Monoglyceride-based organogelator for broad-range oil uptake with high capacity

Article abstract of DOI:10.1021/acs.langmuir.5b00053

Oil/water separation has been a worldwide subject because of increasing release of oil-containing wastewater as well as several marine oil spills. The phase-selective organogelators (PSOGs) are thought to offer a potential and effective implement for addressing this issue. An ideal PSOG for oil adsorption must fulfill some requirements involving effective gelation, easy synthesis, low cost, and recyclable for reuse. However, beyond those, the ability of gelation for a broad-range oil phase without selectivity is also important. However, most of the reported PSOGs have limitation in this respect thus far. In this paper, a new class of saturated 1-monoglyceride-derived organogelators can efficiently uptake almost all of the common fuel oils from water and gelate organic solvents with extremely low minimum gelation concentration (MGC). In addition, the oils in the gel and gelator molecules can be recovered quantitatively through simple vacuum distillation.

Full text of DOI:10.1021/acs.langmuir.5b00053

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Article
Monoglyceride Based Organogelator for
Broad-range Oil Uptake with High Capacity
Dong Wang, Jian Niu, Zhenggong Wang, and Jian Jin
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00053 • Publication Date (Web): 20 Jan 2015
Downloaded from http://pubs.acs.org on January 22, 2015
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Monoglyceride Based Organogelator for
Broadꢀrange Oil Uptake with High Capacity
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Dong Wang,† Jian Niu,† Zhenggong Wang,†,‡ and Jian Jin†,*
†NanoꢀBionics Division and iꢀLab, Suzhou Institute of NanoꢀTech and NanoꢀBionics,
Chinese Academy of Sciences, Suzhou 215123, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
Abstract:
Oil/water separation has been a worldwide subject because of increasing release of
oilꢀcontaining wastewater as well as several marine oil spills. The phase selective
organogelators (PSOGs) are thought to offer a potential and effective implement for
addressing this issue. An ideal PSOG for oil adsorption must fulfill some
requirements involving effective gelation, easy synthesis, low cost, and recyclable for
reuse. But beyond those, the ability of gelation for a broadꢀrange oil phase without
selectivity is also important. However, most of the reported PSOGs have limitation in
this respect so far. In this paper, a new class of saturated 1ꢀmonoglyceride derived
organogelators which can efficiently uptake almost all of the common fuel oils from
water and gelate organic solvents with extremely low minimum gelation
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concentration (MGC). In addition, the oils in the gel and gelator molecules can be
recovered quantitatively through simple vacuum distillation.
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1. INTRODUCTION
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Organogels are formed via weak intermolecular interactions of molecules in a variety
of organic liquids, among which low molecular weight organogelators (LMOGs) have
attracted considerable interest since they provide not only a deep insight into the
understanding of supramolecular assembly processes but also possess unique
properties and a wide range of potential applications, such as oil recovery, toxicity
remediation devices, electroꢀoptical displays, drug delivery systems and as templates
for the fabrication of nanomaterials.1–8 Oil/water separation has been a worldwide
subject because of increasing release of oilꢀcontaining wastewater as well as several
marine oil spills, which cause irrecoverable damage to the environment and
ecosystem. The existing methods of uptaking oils by using dispersants, sorbents, or
solidifiers still have some limitations to contain oil spills from a broad range of
mixtures of water and oil and reclaim the oil.9–13 An effective and broadly applicable
material for highꢀefficient oil adsorption is highly desired.
The phase selective organogelators (PSOGs) are thought to offer a potential and
effective implement for addressing this issue.14 The first example of PSOG was
demonstrated by Bhattacharya and KrishnanꢀGhosh with amino acid amphiphiles.15
After that, various types of PSOGs have been reported.16–19 An ideal PSOGs for oil
adsorption must fulfill some requirements involving effective gelation, easy synthesis,
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low cost, and recyclable for reuse. But beyond those, the ability of gelation for a
broadꢀrange oil phase without selectivity is also important since the oils spilled or
discharged out are often speciesꢀdiverse and have complicated features. However,
most of the reported PSOGs have limitation in this respect so far. For instance, some
of gelators can gelate only aliphatic hydrocarbons, but not aromatic hydrocarbons
including benzene, toluene, and etc., and the gelation behavior is opposite for
others.11, 17, 20–24 The recently reported sugar alcoholꢀderived gelators seem to be an
allꢀrounder which can simultaneously gelate most of aliphatic and aromatic
hydrocarbons, but are not effective to chloroform, tetrahydrofuran, and methanol.2
Herein, we present a new class of saturated 1ꢀmonoglyceride derived amphiphilic
organogelators which can efficiently uptake almost all of the common fuel oils from
water and gelate organic solvents with extremely low minimum gelation
concentration (MGC) and high oil capacity. It shows a great potential to be used for
remediation of spilled oil and cleansing toxic organic liquids from wastewater
produced in industry and daily life.
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2. EXPERIMENTAL SECTION
2.1 Materials.
1ꢀDodecanol (98%, Alfa), 1ꢀhexadecanol (98%, Alfa), 1ꢀoctadecanol (98%, Alfa),
itaconic anhydride (95%, SigmaꢀAldrich), maleic anhydride (95%, SigmaꢀAldrich),
glycidol (96%, SigmaꢀAldrich), ethyl alcohol absolute (≥99.7%, Sinopharm Chemical
Reagent), nꢀhexane (AR, Sinopharm Chemical Reagent), acetic ether (AR, Sinopharm
Chemical Reagent), acetone (AR, Sinopharm Chemical Reagent), chloroform (AR,
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Sinopharm Chemical Reagent), toluene (AR, Sinopharm Chemical Reagent),
tetrahydrofuran (AR, Sinopharm Chemical Reagent), chlorobenzol (AR, Sinopharm
Chemical Reagent), ethylene glycol (AR, Sinopharm Chemical Reagent),
cyclohexane (AR, Sinopharm Chemical Reagent), octane (AR, Sinopharm Chemical
Reagent), undecane (AR, Sinopharm Chemical Reagent), dodecane (AR, Sinopharm
Chemical Reagent), diesel (CP, Sinopharm Chemical Reagent), kerosine (AR,
Aladdin), paraffin (CP, Sinopharm Chemical Reagent), olive oil (CP, Sinopharm
Chemical Reagent), and deuterochloroform (CDCl3, Sinopharm Chemical Reagent)
were used as received. Millipore deionized water was used in the experiments.
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2.2 Synthesis. DGI were synthesized according to previous report as shown in Figure
S1[1]. Typically, itaconic anhydride (25 g, 0.22 mol) was mixed and reacted with
o
dodecanol (40 g, 0.22 mol) at 116 C for 1 h to obtain dodecyl itaconate. After the
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reaction was completed, 100 ml of hexane was added to the above mixture at 80 C
under a vigorous mechanical stirring and the crude product of white crystal was
precipitated. The crude sample of dodecyl itaconate was recrystallized from ethanol
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until the melting point of the sample was close to 76–78 C. The obtained dodecyl
itaconate (10 g, 0.034 mol) was dissolved in toluene (10 ml) and reacted with glycidol
(7.5 g, 0.102 mol) at 105 oC with 20 mg (79.6 ꢁmol) pyridinium pꢀtoluenesulphonate
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as a catalyst. The mixture was refluxed for 5 h at 105 C. When the reaction was
finished, the crude product was applied to a silicaꢀgel column and eluted with a
hexane/ethyl acetate mixture (3/2 by volume). The collected DGI fraction was
purified several times by recrystallization from an acetone/hexane mixture (1/1 by
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volume). The final pure product has a melting point of 76–78 C. The synthesis
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procedures of HGI, OGI, and HGM were same as that of DGI.
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2.3 Characterization.
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Scanning electron microscopy images were obtained on a fieldemission scanning
electron microscope (Hitachi S4800, Japan). Xꢀray Diffraction was collected on a
Bruke D8. Optical microscopy images were obtained on an OLYMPUS BX51. NMR
analysis results were obtained on a Varian 400 MHz. Fourier transform infrared
spectroscopy (FTIR) spectra were collected by using a Nicolet 6700 FTIR
spectrometer. Rheological studies were performed on AR2000 rheometer.
3. RESULTS AND DISCUSSION
3.1 Formation and characterization of gels
Three gelators with different alkyl chain lengths, namely dodecylglyceryl itaconate
(DGI), hexadecylglyceryl itaconate (HGI), and octadecylglyceryl itaconate (OGI)
were synthesized (Figure S1). During phase selective gelation process, the
amphiphilic molecule is prone to selfꢀassemble into nanofibers in liquid and
sequentially form networks above the MGC and thus convert the liquid to opaque gel
as shown in Figure 1a. In order to recognize the process of gelation, morphological
feathers of the organogels (5% wt/v hexane gels) was investigated using optical
polarized microscope (OM) and field emission scanning electron microscopy
(FEꢀSEM) (HGI is taken as an example). The results as shown in Figure 1b and 1c
indicate the formation of oneꢀdimensional fibrous network structures in the organogel.
The interwoven threeꢀdimensional network structure could be clearly observed from
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the assembly of these nanofibers and the network structure is responsible for the
entrapment of organic solvents. The individual fiber obtained from hexane has several
hundreds of micrometers in length and about 20 ꢁm in width. The strength of the
organogels was studied by rheology test. Figure 1d shows the frequency response of a
gel of 5% wt/v HGI in diesel. The elastic modulus G′ is much higher than the viscous
modulus G′′ over the frequency range. This represents a nature of typical gels as it
shows that the sample does not relax over long time scale. The value of G′ is a
measure of the gel stiffness and its value of about 45 kPa indicates a gel with strong
strength. Fig. 1e is the plots of moduli G′ and G′′ as a function of stress amplitude for
the same sample. The stress amplitude at which the G′ and G′′ crosses each other is
the yield stress of the gel, and its value of about 100 Pa is sufficiently high for the gel
to support its own weight in an inverted vial. We then evaluated the ability of HGI to
phaseꢀselectively gelate oil in the presence of water. First, a high concentration of
HGI was dissolved in hexane, and then an aliquot of the solution was added to a
mixture of hexane and water in a vial. After a short time (~ 60 s), the gelation of oil
phase was observed while the aqueous phase was left intact. In the photograph of an
inverted vial in Fig. 1a, we can see that the oil gel is strong enough to hold not only its
own weight but also the weight of the aqueous phase on the top, which is coincidence
with the rheology test. Such efficient phaseꢀselective gelation could be observed in
the cases of many fuel oils such as diesel, kerosene and paraffin, and also mixtures of
them, thereby indicating potential applicability to real oilꢀspill situation and treatment
of refinery effluent.
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Figure 1. (a) Mechanism of gelation (The right is photograph of HGI organogel). (b)
Polarized optical microscope image of selfꢀassembled aggregates of HGI in hexane.
(c) SEM image of the corresponding HGI gel in hexane. Dynamic rheology of
frequency sweep (d) and oscillatory stress sweep (e) of HGI gel in diesel.
3.2 Evaluation of gelling ability of gelators
The value of MGC is an essential indicator for evaluating gelling ability of gelators.
The MGCs of DGI, HGI, and OGI molecules are summarized in Table 1. It can be
seen that with the increase of alkyl chain length, their MGCs decrease accordingly,
indicating longer alkyl chain gives better gelation performance. Among them, DGI
can only gelate aliphatic hydrocarbons with ordinary MGCs. However, OGI could
gelate almost all organic liquids including aliphatic and aromatic hydrocarbons with
extremely low MGC except paraffin. The MGCs of OGI for diesel, nꢀhexane,
undecane, toluene, ethanol, and chloroform are as low as 0.56, 0.54, 0.41, 1.2, 1.5,
and 5.6% wt/v, respectively. These MGCs values rank in the top level of PSOGs
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reported so far and demonstrate powerful gelling ability of OGI for various organic
liquids. HGI seems to be a real allꢀrounder, which can gelate all tested organic liquids
such as paraffin, diesel, and kerosene with extremely low MGCs ranging from 0.4 to
0.7% wt/v. In other words, it could immobilize organic liquid more than 200 times its
own dry weight. More importantly, besides common aliphatic and aromatic fuel oils,
the gelators HGI and OGI could gelate even polar organic solvents such as
chloroform, ethanol, and THF with similar low MGCs. Our gelators show outstanding
versatility towards gelling organic liquids with high oil uptake capacity. In addition,
all the gels are thermoreversible with Tgel of 38ꢀ55 °C corresponding to different
organic solvents. The gels are stable and could be maintained for several months with
no change. In addition, the MGCs of three representative organic liquids, diesel,
toluene, ethyl acetate, from water solution containing 3.5 wt% NaCl was also
measured in order to evaluate the gelation ability of our monoglyceride based
organogelators under the condition of seawater. As shown in Table 1, the addition of
NaCl has no effect on MGC values. The oil contents in water after gelation are
measured by total organic carbon (TOC) analyser. The results show that for all the
fuel oils and organic solvents tested in this work the oil contents in water after
gelation are less than 50 ppm.
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MGCs (% wt/v, mg/100 μL)
Liquid or oil
Top level reported
DGI HGI OGI
by others
n-Hexane
Cyclohexane
Octane
2.0 0.65 0.54 1.0711, 0.715, 0.4120
1.8 0.51 0.43
3.0 0.73 0.60
1.3 0.52 0.41
1.5 0.51 0.43
2.52, 0.415, 0.6220
0.5111, 0.2415
0.3320
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Undecane
Dodecane
Diesel
0.2715, 0.3520
2.3 0.70 0.56 2.52, 1.0311, 0.4120
0.70
Diesel/brine
Kerosine
Paraffin
1.7 0.60 0.54 1.5511, 0.3215, 0.520
1.4 0.40
I
2.02, 0.5115, 0.4320
Olive oil
Chloroform
Toluene
2.2 0.70 0.58
1.02, 3.114
P
7.5
6.0 1.6
1.6
5.6
1.2
2.713
1.52, 0.23, 0.813
Toluene/brine
Ethyl acetate
P
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2.1
7.8
1.3
2.02, 0.913
Ethyl
acetate/brine
Tetrahydrofuran
Ethylene glycol
Ethanol
P
5.0
1.0
1.5
2.0
1.4
4.0 1.1
P
P
P
2.0
2.6
2.0
1.013
1.413
0.23, 0.913, 3.714
Acetone
Chlorobenzol
Table 1. The MGCs of our monoglyceride based organogelators (DGI, HGI, and
OGI) in various fuel oils and organic solvents. The given values are minimum
gelation concentration of gelator (% wt/v, mg/100 ꢁL). I = insoluble; P = precipitate.
Brine is water solution containing 3.5 wt% NaCl.
3.3 Gelation-forming process of gelators
The inꢀdepth investigation of the intermolecular packing of the selfꢀassembled
1ꢀmonoglyceride derived amphiphilic organogelators in the gel state was carried out
by Xꢀray diffraction (XRD) diffraction and FTꢀIR spectra. Figure 2a shows the XRD
pattern of the diesel organogel of HGI in wide angle and small angle (the inset)
regions. In small angle region, it gives a distinct peak at 2θ = 2.11°, which
corresponds to the d spacing value of 4.2 nm. Considering the molecular length of
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HGI is around 2.8 nm, the d spacing fits well a bilayer structure with a 41° tilt angle
of alkyl chains (Figure 2c). The wide angle XRD pattern of the organogel shows
periodical diffraction peaks, indicating that HGI molecules are selfꢀassembled into a
highly ordered structure. The periodic peaks at 7.51° (D/4), 9.39° (D/5), 11.29° (D/6),
13.19° (D/7), and 15.09° (D/8) indicate the existence of oneꢀdimensional lamellar
structure in the gel state.25 The peak at 2θ = 19.75° (d = 4.49 Å) corresponds to
hydrogen bonding distance of gelator molecules in the gel state.11 The XRD results
reveal that the HGI fibers are composed of repeating layered structural units of HGI
bilayer. We therefore speculate that the HGI molecules firstly assemble into lamellar
structure and the lamellae then stack each other under driven by van der Waals force
to give a fibrous structure. Such an assembly is believed to be driven by
intermolecular hydrogen bonding between the hydroxyl groups of adjacent molecules.
The achievement of oil adsorption is ascribed to the migration of oil molecules into
the interlamellar spaces through hydrophobic interaction between oil and alkyl chains
of HGI. To further characterize the intermolecular interaction, FTꢀIR of HGI
organosol and HGI organogel were recorded as shown in Figure 2b. The peak at 3413
cmꢀ1 ascribed to the –OH stretching band for HGI in solution state shifts to 3280 cmꢀ1
in the corresponding gel state, and the peak at 1716 cmꢀ1 ascribed to the >C=O
stretching band for HGI in solution state shifts to 1702 cmꢀ1 in the corresponding gel
state. These variations suggest the formation of strong hydrogen bond between HGI
molecules after gelation. To further confirm the rationality of such packing, Forcite
with the Dreiding force field and Gasteiger charges were used to calculate the packing
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of HGI molecules. As shown in Figure 2c, the result of theoretical calculation gives
the optimized spacing of adjacent HGI molecules of 4.4 Å, which is in agreement
with the XRD result. From the above discussions, we know that the transformation
from lamellar phase units to the fibers is accompanied by the formation of
intermolecular hydrogen bonds, and the hydrogen bonds network is proposed in
Figure 2d. To further understand the relationship between the structure and the gel
ability, a control experiment was carried out where hexadecylglyceryl maleate (HGM)
was used to gelate oils (Figure S1 and S5). HGM has similar molecular structure to
HGI but with a little different head group. Our results reveal that HGM cannot form
stable organogel in common fuel oils and organic solvents. This is because that
different from HGI, it is difficult to form hydrogen bonds between adjacent HGM
head groups since the small volume of HGM head group is insufficient to form
hydrogen bond in HGM bilayer (Figure S6), which has been confirmed by our
previous reports.7 It is concluded that the strong hydrogen bonding network formed
among HGI head groups contribute to the broad range gelꢀforming ability and to the
low MGCs.
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Figure 2. (a) XRD pattern of HGI diesel gel in wide angle and low angle regions. (b)
FTꢀIR spectra of HGI organosol and HGI organogel in diesel. (c) The simulation of
packing structure of HGI molecules in organogel state. (d) Proposed hydrogen
bonding network among HGI molecules correspondingly.
3.4 Recovery of gelators and oils
The recovery of oils from the organogels and the recycling of the gelators are feasible
in our system. Taking diesel as an example, the recovery of diesel and recycling of
gelator from HGI gel is demonstrated in Figure 3. The gelation of 10 mL diesel in the
presence of the same volume of water was firstly performed by adding a small amount
of HGI in ethanol to the above diesel/water mixture with the final concentration of
HGI 5% wt/v. The gelation of diesel occurred immediately, and the organogel was
strong enough to keep stable state of diesel gel in inverted vial within one day.
Afterwards, water was removed by syringe and diesel was collected almost
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quantitatively by vacuum distillation of the gel. The HGI as residue after vacuum
distillation was then left at the bottom of the container and can be reused for oil
uptake next time. Thin layer chromatography has proved that molecular structure of
HGI was maintained during such a recycling process.
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Figure 3. Phase selective gelation and recovery of diesel: (1) selective gelation of
diesel after adding HGI gelator, (2) invert the flask, (3) remove water by syringe, (4)
collect the distilled diesel, and (5) recycling HGI gelator.
4. CONCLUSIONS
In summary, we have developed a new type of amphiphilic organogelators that is able
to gel various fuel oils and organic liquids with high capacity. The gelators showed
phaseꢀselective gelation behavior and can separate the liquid oils or organic solvents
by gelling the oil phase from the biphasic mixtures of oilꢀwater at room temperature.
The oil in the gel can be recovered quantitatively through simple vacuum distillation.
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Taking the advantages of easy preparation, environmental benign, versatility, and
reusability into account, the molecular gelators holds future promise for remediation
of oil spill and treatment of toxic organic liquid reagents.
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ASSOCIATED CONTENT
Supporting Information: Characterization data of gelators, preparation of gels, and
analysis of gelꢀforming ability of gelators. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: jjin2009@sinano.ac.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Key Project of National Natural Science Foundation
(No. 21433012), the National Basic Research Program of China (No.
2013CB933000), the National Natural Science Foundation of China (Grant No.
21473239), the Key Development Project of Chinese Academy of Sciences (No.
KJZDꢀEWꢀM01ꢀ3), and the Natural Science Foundation of Jiangsu Province (No.
BK20130007).
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Table of Contents Graphic and Synopsis
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Saturated 1ꢀmonoglycerideꢀbased phase selective organogelators are successfully
developed. These low molecular weight organogelators are able to efficiently uptake
almost all of the common fuel oils and organic solvents from water with low
minimum gelation concentration and high oil capacity.
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