Low molecular weight gels: potential in remediation of crude oil spillage and recovery

Abstract

A novel amino acid based gelator compound was developed for the phase-selective gelation of hydrocarbon solvents in a biphasic mixture with water/sea water including crude oils. With the crude oils of varying API gravity ranging from 18.7 to 40.5, the organogelator exhibited an uptake capacity ranging from 50 to 100 times. Thus, a wide range of applicabilities of the gelator to contain and mitigate oil spills of any type of crude oil covering the entire crude basket around the globe is proven for the first time.

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Introduction

The recent oil spill in the Gulf of Mexico has initiated researchers around the globe to explore different possible routes to contain oil spills to prevent the marine ecosystem being destroyed. It is estimated that the total discharge of crude oil into the sea was ∼4.9 million barrels.¹ The respective oil company had to pay ∼$18.7 billion as compensation which was the largest corporate settlement in U.S. history.² The oil spillage has become a common problem from well to wheel i.e., from drilling of oil to different stages of transportation till the oil reaches the refinery for further processing and subsequently from the vehicles or during the transfer (loading/unloading). Development of new methods for containing oil spills is the need of the hour both in terms of a valuable economy and also from the environmental view point. In this regard, low molecular weight organogelators (LMOGs),^3–11 have attracted much attention in recent times owing to their excellent selective gelation properties towards the oil phase compared to the aqueous/water phase^12–22 and many other applications.^23–35 Small molecule gelators are recognized for their ability to immobilize solvent molecules into their self-assembled fibrillar networks (SAFINs).^3–11 Non-covalent interactions such as electrostatic, dipole–dipole, hydrogen bonding (H-bonding), π–π stacking, and van der Waals interactions play a crucial role in the self-assembled gelation process.^3–11 This thorough understanding of the supramolecular gelation mechanism led to a noteworthy increase in rationally designed small molecule gelators with desired functionality to serve a specific task. In this regard, amino acid-based LMOGs^31–35 are most frequently used due to their easy availability, low cost, simple and well established synthetic methodologies. Research involving these materials has increased and gained remarkable interests since the last decade owing to their potential applications in oil spill recovery.^12–22 Thus, the design and development of novel organogelators with high affinity for a wide spectrum of organic solvents, more significantly, their capability of exhibiting phase selective gelation properties is always of high demand.^27,28 However for the realistic application of the LMOGs in oil spill/recovery, the gelator must be capable of selectively rigidifying the crude oil and other distillates such as cracked run naphtha (CRN), straight run naphtha (SRN), diesel and so forth from a biphasic mixture of the hydrocarbons with sea water. Most of the previous reports have illustrated examples of phase selective gelation of organic solvents such as toluene, xylene, hexane etc. (model experiments).^12–22 To the best of our knowledge, there are only very few reports focusing on the gelating abilities of refinery products (kerosene and diesel).^36–40 The number of reports underlining the phase selective gelation of crude oil from a biphasic mixture using organogelators in literature are very few.^41–43 There are no gelation studies reported on biphasic mixture of crude oil/refinery products with sea-water on the peptide-based organogelator. The gelating ability with different crude oils or refinery products will give more insights into the oil spill mimicking applications.

Herein, the present work reports a dipeptide-based amphiphilic LMOG (Fig. 1) for selective gelation of crude oils of varying API (American petroleum institute) gravities and other refinery products (SRN, CRN and diesel) from a biphasic mixture of oil/water.

Fig. 1 Structure of the gelator.

Result and discussion

Gelation studies

In the present work, a dipeptide-based amphiphilic LMOG as presented in Fig. 1 was synthesized and examined for phase selective gelation of refinery products (SRN, CRN and diesel) and different crude oils. The gelator compound comprising a long hydrocarbon tail at the N terminus (hydrophobic group), followed by two amino acid residues and one free carboxylic acid group (COOH) at the C terminus (hydrophilic segment) was synthesized following the procedure as reported earlier.³⁵

The elation ability of the compound was examined in different solvents (n-paraffins, alkene, aromatics), refinery fractions like SRN, CRN and diesel, crude and vegetable oils by ‘stable to inversion of glass vial’ (Table 1). The MGC (minimum gelation concentration) of the gelator in different solvents was found to vary from 0.6–2% w/v (Table 1).

Table 1 Uptake of different hydrocarbons including crude oil by the gelator

Solvent system	MGC (% w/v)	MUC^a
a MUC: minimum uptake capability: defined as the ratio of weight of the hydrocarbon consumed by the weight of organogelator (w/w).
n-Hexane	1.1	90
Cyclohexane	0.7	142
Octane	0.8	125
Decane	0.75	133
Dodecane	0.7	142
Hexadecane	0.6	166
1-Octene	0.9	111
Toluene	0.7	142
Xylene	0.7	142
CRN	1	100
SRN	0.8	128
Diesel	1	100
Crude oil (C4)	2	50
Vegetable oil	0.9	111

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The gelator exhibited comparable gelation properties for higher paraffins, alkenes and aromatic solvents (0.7–0.9% w/v, Table 1). In case of refinery/petroleum products, highest gelation efficiency was observed for SRN (0.8% w/v). However, the MGC for crude oil was found to be higher (2% w/v, Table 1) compared to other petroleum fractions. Higher MGC value for crude oil may be attributed to its complex composition having a mixture of hydrocarbons including inorganic impurities such as S, V, Ni and so forth. Furthermore, the gelator was found to efficiently gelate vegetable oil with MGC 0.9% w/v (Table 1) which was in found to be in between that of SRN and diesel. Thus, the gelator has absorbed CRN or diesel about 100 times its original weight, represented by the MUC values in Table 1. The uptake of the refinery fractions followed the order: SRN > CRN ∼ diesel > crude. The uptake of vegetable oil (111 times) was in between SRN and diesel. The images of the gels obtained with different refinery fractions (SRN, CRN and diesel), crude and vegetable oils in biphasic mixture with water are presented in Fig. 2A. The inversion images of the gels obtained are presented to demonstrate the integrity of the gels.

Fig. 2 Photographs of gels obtained with (A) with different oils in a biphasic mixture with water, (B) with the water phase over SRN using sodium salt of the gelator.

The gelator was not soluble in water. Also it did not exhibit any gelation ability for the polar organic solvents such as alcohols (such as methanol, ethanol, propanol etc.), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) up to 5% (w/v). Nevertheless, its property could be tuned from hydrophobic to hydrophilic nature, i.e., the water selective gelation ability, with a small change in the chemical structure. To substantiate this fact, the gelator was converted to its sodium salt which exhibited the reverse gelation property of capturing water/aqueous phase in presence of hydrocarbon mixture as presented in Fig. 2B. This shows the phase selective gelation of aqueous phase in preference to one of the refinery products, SRN, paving way for wide variety of applications involving water/oil mixtures for the reverse gelation process. Selective gelation of petroleum products from a biphasic mixture of oil and water/sea water was performed (Table 2).

Table 2 Gelation abilities of gelator compound in various biphasic mixtures (oil volume: 0.5 mL, aq. phase volume: 0.5 mL)

Hydrocarbon–water mixture	Gelator quantity (mg)	MGC (%, w/v)	MUC
Crude–water	10	2	50
CRN–water	6	1.2	83
SRN–water	5	1	100
Diesel–water	6	1.2	83
Crude–sea water	10	2	50
CRN–sea water	6	1.2	83
SRN–sea water	5	1	100
Diesel–sea water	6	1.2	80

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The gelator was found to immobilize the oil phase selectively leaving the water/sea water phase in its fluid state during gelation experiments. It was quite remarkable that even under highly saline conditions, negligible changes/effect on gelation properties (MCG & MUC) were observed indicating compatibility of the organogelator under variable conditions. MGCs for CRN, SRN and diesel were increased only by 0.2% (w/v) from their respective individual/single phase studies. However, the MGC remained unchanged for crude oil. Gelation ability of the dipeptide compound under harsh conditions has encouraged us to study its practical application in the oil-spill and recovery using crude oils. Crude oils are classified according to the specific gravity based on American Petroleum Institute (API) Standard. Light oil is defined as having an API gravity greater than 31.1°, medium oil is defined as having API gravity between 22.3 and 31.3°; heavy oil has API gravity between 10 and 22.3°. Thus, it could be understood that the crude oils with varying API gravities have different compositions. The crude with lower API has heavier compounds such as resins and asphaltenes in larger quantities compared to the crude with higher API gravity. In this regard, experiments were conducted with crudes with varying API gravities ranging from very low API (C5, 18.8°) to high API (C1, 40.5°) to investigate the effect of crude composition over gelation ability.

The gels obtained over different crudes in a biphasic mixture with sea water are presented in Fig. 3A as inversion images. The effect of API gravity (crude composition) on the minimum uptake capability (MGC) is plotted and presented in Fig. 3B. From the plot (Fig. 3B), it could be envisaged that the uptake capability of lighter crude (higher API) was higher than that of heavier crude (lower API) and the uptake capability decreased with increase in API gravity. The results indicated that composition of crude played a major role influencing the uptake during the gelation process. The ease of formation of gel with lighter crude could be attributed to its higher paraffinic nature. Nevertheless, the gelator was found to be efficient for all the five crudes under studied and MGC for very low API (heaviest) crude of 2.1 wt% is very significant. This study clearly indicated that the gelator could be used for the most of the crudes covering the wide spectrum of crude basket available from different parts of the globe.

Fig. 3 (A) Gelation with different crude oils–sea water biphasic mixture denoted by C1 to C5 (C5: API 18.8, C4: API 27.1; C3: API 28.1, C2: API 35.5; and C1: API 40.5), (B) variation in MGC with API gravity of crude oils, (C) photograph of crude oil layer over water before gelation and (D) after gelation.

Oil spill recovery by phase selective gelation of oil phase in presence of water is generally performed either by (i) dissolving the sample in oil by means of heating and again cooling to form gel³¹ or by (ii) dissolving the gelator in soluble solvents e.g. alcohols, ethers such as tetrahydrofuran (THF) and adding the solution over oil.^34,38 However, both processes have certain limitations as the heating-cooling processes is practically impossible on the sea level and water miscible alcohol and ether solvents would be incompatible or even detrimental for marine eco-system.⁴³ Hence, considering the limitations different gelation process mainly applying xerogels i.e. dried gels were reported.⁴⁴ However the use of xerogels have certain limitations owing to their low absorption capabilities.³³ Other alternative routes are dissolution of the gelator in the same liquid that to be gelated or in a suitable solvent having concentration much higher than MGC for both cases.^26,32 Here, experiments were conducted to eliminate solubility issues and to focus on more realistic application in oil spillage recovery from sea water. The gelator was dissolved in a suitable organic solvent such as toluene, in excess, and the hot solution was added over crude oil–sea water mixture. Within a few minutes, crude oil along with the aromatic solvent were transformed to the gel phase. The prototype experiment is depicted in Fig. 3C and D, where 3C is the image of the crude oil before gelation and 3D is the image after gelation. The gel phase was collected from the water surface by scooping for further recovery of oil. The effectiveness of gelator towards the recovery of oil from gel phase was performed using diesel gel and quantitative recovery of ∼95% of diesel was achieved. Additionally, the gel obtained from C1 crude oil was subjected to recovery experiment. After distillation, the recovery of the oil fraction was found to be 65 ± 5% up to boiling range 350 °C.

Microscopic study

The morphology of the native supramolecular organogels was characterized by scanning electron microscopy (SEM) and the results are presented in Fig. 4. The SEM images of the xerogels (dried gels) prepared from various petroleum fractions revealed the formation of self-assembled fibrillar networks. Similar properties were observed in earlier literature.^31–38

Fig. 4 SEM images of xerogels obtained from (A) CRN, (B) diesel and (C) SRN.

For example, xerogel from CRN showed the presence of entangled fibrillar networks with fiber diameter of 50–100 nm and length of few micrometres. Few fibers were associated with each other to form thicker fibers of ∼500 nm to ∼1 nm (Fig. 4A) and trapped solvent via surface tension. The xerogel (obtained using diesel) also exhibited an entangled fibrillar network with a thickness of 100–200 nm and lengths of several micrometers (Fig. 4B). The SEM image of xerogel from SRN-gel also confirmed the presence of similar fibrillar networks (Fig. 4C). Thus, the self-assembled fibrillar networks entrapped solvent molecules by means of capillary forces and results in the formation of supramolecular gel.⁴⁵

FTIR study

FT-IR spectroscopy was used for studying the diverse non-covalent interactions involved in physical gelation.^31–38 FT-IR spectra of the gelator, xerogels prepared from crude oil and other distillates, and the non-self-assembled state in chloroform were recorded. In chloroform solution, the neat compound displayed characteristic non-hydrogen-bonded transmittance bands centered at ∼3417, ∼1725, ∼1645, and 1521 cm⁻¹ for amide ν_N–H (amide A), carboxylic acid ν_C=O, amide ν_C=O (amide I), and ν_N–H (amide II), respectively (Fig. 5A).

Fig. 5 FT-IR spectra of gelator (A) in chloroform and (B) xerogel obtained from diesel and CRN.

The IR spectra of xerogels prepared from diesel, and CRN exhibited transmission bands centered at 3285, 1710, 1637, and ∼1538 cm⁻¹. These bands are characteristic for intermolecular hydrogen-bonded amide N–H stretching, C=O stretching (carboxylic acid), C=O stretching (amide I), and N–H bending (amide II), respectively. The shifts in IR bands in the respective finger-print regions confirm the presence of strong intermolecular hydrogen bonding between the amide and carboxylic acid group in the supramolecular gel network. The shifting of antisymmetric (ν_as) and symmetric (ν_s) stretching frequency bands of methylene groups of long hydrocarbon chain were observed from 2926 cm⁻¹ and 2855 cm⁻¹ in the solution phase to 2918 cm⁻¹ and 2850 cm⁻¹ in the gel state, respectively. This particular shift in the CH₂ stretching frequency indicated the decrease in the fluidity of the hydrophobic chains due to the formation of strong aggregates via van der Waals interactions.

Rheological study

The organogels were subjected to rheological studies in oscillatory mode to investigate their stiffness. These experiments provide information about two main parameters namely storage modulus (G′) and loss modulus (G′′) where G′ corresponds to the ability of the deformed material to store energy and G′′ signifies the flow behaviour of the material under stress. Both frequency dependent and amplitude dependent rheological experiments were performed taking 2% w/v gels to study their viscoelastic properties and the results are presented in Fig. 6. In general, the gel state is signified by G′ > G′′ and in the sol state is signified by G′ < G′′; hence value of G′ and G′′ dictate the state of the material under stress. In a typical frequency sweep experiment, the variations of storage modulus (G′) and loss modulus (G′′) was monitored as a function of applied angular frequency under a constant strain 0.01%.¹⁷ For the organogels under study G′ was found to be higher than G′′ and they did not cross each other throughout the experimental region (10 to 500 rad s⁻¹) (Fig. 6A). This observation suggests the formation of viscoelastic gel materials^34,35 by the organogelator with different oils. The storage modulus of all the organogels was found to be in the order of 10–100 kPa which indicated the higher mechanical strength of the organogels and good tolerance towards the external forces.^46–48

Fig. 6 Dynamic rheology of the organogels obtained from different oils: (A) as a function of angular frequency and (B) as a function of oscillatory stress at 25 °C.

The highest storage modulus for SRN gel indicated best gelation efficiency among three oil samples. We have also carried out oscillatory stress sweep experiments for the organogels where G′ and G′′ were measured as a function of oscillatory stress at a constant oscillation frequency.⁴⁸ with a gradual increase in applied stress, both G′ and G′′ was found to decrease. Higher value of G′ compared to that of G′′ again signified the viscoelastic behaviour of the gels. It is evident from the Fig. 6B that the mechanical strength of the gels followed the trend: SRN > diesel > crude oil, as also inferred from the frequency sweep experiments.

Conclusions

In conclusion, we report, a dipeptide-based molecular organogelator compound, a soft organic material, with remarkable gelation ability for different petroleum fractions for the mitigating/containing the oil spillage and expanding its applications in a broader perspective. The organic soft material was capable of gelating different solvents, vegetable oil, different refinery fractions (SRN, CRN and diesel) and several crude oils with varying APIs, either as individual or as a mixture with water or sea-water. The uptake capacity ranged from 50–125 times for different petroleum products and even better capacity was observed for the hydrophobic organic solvents. The uptake for lighter crude oil was higher compared to the heavy crude oil, however a minimum uptake capacity of 50 times was observed for all crude oils. Strong intermolecular hydrogen bonding leading to self-assembled fibrillar network was the prime factor for the gel formation as evidenced from IR and SEM studies. The mechanical strengths of the gels calculated from rheology studies follow the order: SRN > diesel > crude. These observations were confirmed by the MGC and MUC values obtained for the different oils. Thus, lower MGC (Higher MUC), the mechanical strength of the gel was higher. The same gelator with slight structural modification could be used for the removal of water/aqueous phase from oil–water mixture. The potential and scope of using these organogelator for the oil spillage remediation and recovery are enormous. The gelation with refinery fractions, vegetable oil and crude oils with varying APIs covering almost the entire crude basket in a biphasic mixture with sea water and the reverse gelation is reported for the first time thereby widening the scope of applications of these soft organic materials.

Experimental procedure

Materials

Reagent- or analytical grade chemicals were purchased from Sigma-Aldrich and were used without further modifications. Synthesis grade solvents were purchased from Merck-India and dried according to standard procedures whenever required.

Instruments and characterization

¹H NMR spectra were recorded on Bruker Avance500 MHz spectrometer. Deuteriated solvents for NMR experiments were obtained from Aldrich Chemical Co. Mass spectrometric data were acquired by an electron spray ionization (ESI) technique on a Q-tof-micro quadruple mass spectrometer (Micromass). Elemental analyses were performed on Perkin-Elmer 2400 CHN analyzer. FT-IR spectroscopy was performed using Nicolate 380 FT-IR spectrophotometer (Thermo Scientific) making pallet with KBr.

Experimental procedure

Amino acid based amphiphiles were synthesized by conventional solution phase methodology. Briefly, the methyl ester of L-methionine was coupled with palmitoyl chloride in dry chloroform in presence of triethyhl amine (Et₃N). The ester-protected long-chain amide was further purified through column chromatography using 60–120 mesh silica gel and ethyl acetate/hexane as the eluent. The ester was hydrolyzed using 1 N NaOH (1.1 equiv.) in MeOH for 6 h at 25 °C with constant stirring. Solvents were removed on a rotary evaporator, and the mixture was extracted in dichloromethane (DCM) followed by acidification with 1 N HCl to get the corresponding carboxylic acid. The product acid was then coupled with another methyl ester protected L-methionine using dicyclohexylcarbodiimide (DCC, 1 equiv.) in dry DCM in presence of 1-hydroxybenzotriazole (HOBT, 1 equiv.) and 4-N,N-(dimethyl)aminopyridine (DMAP, 1 equiv.). The purified ester was obtained by column chromatography using 60–120 mesh silica gel and ethyl acetate/toluene as the eluent. The ester was then subjected to hydrolysis using 1 N NaOH (1.1 equiv.) in MeOH for 6 h to obtain the final carboxylic acid product following the same procedure as described above. The acid was purified by column chromatography using methanol/chloroform as the eluent. Yield obtained was 67%. The product was characterized by ¹H and ¹³C NMR and mass spectrometry. ¹H NMR (500 MHz, CDCl₃, rt): δ = 7.33 [d, 1H, J = 7 Hz, CONHCH], 6.55 [d, 1H, J = 7 Hz, CONHCH], 4.74–4.68 [m, 2H, NHCH(CH₂)CO], 2.58–2.52 [m, 4H, CHCH₂CH₂S], 2.22 [t, 2H, J = 7.5 Hz, CHCH₂CH₂S], 2.09 [s, 6H, SCH₃], 2.07 [t, 2H, J = 7.5 Hz, CHCH₂CH₂S], 1.96 [t, 2H, J = 6.5 Hz, COCH₂CH₂], 1.63–1.61 [m, 2H, COCH₂CH₂], 1.28–1.24 [m, 24H, CH₃(CH₂)₁₂CH₂], 0.87 [t, 3H, J = 6 Hz, CH₃(CH₂)₁₂]; ¹³C NMR (125 MHz, CDCl₃, rt): δ = 173.38, 155.45, 155.35, 56.62, 54.20, 32.08, 31.98, 29.61, 29.15, 28.94, 28.89, 28.36, 25.07, 25.00, 24.83, 14.80; IR (KBr pellet, ν_max/cm⁻¹):3285, 3037, 2917, 2849, 1709, 1642, 15 410, 1470, 1436, 1278, 1225, 1113, 936, 719; ESI-MS: m/z: 541.4296 (M⁺ = C₂₆H₅₀N₂O₄S₂Na⁺), m/z (calculated): 541.3065 (M⁺ = C₂₆H₅₀N₂O₄S₂Na⁺); E.A: calculated for C₂₆H₅₀N₂O₄S₂: C, 60.19; H, 9.71; N, 5.40. Found: 60.10; 9.79; 5.33.

Gelation experiment

Gelation property of the gelator in different solvents was studied by conventional heating-cooling process. In a typical procedure, 0.5 mL of a solvent/oil was taken in a glass vial with an internal diameter of 10 mm and required amount of gelator was added to the solvent/oil. The mixture was warmed gently to dissolve the solid compound. The solution was allowed to cool slowly to room temperature without disturbance. After few minutes, the solid aggregate mass was found to be stable to inversion of the glass vial and then the compound was recognized to form a gel.

Microscopy study

SEM experiments were performed on a Zeiss Scanning Microscope Gemini SEM300 with gold coating. A drop of gel was placed on a piece of cover slip and desiccated for few hours under vacuum before imaging.

Rheological studies

The rheological experiments were carried out in cone and plate geometry (diameter 40 mm) on the rheometer plate using an Anton Paar MCR 302 rheometer. The gels were scooped on the rheometer plate so that there was no air gap with the cone. Both frequency and oscillatory stress sweep experiment was performed at 25 °C.

Phase selective gelation study and recovery

Initially the gelator solution was prepared by dissolving 25 mg of gelator in toluene. This aliquot was added to a mixture of crude oil/different refinery distillates and water (having 0.5 mL of each solvent) in a glass vial. The mixture was then kept undisturbed. In the course of the process, the oil phase was converted to a gel in less than a minute, keeping the water phase intact.

Phase-selective gelation of C5 crude oil was also performed on a thin layer of oil floating on a large pool of water (in a Petri dish) (ESI†).

For recovery study, 100 mL of crude oil and 100 mL of water was taken in a 500 mL beaker. Required amount of gelator dissolved in toluene was added. The resulting gel was scooped out, and the oil phase was recovered through vacuum distillation.

Acknowledgements

Crude testing lab, Corporate R&D HPCL, is acknowledged for providing crude oils with varying APIs. The support and encouragement from HPCL Management is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Gelation process is given in the supporting information as a video file. See DOI: 10.1039/c6ra10462b

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