Second, as highlighted by a purple oval shape in Figure 2b, supergelation with BMGCs of ≤ 1% w/v for any of four crude oils by any of the four gelators studied herein can be effectively achieved using ≤ 10 vol% of ethyl acetate in oil, with BMGCs as low as 0.36% w/v in the case of gelling Arab Light by Ac-Ile-C8 at 2.9 vol% of ethyl acetate. As highlighted in red in Table S3, a total of 16 lowest BMGCs (e.g., four oils × four gelators) vary from 0.42 to 0.75% w/v, values that are remarkably much lower than those of 16 MGCs (0.42 – 2.40 % w/v) and 16 bMGCs (1.2 – 3.9 % w/v) for the same four crude oils (Table S1 and also insert in Figure 2b). An inspection of Figure 2b and Table S3 suggests preferred gelators for various crude oils are Z-Ile-C4 for Grissik and Arab Heavy, Ac-Ile-C8 for Arab Light, and AcIle-C8 and Z-Ile-C4 for Ratawi. While rapid gelation readily occurs with shaking at room temperature, we have also carried out gelation in the absence of shaking. We found that gelation time in the absence of shaking strongly depends on the thickness of crude oil in water, and generally ranges from 5-20 mins for oil of < 3 m in thickness. Exceptionally high gelling capacity of four PSOGs toward (un)weathered crude oils induced by 5 vol% of ethyl acetate. To compare the relative gelling ability of gelators toward oils in somewhat more quantitative manner, BMGCs were determined by adding gelator-containing ethyl acetate (5 vol%) into oil (solid lines, Figure 2c and Table S4). To check the effect of ethyl acetate diluted into crude oil, these BMGCs were further compared to MGCs determined by adding gelator in the powder form into oil containing 5 vol% of ethyl acetate, followed by a heating-cooling process (dotted lines, Figure 2c). We found that a simple addition of 5 vol% of ethyl acetate into oil does not make roomtemperature oil gelation possible for any of the four PSOGs, and a heating-cooling process is still required in order to achieve efficient gelation of oil premixed with ethyl acetate at 5 vol%. In contrast, prior dissolution of gelator in 5 vol% of ethyl acetate
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Table 1. MGC, bMGC, PMGC and BMGC values (% w/v, mg/100 μL) as well as solvent effects for oil gelation by Ac-Ile-C8. BMGCc,d
Viscositya With EA (5 vol%)
No EA
bMGC43
PMGCb EA
DMK
ACN
THF
DCM
Grissik
0.7
0.6
2.16
1.8
8d
0.59
0.70
0.65
0.68
0.75
Arab Light
2.7
2.6
1.39
2.2
12
0.40
0.68
0.60
0.70
1.20
2.6
14
0.46
0.77
0.78
0.80
1.26
3.5
16
0.38
0.90
1.20
1.00
1.38
5.9
35
0.48
1.18
1.35
1.28
1.45
d
WAL
11.1
8.1
1.45
Arab Heavy
42.5
36.6
2.40
WAH
a
MGC42
334.2
d
110.5
2.83
d
Ratawi
81.9
62.8
0.82
3.8
20
0.48
0.62
0.60
0.75
1.16
WRd
343.0
151.6
1.50
2.7
38d
0.65
0.70
0.80
0.82
1.25
o
b
14 c
In the unit of mPas at 25 C. PMGC values needed for wetted powder gelator to gel oil within 10 min. Determined using 5% ethyl acetate as the cosolvent in a biphasic system. d This work. EA = ethyl acetate, DMK = acetone, ACN = acetonitrile, DCM = dichloromethane, WAL = weathered Arab Light, WAH = weathered Arab Heavy and WR = weathered Ratawi.
readily eliminates such heating-cooling requirement. From Figure 2c and Table S4, it is clear that all 28 BMGC values (e.g., four PSOGs against seven oils), which range from 0.24 – 0.90 % w/v, are always lower than the corresponding 28 MGC values of 0.42 – 1.26 % w/v, and in most cases lowered by 0.3% w/t. As expected, these BMGC values are notably better than the aforementioned MGCs (0.42 – 2.40 % w/v)44 and bMGCs (1.2 – 3.9 % w/v)45 summarized in Table S1 and insert of Figure 2b. To our great delight, highly weathered Arab Light and Arab Heavy46 also can be efficiently gelled with BMGCs of 0.40 - 0.48% w/v by both Ac-Ile-C8 and Z-Ile- C4. By carefully choosing different PSOGs, the lowest BMGCs for seven oils including six crude oils range from 0.24 to 0.47 % w/v, i.e., 0.24 (Diessel), 0.47 (Grissik), 0.40 (Arab Light), 0.40 (WAL), 0.38 (Arab Heavy), 0.42 (WAH) and 0.46% w/v (Ratawi), respectively. More significantly, the BMGCs of four PSOGs are 0.40 – 0.90% w/v for WAL and WAH, values that differ insignificantly from the BMGCs of 0.38 - 0.73% w/v obtained for the same but unweathered crude oils by the same four PSOGs. This is important because oil weathering on the sea surface, which begins the moment it is spilled, poses an outstanding challenge to existing oil-controlling materials but likely exerts much less influence in the capacity of our gelators in oil gelation. For instance, the “window of opportunity” for the successful application of widely used dispersants generally is limited to the first two hours after an oil spill, and beyond the two hour window, the dispersants might not function properly. Given the vigorous oil weathering process we have adopted,46 the “window of opportunity” for both Ac-Ile-C8 and Z-Ile-C4, when applied using 5 vol% ethyl acetate, could be days or longer, leaving ample response times for oil spill treatment. Focused investigations on Ac-Ile-C8. From Figure 2c, it can be concluded that Ac-Ile-C8 and Z-Ile-C4 are comparable to each other in gelling oils of various types at room temperature, and both are considerably better than F-Leu-C4 and F-Leu-C6 in gelling ability, especially for heavy crude oils. Unlike Z-Ile-C4
Gelator Addition
Vortexing
Emulsified oil
Oil Gelation
Oil-Water Separation
Figure 3. Phase-selective gelation of emulsified Arab Light using Ac-IleC8 at 1% w/v at room temperature. Vigorous agitation was used to create emulsified oily water right before addition of gelator-containing ethyl acetate into crude oil.
that could only serve as the solution-based gelator, Ac-Ile-C8 also functions in the powder form. Additionally, Ac-Ile-C8 has a better solubility in various solvents than Z-Ile-C4. Thus, Ac-Ile-C8 with a dual function was chosen for further investigation. Table 1 summarizes the solvent-enhanced biphasic gelling abilities of Ac-Ile-C8 toward seven types of oils including highlyweathered crude oils (WAL, WAH and WR). BMGCs determined at 5 vol% of any one of the five polar solvents (ethyl acetate, acetone, acetonitrile, tetrahydrofuran and dichloromethane, Table 1) are consistently much better than both MGCs44 and bMGCs45, and far better than PMGCs. With four exceptions, supergelation of both weathered and unweathered crude oils can be achieved using ethyl acetate, acetone, acetonitrile or tetrahydrofuran. Especially at 5 vol% of ethyl acetate the BMGC values of 0.38 – 0.65% w/v over seven crude
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Table 2. MGCa and BMGCb values (% w/v, mg/100 μL) determined for gelators 1-3 in seven crude oils using ethyl acetate or acetone as the cosolvent. Gelator 1 BMGC
Gelator 2 b
BMGC
Gelator 3 b
DMK (5 vol%)
MGCa
EA (5 vol %)c
DMK (5 vol%)c
MGCa
BMGCb (5 vol% EA)
2.58
2.37
1.25
1.00
0.92
5.20
1.68
5.28
4.20
3.16
3.56
1.66
1.45
4.80
1.65
WAL
5.75
4.00
3.30
5.26
3.58
2.28
5.26
1.86
Arab Heavy
3.60
2.66
2.00
5.80
3.50
2.80
3.63
1.48
WAH
4.25
2.40
2.25
7.20
3.65
3.26
4.68
2.45
Ratawi
3.66
2.20
2.08
6.15
3.68
3.45
3.88
1.20
WR
3.82
2.00
1.90
6.46
3.20
3.00
4.25
1.68
MGCa
EA (5 vol%)
Grissik
4.40
Arab Light
Determined by adding gelator in the powder form into the oil, followed by a heating-cooling process. b Determined by dissolving gelator in 25 L (for 1 and 3) or 75 L (for 2) ethyl acetate or acetone, and adding this gelator-containing solution into 500 L oil in the presence of 2 mL water. c Use of ≤ 10 vol% of either ethyl acetate or acetone results in gelator 2 precipitating from oil without oil gelation, and so all the BMGC values were determine at 15 vol% of cosolvent. EA = ethyl acetate, DMK = acetone, WAL = weathered Arab Light, WAH = weathered Arab Heavy and WR = weathered Ratawi. a
Figure 4. Structures of PSOGs 1-3.
oils from Grissik to weathered Ratawi (Table 1), are signi ficantly much better than the MGCs (0.82 – 2.83% w/v) by 0.7 to 4.3 folds, bMGCs (1.8 – 5.9% w/v) by 2.1 to 11.3 folds and PMGCs (8 - 38% w/v) by 13 to 72 folds. Further note that these BMGCs at 5 vol% have not been optimized yet. That is, some of these BMGCs might be even lower at volumetric ratios other than 5 vol%. Once again, the difference in BMGCs between unweathered and highly weathered crude oils is negligible. From Table 1, it can also be seen that addition of 5 vol% ethyl acetate significantly reduces the viscosities of heavy or weathered crude oils. As discussed above, this reduction in viscosity is one of many factors that might contribute to the enhanced gelling capacity of PSOGs in oil, but apparently not the sole one as evidenced by the fact that the determined BMGC values (solid lines, Figure 2c) are much smaller than MGC values (dotted lines, Figure 2c). And the exact mechanism underlying these noticeable enhancements in gelling capacity requires a further investigation. The rheological study of gels formed using Ac-Ile-C8 at
0.6% w/v reveals high G’ values of 1.1×105, 3.5×10 4, 3.1×105 and 3.7×104 for gels formed from Diesel, Arab Light, Arab Heavy and Ratawi, respectively, demonstrating remarkable stiffness and strength of the formed 3D fibrinous network (Figures S1 and S2). To examine the efficiency of gelator to gel emulsified oil in water, which may result from strong wave currents or oil aging, Arab Light (2 mL) was placed on top of 10 mL water, and Vortex Mixer was used to create highly emulsified oily water (Figure 3 and Movie S1). 200 L ethyl acetate containing 20 mg Ac-Ile-C8 was then added into this emulsified oily water, followed by vigorous shaking for 5 s. Two minutes later, > 98% of oil was fully gelled, producing essentially oil-free water with yellow color that might indicate the presence of residual emulsified oil droplets or gelled oil particles of small sizes. Polar solvent-enhanced gelling capacity in other types of PSOGs. Further application of this polar solvent-assisted protocol on structurally different PSOGs 1[25] and 2 [33] (Figure 4) shows that the averaged gelling capacities of 1 and 2 toward seven crude oils both increase by about one fold using acetone (See BMGCs vs MGCs in Table 2), thus establishing the generality of the strategy in boosting the gelling ability of other types of PSOGs. Most importantly, a heating-cooling process, which is required for 1 or 2 to gel the oil, is now not needed. That is, room-temperature gelation of all seven crude oils by both 1 and 2 become readily realized with the use of ethyl acetate or acetone as the carrier solvent. Presumably as a result of a relatively poor solubility in orgaic solvents in the case of 2 or very low gelling abilities of 4.39 and 5.10 % w/v for 1 and 2 toward crude oils, 7 enhancements in gelling ability for 1 and 2 are not as exceptional as those seen in our four highly soluble PSOGs (Figure 1a) whose gelling property and solubility in organic solvent has been carefully and combinatorially optimized and selected from 84 PSOGs. 14-16
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Additionally, for both 1 and 2, use of acetone leads to the averaged gelling capacity additionally enhanced by about 15% relative to those obtained from ethyl acetate. This suggests ethyl acetate, which is the optimum polar solvent for four PSOGs developed by us, is not necessarily the best solvent for organogelators developed by others. Nevertheless, the same strategy allows the polar solvent-enhanced gelling capacity to be observed in other types of gelators even though the extent of enhancements varies among gelators of different types. To further examine the applicability of the polar solvent-assisted principle in enhancing gelator’s gelling capacity, we have also prepared another monopeptide-based gelator 3 (Figure 4), which contains a more flexible linker than the four PSOGs shown in Figure 1a and a methyl group at the para-position. Compared to the MGCs of 0.42 – 2.40% w/v for four PSOGs toward four crude oils shown in Table S1, introducing such a flexible linker and a methyl group in 3 greatly decreases its gelling capacity to 3.63 – 5.20% w/v toward the same four types of crude oils (Table 2). Nevertheless, using 5 vol% ethyl acetate, its gelling capacity toward seven crude oils was dramatically augmented by 0.9 – 2.1 fold.
CONCLUSIONS In summary, we have demonstrated here a conceptually novel polar solvent-assisted principle to substantially boost the gelling capacities of different types of organogelators without resorting to any structural alteration, which may come in a laborious, difficultto-predict and unproductive fashion. Its generality and reliability toward organogelators of diverse types could be established by its remarkable consistency in delivering the substantially enhanced gelling capacities in seven structurally different orgagogelators toward all seven types of (un)weathered crude oils of widely ranging viscosities. Moreover, using any of four polar but nonstrongly H-bonding organic solvents (e.g., ethyl acetate, acetone, acetonitrile, and tetrahydrofuran) at 5 vol% of oil to be gelled, rapid room-temperature supergelation of seven crude oils can be effectively achieved in 24 out of 28 combinatorial cases (Table 1). Particularly at 5 vol% of essentially non-toxic ethyl acetate with Ac-Ile-C8 as the non-toxic gelling material,16 the determined BMGC values over all seven crude oils range from 0.38 – 0.65% w/v (e.g., 3.8 – 6.5 g of gelling material per liter of crude oil). This incredibly low value is much lower than the MGCs (0.82 – 2.83% w/v)[16,42] by 0.7 to 4.3 folds, bMGCs (1.8 – 5.9% w/v)16,45 by 2.1 to 11.3 folds and PMGCs (8 - 38% w/v) by 13 to 72 folds14 previously reported for the same gelator. These exceptional enhancements in room-temperature gelling ability not only, for the very first time, make rapid supergelation of both unweathered and highly weathered crude oils readily take place at room temperature,7 but also translate into an averaged reduction in the material cost of Ac-Ile-C8 by 85 and 97%, respectively. This results in a very low material cost of < 10¢ per liter of either unweathered or highly weathered crude oils.47 While providing a fundamentally novel, general and reliable principle to further increase the intrinsic gelling ability of gelators with an unaltered structure, our current work certainly represents a significant leap
toward practical application of solution-based PSOGs in largescale marine oil spill treatment in the future.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. MGC determination, gelling data, rheology study, SEM, material cost estimation for Ac-Ile-C8 and 1H NMR spectra for gelators 1-3.
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected];
[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), the National Natural Science Foundation of China (61671162) and Technology Plan of Guangdong Province (2016A010103031).
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