Synthesis-Free Phase-Selective Gelator for Oil-Spill Remediation

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A Synthesis Free Phase-Selective Gelator for Oil-spill Remediation Yaowen Cui, Mei-Chun Li, Qinglin Wu, John A Pojman, and Daniel G Kuroda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10009 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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ACS Applied Materials & Interfaces

A Synthesis Free Phase-Selective Gelator for Oilspill Remediation Yaowen Cui,† Mei-Chun Li,‡ Qinglin Wu, ‡ John A. Pojman,† Daniel G. Kuroda*,† † Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA. ‡School of Renewable Natural Resources, Louisiana State University, Agricultural Center, Baton Rouge, Louisiana 70803, USA. *Address correspondence to [email protected] KEYWORDS Phase-selective gelator, deep eutectic solvent, DES lauric acid and Nmethylacetamide, lauric acid, N-methyl acetamide, oil gelation, oil-spill remediation.

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ABSTRACT A new deep eutectic solvent (DES) was developed as a phase-selective gelator for oil-spill remediation. The newly designed non-ionic DES is based on a combination of an amide (N-methylacetamide) and a long chain carboxylic acid (lauric acid) and does not require any synthetic procedure besides mixing. Our studies show that the DES works as gelator by forming a gel between lauric acid and the hydrocarbon, while the amide serves to form the DES and dissolves in water during the gelation process. In addition, the DES material has gelation properties comparable to those considered as state-of-the-art. Overall, the new developed material shows a promising future in oil recovery methodologies.


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Hydrocarbons are a value feedstock of the chemical industry. However, the utilization of hydrocarbons comes with its own drawbacks. For example, vast amounts of oil are spilled yearly into the sea.1 While not all oil spills have the same impact in the ecosystems, some of them have extensive negative effects on both flora and fauna of marine and land environments, which in some cases persist for years.1 Nowadays, the most utilized methods for oil spill remediation consist of a combination of recovery and dispersion. The dispersion methodology involves the use of oil dispersants to break large pools of oil into small droplets, such that the oil droplets can be degraded by microbes. Unfortunately, commonly used solvents in oil dispersants are low molecular weight petroleum distillates, which further adds to the release of hydrocarbons in the environment. In contrast, oil recovery consist of using mechanical or chemical approaches to skim the oil. The mechanical oil recovery utilizes devices, such as containment booms and skimmers, to retain and remove oil, and they are severely affected by wind and waves and cannot be used over large areas. Chemical recovery consists of utilizing a phase-selective gelator (PSG). PSGs works by preferentially forming a gel of the oil phase that can be skimmed from the sea surface. Although chemical recovery has been proposed as a viable method for oil spill remediation, its high cost and availability have made it economically not competitive. Chemical oil gelators work by creating entangled macroscopic molecular networks within the oil that turn the liquid oil into a gel.2 Different gelators have been synthesized from different organic precursors, such as sugars3-6 aromatic amino acids,7-11 cholesterol,12 and polysiloxane.13 In all these cases, the gelator synthesis involves one or more reactions,3-4, 7-8, 10 and so, the gelator preparation is limited by the reaction yield and its purification. In addition, synthetic gelators are solids that require a carrier solvent for high delivery efficiency.8 Thus, the chemical design of the gelator must take into consideration its solubility in a carrier solvent as well as in the oil. In


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summary, synthetic gelators usually have high commodity values and low availability due to their custom synthesis, which severely limits their applicability in large oils spills. Here, we present a new class of phase selective gelator based on deep eutectic solvents (DESs) that does not have the aforementioned drawbacks of synthetic gelators.

Scheme 1. Structure of lauric acid (left) and N-methylacetamide (right). DESs are formed when a mixture of two high melting point compounds melt into a solution with a fusion point lower than that of either individual component.14-17 DESs are obtained by mixing a hydrogen-bond acceptor (HBA) with a hydrogen-bond donor (HBD).16, 18 In a DES, the HBD solvates the HBA and partially screens certain interactions, such as hydrogen-bonds, which macroscopically translates to a freezing point depression of the mixture. DESs have been proposed and used in a wide variety of applications in areas such as electrochemistry,19-20 catalysis,21 carbon dioxide capture,22 and purification.23 Moreover, they have interesting physical chemical properties of conductivity,17 low vapor pressure,24-25 and extraordinary dissolution power, which further expand the pool of possible applications.26 DESs are very easy to synthesize because they are prepared by mixing two solid compounds.15, 18 This last distinction is what give DESs a considerable advantage over custom synthesized organo-gelators in oil spill remediation. For example, it is possible to choose chemical feedstock materials to eliminate the high cost and low availability drawbacks of custom synthesized organo-gelators. Moreover, the materials can be selected such that they have low toxicity. Finally, the ratio of the components can be modified to tailor the properties of the DES according to its use.15-16, 18


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Figure 1. DES mixed with the oil-water mixture. The samples consist of 1mL of water, 240 mg of DES, and different volumes of oil (i.e., 200 µL, 400 µL, 600 µL, 800 µL, 900 µL, 1000 µL, and 1500 µL from left to right). Here, we present a new DES composed of lauric acid (LA) and N-methylacetamide (NMA), shown in Scheme 1, as a gelling agent for oil spills. It is important to note that both compounds have low toxicity and are biodegradable.27-28 The mixture of LA-NMA has its eutectic point at a molar ratio of 1:4, respectively (see SI, Figure S1). However, we have selected a mixture with a molar ratio of 1:2 for the experiments presented here, which corresponds to a “solvated” DES. The selection of the 1:2 ratio is based on the higher molar ratio of the gelling agent (LA) and the fusion point being almost 20 oC below room temperature (see SI, Figure S1). The gelator properties of the presented DES were tested with different amounts of regular pump oil (200 µL - 1500 µL) in the presence of large quantities of water (~10 mL). The test results show that the DES can gel oil at a minimum ratio of 240 mg of DES per 1 mL of oil (Figure 1). Interestingly, small quantities of oil (200 µL) with 240 mg of DES produced many small floating particles, but the floating particles aggregate to one spherical-like and firm cluster when the amount of the oil is increased. Note that this formed clusters can be scooped out of the


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solution (see SI). As the ratio of oil is increased, the floating cluster turns into a soft gel for a DES to oil ratio closer to 240 mg per 1 mL, but its consistency is maintained. At DES to oil ratios lower than 240 mg/mL, the top layer loses its gel-like consistency, and the top phase behaves like a viscous liquid. The observed results are not altered if the oil-water sample is shaken before the addition of the DES, indicating that the method might also work in weathered oil. Furthermore, the water layer (bottom) becomes clear after standing overnight for all the tested conditions, which indicates that the percentage of oil in this phase is relatively low. Note that the presence of oil in the water phase is not detected by IR (see SI, Figure S2). The molecular process behind the oil gelation was studied via FTIR spectroscopy. We chose FTIR because each component of the mixture has various vibrational modes with contrasting associated frequencies (see SI, Figure S3); i.e., amide (1600-1700 cm-1), carboxylic acid (17001800 cm-1), hydrocarbons (2800-3000 cm-1), and water (3000-3500 cm-1). The FTIR spectra of the gel formed with a DES to oil ratio of 240 mg per mL, or higher, (Figure 2) show only two set of bands located at 1700 cm-1 and 2900 cm-1 corresponding to the LA and the oil, which indicates that the gel is primarily composed of oil and lauric acid. Moreover, the IR spectra also show that as the amount of oil is increased the carbonyl stretch band narrows and shifts to higher frequencies, which agrees with the acid having less hydrogen bonds per carbonyl and having a larger participation in the formation of the gel.29


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Figure 2. Normalized ATR-FTIR spectra of DES, LA, oil, and water-oil-DES mixtures for different volumes of oil. The pink, green, and dashed black lines correspond to the IR spectra of pure DES, LA, and oil, respectively. The FTIR spectra of the gel recovered from the different DES/water/oil mixture having 900 µL, 400 µL, and 200 µL of oil, 240 mg of DES, and 1mL of water are represented by the black, red, and blue lines, respectively. The lack of an amide I band in the FTIR spectra of the gel demonstrates that the NMA serves as a hydrogen bond donor to form the DES.18 However, the strong interaction between NMA and water favors its dissolution at the expense of leaving an insoluble layer of LA that is responsible for forming the gel with the oil. In addition, the FTIR spectrum of the top phase when the DES is directly added to pure water shows that the floating solid is mostly composed of LA (see SI, Figure S4), which confirms our previous hypothesis in which the formation of the gel is only related to oil–LA interaction. Finally, the addition of pure LA alone does not produce the same effect. Although the oil phase in this mixture shows signs of gelation, the addition of pure LA in


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the same amount as in the DES does not completely gels the oil (see SI, Figure S5). Moreover, some of the pure lauric acid remain as solid pieces within the oil phase. This last observation is related to the sponge-like morphology of the LA precipitate formed by the addition of DES to water compared to the crystalline structure of the pure LA solid as seen in Figure 3. The formation of a gel between LA and oil was further confirmed via rheology. The rheology measurements show that the frequency dependence of the storage modulus (G’) and the loss modulus (G’’) of a gel sample recovered from the DES-Oil-water mixture (240 mg/mL ratio) has a linear dependence of G’ and G’’ in the log-log plot (see SI), in which G’ > G’’. The results indicate that the material has viscoelastic properties of a “weak” gel, and it is in agreement with the idea of the gel being formed by a network of hydrogen bonded LAs in the dispersion medium created by the oil. Furthermore, the recovered supernatant formed at high DES-Oil ratios (1200 mg/mL) displays similar values for G’ and G’’, but much larger in magnitude that those observed for the gel. The increase of storage modulus with LA concentration shows that strength of the material increases with greater amounts of LA due to a higher number of hydrogen bonds, which increases the cross-linking in the gel. However, at 1200 mg/mL LA concentration the floating cluster behaves more like a solid than a gel (see SI) confirming the idea that at low oil concentrations the oil dispersion medium is not enough to produce a “pure” gel.


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Figure 3. Morphological differences between pure LA and LA precipitated from the DES. Top and middle panels display the structure of LA observed on a polarization microscope for the crystalline LA and that obtained from adding DES to water, respectively; where the left and right panels show the image under parallel and perpendicular light polarization. Bottom panels show the SEM images of the sponge-like structure of LA formed by adding the NMA-LA DES to water. The gelation due to the oil-LA interaction is tested by adding different amounts of pure LA to 1 mL of pure oil. At concentrations equal to or higher than 100 mg of LA per 1 mL of oil, the mixture forms a gel that is stable for weeks. In addition, the gel having 100 mg/mL of LA also presents viscoelastic properties expected for a “weak” gel formed by hydrogen bonds (see SI). Note that the 100 mg/mL (or 10% w/v) concentration is comparable to the amount needed for state of the art synthetic gelators.3, 6-7 The mechanism of gel formation is investigated via FTIR


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through the carbonyl stretch of the acid stretch located at ~1700 cm-1 (Figure 4). At concentrations lower than 100 mg/mL, LA does not form a gel, and its FTIR spectrum exhibits a single band in the carbonyl stretch region, which corresponds to LA dissolved in oil (see Scheme 2). However, at higher LA concentrations the carbonyl stretch of the acid starts to show a low frequency band (~1680 cm-1) that increases as the concentration of the acid is increased. While the carbonyl stretch at 1715 cm-1 corresponds to the “free” acid in the oil, the shoulder at 1680 cm-1 appearing at higher concentrations of LA is assigned to the carbonyl groups of the acid being hydrogen bonded to other acids (see Scheme 2). This is in agreement with the expected carbonyl stretch behavior and it is confirmed in the FTIR spectra of LA in different solvents (see SI, Figure S6). The presence of the low frequency shoulder confirms the mechanism of gel formation since the acid is cross-linked with other acids via hydrogen bonds to form a network in which the oil is anchored by its interaction with the LA hydrocarbon tails (see Scheme 2). Moreover, at very high LA concentrations (1200 mg/mL), the FTIR of the supernatant in the 1700 cm-1 region is very similar to that of pure LA suggesting that the floating material is not composed of “pure” gel, but of a mixture of solid LA and gelled oil (see Scheme 2). This result is in agreement with the rheology measurements as previously described.


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Scheme 2. Cartoon representation of the gel formation mechanism of LA-oil. Bottom structures show the change in the hydrogen bonds as function of lauric acid concentration. The proposed gelation mechanism in which LA acts as cross linkers via hydrogen bonds between the carboxylates groups is supported by the rheology measurements, where an increase in crossover angular frequency for the G’ and G’’ is observed when the LA concentration is higher in the gel (see SI). The crossover frequency represents the average relaxation time of entanglements among gel cross linkers; thus, higher concentration of LA (cross linker) produced more hydrogen bonds (entanglement between cross-linkers). The observed result further supports the idea that LA acts as a cross linker (Scheme 2) and is responsible for the formation of the gel. In addition, the gelation of the LA-oil mixture is verified by scanning electron microscopy (SEM). The SEM images (Figure 4) show the layered structure of the gel observed when scanning thick portions of the sample and its heterogeneous microstructure when the sample thickness is very thin.


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Figure 4. Gel formation between oil and LA. Top panels show the formation of the gel as a function of the LA concentration. Middle panels shows the corresponding IR spectra in the carbonyl stretch region for the LA-oil mixtures. The concentrations of LA in the oil is 50 mg/mL (red line), 100 mg/mL (blue line), 150 mg/mL (pink line), and 200 mg/mL (green line) from left to right. The spectra of LA (black line) and oil (dashed black) are presented as reference. Bottom panel shows the SEM image of the gel form between 1 mL of oil and 100 mg of LA. It is now clear that the DES works as a gelator because the LA acts as gelling agent while the NMA serves as a carrier solvent. However, it is not clear if the DES is a solvent or if it can be


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replaced by more conventional solvents like ethanol. To this end, the use of DES as a carrier solvent was compared with ethanol. In this case, the molar ratio of ethanol to LA used is 4:1, respectively, due to solubility limits of LA in ethanol at room temperature which makes any lower ratio a solid under the same conditions. As expected, LA-ethanol mixtures also produce a gel when mixed with oil and water. Moreover, the FTIR spectrum of the formed gel shows the same expected behavior; i.e., the presence of shoulder at the low frequency side of the carbonyl stretch of the acid (see SI, Figure S7). However, the overall amount of LA needed to produce the same gel with the ethanol solution is larger than that of the DES, which is estimated to be on the order of 30%. The larger amount of LA needed compared to the DES is likely due to the lower polarity (dielectric constant) of ethanol compared to NMA, which helps to dissolve the acid in water. The solubility phenomenon is observed by comparing the supernatant produced by the DES and the ethanol solution (see SI, Figure S8). In the former case, the acid shows a very small shoulder on its low frequency side indicating a relatively low interaction between the water and the acid and the absence of NMA in the solid. In contrast, the supernatant produced by the ethanol solution shows a significant shoulder displaying the interaction between ethanol and LA that makes it more soluble in water. Our previous experiments show that the solvated DES is a better carrier of LA than solutions with ethanol because lower amounts of LA are needed to achieve the same gelator effect. However, even in the case that both DES and ethanol solutions produce the same effect per mol of LA added, there is an intrinsic advantage of the DES due to its higher density, which further supports the selection of the DES as a carrier. The densities of the DES and the ethanol solution are 0.95 g/mL and 0.86 g/mL, respectively; and their corresponding LA molar concentration are


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2.7 M and 2.2 M. Thus, the DES will deliver more than 20% moles of LA compared to an equivalent volume of a LA ethanol solution. A new gelator for oil-spill remediation, which selectively gels the oil phase, is presented. Compared with the traditional phase selective gelators, this new gelator does not require the use of organic synthesis for its preparation. Moreover, our gelator uses feedstock chemicals, which makes it inexpensive and readily available. In addition, the selected components are biodegradable and much less toxic than currently proposed or used ones. Finally, it is shown that the performance of our gelator is similar to those considered as state-of-the-art. ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge on the ACS Publications website at DOI: The following files are available free of charge. Materials and methods, and Figures S1 to S8 (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: (+1) 225-578-1780 ORCID Daniel G. Kuroda: 0000-0002-4752-7024 John A. Pojman: 0000-0003-4788-8767 Qingli Wu: 0000-0001-5256-4199 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We would like to thank Dr. Kristen Fulfer for helping with the manuscript and Dr. Clayton Loehn for helping with the SEM experiments. The current work was funded in part by Louisiana Board of Regents - Research Competitiveness Subprogram (RCS) and start-up funds provided by the Department of Chemistry.

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Synthesis-Free Phase-Selective Gelator for Oil-Spill Remediation

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