Article
pubs.acs.org/Langmuir
Phase-Selective Sorbent Xerogels as Reclamation Agents for Oil
Spills
Phillip Lee and Michael A. Rogers*
Department of Food Science, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08901, United
States
ABSTRACT: 12-Hydroxystearic acid (12-HSA) xerogels derived from 12-HSA−
acetronitrile organogels are highly effective sorbent materials capable of adsorbing
apolar, spilled materials in aqueous environments. 12-HSA xerogels made from 12HSA−acetronitrile organogels are more effective than 12-HSA xerogels made from 12HSA−pentane organogels because of the highly branched fibrillar networks established
in acetonitrile molecular gels. This difference arises because of dissimilarities in the
network structure between 12-HSA in various solvents. These xerogels, being
thermoreversible, allow for both the spilled oil to be reclaimed but also the gelator
may be reused to engineer new xerogels for oil spill containment and cleanup.
■
■
INTRODUCTION
Waterways facilitate the transport of the world’s petrochemical
supply; however, there are associated risks with this mode of
transport, as witnessed by the over 30 significant oil spills (i.e.,
>7000 tons of crude oil released per incident) since 2000.1
Confounding the issue, underwater exploration and offshore
drilling increase the risks of petrochemicals entering our
waterways. In 2010, nearly 5 billion barrels of crude oil were
released in the Gulf of Mexico.2 Each incident has a devastating
effect on Earth’s delicate ecosystems. The presence of both
crude oil and the chemical dispersants, used to disperse the oil,
can disrupt sensitive food webs and create situations of toxin
bioaccumulation.
Current materials used to clean spilled oil are subdivided into
three primary categories, including dispersants, sorbents, and
solidifiers.3 Dispersants emulsify the oil into small finely divided
droplets and disperse them into the environment.4,5 Sorbents
are typically powders that selectively absorb the oil via capillary
forces within a superhydrophobic matrix.6−8 Solidifiers gel the
material on the surface of the water using either polymeric or
monomeric gelators.3,9,10 Ideally, any material used to treat
spilled oil in our waterways must selectively remove the oil
phase from water, be nontoxic, and allow oil to be reclaimed,
and the material should be recyclable or reusable.3 A recent
surge in interest using amphiphilic molecular gelators to solidify
spilled oil includes sugar alcohols,3 amino acid amphiphiles,11
aromatic amino acids,9 and modified 12-hydroxy-N-alkyloctadecanamide.10 A different, novel approach to remove spilled oil
from water is to use a xerogel, which will adsorb the oil from
water. The separation ability of a xerogel arises from the
hydrophobicity of the gelator, while the driving force of
adsorption is the capillary forces arising from the void volumes
within the xerogel created by the fibrillar aggregates. Hence, the
ability to form a highly porous xerogel, using the simplest
method, is central to developing a low-cost sorbent-based
xerogel to be used as an oil reclamation agent.
© 2013 American Chemical Society
MATERIALS AND METHODS
Organogel and Xerogel Preparation. The samples were
prepared in 10 mL vials by adding 0.125 g of 12-hydroxyoctadecanoic
acid (12-HSA) to 5 g of the organic solvent. Solvents were selected on
the basis of their ability to be gelled by 12-HSA and their high volatility
at atmosphere pressure. The three organic solvents tested were
acetonitrile, pentane, and diethyl ether. Each of the vials were capped
and heated to 80 °C for 20 min to allow for 12-HSA to dissolve into
the solvent. The sols were cooled and stored at 25 °C for 24 h,
allowing the gel to form. After organogelation, the vials were uncapped
under atmospheric pressure for at least 24 h, allowing for the solvent
to evaporate from the organogel, leaving behind the xerogel. Samples
were stored below the melting point (∼60 and 65 °C) of the xerogel in
an incubator set at 40 °C.
Oil Spill Simulation. Two different setups were used to simulate
the oil spill. The first setup used 10 mL beakers each containing 3 g of
distilled water and 3 g of 10W-40 motor oil. The second setup used 3
g of sterile seawater (Sigma-Aldrich, lot 076k8431) and 3 g of diesel
fuel (Exxon Mobile) or 3 g of regular gasoline (87 octane, Exxon
Mobile). The mass of the oil/fuel, water, and beaker system was then
weighed and recorded. A known amount of xerogel was placed onto
the top of a simulated oil spill and left quiescent for 1 h, after which
time the sample was removed and blotted dry using Kimwipes to
remove residual surface oil. To obtain the kinetics of adsorption, gels
were removed from the beaker and the weight of the gels was recorded
every min for the first 5 min, every 5 min from 5 to 30 min, and then
every 15 min from 30 min to 1 h for the 10W-40 motor oil in distilled
water. From these data, the mass of the oil absorbed from the different
gels could be calculated, and the information was used to calculate the
percent of oil absorbed with respect to the mass of the xerogel. A
second set of experiments determined the amount of diesel or gasoline
adsorbed by the gel from seawater after 60 min and is presented as a
percent weight change of the xerogel.
Microscopy. Brightfield micrographs were obtained by placing the
sample onto a 25 × 75 × 1 mm glass slide, and a coverslip was applied
to the sample (Fisher Scientific, Pittsburgh, PA) after being heated to
Received: March 3, 2013
Revised: April 9, 2013
Published: April 16, 2013
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Figure 1. XRD patterns for the short angle (A and C) and wide angle (B and D) for pentane (A and B) and acetonitrile (C and D).
80 °C. Samples were imaged on a Linkham microscope (Linkham
Scientific Instruments, Surrey, U.K.) with a 10× objective lens (N.A.
0.25). A charge-coupled device (CCD) color camera (Olympus,
Center Valley, PA) acquired images as uncompressed 8-bit (256 grays)
gray-scale TIFF files with a 1280 × 1024 spatial resolution.
X-ray Diffraction (XRD). The XRD or wide-angle X-ray scattering
(WAXS) patterns of 12-HSA gels in different solvents were obtained
by use of a Bruker HiStar area detector and an Enraf-Nonius FR571
rotating anode X-ray generator equipped with a Rigaku Osmic mirror
optic system (∼0.06° 2θ nominal dispersion for Cu Kα; l = 1.5418 Å)
operating at 40 kV and 40 mA. All of the data were collected at room
temperature over a period of about 300 s. The sample−detector
distance was 10.0 cm, and the standard spatial calibration was
performed at that distance. Scans were 4° wide in ω with a fixed
detector or Bragg angle (2θ) of 0° and fixed platform (f and c) angles
of 0° and 45°, respectively. In all cases, the count rate for the area
detector did not exceed 100 000 counts per second (cps).
Rheology. An AR-G2 hybrid rheometer (TA Instruments, New
Castle, DE) was used to probe the macroscopic properties of the gels.
An 8 mm flat, serrated, parallel plate (TA Instruments, New Castle,
DE) was used to carry out the oscillatory measurements. Small
deformation oscillatory rheology was employed to determine the
complexes G′ and G″ within the linear viscoelastic region (LVR) of the
gel at 1 Hz by carrying out a stress sweep at 10−500 Pa.
Upon cooling the sol, gelator molecules phase-separate and
interact via non-covalent interactions; in the case of 12-HSA,
dimerization occurs between the carboxylic acid groups and
longitudinal growth is promoted by the hydroxyl group at
position 12, forming hydrogen bonds between adjacent 12HSA molecules (Figure 2).15−19 SAFiNs, in organic solvents,
■
RESULTS AND DISCUSSION
Numerous solvents are structured into organogels when 12HSA, a hydrogenated component of castor seed oil, is added in
low concentrations.12 The hydrogenated derivative of castor
seed oil is enantiomerically pure, which allows for gelation at
much lower concentrations.13 Dependent upon the solvent, the
critical gelation concentration (CGC) varies between 0.5% for
alkanes (i.e., pentane, hexane, etc.) and 2.3% for nitriles (i.e.,
ethyl nitrile and butyl nitrile).12 The difference in the CGC
between alkanes and nitriles pertains to the polymorphic form
of the molecular gel. In alkanes, a hexagonal subcell spacing
(∼4.1 Å) and a multilamellar crystal morphology, with a
distance between lamella greater than the bimolecular length of
12-HSA, corresponded to molecular gels with the CGC less
than 1 wt % (Figure 1A).14 12-HSA molecular gels in nitriles
have a much higher CGC corresponding to a triclinic parallel
subcell (∼4.6, 3.9, and 3.8 Å) and interdigitation in the lamella
(Figure 1B).
Figure 2. Schematic representation of the different polymorphic
nanoscale interactions found in 12-HSA−pentane and 12-HSA−
acetonitrile organogels.
require a meticulous balance between contrasting parameters,
including solubility and those intermolecular forces that control
epitaxial growth into axially symmetric elongated aggregates.20
The precise ratio of gelator−gelator interactions to gelator−
solvent interactions is established to play a central role in the
formation of an organogel and the length of the fibrillar
aggregates.21−23 12-HSA−acetonitrile molecular gels have a
drastically different microstructure than the 12-HSA−pentane
microstructure (Figure 3). In acetonitrile, numerous fibers
radiate from a central nuclei, forming more numerous, shorter
fibers compared to 12-HSA in pentane. The differences in the
crystalline structure will give rise to the difference in the
number and size of the pores and, hence, the magnitude of the
capillary forces. Because the self-assembly process of organo5618
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at atmospheric conditions and ability to form an organogel at
low concentrations. Using these criteria, three solvents were
selected, pentane, acteonitrile (ethyl nitrile), and diethyl ether.
Diethyl ether formed a very fragile xerogel, which was difficult
to handle, and, therefore, was not analyzed further. The
organogels had a higher elastic modulus than loss modulus
(acetonitrile, G′ = 252 940 ± 14 918 Pa and G″ = 43 124 ±
4564 Pa; pentane, G′ = 322 000 ± 19 312 Pa and G″ = 51 462
± 9414 Pa) indicating that the materials were gels.
The samples were uncapped and allowed to evaporate for 72
h to ensure that the majority of the solvent had evaporated.
Upon evaporation, resulting in the transformation of the
organogel to a xerogel, the elastic modulus increased drastically
(acetonitrile, G′ = 620 629 ± 32 829 Pa and G″ = 47 107 ±
5919 Pa; pentane, G′ = 502 600 ± 11 142 Pa and G″ = 42 662
± 4328 Pa). Dependent upon the solvent volatility, the rate
that the gels dried varied greatly and affected the structure of
the xerogel. As seen in Figure 4, ethyl nitrile (acetonitrile) had
very little collapse in the structure, maintaining a large volume,
while pentane and more obviously diethyl ether collapsed into
very dense xerogels, which can be correlated to the differences
in the microstructure and ultimately differences in the
nanostructure of the crystalline phase. Each of these systems
was placed into the simulated oil spill, and the mass gained after
1 h (Figure 5) was measured to see if the capillary forces were
sufficient to draw the hydrophobic solvent into the empty pores
of the xerogels. The 12-HSA xerogel produced from diethyl
ether did not adsorb solvent during the experiment; the xerogel
produced using pentane increased its mass by 93 ± 9.2 wt %,
while the xerogel made from 12-HSA in acetronitrile increased
its mass by 387 ± 21 wt % in distilled water and 10W-40 motor
oil (Figure 5). To ensure that the mass gained was from motor
oil and not water, the 12-HSA xerogel produced using
acetronitrile was placed in water. The mass gained after 1 h
was 3.85 ± 0.5 wt % in distilled water and 5.23 ± 1.2 wt % in
seawater, effectively demonstrating that the xerogel was phaseselective in adsorbing the oil. The 12-HSA xerogel made using
acetronitrile as the solvent was also tested on diesel in seawater
and regular gasoline in seawater. After exposure for 1 h of the
Figure 3. Brightfield micrographs of (A) 2.5 wt % 12-HSA−
acetonitrile and (B) 2.5 wt % 12-HSA−pentane organogels. The
width of the micrograph is 120 μm.
gelators requires such an important balance of solvent polarity,
water content, and additives to effectively immobilize the
continuous phase, our aim is to minimize the impact of the
solvent when containing spilled materials. Recently, a group of
sugar-derived gelators showed promise to contain oil spills
because of their robust ability to form gels in numerous
environments.24,25 Our strategy is distinctively different, where
the organogel is formed in a very controlled environment and
then the solvent is removed, creating a xerogel (Figure 3).
Solvents were selected on the basis of their low vapor pressure
Figure 4. Oranogels produced in three different solvents and the resulting 0.125 g of xerogels after 72 h of drying. The simulated oil spill was
performed in triplicate for the weight gain after 1 h and the time-resolved adsorption.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
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459 ± 33 wt % in diesel and 583 ± 42 wt % in gasoline. This
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previously been shown that similar materials (i.e., silica
aerogels) are far less effective (∼0.1 wt % gain) than 12-HSA
xerogels (∼583 wt % gain).26 However, 12-HSA xerogels
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cost castor seed oil.
It is obvious from the weight gained and oil recovered that
the xerogels made from 12-HSA and acetronitrile are very
effective phase-selective sorbent materials. The major advantage
of using the xerogel, as opposed to forming an organogel in the
environment, is that this system is not dependent upon
environmental factors. As long as the material of interest is
apolar, our sorbent xerogel pellets will be an effective mode to
contain and remove the materials from the environment.
■
CONCLUSION
Xerogels formed using 12-HSA and acetronitrile are highly
effective sorbent materials capable of adsorbing apolar, spilled
materials in aqueous environments. 12-HSA organogels in
acetronitrile are more effective at producing xerogels, capable of
adsorbing spilled oil, than from pentane because of the highly
branched fibrillar networks established in acetonitrile molecular
gels. This difference arises because of dissimilarities in the
network structure between 12-HSA in various solvents. These
xerogels, being thermoreversible, allow for both the spilled oil
to be reclaimed but also the gelator may be reused to engineer
new xerogels for oil spill containment and cleanup.
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