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.
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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. © XXXX 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
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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,
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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 organoB
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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. C
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Huijer, K. Trends in Oil Spills from Tanker Ships 1995−2004; International Tanker Owners Pollution Federation (ITOPF): London, U.K.; http://www.itopf.com/_assets/documents/amop05.pdf (accessed Sept 25, 2012). (2) Hoch, M. New Estimate Puts Gulf Oil Leak at 205 Million Gallons; http://www.pbs.org/newshour/rundown/2010/08/new-estimateputs-oil-leak-at-49-million-barrels.html (accessed Sept 25, 2012). (3) Jadhav, R. S.; Vemula, P. K.; Kumar, R.; Raghavan, S. R.; John, G. Sugar-derived phase-selective molecular gelators as model solidifiers for oil spills. Angew. Chem. 2010, 49, 7695−7698. (4) Lessard, R. R.; DeMarco, G. The significance of oil spill dispersants. Spill Sci. Technol. Bull. 2000, 6, 59−68. (5) Chapman, H.; Purnell, K.; Law, R. J.; Kirby, M. F. The use of chemical dispersants to combat oil spills at sea: A review of practice and research needs in Europe. Mar. Pollut. Bull. 2007, 54, 827−838. (6) Yuan, J.; Liu, X.; Akbulut, O.; Hu, J.; Suib, S. L.; Kong, J.; Stellacci, F. Superwetting nanowire membranes for selective absorption. Nat. Nanotechnol. 2008, 3, 332−336. (7) Thanikaivelan, P.; Narayanan, N. T.; Pradhan, B. K.; Ajayan, P. M. Collagen based magnetic nanocomposites for oil removal applications. Sci. Rep. 2012, 2, 230−237. (8) Karakutuk, I.; Okay, O. Macroporous rubber gels as reusable sorbents for the removal of oil from surface waters. React. Funct. Polym. 2010, 70, 585−595. (9) Basak, S.; Nanda, J.; Banerjee, A. A new aromatic amino acid based organogel for oil spill recovery. J. Mater. Chem. 2012, 22, 11568−11664. (10) Mallia, V. A.; Weiss, R. G. Low molecular weight gelators for crude oil, petrolium product or chemical spill containment. Patent WO/2012/047251, 2012. (11) Bhattacharya, S.; Krishnan-Ghosh, Y. First report of phase selective gelation of oil from oil/water mixtures. Possible implication toward containing oil spills. Chem. Commun. 2001, 185−186. (12) Gao, J.; Wu, S.; Rogers, M. A. Harnessing Hansen solubility parameters to predict organogel formation. J. Mater. Chem. 2012, 22, 12651−12658. (13) Grahame, D. A. S.; Olauson, C.; Lam, R. S. H.; Pedersen, T.; Borondics, F.; Abraham, S.; Weiss, R. G.; Rogers, M. A. Influence of chirality on the modes of self-assembly of 12-hydroxystearic acid in molecular gels of mineral oil. Soft Matter 2011, 7, 7359−7365. (14) Wu, S.; Gao, J.; Emge, T.; Rogers, M. A. Solvent induced polymorphic nanoscale transitions for 12-hydroxyoctadecanoic acid molecular gels. Cryst. Growth Des. 2013, 13, 1360−1366. (15) Abraham, S.; Lan, Y.; Lam, R. S. H.; Grahame, D. A. S.; Kim, J. J. H.; Weiss, R. G.; Rogers, M. A. Influence of positional isomers on the macroscale and nanoscale architectures of aggregates of racemic hydroxyoctadecanoic acids in their molecular gel, dispersion, and solid states. Langmuir 2012, 28, 4955−4964. (16) Fameau, A. L.; Houinsou-Houssou, B.; Novales, B.; Navailles, L.; Nallet, F.; Douliez, J. P. 12-Hydroxystearic acid lipid tubes under various experimental conditions. J. Colloid Interface Sci. 2010, 341 (1), 38−47. (17) Mallia, V. A.; George, M.; Blair, D. L.; Weiss, R. G. Robust organogels from nitrogen-containing derivatives of (R)-12-hydroxystearic acid as gelators: Comparisons with gels from stearic acid derivatives. Langmuir 2009, 25 (15), 8615−8625. (18) Rogers, M. A.; Pedersen, T.; Quaroni, L. Hydrogen-bonding density of supramolecular self-assembled fibrillar networks probed using synchrotron infrared spectromicroscopy. Cryst. Growth Des. 2009, 9 (8), 3621−3625.
Figure 5. (A) Weight gain by the xerogel in the simulated oil spill versus time and (B) oil remaining on the water from the exposure of 0.1 g of xerogel to 3 g of spilled oil.
xerogel to the simulated spill, the xerogel increased in weight by 459 ± 33 wt % in diesel and 583 ± 42 wt % in gasoline. This illustrates that the 12-HSA xerogel is not only effective but also very versatile in its applications of reclaiming spilled oil. It has 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 absorb similar amounts of oil as a Bregoil sponge (made from waste-wood fibers) (weight gain of ∼700 wt %),27 polypropylene fibers (weight gain ranges from 200 to 1000 wt %),28−30 and cellulose fibers (weight gain of 375 wt %).31 Although, the 12-HSA xerogels do not absorb as much oil as the most advanced materials (i.e., superwetting cryptomelanetype manganese oxide nanowires),20 12-HSA xerogels are as selective for the apolar phase and are derived from much lower 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.
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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. D
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(19) Sakurai, T.; Masuda, Y.; Sato, H.; Yamagishi, A.; Kawaji, H.; Atake, T.; Hori, K. A comparative study on chiral and racemic 12hydroxyoctadecanoic acids in the slutions and aggregation states: Does the racemic form really form a gel? Bull. Chem. Soc. Jpn. 2010, 83 (2), 145−149. (20) Yuan, B.; Li, J.-L.; Liu, X. Y.; Ma, Y.-Q.; Wang, Y. J. Size invariance of fibrous networks of supramolecular soft materials during formation under critical volume confinement. Soft Matter 2012, 8, 5187−5193. (21) Weiss, R. G.; Terech, P. Introduction. In Molecular Gels: Materials with Self-Assebled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006; pp 1−13. (22) Suzuki, M.; Nakajima, Y.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Effects of hydrogen bonding and van der Waals interactions on organogelation using designed low-molecular-weight gelators and gel formation at room temperature. Langmuir 2003, 19 (21), 8622−8624. (23) Rogers, M. A.; Marangoni, A. G. Solvent-modulated nucleation and crystallization kinetics of 12-hydroxystearic acid: A nonisothermal approach. Langmuir 2009, 25 (15), 8556−8566. (24) Wu, Y.; Wu, S.; Zou, G.; Zhang, Q. Solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel. Soft Matter 2011, 7, 9177−9183. (25) Hirst, A. R.; Coates, I. A.; Boucheteau, T. R.; Miravet, J. F.; Escuder, B.; Castelletto, V.; Hamley, I. W.; Smith, D. K. Lowmolecular-weight gelators: Elucidating the principles of gelation based on gelator solubility and a cooperative self-assembly model. J. Am. Chem. Soc. 2008, 130, 9113−9121. (26) Reynolds, J. G.; Coronado, P. R.; Hrubesh, L. W. Hydrophobic aerogels for oil-spill cleanupIntrinsic absorbing properties. Energy Sources 2001, 23, 831−843. (27) Faudree, T. L. Foamaceous hydrocarbon adsorption medium and method and system for making same. U.S. Patent 4,230,566, 1980. (28) Choi, H. M.; Cloud, R. M. Natural sorbents in oil spill cleanup. Environ. Sci. Technol. 1992, 26, 722−776. (29) Choi, H.-M. Needle punched cotton nonwovens and other natural fibers as oil cleanup Sorbents. J. Environ. Sci. Health, Part A: Environ. Sci. Eng. Toxic Hazard. Subst. 1996, A31, 1441−1457. (30) Wei, Q. F.; Mather, R. R.; Fotheringham, A. F.; Yan, R. D. Evaluation of nonwoven polypropylene oil sorbents in marine oil-spill recovery. Mar. Pollut. Bull. 2003, 46, 780−783. (31) Teas, C.; Kalligeros, S.; Zanikos, F.; Stournas, S.; Lois, E.; Anastopoulos, G. Investigation of the effectiveness of absorbent materials in oil spills clean up. Desalination 2001, 140, 259−264.
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