Sugar-derived phase-selective molecular gelators as model solidifiers for oil spills

Article abstract of DOI:10.1002/anie.201002095

Mopping up: Phase-selective gelators were synthesized from sugar alcohols using biocatalysis. The gelators exhibited an unprecedented ability to phase-selectively gel only organic liquids including crude-oil fractions in the presence of water at room temp

Full text of DOI:10.1002/anie.201002095

Angewandte
Chemie
DOI: 10.1002/anie.201002095
Molecular Gelators
Sugar-Derived Phase-Selective Molecular Gelators as Model Solidifiers
for Oil Spills**
Swapnil R. Jadhav, Praveen Kumar Vemula, Rakesh Kumar, Srinivasa R. Raghavan, and
George John*
The world has witnessed several marine oil spills, including
the recent one caused by the blowout of an oil well in the Gulf
of Mexico.[1] Such oil spills cause irrecoverable damage to the
environment and the ecosystem.[2] There is clearly a need for
materials to contain oil spills and reclaim the oil. Current
materials for this purpose are classified as dispersants,
sorbents, and solidifiers.[3] Typically, dispersants emulsify the
oil,[4] sorbents are solid powders that absorb the oil,[5] and
solidifiers are usually polymeric materials that gel the oil
layer.[6] However, all current materials have limitations, both
in containing the oil spread and in allowing recovery of the
spilled oil. For example, polymeric solidifiers cannot be
blended easily with viscous oils, and the recovery of oil from
polymer gels is cumbersome.
An effective and ideal solidifier for oil spills must:
1) selectively and efficiently gel the oil phase in the presence
of water at room temperature; 2) be synthesized easily and at
low cost; 3) be environmentally benign; 4) facilitate recovery
of the oil from the gel; and 5) be recyclable and reusable.
Herein, we describe a new class of amphiphilic solidifiers
based on derivatives of naturally occurring sugar alcohols.
These amphiphiles can function as phase-selective gelators
(PSGs) of the oil phase from a mixture of oil and water.[7,8]
They can potentially satisfy each of the above criteria and
therefore are promising model candidates for oil-spill reme-
diation.
hydrogen bonds and van der Waals interactions. Thus,
molecular gels tend to be thermoreversible; that is, the gel
can be liquefied by heating beyond a temperature Tgel (gel-to-
sol transition temperature) and reverted to the gel state by
cooling below Tgel. We exploit the thermoreversibility of gels
formed by our PSGs for the recovery of gelled oil through
simple distillation.
The first example of phase-selective gelation was demon-
strated by Bhattacharya and Krishnan-Ghosh.[7] with amino
acid amphiphiles. Molecular gelators based on naturally
occurring compounds[16,17] including closed-chain sugars
have been synthesized previously; however, gelators based
on open-chain sugars (sugar alcohols) remain almost unex-
plored.[18–20] The PSGs reported here are dialkanoate deriv-
atives of the sugar alcohols mannitol (Man) and sorbitol (Sor)
(Figure 1a). Conventional routes to such compounds involve
The PSGs are examples of molecular gelators, in other
words, small molecules that self-assemble into nanoarchitec-
tures (like fibers) in liquids.[9–12] Above the minimum gelation
concentration (MGC), the fibers become entangled into a
network, thereby converting the liquid into a coherent
gel.[13–15] Intermolecular interactions between gelator mole-
cules are noncovalent and hence relatively weak, for example
[*] S. R. Jadhav, Dr. P. K. Vemula, Prof. G. John
Department of Chemistry, The City College of New York, and The
Graduate School and University Center of The City University of
New York, 160 Convent Avenue, New York, NY 10031 (USA)
Fax: (+1)212-650-6107
E-mail: john@sci.ccny.cuny.edu
Homepage: http://www.sci.ccny.cuny.edu/~john/index.html
R. Kumar, Prof. S. R. Raghavan
Department of Chemical and Biomolecular Engineering
University of Maryland, College Park, MD 20742-2111 (USA)
Figure 1. a) Chemical structure of open-chain sugar amphiphiles, their
enzyme-mediated synthesis, and the structure of the benzylidine-
protected sugar amphiphile, protected Man-8. b) Image of self-assem-
bled aggregates of Man-8 in diesel recorded with an optical polarized
microscope. c) Image of a toluene gel recorded with a scanning
electron microscope (SEM). (An SEM image of a diesel gel could not
be obtained; it was difficult to prepare xerogels of diesel gel owing to
the high boiling point of diesel.)
[**] We thank the Science Interdepartmental Imaging Center at CCNY.
This work has been partially supported by the GCI-PRF Grant 48124-
GCI to G.J. P.K.V. thanks the Ewing Marion Kauffman Foundation for
Entrepreneur Postdoc Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002095.
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7695

Communications
multiple steps with protection and deprotection protocols.
Here we prepared PSGs in a single step by employing
regiospecific enzyme catalysis (see the Supporting Informa-
tion for details). The ease of synthesis and the abundance of
sugar alcohols in nature ensure that the PSGs are inexpensive,
nontoxic, and biodegradable.
The gelling abilities of the sugar gelators are summarized
in Table 1. Man-4, Man-8, Sor-4,and Sor-8 gel numerous
organic liquids including diesel and mineral and silicone oils,
Table 1: Gelation abilities (MGC values in % wt/v) of sugar amphiphiles
Man-4, Man-8, Sor-4, and Sor-8 in various organic liquids and oils.[a]
Liquid or oil
Man-4
Man-8
Sor-4
Sor-8
coconut oil
olive oil
soybean oil
grape seed oil
canola oil
cooked sunflower oil
cyclohexane
toluene
G (4.0) G (1.3)
G (2.5) G (1.0)
G (4.0) G (1.3)
G (4.0) G (1.3)
G (4.0) G (1.0)
G (4.0) G (1.0)
G (3.0) G (2.5)
G (2.5) G (1.5)
G (2.5) G (1.5)
G (5.0)
G (3.0)
G (5.0)
G (5.0)
G (5.0)
G (5.0)
G (3.0)
G (3.0)
G (3.0)
G (1.5)
G (3.0)
G (3.0)
G (3.0)
G (3.0)
G (2.5)
G (2.5)
Figure 2. Dynamic rheology of diesel gel and a model for self-assembly
of amphiphiles. a) Frequency sweep (w) and b) stress sweep (s0) of
5% wt/v Man-8 gel in diesel. c) Possible hydrogen-bonding network in
Man-8 amphiphiles with schematic representation of postulated
molecular packing model for organogels of Man-8. d) The energy-
minimized structure of Man-8.
benzene
G (3.25) G (3.0)
chloroform
ethyl acetate
tetrahydrofuran
methanol
S
S
S
S
S
S
I
S
S
S
S
G (2.5) G (2.0)
S
S
I
S
S
I
water
I
hexane
heptane
diesel
mineral oil
paraffin oil
pump oil
–
–
–
–
–
–
–
–
P
P
–
–
–
–
–
–
–
–
P
P
5% wt/v Man-8 in diesel. The elastic modulus G’ is inde-
pendent of frequency and much higher than the viscous
modulus G’’ over the frequency range. This response is typical
of gels as it shows that the sample does not relax over long
time scales. The value of G’ is a measure of the gel stiffness,
and its value here (ca. 4000 Pa) indicates a gel of moderate
strength. Figure 2b shows a plot of the moduli G’ and G’’ as a
function of the stress amplitude s0 for the same sample. The
stress amplitude s0 at which a sharp decrease in moduli occurs
is the yield stress of the gel, and its value ( ꢀ 30 Pa) is
sufficiently high for the gel to support its own weight in an
inverted vial.
G (2.5)
G (1.2)
G (2.0)
G (4.0)
TG (5.0)
G (2.5)
G (3.5)
G (1.5)
G (2.5)
G (5.0)
TG (5.0)
G (3.5)
silicone oil
mixture of hydrocarbons[b]
[a] Values in parenthesis are minimum gelation concentration of gelator
(% wt/v, mg/100 mL). G=opaque gel; TG=transparent gel; S=soluble;
I=insoluble; P=precipitation. [b] Mixture of aliphatic (pentane, hexane,
octane) and aromatic (benzene, toluene, xylene) solvents in 1:1 volume
ratio.
We then evaluated the ability of Man-8 and Sor-8 to
phase-selectively gel oil in the presence of water. Since a
required heating step in phase-selective gelation would be
impractical for high-volume applications like oil-spill recov-
ery, a room-temperature gelation protocol was formulated.
First, we dissolved a high concentration of the gelator in a
water-miscible solvent (ethanol). An aliquot of this solution
was then added to a 1:1 mixture of oil (diesel) and water in a
vial or flask. Spontaneous partitioning of the aliquot solvent
into water and the gelator into the oil phase was observed;
gelation of oil phase occurred while the aqueous phase was
left intact. Note the photograph of an inverted vial in
Figure 3a: the oil gel is strong enough to hold not only its
weight but also the weight of the aqueous solution on top (the
rheology of the oil gel was identical to that in Figure 2). Such
efficient phase-selective gelation was observed with many oils
including diesel, gasoline, pump oil, crude-oil fractions
(alkanes with n > 9 carbon atoms) and mixture of hydro-
carbon solvents (aliphatic and aromatic), thereby indicating
potential applicability to real oil-spill situations and treatment
with minimum gelation concentrations (MGCs) ranging from
1.5 to 5% wt/v. For example, Man-8 gels diesel at 2.5% wt/v;
in other words, it immobilizes diesel to roughly 32 times its
own dry weight. All gels were thermoreversible and their Tgel
values ranged from 82–1258C and 38–698C for Man-8 and
Sor-8 (5% wt/v), respectively (see Table S1 in the Supporting
Information). They were stable for several months, indicating
their temporal stability. Note that mannitol and sorbitol have
similar chemical structures, except for the orientation of the
hydroxy group at the C2 stereocenter (Figure 1a). This subtle
change in chirality significantly impacts gelling ability—the
mannitol derivatives formed stronger gels and had higher Tgel
values. Alkyl chain length also influences gelling ability—the
MGCs of dioctanoate derivatives were much lower than those
of the dibutanoates. Together, the results indicate that gelling
can be modulated by altering either the hydrophilic sugar or
the hydrophobic fatty acid chain of these gelators.
The strength of the organogels was then studied by
rheology. Figure 2a shows the frequency response of a gel of
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hydrogen bonding of Man-8 and Sor-8 in the oil phase,
possibly because the hydrophobic tails on either side of the
sugar head ensure that the molecules remain in the nonpolar
oil layer. In contrast, Man-8 and Sor-8 did not gel protic
solvents like ethanol, presumably because these solvents
compete for hydrogen bonding with the hydroxy groups on
the gelators. Importantly, the properties of gels obtained by
the aliquot method and by conventional heating were similar
(see Figure S4 in the Supporting Information).
In the few existing examples of low-molecular-weight
phase-selective oil gelators, the recovery of oil from the gels
and the recycling of the gelator were not demonstrated. For
application in oil remediation, these aspects must be
addressed. We therefore performed gelation of diesel
(20 mL) in the presence of 40 mL of water (Figure 3b). A
solution of Man-8 in ethanol was added by syringe to this
mixture such that its concentration in diesel was 5% wt/v. The
diesel gelled instantly, and within 1 h the gel was strong
enough to bear its weight plus that of 40 mL water (see
Video S1 in the Supporting Information). Subsequently,
water was removed by syringe, and the gel was subjected to
distillation by heating it to 1258C (above Tgel), whereupon the
gel liquefied and the diesel distilled off. Diesel could thus be
recovered almost quantitatively. The residue in the flask was
characterized by thin layer chromatography, and Man-8 was
found to be intact. Recycled Man-8 could gel a fresh batch of
diesel, confirming its reusability. Phase-selective gelation of
oil was also performed on a thin layer (< 1 mm) of diesel
floating on a large pool of water (in a Petri dish). The resulting
gel could be scooped out with a spatula, and the oil in it could
again be recovered through distillation. This example mimics
the real scenario of an oil spill.
Figure 3. Phase-selective gelation and diesel recovery from a two-
phase system. a) Phase-selective gelation of an organic liquid in the
presence of water. If a smaller amount of gel forms it will float
(picture 2); at higher concentrations of gel, the flow of water is
stopped upon inversion of the vial (picture 3). b) Gelation of bulk
diesel in the presence of water, and its quantitative recovery through
vacuum distillation. Photographs: 1) diesel and water form a two-
phase system; 2) gel forms instantaneously upon addition of gelator
by syringe; 3) Owing to the strength of the diesel gel, the flow of water
is stopped upon inversion of flask; 4) Diesel gel remains after removal
of the bottom water layer; 5) The entrapped diesel is recovered by
vacuum distillation; 6) Recovered diesel.
In conclusion, we have demonstrated a new class of sugar-
gelators that can selectively gel (solidify) the oil phase from
an oil–water mixture at room temperature. Quantitative
recovery of oil from the gel has been achieved through simple
vacuum distillation. The gelators are easily synthesized and
environmentally benign, and can be recovered and reused
multiple times. We believe this is a promising approach for the
containment and treatment of oil spills.
of refinery effluent. Importantly, even impure compounds
exhibited phase-selective gelation. Also, the oil to water ratio,
type of water (river water from Hudson River, New York
City, USA and sea water from Cooney Island, Brooklyn,
USA) and nature of the aqueous solution (acidic, basic,
neutral, saturated NaCl and saturated CaCl2 solution) did not
alter the phase-selective gelation, which indicates the robust-
ness of the phenomenon.
Optical and scanning electron microscope (SEM) images
(Figure 1c and Figure S1 in the Supporting Information)
show networks of entangled fibers in the gels. X-ray
diffraction (XRD) studies (see Figure S2 in the Supporting
Information) suggest that the fibers are stacks of gelator
molecules with the tails tilted relative to the fiber axis
(Figure 2d). Fiber assembly, in turn, is believed to be driven
by intermolecular hydrogen bonding between the hydroxy
groups of adjacent gelators (Figure 2c). Proof for the hydro-
gen bonding in the gel state comes from Fourier transform
infrared spectroscopy (FTIR) (see Figure S3 in the Support-
ing Information). The importance of hydrogen bonding was
also shown by the synthesis of a hydroxy-protected benzyl-
idene derivative of Man-8 (Figure 1a), which did not gel any
of the oils (see the Supporting Information for synthesis and
characterization). In the case of the phase-selective gelation,
it is clear that the presence of water does not disrupt the
Experimental Section
Room-temperature gelation (aliquot method): An aliquot of gelator
was prepared in a hydrophilic solvent (alcohols, dioxane, and
tetrahydrofuran). A specific amount of prepared aliquot, capable of
delivering the MGC concentration, was injected at the interface. In a
typical phase-selective gelation experiment conducted at 25–308C,
25 mg of Man-8 was dissolved in 400 mL of ethanol. This aliquot was
injected at the interface of the diesel–water mixture (1 mL each). The
mixture was allowed to set to obtain gel of only diesel phase.
Details of the synthesis, characterization, gel preparation, and gel
characterization (gel melting temperature, optical microscopy, elec-
tron microscopy, XRD, IR, and rheology) are included in the
Supporting Information.
Received: April 9, 2010
Published online: July 15, 2010
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Keywords: biocatalysis · molecular gelators · oil pollution ·
self-assembly · solidifiers
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