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Interface-Rich Materials and Assemblies
Highly Efficient Recovery of Oils in Water via Serine-based Organogelators Yimeng Zhang, tianxiang luan, qiuhong cheng, Wei An, ruipeng tang, pengyao xing, and Aiyou Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00038 • Publication Date (Web): 24 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019
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Langmuir
Highly Efficient Recovery of Oils in Water
mannitol based gelators,29 biscalix[4]-arene derivative,30 D-/L-arabinose
gelators,31
N-acetylglucosamine-based
via Serine-based Organogelators
gelators,32
Yimeng Zhang, Tianxiang Luan, Qiuhong Cheng, Wei An, Ruipeng Tang, Pengyao Xing and Aiyou Hao*a
applications, issues like low gelling ability or difficulty to
a
and many others showed excellent performance in
oil treatment. Nevertheless, while it collectively comes to
Key Laboratory of Colloid and Interface Chemistry of Ministry of Education,
School of Chemistry and Chemical Engineering, Shandong University, Jinan,
carry out in bulky oil treatment or taking out oils incapability, as well as toxic/hot solvents for solubilizing gelators arise. Powder PSOGs advantaged with their ability to gelate oil in the form of powder have reported rencently.40-42 Sureshan reported a class of glucose-derived PSOGs which can be
250100, P.R. China. E-mail:
[email protected]; Fax: +86-531-88564464; Tel: +86-531-88363306
operated by aerial spraying in the powder form followed by oil recovery.40 Naphthalene diimide based gelators reported by
ABSTRACT: We report here the gelation of a series of petroleum products by serine derivatives. Among Fmoc (9-fluorenylmethoxycarbonyl) amino acids modified by long chain
amines
with
different
substituent,
serine-based
compounds exhibit excellent performance in gel formation. Further studies on the variation of serine-based gelators demonstrate
considerable
structure-property
relationship
between oil gelation performance and molecular structure. Oils could be separated and collected by acid and distillation. It provides a potential effective treatment of oil-containing water produced from frequent marine oil spills. Key words: Fmoc amino acid derivatives; recovery of oils; phase-selective organogels; marine oil spills
vesicles, micelles, fibers and nanotubes could be realized facilely through self-assembly.1-2 The control over structure and morphology is important in the design of intelligent materials due to the structure-property relationship.3-7 Low molecular mass organogels (LMOGs) as supramolecular attracted
considerable
interest
and
tremendous progress has been made, such as stimuli responsiveness, light harvesting, molecular recognition, chirality transfer and so on.8-18 Oil/water separation and recovery have been a major challenge due to increasing release of oily wastewater and marine oil pollution, which lead to irreversible damages to ecosystem and environment.19-20 Therefore environmental oil spills and destruction mainly remain for a long period of time. Phase-selective
dramatically enhance the gelation speed.42 But powder PSOGs also come with a series of demerits, including slowing gelling action and the increased risk of secondary pollution during and after gelation. Jin34 group reported monoglyceride-based organogelator showed outstanding versatility toward gelling organic liquids with a broad-range. Regardless of these progresses, developing novel gelator-based effective oil recovery agents is still rather challenging. Moreover, prediction of gel formation from pre-designed molecular topology remains considerate challenges. To work out this intriguing problem, herein, we report a researched their self-assembly behavior in a various oils
Diverse structure and morphology of aggregates such as
have
instantaneously.41 Zeng reported a wetting strategy which
class of PSOGs derivate from Fmoc amino acids and
INTRODUCTION
aggregates
Chaudhuri can gelate heavy crude oil in water near
organogelators
(PSOGs)
derived
from
LMOGs, which could selectively gelate oils in the presence of water for easy collection and recovery, have reported as candidates as “smart” oil-scavenging materials recently.21-39 A fatty acid derived amino acid acted as PSOGs was reported for the first time in 2001.27 After that, gallic ester-based
which showed diverse assemblies. Gels and precipitates with different properties are obtained by tuning substituent. The optimized serine-based gelators reported enable efficient room-temperature gelation of various type oils in the presence of water. Properties of serine gelators with different length of alkyl chain are further compared. A shorter alkyl chain gives better strength and shorter gelation time. Remarkably, these excellent gels could be collected and transformed into solution state by heating or acidification. Oils could further be separated by distillation. It demonstrates potential to be applied in recovery of oil pollution and eliminating oily wastewater generated from daily life or industry. RESULTS AND DISCUSSION Self-assembly of Fmoc amino acid derivatives Fmoc amino acids derivates were synthesized via a simple step from Fmoc amino acids and alkyl amines (Scheme 1). Condensation
reaction
was
carried
out
under
the
benzotriazol-1-yloxytris(dimethylamino)-phosphonium hexafluorophosphate
(BOP
reagent)
N,N-Diisopropylethylamine (DIEA) condition.
gelators,21 carbohydrate gelators,22 sugar-derived gelators,28 ACS Paragon Plus Environment
and
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O R O N OH HO
H2N
BOP, DIEA 9
CH2Cl2/DMF
R O O N H N HO
Page 2 of 18
Ser10 and oil after heating or acidification; h) clean CuSO4 solution after gel taken out by tweezers. 9
Ala10 R=H Phe10 R=Ph Ser10 R=OH
Regarding the self-assembly, effects of block B variation of these three structure features were studied first. Ala10, Phe10, and Ser10 derived from alanine, phenylalanine and serine were prepared with n-decylamine. The gelation abilities in oils for the designed molecules were then examined by the ‘tube inversion’ method in six oils, including diesel, petrolatum, petroleum ether, gasoline, decane and methyl cyclohexane. Except that Phe10 was assembled into precipitates, both Ala10 and Ser10 could gelate all tested oils. The difference was that large white particles were precipitated from Ala10 gel after aged for five days, while the Ser10 gel
Scheme 1 Synthesis of Fmoc amino acids modified by n-decylamine
was stable. Worth mentioning is that Ser10 enabled stable
and chemical structures of Ala10, Phe10 and Ser10.
gelation even in the presence of water and the gel could stay steady until it dried at room temperature. The excellent gel
These building blocks contain three segments (A-C) with variable structure features. A robust planar conjugated
could be collected by tweezers and transformed into solution state via heating or acidification.
aromatic Fmoc groups (block A) immensely enhances the possibility of assembly by rigid aromatic π-π stacking interactions. Diverse amino acid substituent (block B) provide H-bonds to enhance the stability. Two pairs of amide H atoms and carbonyl O atoms carried by two secondary amide bonds produce effective intermolecular H-bonds. And the alkyl chain with adjustable length (block C) capable of forming van der Waals interactions may help fix the molecular skeleton to a predisposed conformation with reduced entropic penalties. In line with this vision, the structures could be tuned by systematically combining the three blocks. And we expect the corresponding properties and functions can further be tuned.
Figure 1 SEM images of the supramolecular assemblies formed from Scheme 2 Schematic representation of assemblies from Fmoc amino
decane and a,b) Ala10; c,d) Ala10 (aged for 5d); e,f) Phe10; g,h)
acids derivates and a) oils: diesel, petrolatum, petroleum ether,
Ser10.
gasoline, decane, methyl cyclohexane; b) gel from Ala10; c) precipitation from Phe10; d) gel from Ser10; e) phase-selective
To recognize the process of self-assembly, morphology of
gelation from Ser10 in presence of CuSO4 solution (for contrast); f)
the assemblies was investigated under SEM as shown in
Figure 1. The results in Figure 1a,b suggest the existence of ACS Paragon Plus Environment
collapse aggregation of Ala10 after aged for 5days; g) solution of
Page 3 of 18
entangled networks from Ala10. The individual fiber obtained
3438 cm-1, 3308 cm-1 and 3421 cm-1, 3304 cm-1 in the
is interwoven and about 100-200 nm in width. Figure 1c
corresponding assembled state, respectively. The amide I
shows one whole white particle from aged Ala10 gels, and
bands at 1640-1690 cm-1 in C=O stretching vibration and
Figure 1d is the partial enlarged detail. 3D fibrous network
amide II bands around 1530 cm-1 in N-H bending mode also
can also be observed but the sizes of the fibers have been
exhibit wavenumber shifts obtained in monomer state and
increased. As the lack of interactions at the substituent from
assembled state. These variations suggest the formation of
block B, the fibrous networks unable to sufficiently entrap
efficient intermolecular H-bonds of inter-carboxyl and
solvents, which result in the precipitates formation. Rod-like
amide-amide after self-assembly. All the applied molecules
structure with widths ranging from 1 to 2 μm was obtained
are able to self-assemble into ordered structures via
(Figure 1e,f). The morphology of assemblies from Phe10
complementary H-bonds.
features a typical precipitate structure. Because of the
Fluorescence emission spectra detected the interactions
flexibility of π-π stacking brought from extra presence of
between aromatic moieties of building units in Figure 2b,d,f.
benzene ring, three dimensional molecular growth is more
The emission maximum of Ala10 in gel state shows a
favorable. Figure 1g,h demonstrate the existence of
blue-shift compared with the peak of molecular state in
nanofibers with diameter around 30 nm with significant
solution. This indicates aromatic π-π stacking interactions
capillary forces to entrap oils. Ser10 possesses specific
among Fmoc groups which are arranged in a shoulder to
L-chiral centers and H-bond sites, the fibers twist for chirality
shoulder J-type mode.43,44 The emission peaks of Phe10
transferring from the chiral center to the supramolecular gels.
precipitate and Ser10 gel are red-shifted compared to monomers by 13 nm and 5 nm, respectively. Such red shifts are indicative of π-π stacking between Fmoc groups via head
Mechanism studies regarding gel formation in oils
to head H-type modes. These results all proved that the rigid π-π stacking interactions facilitate the formation of nanoscale networks as well as the gelation in oils.
1
Intensity/a.u.
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
2 3 1 2
3
4
Ser10
5 4
5
Ala10
1
5
2
3
4
10
15
20
Phe10 25
30
2θ/degree Figure 3 XRD pattern of the dried self-assembly aggregates of Ser10, Ala10 and Phe10 in Toluene/Decane. Figure 2 a,c,e) FTIR spectroscopy and b,d,f) Fluorescence emission
Further analyses of the intermolecular packing of the
spectra of Ala10 (a,b), Phe10 (c,d) and Ser10 (e,f) in monomer state
self-assemblies were carried out by XRD pattern. Figure 3
and assembled state (Toluene/Decane (5/5, v/v)), c= 0.5 wt%,
shows the XRD patterns of Ser10, Ala10 and Phe10
excitation wavelength: 280 nm.
aggregates. Evident peaks at 2θ = 4.0o, 2.6o and 4.8o appear in the small-angle region, for these three self-assemblies
To characterize the intermolecular interactions, infrared
respectively. According to the basis of Bragg’s equation, the
spectroscopy and fluorescence emission of Ala10, Phe10 and
first intense reflection corresponds to the d spacing values of
Ser10 were recorded, as shown in Figure 2. The peaks for
2.2 nm, 3.4 nm and 1.8 nm of the basic units. In consideration
secondary N-H stretching vibrations at 3438 (Ala10), 3451 cm-1
cm-1,
3313
cm-1
cm-1,
(Phe10) and 3447
(Ser10) in solution state shift to 3451
cm-1,
3303
cm-1
of the value of around 1.8 nm of Fmoc amino acid derivates’
cm-1,
3295
molecular length according to the geometry optimized
cm-1;
3303 structure, the d spacing fits bilayer structures with overlapped ACS Paragon Plus Environment
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Page 4 of 18
segments for Ser10 and Ala10 gels, yet Phe10 adopts a
of π-π stacking interactions from the extra plane, the
different packing mode.
self-assemblies
in
two
or
multiple
dimensions
may
In the wide-angle region of assemblies, XRD patterns
demonstrate ordered structures. It is favored with the growth
show regular peaks, which are close to the ratio of 1:2:3,
of Phe10 into each dimension at a more or less relative rate.
indicating the existence of lamellar structures in all assemblies.
These extensively aggregated assemblies are expectedly
In addition, the peaks at 2θ=21.5
o,
21.4
o,
20.7 corresponds
to the π-π stacking distance in the assembly
o
state.45,46
incapable to soluble oils and hence produce gel network.
This is
further evidence that the formation of assembled structures
Gelation behaviors of Ser10
give the effective credit to the rigid π-π stacking interactions.
In consideration of the property of Ser10 in gelling oils, the behavior of phase-selectively gelling oils in water was estimated. Solution at a 1 wt% concentration of Ser10 in toluene (1 mL) was added to a hybrid of decane (1 mL) and water (5 mL). After one second, the solidification of oil layer can be obtained afloat, and the aqueous layer was left intact in Scheme 2e with a tilted vial. Similar phase-selectively gelling process are obtained regarding various oils even heavy oils, such as petroleum ether, methyl cyclohexane, petrolatum, diesel, and gasoline, and also mixtures of them. However, some oxygenated oils like DBE (Dibasic Esters), soybean oil, olive oil, biodiesel and camellia oil could not be gelated (Table S1). Presumably the solvents containing polar domains resulted in a breakdown of the H-bonded structures accountable for the formation of fibers. The value of CGC (critical gelator concentration) and gelation time were also measured as shown in Table S2.
Scheme 3 Proposed molecular packing structures of a) Ala10 and b)
Ser10 acted as supergelators for all tested oils, according to
Ser10 molecules in organogel state.
the conventional definition, which could fix solvents to over 100 times their own weight. For instance, decane could be
To further understand the rationality of molecular packing
gelled with an extremely low CGC of 1 mg mL-1
structure, Material Studio 5.0 software was used to establish
(gelator/organic solvent). In order words, Ser10 could
the packing model in Scheme 3. Manifested by the
immobilize decane more than 1000 times its own weight.
fluorescence emission spectra, one key difference between
Compared
Ala10 and Ser10 gels is shown in the relative orientation of
oils22,25-29,33-34,37,44,
Fmoc groups. For Ala10, the Fmoc groups are aligned in
and exhibit the highly efficient gelling ability of Ser10.
with
the
previously
reported
CGC
for
the values are among the best of PSOGs
parallel with each other, and the long alkyl chains arrange in
We further tested the gelling ability of Ser10 regarding
the outside of the bilayer structures. In Ser10, the Fmoc
benign solvent factor. Table S3 listed various benign solvents
groups are perpendicular to each other with the long alkyl
in which Ser10 gelated decane. Alcohols, esters, ethers,
chains extending inside of bilayer arrays. In both structures,
arenes and some other commonly used solvents were tested.
the primary driving forces are provided by both H-bonds and
Among them, only with the help of arenes such as toluene,
π-π stacking interactions, which direct the gelator molecules
ethylbenzene and styrene, Ser10 can gelate all mentioned oils
to pack directional to create a well-ordered one-dimensionally
above.
(1D) stack. Weak van der Waals forces adjust the packing,
In consideration of the toxicity of toluene, gelling ability
and might interact with oil molecules together. The
of Ser10 in various ratios of Toluene/Decane was studied.
sub-structures further self-associate to generate an intertwined
Gels formation were occurred via blending decane and a
fiber network for oils entrapment.
toluene solution of the Ser10 molecules (v/ v = 1: 9 to 9: 1),
With regard to the nongelling precipitate formed Phe10,
which are shown in Table S4. The addition of the toluene
extensive well-aligned structures are not easily formed
solution into decane led to an instant gelation within one
resulted from the existence of additional benzene ring. This is
second in the system with a 1/9 ratio of Toluene/Decane. With
the fraction of decane decreased, the time for gelation a common phenomenon. Because of the flexible orientations ACS Paragon Plus Environment
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Langmuir
progress increased. When solvent ratios of Toluene/Decane
The mechanical strength of the organogels with different
reached 8/2 or 9/1, the gelation time required about 1 day or
chain length (Ser3, Ser6, Ser10 and Ser16) was researched
longer. Our additional test showed that, under extremely
by rheological measurement at room temperature. Figure 5a
vigorous shaking, Ser10 even in powder form could gelate
demonstrates oscillation frequency responses from gels. The
decane. The results show that, even no solvent is required for
elastic storage modulus G’ (contribution of elastic) is much
dissolving the required amount of the gelator.
larger than the elastic loss modulus G’’ (contribution of
In addition, all the gels are thermo-reversible. And almost
viscous) in the whole frequency range, which illustrates the
all the gelated oils could be entirely taken out by tweezers
characteristic of a representative gel. The reason is that it
which could remain stable even for several months with no
demonstrates the samples not relaxing during the whole tested
change at room temperature. The gelling ability of Ser10 was
time scale. Except Ser16 is relatively weak, Ser3, Ser6 and
also tested under the existence of water solution containing
Ser10 with over 10000 Pa values of G’ indicate remarkable
saturated
Na+,
K+,
Ca2+,
Cu2+
and
Mg2+
and also mixtures of
them. Obviously, the addition of inorganic salts had no effect on gelation as shown in Figure S2.
stiffness and excellent strength. The yield stress of gels can be obtained from the value where G’ and G’’ inter-cross in Figure 5b. High values of over 200 Pa were observed except for Ser16, exhibiting the
Serine-based Gelators with varied alkyl chain length
ability of these gels to withstand high applied stress. Chain length-dependent behaviors are observed with mechanical strength in this system. With the increase of the alkyl chain length, their values of yield stress correspondingly reduce and exhibit shorter carbon chain with stronger strength.
Figure 4 Structures of Fmoc-L-serine (Ser) and serine-based PSOGs
Recovery of Gelators and Oils
of Ser3, Ser6, Ser10 and Ser16.
The chemical stability of gels was tested under thermal, acid Ser3, Ser6, Ser10, Ser16 derived from block C were prepared
and alkali conditions. Heating or adding acids like acetic acid
by coupling Fmoc-L-serine (Ser) and four different alkyl
and hydrochloric acid solution led to the gel collapse to
amines which varies in the carbon chains length. Ser as a
solution. Ser10 completely converted into solution by 150
control compound that lacked the alkyl chain, was also
microliter of pure acetic acid after 20 minutes was shown in
prepared to evidence the role played by van der Waals
Figure S3. But alkalis such as triethylamine and sodium
interactions. The results in Table S5 where displayed that
hydroxide solution, failed to transform the gel structure. This
Ser3, Ser6, Ser10, Ser16 enabled room-temperature gelation
phenomenon might result from the destruction of the H-bonds
but Ser could not gelate any tested oils proved the necessity of
structures after addition of acids or increased energy from
van der Waals interactions from alkyl chains. As the length of
heating, thus give rise to reconstruction of the H-bonds system,
alkyl chain increased, their gelation time of 1 s, 5 s, 1 min and
which was explained in detailed in our previous report.47
2 h grew according gelators. The growing gelation time
Thereby it indicates a potential possibility to a practical oil
indicates that gels with a longer alkyl chain take longer time
spilled circumstance and recovery of treated oils.
to achieve stable self-assembly due to more complex van der Waals interactions between gelators and solvents. These tests were also confirmed in the presence of salts solutions.
Figure 5 a) Frequency sweep and b) dynamic oscillatory stress
Figure 6 Phase-selectively gelation of diesel in water and its
sweep of gels from Ser3, Ser6, Ser10, Ser16 and decane, c = 0.5
recovery: a) saturated salt solution; b) diesel/aqueous mixture; c)
wt%.
gelation of diesel in water; d) diesel gel collected by tweezers; e) thermal decomposed diesel solution; f) fractions of oil via
ACS Paragon Plus Environment
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distillation.
Page 6 of 18
1973–2129. [3] Higashiguchi, K.; Taira, G.; Kitai, J.; Hirose, T.; Matsuda,
The uptake and recovery of gelated oils is another
K.; Photoinduced macroscopic morphological transformation
significant applied factor which might favor eliminating
of an amphiphilic diarylethene assembly: reversible dynamic
possible damages caused by the spilled oils to biological,
motion. J. Am. Chem. Soc. 2015, 137, 2722−2729.
physical and chemical integrity of the environment and
[4] Du, X.; Zhou, J.; Shi, J.; Xu, B. Supramolecular
ecosystem. Figure 6 demonstrates the collection of treated
hydrogelators and hydrogels: from soft matter to molecular
diesel from water and the recycle of gelators. 1 mL of diesel
biomaterials. Chem. Rev. 2015, 115, 13165–13307.
was firstly gelated in the saturated salt solution, followed by
[5] Dawn, A.; Shiraki, T.; Haraguchi, S.; Tamaru, S.-i.;
the addition of a toluene solution of Ser10 (10 mg amount in
Shinkai, S. What kind of “soft materials” can we design from
1 mL) to the above diesel/aqueous solution. The gelling
molecular gels? Chem.-Asian J. 2011, 6, 266−282.
progress of diesel was observed in one second, and the diesel
[6] Dastidar, P. Supramolecular gelling agents: Can they be
gel maintains a stable state within several days. Then diesel
designed? Chem. Soc. Rev., 2008, 37, 2699−2715.
was collected even by tweezers and transformed into solution
[7] Sangeetha, N. M.; Maitra, U. Supramolecular gels:
state by heating. Oil could further be almost quantitatively
Functions and uses. Chem. Soc. Rev., 2005, 34, 821−836.
separated and collected by vacuum distillation. Ser10 as a
[8] Sandeep, A.; Praveen, V. K.; Kartha, K. K.; Karunakaran,
high boiling part after distillation was left and could be reused
V.; Ajayaghosh, A. Supercoiled fibres of self-sorted
for oil gelation next time. The aqueous solution was clean
donor-acceptor stacks: a turn-off/turn-on platform for sensing
again.
volatile aromatic compounds. Chem. Sci. 2016, 7, 4460-4467. [9] Kartha, K. K.; Sandeep, A.; Praveen, V. K.; Ajayaghosh,
CONCLUSION
A. Detection of nitroaromatic explosives with fluorescent
We designed and synthesized a class of modularly tunable
molecular assemblies and π-gels. Chem. Rec. 2015, 15,
PSOGs derivated from Fmoc amino acids. Amino acid
252–265.
substituent tuned molecules assembled to gels and precipitates
[10]
with different properties. Ser10 with adjustable alkyl chain
White-light-emitting supramolecular gels. Angew. Chem., Int.
length showed outstanding versatility towards gelling oils
Ed. 2014, 53, 365–368.
with high phase-selectivity at room temperature. The excellent
[11] Weiss, R. G. The past, present, and future of molecular
prospect of removal and recovery of the treated oils, makes
gels. what is the status of the field, and where is it going? J.
the gelators more closer to their eventual practical uses in oil
Am. Chem. Soc. 2014, 136, 7519−7530.
spill treatment.
[12] Buerkle, L. E.; Rowan, S. J. Supramolecular gels formed
Praveen,
V.
K.;
Ranjith,
C.;
Armaroli,
N.
from multi-component low molecular weight species. Chem. EXPERIMENTAL SECTION
Soc. Rev. 2012, 41, 6089−6102.
Materials and synthesis, preparation of samples and
[13] Okesola, B. O.; Smith, D. K. Applying low-molecular
characterization could be found in supporting information.
weight
supramolecular
gelators
in
an
environmental
setting-self-assembled gels as smart materials for pollutant CONFLICTS OF INTEREST
removal. Chem. Soc. Rev. 2016, 45, 4226−4251.
There are no conflicts of interest to declare.
[14] Khatua, D.; Dey, J. Spontaneous formation of gel emulsions in organic solvents and commercial fuels induced
ACKNOWLEDGEMENTS
by a novel class of amino acid derivatized surfactants.
This work is supported by the National Natural Science
Langmuir 2005, 21, 109−114.
Foundation of China (21872087).
[15] Kar, T.; Debnath, S.; Das, D.; Shome, A.; Das, P. K. Organogelation and hydrogelation of low-molecular-weight
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Table of contents
Among Fmoc amino acids derivates with different substituent, serine-based compounds exhibit excellent performance in gelation of various petroleum products.
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Scheme 1 Synthesis of Fmoc amino acids modified by n-decylamine and chemical structures of Ala10, Phe10 and Ser10. 210x173mm (150 x 150 DPI)
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Scheme 2 Schematic representation of assemblies from Fmoc amino acids derivates and a) oils: diesel, petrolatum, petroleum ether, gasoline, decane, methyl cyclohexane; b) gel from Ala10; c) precipitation from Phe10; d) gel from Ser10; e) phase-selective gelation from Ser10 in presence of CuSO4 solution (for contrast); f) collapse aggregation of Ala10 after aged for 5days; g) solution of Ser10 and oil after heating or acidification; h) clean CuSO4 solution after gel taken out by tweezers. 255x166mm (150 x 150 DPI)
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Figure 1 SEM images of the supramolecular assemblies formed from decane and a,b) Ala10; c,d) Ala10 (aged for 5d); e,f) Phe10; g,h) Ser10. 221x299mm (150 x 150 DPI)
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Figure 2 a,c,e) FTIR spectroscopy and b,d,f) Fluorescence emission spectra of Ala10 (a,b), Phe10 (c,d) and Ser10 (e,f) in monomer state and assembled state (toluene/decane (5/5, v/v)), c= 0.5 wt%, excitation wavelength: 280 nm. 226x230mm (150 x 150 DPI)
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Figure 3 XRD pattern of the dried self-assembly aggregates of Ser10, Ala10 and Phe10 in toluene/decane. 204x144mm (150 x 150 DPI)
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Scheme 3 Proposed molecular packing structures of a) Ala10 and b) Ser10 molecules in organogel state. 150x142mm (150 x 150 DPI)
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Figure 4 Structures of Fmoc-L-serine (Ser) and serine-based PSOGs of Ser3, Ser6, Ser10 and Ser16. 211x40mm (150 x 150 DPI)
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Figure 4 Structures of Fmoc-L-serine (Ser) and serine-based PSOGs of Ser3, Ser6, Ser10 and Ser16. 280x100mm (150 x 150 DPI)
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Figure 6 Phase-selectively gelation of diesel in water and its recovery: a) saturated salt solution; b) diesel/aqueous mixture; c) gelation of diesel in water; d) diesel gel collected by tweezers; e) thermal decomposed diesel solution; f) fractions of oil via distillation. 255x134mm (150 x 150 DPI)
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