Phase-Selective Sorbent Xerogels as Reclamation Agents for OilSpillsPhillip Lee and Michael A. Rogers*

Department of Food Science, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08901, UnitedStates

ABSTRACT: 12-Hydroxystearic acid (12-HSA) xerogels derived from 12-HSA−acetronitrile organogels are highly effective sorbent materials capable of adsorbingapolar, spilled materials in aqueous environments. 12-HSA xerogels made from 12-HSA−acetronitrile organogels are more effective than 12-HSA xerogels made from 12-HSA−pentane organogels because of the highly branched fibrillar networks establishedin acetonitrile molecular gels. This difference arises because of dissimilarities in thenetwork structure between 12-HSA in various solvents. These xerogels, beingthermoreversible, allow for both the spilled oil to be reclaimed but also the gelatormay be reused to engineer new xerogels for oil spill containment and cleanup.

■ INTRODUCTION

Waterways facilitate the transport of the world’s petrochemicalsupply; however, there are associated risks with this mode oftransport, 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 offshoredrilling increase the risks of petrochemicals entering ourwaterways. In 2010, nearly 5 billion barrels of crude oil werereleased in the Gulf of Mexico.2 Each incident has a devastatingeffect on Earth’s delicate ecosystems. The presence of bothcrude oil and the chemical dispersants, used to disperse the oil,can disrupt sensitive food webs and create situations of toxinbioaccumulation.Current materials used to clean spilled oil are subdivided into

three primary categories, including dispersants, sorbents, andsolidifiers.3 Dispersants emulsify the oil into small finely divideddroplets and disperse them into the environment.4,5 Sorbentsare typically powders that selectively absorb the oil via capillaryforces within a superhydrophobic matrix.6−8 Solidifiers gel thematerial on the surface of the water using either polymeric ormonomeric gelators.3,9,10 Ideally, any material used to treatspilled oil in our waterways must selectively remove the oilphase from water, be nontoxic, and allow oil to be reclaimed,and the material should be recyclable or reusable.3 A recentsurge in interest using amphiphilic molecular gelators to solidifyspilled oil includes sugar alcohols,3 amino acid amphiphiles,11

aromatic amino acids,9 and modified 12-hydroxy-N-alkylocta-decanamide.10 A different, novel approach to remove spilled oilfrom water is to use a xerogel, which will adsorb the oil fromwater. The separation ability of a xerogel arises from thehydrophobicity of the gelator, while the driving force ofadsorption is the capillary forces arising from the void volumeswithin the xerogel created by the fibrillar aggregates. Hence, theability to form a highly porous xerogel, using the simplestmethod, is central to developing a low-cost sorbent-basedxerogel to be used as an oil reclamation agent.

■ MATERIALS AND METHODSOrganogel and Xerogel Preparation. The samples were

prepared in 10 mL vials by adding 0.125 g of 12-hydroxyoctadecanoicacid (12-HSA) to 5 g of the organic solvent. Solvents were selected onthe basis of their ability to be gelled by 12-HSA and their high volatilityat atmosphere pressure. The three organic solvents tested wereacetonitrile, pentane, and diethyl ether. Each of the vials were cappedand heated to 80 °C for 20 min to allow for 12-HSA to dissolve intothe solvent. The sols were cooled and stored at 25 °C for 24 h,allowing the gel to form. After organogelation, the vials were uncappedunder atmospheric pressure for at least 24 h, allowing for the solventto evaporate from the organogel, leaving behind the xerogel. Sampleswere stored below the melting point (∼60 and 65 °C) of the xerogel inan incubator set at 40 °C.

Oil Spill Simulation. Two different setups were used to simulatethe oil spill. The first setup used 10 mL beakers each containing 3 g ofdistilled water and 3 g of 10W-40 motor oil. The second setup used 3g of sterile seawater (Sigma-Aldrich, lot 076k8431) and 3 g of dieselfuel (Exxon Mobile) or 3 g of regular gasoline (87 octane, ExxonMobile). The mass of the oil/fuel, water, and beaker system was thenweighed and recorded. A known amount of xerogel was placed ontothe top of a simulated oil spill and left quiescent for 1 h, after whichtime the sample was removed and blotted dry using Kimwipes toremove residual surface oil. To obtain the kinetics of adsorption, gelswere removed from the beaker and the weight of the gels was recordedevery min for the first 5 min, every 5 min from 5 to 30 min, and thenevery 15 min from 30 min to 1 h for the 10W-40 motor oil in distilledwater. From these data, the mass of the oil absorbed from the differentgels could be calculated, and the information was used to calculate thepercent of oil absorbed with respect to the mass of the xerogel. Asecond set of experiments determined the amount of diesel or gasolineadsorbed by the gel from seawater after 60 min and is presented as apercent weight change of the xerogel.

Microscopy. Brightfield micrographs were obtained by placing thesample onto a 25 × 75 × 1 mm glass slide, and a coverslip was appliedto the sample (Fisher Scientific, Pittsburgh, PA) after being heated to

Received: March 3, 2013Revised: April 9, 2013Published: April 16, 2013

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80 °C. Samples were imaged on a Linkham microscope (LinkhamScientific 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 obtainedby use of a Bruker HiStar area detector and an Enraf-Nonius FR571rotating anode X-ray generator equipped with a Rigaku Osmic mirroroptic 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 roomtemperature over a period of about 300 s. The sample−detectordistance was 10.0 cm, and the standard spatial calibration wasperformed at that distance. Scans were 4° wide in ω with a fixeddetector or Bragg angle (2θ) of 0° and fixed platform (f and c) anglesof 0° and 45°, respectively. In all cases, the count rate for the areadetector 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. Smalldeformation oscillatory rheology was employed to determine thecomplexes G′ and G″ within the linear viscoelastic region (LVR) of thegel at 1 Hz by carrying out a stress sweep at 10−500 Pa.

■ RESULTS AND DISCUSSION

Numerous solvents are structured into organogels when 12-HSA, a hydrogenated component of castor seed oil, is added inlow concentrations.12 The hydrogenated derivative of castorseed oil is enantiomerically pure, which allows for gelation atmuch lower concentrations.13 Dependent upon the solvent, thecritical gelation concentration (CGC) varies between 0.5% foralkanes (i.e., pentane, hexane, etc.) and 2.3% for nitriles (i.e.,ethyl nitrile and butyl nitrile).12 The difference in the CGCbetween alkanes and nitriles pertains to the polymorphic formof the molecular gel. In alkanes, a hexagonal subcell spacing(∼4.1 Å) and a multilamellar crystal morphology, with adistance between lamella greater than the bimolecular length of12-HSA, corresponded to molecular gels with the CGC lessthan 1 wt % (Figure 1A).14 12-HSA molecular gels in nitrileshave a much higher CGC corresponding to a triclinic parallelsubcell (∼4.6, 3.9, and 3.8 Å) and interdigitation in the lamella(Figure 1B).

Upon cooling the sol, gelator molecules phase-separate andinteract via non-covalent interactions; in the case of 12-HSA,dimerization occurs between the carboxylic acid groups andlongitudinal growth is promoted by the hydroxyl group atposition 12, forming hydrogen bonds between adjacent 12-HSA molecules (Figure 2).15−19 SAFiNs, in organic solvents,

require a meticulous balance between contrasting parameters,including solubility and those intermolecular forces that controlepitaxial 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 theformation of an organogel and the length of the fibrillaraggregates.21−23 12-HSA−acetonitrile molecular gels have adrastically different microstructure than the 12-HSA−pentanemicrostructure (Figure 3). In acetonitrile, numerous fibersradiate from a central nuclei, forming more numerous, shorterfibers compared to 12-HSA in pentane. The differences in thecrystalline structure will give rise to the difference in thenumber and size of the pores and, hence, the magnitude of thecapillary forces. Because the self-assembly process of organo-

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).

Figure 2. Schematic representation of the different polymorphicnanoscale interactions found in 12-HSA−pentane and 12-HSA−acetonitrile organogels.

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gelators requires such an important balance of solvent polarity,water content, and additives to effectively immobilize thecontinuous phase, our aim is to minimize the impact of thesolvent when containing spilled materials. Recently, a group ofsugar-derived gelators showed promise to contain oil spillsbecause of their robust ability to form gels in numerousenvironments.24,25 Our strategy is distinctively different, wherethe organogel is formed in a very controlled environment andthen the solvent is removed, creating a xerogel (Figure 3).Solvents were selected on the basis of their low vapor pressure

at atmospheric conditions and ability to form an organogel atlow concentrations. Using these criteria, three solvents wereselected, pentane, acteonitrile (ethyl nitrile), and diethyl ether.Diethyl ether formed a very fragile xerogel, which was difficultto handle, and, therefore, was not analyzed further. Theorganogels 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 theorganogel 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 ratethat the gels dried varied greatly and affected the structure ofthe xerogel. As seen in Figure 4, ethyl nitrile (acetonitrile) hadvery little collapse in the structure, maintaining a large volume,while pentane and more obviously diethyl ether collapsed intovery dense xerogels, which can be correlated to the differencesin the microstructure and ultimately differences in thenanostructure of the crystalline phase. Each of these systemswas placed into the simulated oil spill, and the mass gained after1 h (Figure 5) was measured to see if the capillary forces weresufficient to draw the hydrophobic solvent into the empty poresof the xerogels. The 12-HSA xerogel produced from diethylether did not adsorb solvent during the experiment; the xerogelproduced using pentane increased its mass by 93 ± 9.2 wt %,while the xerogel made from 12-HSA in acetronitrile increasedits mass by 387 ± 21 wt % in distilled water and 10W-40 motoroil (Figure 5). To ensure that the mass gained was from motoroil and not water, the 12-HSA xerogel produced usingacetronitrile was placed in water. The mass gained after 1 hwas 3.85 ± 0.5 wt % in distilled water and 5.23 ± 1.2 wt % inseawater, effectively demonstrating that the xerogel was phase-selective in adsorbing the oil. The 12-HSA xerogel made usingacetronitrile as the solvent was also tested on diesel in seawaterand 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. Thewidth of the micrograph is 120 μm.

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 wasperformed in triplicate for the weight gain after 1 h and the time-resolved adsorption.

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xerogel to the simulated spill, the xerogel increased in weight by459 ± 33 wt % in diesel and 583 ± 42 wt % in gasoline. Thisillustrates that the 12-HSA xerogel is not only effective but alsovery versatile in its applications of reclaiming spilled oil. It haspreviously been shown that similar materials (i.e., silicaaerogels) are far less effective (∼0.1 wt % gain) than 12-HSAxerogels (∼583 wt % gain).26 However, 12-HSA xerogelsabsorb similar amounts of oil as a Bregoil sponge (made fromwaste-wood fibers) (weight gain of ∼700 wt %),27 poly-propylene 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 asthe most advanced materials (i.e., superwetting cryptomelane-type manganese oxide nanowires),20 12-HSA xerogels are asselective for the apolar phase and are derived from much lowercost castor seed oil.It is obvious from the weight gained and oil recovered that

the xerogels made from 12-HSA and acetronitrile are veryeffective phase-selective sorbent materials. The major advantageof using the xerogel, as opposed to forming an organogel in theenvironment, is that this system is not dependent uponenvironmental factors. As long as the material of interest isapolar, our sorbent xerogel pellets will be an effective mode tocontain and remove the materials from the environment.

■ CONCLUSIONXerogels formed using 12-HSA and acetronitrile are highlyeffective sorbent materials capable of adsorbing apolar, spilledmaterials in aqueous environments. 12-HSA organogels inacetronitrile are more effective at producing xerogels, capable ofadsorbing spilled oil, than from pentane because of the highlybranched fibrillar networks established in acetonitrile moleculargels. This difference arises because of dissimilarities in thenetwork structure between 12-HSA in various solvents. Thesexerogels, being thermoreversible, allow for both the spilled oilto be reclaimed but also the gelator may be reused to engineernew xerogels for oil spill containment and cleanup.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: rogers@aesop.rutgers.edu.

NotesThe authors declare no competing financial interest.

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Figure 5. (A) Weight gain by the xerogel in the simulated oil spillversus time and (B) oil remaining on the water from the exposure of0.1 g of xerogel to 3 g of spilled oil.

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