Arabinose based gelators: rheological characterization of the gels and phase selective organogelation of crude-oil

Abstract

Oil-spills, whether marine or terrestrial, result in major devastation to the environment and ecosystem, loss of habitat and damage to the economy. While inevitable in the current scenario, oil spillages necessitate prompt remedial action to limit the damage caused. A recent strategy towards this is to congeal the spilled oil into a gel through gelation (for terrestrial oil-spills) or through Phase Selective Organogelation (PSOG) for marine oil-spills which can stem the spread of the oil and also provide for the easy removal and reclamation of the gelled oil. Herein, we report two triazolylarabinoside derivatives that are stable, completely insoluble in water and can carry out the gelation as well as the PSOG of hydrocarbon based organic solvents, petrol, diesel and also crude-oil. The gels have been characterized thoroughly by optical microscopy, FESEM, AFM, XRD and rheological experiments. The gelation of crude-oil has also been unequivocally established via rheological studies. The easily synthesized and cheap triazolylarabinosides could provide simple gelators for the remediation of marine/terrestrial oil-spills.

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Introduction

Studies on a relatively new class of soft materials based on the self-assembly properties of a class of small molecules called Low Molecular Weight Gelators (LMWGs) have drawn immense attention from researchers recently due to the potentially wide applications of such materials.^1–15 Although structurally very diverse, many of the LMWGs are based on some specific motifs such as steroids,¹⁶ amino acids and peptides,^17,18 polyaromatic hydrocarbons and conjugated polyenes,⁵ ureas,¹⁹ and carbohydrates.²⁰ Each of these structural moieties is associated with some unique properties that result in the self-assembly of the molecules resulting in the formation of crosslinked networks for gelation. For example, with steroidal gelators, the flat topology of the molecules result in the self-assembly through van der Waals forces, while in polyaromatic hydrocarbons the self-assembly occurs through π–π stacking. Again, in amino acid, peptide and carbohydrate based gelators, the aggregation is the result of H-bonding. Often however, the gelators include more than one of these structural features that enable stronger self-assembly leading to stronger gels. Amongst carbohydrates, most of the gelators contain two or more free hydroxyl groups which enables them to form the aggregates necessary for gelation through H-bonding based self-assembly. Structurally, most of the carbohydrate based LMWGs are based on glucose, galactose and glucosamine; although, a few examples of organogelators derived from mannose and xylose have also been reported.²⁰ The unique advantage that carbohydrate based LMWGs offer is that they are easily synthesized from commercially available starting materials through only a few synthetic steps most often involving protection/deprotection reactions.

With respect to the applications of such supramolecular gels, a recent one is towards the remediation of environmental problems associated with human activities.²¹ Amongst such environmental issues that could be potentially remedied by the use of gelators are marine oil-spills which pose a severe threat to the environment and to a lesser extent is a cause of major financial impositions on the oil producing companies. For example, the oil-spill resulting from the explosion on the Deepwater Horizon oil rig (the BP oil-spill) in 2010 in the Gulf of Mexico resulted in massive destruction of the ecosystem, habitat and tourism industry in spite of the massive remediation activity that was sustained for a long time.²² In addition to the immediate aftermath,²³ the environmental damage caused by the disaster can be estimated from the fact that toxic effects could be detected in aquatic life as late as in 2013 (ref. 24) and on the coastal life even more recently.²⁵ Again, the environmental impact is just not limited to the oceanic and coastal areas but also spills on to the atmosphere due to the evaporation of the volatile fractions of the crude-oil. In addition, the oil-spill also resulted in financial penalties to BP. In the light of this and many other recent oil-spills, it is essential that effective strategies be developed to remediate oil-spills as quickly and as efficiently as possible to mitigate the impact of the damage caused. A very recent strategy towards this objective is to convert the marine surface upon gelation as a solid or semi-solid (Fig. 1).²⁶ For such gelation of oil, several features have to be taken into account such as the (i) gelation in the presence of water (Phase Selective Organogelation, PSOG), (ii) gelation time, (iii) the mechanical stability of the gelled oil (iv) non-toxic and environmental friendly nature of the gelator (v) its easy accessibility via a short synthetic sequence and (vi) the stability, recoverability and the recyclability of the gelator.

Fig. 1 An illustration depicting the environmental impact of marine oil-spill and its remediation based on the concept of Phase Selective Organogelation (PSOG) of crude-oil.

Many gelators for phase selective gelation of various distilled oil fractions such as petrol, diesel, kerosene, etc. in the presence of water have been reported by several authors^27–39 since the initial report of Bhattacharya and co-workers.⁴⁰ However, as often the case is, the major oil-spills that have caused havoc on the environment are those that have to do with the spill of crude-oil. We reported the first instance of the PSOG of crude-oil using the arabinose based LMOG 1a.⁴¹ Very recently, Sureshan and co-workers⁴² also reported a similar benzylidine protected sugar based LMOG 1b for the PSOG of crude-oil. Although, very effective towards the PSOG of crude-oil in aqueous environments, both the compounds 1a and 1b suffer from a major hindrance in the form of the acid labile acetonide (for 1a) and benzylidine (for 1b) protecting groups which could render them ineffective for the PSOG under acidic conditions, for example, in cases dealing with the gelation of sour crude which may contain a fair amount of hydrogen sulphide. Recently, Zeng and co-workers also reported Fmoc protected amino acid derivatives (2) as potential gelators for PSOG of crude oil.⁴³ In this work, we present a second generation arabinose based gelator with a triazolyl group that is very effective for the gelation of crude-oil including its PSOG in the presence of water (Fig. 2).

Fig. 2 Phase selective crude-oil gelators reported (a) previously and (b) in the current work.

Results and discussion

Synthesis of the triazolylarabinosides and preliminary gelation studies

We started towards our objective of designing an acid stable gelator by considering the option of incorporating a triazolyl glycoside on the arabinose moiety since it has been previously reported to be effective when appended to various sugar monosaccharides.^44,45 We synthesized a series of triazolyarabinosides (3a–g) containing various substituents on the triazole moiety using very simple and high yielding steps starting from commercially available D-arabinose. The syntheses first involved conversion of D-arabinose to the per-O-acetylated derivative followed by conversion to the anomeric bromide using PBr₃ according to the procedure reported previously.⁴⁶ This was followed by conversion to the anomeric azide by simple nucleophilic substitution reaction using sodium azide. These reactions were carried out at multi-gram scales followed by the Cu(II) catalyzed “click reactions” with appropriate acetylenes to yield the triazolylarabinosides 3a–g (Scheme 1).

Scheme 1 Synthesis of the triazolyarabinosides.

Initial gelation tests for all the compounds were carried out in a variety of solvents after which it was found that only the phenyltriazolyl derivative 3a and the carbomethoxytriazolyl compound 3g were effective organogelators for common hydrocarbon based organic solvents (Table 1). In addition, the gelator 3a was able to gel EtOH. However both 3a and 3g were unable to gel other solvents. An exhaustive list of the solvents tested for gelation with 3a–g is presented in the ESI (Table S1†). In general, the compound 3g had lower Minimum Gelator Concentrations (MGCs, typically 0.5–0.7%, w/v) than compound 3a (typically 0.7–1.0%, w/v) for the gel-able common solvents. Additionally the gelation of petrol and diesel with both 3a and 3g proceeded with MGCs of 0.3% (w/v), which was much less than those for the other solvents and is comparable with the lowest values reported previously. Therefore, both 3a and 3g are supergelators with respect to non-polar solvents, kerosene, petrol and diesel. All the gels were thermally reversible. The T_g for each of the gels was determined at the MGC and at higher concentrations. The increase of T_g with increased concentrations was more pronounced for the gels from 3g compared to the corresponding increases for 3a (Fig. S20 and S21, ESI†). Therefore, from these data it may be reasonable concluded that the thermal stabilities of the gels can be controlled via variations in concentration of the gelators.

Table 1 Gelation properties of triazolylarabinosides 3a–g^a

Solvent	3a (MGC^b, Tg)	3b	3c	3d	3e	3f	3g (MGC^b, Tg)
a G = gel; S = solution; I = insoluble.b Minimum gelation concentration (w/v).
Benzene	G, (1.0%, 44–45 °C)	S	S	S	S	S	G, (0.7%, 49–50 °C)
Toluene	G, (1.0%, 46–47 °C)	S	S	S	S	S	G, (0.5%, 53–54 °C)
o-Xylene	G, (0.9%, 47–48 °C)	S	S	S	S	S	G, (0.5%, 57–58 °C)
m-Xylene	G, (0.7%, 48–49 °C)	S	S	S	S	S	G, (0.5%, 52–53 °C)
p-Xylene	G, (0.9%, 46–47 °C)	S	S	S	S	S	G, (0.5%, 51–52 °C)
Chlorobenzene	G, (1.0%, 45–46 °C)	S	S	S	S	S	G, (0.7%, 53–54 °C)
Ethanol	G, (1.0%, 48–49 °C)	S	S	S	S	S	S
Kerosene	G, (0.3%, 69–70 °C)	S	S	S	S	S	G, (0.3%, 71–72 °C)
Petrol	G, (0.3%, 63–64 °C)	S	S	S	S	S	G, (0.3%, 61–62 °C)
Diesel	G, (0.3%, 66–67 °C)	S	S	S	S	S	G, (0.3%, 68–69 °C)

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Morphology and characterization of the gels

To gain insight into the microstructure of the organogels, Field Emission Scanning Electron Micrographs (FESEM) of the corresponding xerogels were also obtained which confirmed the cross-linked fibrillar network of the xerogels (Fig. S23–S39, ESI†). Again the FESEM microstructures for the gels from petrol and diesel were also obtained and are presented in Fig. 3. The general stronger nature of the 1% gels for these solvents compared to the conventional organic solvents is proved by the generally thicker nature and the greater cross-linking for the petrol and diesel gels as can be seen from the corresponding FESEM images. Additionally, the organogels from 3a and 3g with meta-xylene were also characterized by optical microscopy and AFM which reconfirmed the fibrillar nature of these gels.

Fig. 3 FESEM image (a) meta-xylene gel of 3a (b) petrol gel of 3a (c) diesel gel of 3a (d) meta-xylene gel of 3g (e) petrol gel of 3g (f) diesel gel of 3g.

Wide angle X-ray diffraction of the xerogels

To understand the molecular packing and the mechanism of the self-assembly during the gelation, Wide Angle X-ray Diffraction (WXRD) experiments was carried out for the xerogels of 3a and 3g. The diffraction pattern of both the xerogels showed periodical peaks indicating lamellar organization (Fig. 4). For the xerogel 3a, a sharp reflection peak was obtained at a 2θ value of 23.4° corresponding to a d spacing 3.8 Å, suggesting π–π stacking during the self-assembly,⁵ while a sharp peak at 2θ = 19.9° corresponding to d spacing 4.5 Å indicated H-bonding interactions.^14,15 The XRD diffractogram of xerogel 3g also exhibited a sharp peak at 2θ = 24.0° corresponding to a d spacing of 3.7 Å, indicative of π–π stacking. Again, reflection peaks at 2θ = 18.4°, 19.9°, 21.4° corresponding to d spacings of 4.8 Å, 4.5 Å, and 4.1 Å indicated the presence of H-bonding during the self-assembly into aggregates.

Fig. 4 WXRD diffractograms of xerogel of (a) 3a and (b) 3g.

Rheological studies of the gels

To establish the true gel-like nature of the gels, oscillatory rheological experiments were carried out for each of the gels with different solvents. Rheologically, a gel may be defined as a two component system in which the storage modulus (G′) is greater than the loss modulus (G′′).⁴⁷ Typically the rheological experiments were carried out at 1% w/v concentrations which is very near the MGC for each of the gels. For each of the gels of 3a and 3g with different solvents, three separate experiments were carried out viz. (i) The Dynamic Strain Sweep (DSS) experiments (ii) the Dynamic Frequency Sweep (DFS) (iii) the Dynamic Time Sweep (DTS) experiments. The curves obtained for each of the experiments are presented in the accompanying ESI (Fig. S42–S60†) while the results and conclusions drawn from the rheological experiments are summarized in Table 2.

Table 2 Summary of the rheological properties of the gels from 3a and 3g^a

Solvent	3a				3g
	γ (%) × 10^−2	G′ (Pa) × 10^3	G′′ (Pa) × 10^3	tan δ^b	γ (%) × 10^−2	G′ (Pa) × 10^3	G′′ (Pa) × 10^3	tan δ^b
a G′ = storage modulus; G′′ = loss modulus.b tan δ = G′′/G′.
Benzene	3.26	9.42	2.05	0.22	14.49	21.07	6.98	0.35
Toluene	12.96	11.64	2.66	0.23	13.26	23.00	3.48	0.15
o-Xylene	7.92	4.10	1.33	0.40	32.71	3.15	13.90	0.42
m-Xylene	2.63	6.69	1.10	0.20	6.46	23.30	5.16	0.23
p-Xylene	2.57	2.38	1.76	0.52	6.83	13.90	3.87	0.28
PhCl	5.43	23.71	5.12	0.22	4.57	3.39	1.42	0.40
EtOH	7.81	9.19	3.35	0.37	—	—	—	—
Petrol	2.42	4.41	1.38	0.34	7.24	3.44	0.99	0.23
Diesel	11.67	86.13	17.4	0.21	8.23	11.45	1.56	0.14
Crude-oil	1.82	24.36	11.9	0.49	2.63	260.0	82.20	0.31

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The DSS experiments were performed to study the behaviour of the gel towards mechanical stress at constant frequency. The storage modulus G′ and the loss modulus G′′ were determined simultaneously and plotted against increasing strain. All the gels showed the typical curves for G′ and G′′ where the G′ dominated over G′′ at lower strain which is characteristic of gels. However, at higher strains the G′′ dominated over the G′ suggesting sol type nature. From the curves, the point of intersection of the two curves (i.e. G′ = G′′), the transition from the gel state to the sol state takes place and is depicted by γ. The values γ for each of the gels were determined and are presented in Table 2. From the γ values obtained for the gels of 3a, it can be clearly seen that toluene formed the gel with the maximum strain bearing capability with a value of γ = 0.1296%. From the γ values of 3g presented in Table 2, it is evident that ortho-xylene formed the gel which underwent the transition from the gel state to the sol state at the highest strain (γ = 0.3271%) and was the mechanically most stable of all the 3g gels. Again, through a comparison of the γ values of the corresponding gels with different solvents from 3a and 3g, it is clearly evident that the strain withstanding capabilities of the gels from the latter were higher than the former indicating that 3g formed were mechanically more stable gels than 3a.

The DFS experiments, in which curves were obtained for the variation of G′ and G′′ with increase in the frequency of oscillation at constant strain, information about the mechanical strength of the gels with respect to frequency was obtained. The graphs presented in the ESI† show the linear visco-elastic region at which the gels are stable. Again, the ratio of G′′ to G′ which is represented as the tangent of the phase angle δ (tan δ) could be calculated. tan δ provides information about the mechanical strength of the gel over the visco-elastic region – the lower the value of tan δ, greater is the mechanical strength of the gel. From the tan δ values of the gels of 3a presented in Table 2, it can be easily seen that the gels from the gellable solvents other than o-xylene, p-xylene, EtOH and petrol had comparable mechanical strength. The weaker nature of the gels for the afore-mentioned solvents was also evident from the DSS experiments.

The DTS experiments were also performed with all the gels and provided information about the stability of the gels with respect to mechanical stress with time. The graphs shown in the ESI† provide unequivocal evidence of the stability of the gels over the time studied. Further, it was also observed that the gels were quite stable over two months if kept under sealed conditions to avoid solvent loss.

The reversibility of the gel to sol transition under mechanical stress was also demonstrated using the meta-xylene gel as a representative example by the thixotropic test using rheology (Fig. S61, ESI†). During this experiment the gel sample was subjected to a sudden increase in the applied strain from 0.01% to 1.0%. At lower strains, the G′ dominated over the G′′, demonstrating gel-like nature, while at higher strains G′′ dominated over G′ demonstrating transition of the gel to sol. On decrease of the strain the initial G′ to G′′ ratio was regained demonstrating re-transition to the gel state. The cycle was further repeated after a recovery period with the same results which demonstrated the thixotropic nature of the gel.

To establish the nature of the gels with petrol, diesel and crude-oil, the rheological studies were also carried out with the corresponding 1% (w/v) organogels from 3a and 3g (Fig. S49–S51 and S58–S60, ESI†). The results and conclusions from the rheological experiments with these are presented in Table 2. With respect to the gels from petrol, the breaking strain for gel to sol transition (γ) at 1% concentration was at a strain of 0.024% for 3a, while that for the 3g gel was 0.072%. Again the tan δ value for the petrol gel with 3a was 0.34 while that for 3g was 0.23. For diesel, γ for 3a gel was 0.1167% which was comparable to that for 3g gel (γ = 0.0823%). The tan δ for the former was 0.21 which was higher than the corresponding value (0.14) for 3g gel. Therefore, from the above data it may be concluded that the gelator 3g formed stronger gels with both petrol and diesel compared to 3a, which is in general agreement for the corresponding gels of other typical organic solvents.

With crude-oil, the formation of gels was also established by rheological studies (Fig. 5). Fig. 4 shows the rheological cures obtained for pure crude-oil and the crude-oil gels with 3a and 3g. Comparison of the DFS curves for pure crude-oil and the gelled crude-oils with 3a and 3g provide unequivocal evidence for the proof of the gelation. In the pure crude-oil, the loss modulus (G′′) was greater than storage modulus (G′) suggesting fluid like behaviour while in the latter two cases, G′ was greater than G′′ proving gel-like behaviour. The breaking strain (γ) for the crude-oil gel for 3a was 0.18 while that for the gel from 3g was 0.26. The tan δ value for the crude-oil gel with 3a was 0.408 while that for the gel from 3g was 0.308 suggesting that 3g is once again a better gelator for crude-oil.

Fig. 5 Rheology of crude oil and the crude-oil gels with 3a and 3g. (a) DSS of pure crude-oil (b) DFS of pure crude-oil (c) DSS of the crude-oil gels of 3a and 3g (d) DFS of the crude-oil gels of 3a and 3g.

Phase selective organogelation of petrol and diesel

Having established the gelation of petrol, diesel and crude-oil, we next turned our attention towards the phase selective organogelation (PSOG) of these from a biphasic system containing water. For the PSOG experiments with a view to remediation of oil-spills, the addition of a solution of the gelator provides a practical and easy method for the uniform dispersal of the gelator over large surface areas via spraying (ensuring thorough mixing with the oil) compared to the dispersion of the gelator in the solid form which has the added disadvantage of requiring agitation. For the dispersal of our gelators for PSOG of petrol and diesel, we chose concentrated, hot solutions of the gelators in the respective oils. A typical PSOG experiment was carried out by taking 4 mL of water (saline/normal tap water) in a vial, adding 1.5 mL of petrol or diesel to make a biphasic system and then adding 0.5 mL of a hot solution of 10 mg of the gelator in the same oil to the oil water mixture. The process resulted in an overall 0.5% w/v concentration of the gelator in the oil layer. Gelation occurred almost instantaneously which was proven by the inversion test. The gelled oil layer was then scooped out from which about 90% of the oil could be recovered via distillation in a simple set-up on an oil-bath. Around 80–90% of the gelator could also be recovered from the residue left after distillation following a quick filtering column chromatographic separation over silica gel. A pictorial representation of the sequence of events for the PSOG of petrol and the reclamation of the petrol using compound 3a is depicted in the ESI† (PSOG and reclamation photos of petrol and diesel using 3a and 3g are provided in the ESI, Fig. S62–S65†).

For the PSOG of crude-oil and for the gel to withstand the weight of the aqueous layer during the inversion tests of the biphasic system, higher concentrations than the MGC of the gelator molecule was required, which made petrol or diesel unsuitable for the dispersal of the gelators due to the relatively poor solubility of the gelators in these oils. Therefore, for dispersal of our gelators in crude-oil during its PSOG, we chose meta-xylene as the carrier solvent due to the fair solubility of the gelators in it, as well its miscibility with crude-oil. Again, we also considered this as the carrier medium due to its relatively less toxic nature and high boiling point. A typical PSOG experiment with crude oil was carried out by taking 4 mL of water (saline/normal tap water) and 2 mL of crude-oil in a vial (to simulate an oil-spill) followed by addition of 0.5 mL of a hot solution of the gelator in meta-xylene. Gelation occurred within a few minutes and the vial could be inverted so that the gelled crude-oil could withstand the weight of the aqueous layer. Furthermore, the gelled crude-oil could be scooped out demonstrating the feasibility of the process for potential oil-spill remediation. A pictorial representation of the PSOG and reclamation of the congealed oil using gelator 3a is depicted in Fig. 6 and that for 3g is presented in the ESI (Fig. S66†). In practical situations of crude-oil spillages, the recovered gelled crude-oil could be fractionally distilled to yield the consumable petroleum products. The gelator could also be recovered after the fractional distillation process from the residue as we have demonstrated earlier for the PSOG experiments with petrol and diesel.

Fig. 6 PSOG of crude-oil using 3a. (a) Water (b) biphasic mixture of water and crude-oil (c) congealed crude-oil layer after addition of gelator solution (d) floating congealed crude-oil layer (e) removed congealed crude oil (f) residual water.

Experimental

General procedures

Chemicals, solvents and reagents were procured from commercial sources and were used without further purification unless otherwise stated. Anhydrous solvents when used were prepared according to reported procedures.⁴⁸ Reactions were performed under an atmosphere of nitrogen gas maintained by an inflated balloon. Column chromatography was performed by using silica gel (230–400 mesh) under medium pressure. TLC was performed on precoated aluminium plates of silica gel 60-F254. TLC spots were visualized by UV light (254 nm) and by staining with a vaniline solution and subsequent heating on a hot plate. ¹H and ¹³C NMR spectra were recorded on Bruker AC-400 NMR spectrometer at 400 MHz (¹H) and 100 MHz (¹³C) in solutions of CDCl₃ using tetramethylsilane as an internal standard. δ values are reported in parts per million (ppm) and coupling constants (J) in Hertz.

Synthesis of 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside

The arabinosyl bromide was prepared according to literature procedure.⁴⁶ D-Arabinose (6.0 g, 39.9 mmol) was stirred in a solution of acetic anhydride (18.6 mL, 196.7 mmol) and pyridine (75.0 mL, 775.7 mmol) at room temperature for 12 hours. After completion of the reaction, 35 mL of H₂O was added to quench the reaction following which it was extracted with ethyl acetate (EA, 60 mL × 3) three times. The combined ethyl acetate extract was then washed with brine solution and dried over sodium sulfate (Na₂SO₄) and evaporated it in vacuo to yield a syrupy residue of 1,2,3,4-tetra-O-acetyl-D-arabinopyranoside (11.90 g, 93%) which was used further without purification.

Subsequently, the 1,2,3,4-tetra-O-acetyl-D-arabinopyranose (6.0 g, 18.85 mmol) was dissolved in acetic acid (105 mL) followed by the drop wise addition of phosphorus tribromide (17.17 mL, 180.6 mmol) under nitrogen atmosphere. The solution was stirred at room temperature for 4 hours after which the reaction mixture was poured into ice/water (200 mL) and the aqueous phase was extracted thrice with dichloromethane (DCM, 80 mL each). The combined extract was then washed with brine and then dried over anhydrous (Na₂SO₄). Concentration of the extract in vacuo yielded a colorless solid of 2,3,4-tri-O-acetyl-1-bromo-α-D-arabinopyranoside (5.58 g, 87%).

Subsequently, the 2,3,4-tri-O-acetyl-1-bromo-α-D-arabinopyranoside (5.58 g, 16.45 mmol) was dissolved in anhydrous DMF (20 mL) and sodium azide (3.45 g, 52.5 mmol) was added to the reaction mixture. The resulting mixture was heated at 80 °C. The progress of the reaction was monitored by TLC. After 2 h, when the reaction was completed, the mixture was cooled. Then the reaction mixture was quenched with H₂O (50 mL) and extracted thrice with EA (50 mL × 3). The combined organic extract was then dried over Na₂SO₄, filtered and concentrated in vacuo to yield the crude product. The crude product was purified by column chromatography (ethyl acetate : pet ether : 30 : 70) to afford 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (4.10 g, 72%): mp 83–84 °C. [α]³⁰_D = 13.63 (c = 1.0 in CHCl₃). ¹H NMR (400 MHz, CDCl₃)⁴⁹ δ (ppm) 5.30–5.28 (m, 1H), 5.15 (dd, J = 9.7 and 8.0 Hz, 1H), 5.04 (dd, J = 9.7 and 3.4 Hz, 1H), 4.58 (d, J = 8.0 Hz, 1H), 4.10 (dd, J = 13.2 and 2.8 Hz, 1H), 3.74 (dd, J = 13.2 and 1.5 Hz, 1H), 2.15 (s, 3H), 2.08 (s, 3H), 2.02 (s, 3H). ¹³C NMR (100 MHz, CDCl₃)⁴¹ δ (ppm) 170.1, 169.9, 169.3, 88.4, 70.0, 68.3, 67.4, 65.5, 20.8, 20.6, 20.5. IR_(neat) (cm⁻¹) = 2116 (–N₃), 1737 (COCH₃). HRMS calcd for C₁₁H₁₅N₃O₇Na⁺ (M + Na)⁺ 324.0808; found 324.0809.

General procedure for the synthesis of triazolylarabinosides (3a–g)

To a solution of 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (1.0 equiv.) in tert-BuOH : water (1 : 1, 10 mL) was added the alkynyl compound (1 equiv.), sodium ascorbate (2 equiv.) and copper sulphate pentahydrate (0.2 equiv.). The resulting mixture was heated at 70 °C for 2 hour to after which TLC showed the complete disappearance of sugar azide. The reaction mixture was then cooled and the solvents were evaporated in vacuo. The resulting residue was dissolved in chloroform (40 mL), washed with brine thrice (15 × 3 mL) after which it was dried with anhydrous Na₂SO₄. Evaporation of the solvent in vacuo followed by flash column chromatography using chloroform : methanol (90 : 10) to afford the desired product.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(phenyl)-1H-1,2,3-triazole (3a)

The reaction of phenyl acetylene (0.091 mL, 0.829 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.250 g, 0.829 mmol) according to the general procedure followed by column chromatography afforded 3a, (320 mg, 95%) as a colorless solid. Mp 183–185 °C. [α]³⁰_D = −22.72 (c = 1.0 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); δ (ppm) 8.05 (s, 1H), 7.86–7.84 (m, 2H), 7.45–7.41 (m, 2H), 7.37–7.33 (m, 1H), 5.81 (d, J = 9.2 Hz, 1H), 5.68 (dd, J = 9.6 Hz and 9.6, 1H), 5.47–5.46 (m, 1H), 5.27 (dd, J = 10.1 and 3.5 Hz, 1H), 4.21 (dd, J = 13.5 and 2.1 Hz, 1H), 3.98 (dd, J = 13.5 and 1.1 Hz, 1H), 2.25 (s, 3H), 2.04 (s, 3H), 1.91 (s, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.2, 169.9, 169.2, 148.4, 130.1, 128.9, 128.5, 125.9, 117.7, 86.8, 70.6, 68.0, 67.8, 67.4, 21.0, 20.6, 20.3. HRMS calcd for C₁₉H₂₁N₃O₇Na⁺ (M + Na)⁺ 426.1277; found 426.1276.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(1-nepthyloxymethyl)1H-1,2,3-triazole (3b)

Treatment of 2-(ethynyloxy)naphthalene (0.181 g, 1.0 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.300 g, 1.0 mmol) according to the general procedure followed by column chromatography afforded 3b (389 mg, 80%) as a colorless solid. Mp 143–144 °C. [α]³⁰_D = 25.45 (c = 1.25 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); ¹H NMR (400 MHz, CDCl₃); δ (ppm) 8.0 (s, 1H), 7.80–7.75 (m, 3H), 7.48–7.44 (m, 1H), 7.38–7.34 (m, 1H), 7.28–7.20 (m, 2H), 5.80 (d, J = 9.2 Hz, 1H), 5.65 (dd, J = 9.6 and 9.6 Hz, 1H), 5.45 (m, 1H), 5.36 (s, 2H), 5.26 (dd, J = 10.0 and 3.4 Hz, 1H), 4.19 (dd, J = 13.5 and 1.8 Hz, 1H), 3.95 (d, J = 13.3 Hz, 1H), 2.22 (s, 3H), 2.04 (s, 3H), 1.83 (s, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.8, 169.0, 156.0, 144.9, 134.4, 129.5, 129.1, 127.6, 126.9, 126.4, 123.9, 121.1, 118.8, 107.2, 86.6, 70.5, 68.0, 67.6, 67.2, 61.9, 20.9, 20.5, 20.1. HRMS calcd for C₂₄H₂₅N₃O₈H⁺ (M + H)⁺ 484.1720; found 484.1715.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(hexyl)-1H-1,2,3-triazole (3c)

The reaction of 1-octyne (0.148 mL, 0.995 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.300 g, 0.995 mmol) according to the general procedure followed by column chromatography afforded 3c (375 mg, 85%) as a colorless solid. Mp 118–120 °C. [α]³⁰_D = −6.281 (c = 0.67 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); δ 7.56 (s, 1H), 5.72 (d, J = 9.2 Hz, 1H), 5.58 (dd, J = 9.6 and 9.6 Hz, 1H), 5.42–5.41 (m, 1H), 5.22 (dd, J = 10.1 and 3.5 Hz, 1H), 4.15 (dd, J = 13.5 and 2.0 Hz, 1H), 3.93 (dd, J = 13.5 and 1 Hz, 1H), 2.71–2.67 (m, 2H), 2.20 (s, 3H), 2.01 (s, 3H), 1.86 (s, 3H), 1.65–1.63 (m, 2H), 1.33–1.23 (m, 6H), 0.86 (t, J = 6.7 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.8, 169.1, 141.1, 119.0, 86.6, 70.5, 68.0, 67.7, 67.2, 31.5, 29.1, 28.6, 25.7, 22.6, 20.9, 20.5, 20.2, 14.02. HRMS calcd for C₁₉H₂₉N₃O₇Na⁺ (M + Na)⁺ 434.1903; found 434.1902.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(octyl)-1H-1,2,3-triazole (3d)

The reaction of 1-decyne (0.179 mL, 0.995 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.300 g, 0.995 mmol) according to the general procedure followed by column chromatography afforded 3d (425 mg, 91%) as a colorless solid; mp 84–86 °C. [α]³⁰_D = 3.64 (c = 1.25 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); δ (ppm) = 7.56 (s, 1H), 5.72 (d, J = 9.2 Hz, 1H), 5.57 (dd, J = 9.6 and 9.6 Hz, 1H), 5.42–5.40 (m, 1H), 5.21 (dd, J = 10.1 and 3.5 Hz, 1H), 4.14 (dd, J = 13.5 and 1.9 Hz, 1H), 3.92 (d, J = 12.9 Hz, 1H), 2.70–2.67 (m, 2H), 2.19 (s, 3H), 2.00 (s, 3H), 1.85 (s, 3H), 1.64 (m, 2H), 1.28–1.24 (m, 10H). 0.84 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.8, 169.0, 143.8, 118.6, 86.6, 70.6, 67.9, 67.7, 67.2, 31.7, 29.6, 29.2, 29.1, 29.0, 25.5, 22.6, 20.9, 20.5, 20.2, 14.0. HRMS calcd for C₂₁H₃₃N₃O₇Na⁺ (M + Na)⁺ 462.2216; found 462.2218.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(octyl)-1H-1,2,3-triazole (3e)

The reaction of 1-dodecyne (0.212 mL, 0.995 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.300 g, 0.995 mmol) according to the general procedure followed by flash column chromatography afforded 3e (0.389 mg, 79%) as a colorless solid; mp 88–90 °C. [α]³⁰_D = −4.54 (c = 1.0 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); δ (ppm) 7.58 (s, 1H), 5.72 (d, J = 9.2 Hz, 1H), 5.58 (dd, J = 9.6 and 9.6 Hz, 1H), 5.43–5.42 (m, 1H), 5.22 (dd, J = 6.3 and 3.4 Hz, 1H), 4.16 (dd, J = 13.5 and 1.9 Hz, 1H), 3.93 (d, J = 12.8 Hz, 1H), 2.69 (m, 2H), 2.20 (s, 3H), 2.01 (s, 3H), 1.86 (s, 3H), 1.65 (m, 2H), 1.29–1.23 (m, 14H), 0.85 (t, J = 6.84 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.9, 169.1, 144.7, 119.1, 86.7, 70.7, 68.0, 67.8, 67.3, 31.9, 29.6, 29.5, 29.4, 29.2, 29.1, 29.6, 25.6, 22.7, 21.0, 20.6, 20.3, 14.1. HRMS calcd for C₂₃H₃₇N₃O₇H⁺ (M + H)⁺ 468.2710; found 468.2707.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(1-hydroxylmethyl)-1H-1,2,3-triazole (3f)

The reaction of propargyl alcohol (0.038 mL, 0.663 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.200 g, 0.663 mmol) according to the general procedure followed by column chromatography afforded 3f as a colorless solid (150 mg, 63%), white solid; mp 97–99 °C. [α]³⁰_D = −18.18 (c = 1.0 in CHCl₃), ¹H NMR (400 MHz, CDCl₃); δ (ppm) 7.83 (s, 1H), 5.75 (d, J = 9.1 Hz, 1H), 5.55 (dd, J = 9.6 and 9.6, 1H), 5.41–5.40 (m, 1H), 5.24 (dd, J = 10.2 and 3.4 Hz, 1H), 4.75 (s, 2H), 4.15 (dd, J = 13.2 and 1.5 Hz, 1H), 3.94 (d, J = 13.4 Hz, 1H), 2.18 (s, 3H), 2.00 (s, 3H), 1.87 (s, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.9, 169.3, 148.4, 120.2, 86.5, 70.41, 68.2, 67.7, 67.1, 56.2, 20.8, 20.5, 20.2. HRMS calcd for C₁₄H₁₉N₃O₈Na⁺ (M + Na)⁺ 380.1070; found 380.1072.

Spectral data for 1-(2,3,4-tri-O-acetyl-β-D-arabinopyranosyl)-4-(methyl methanoate)-1H-1,2,3-triazole (3g)

The reaction of methyl propiolate (0.058 mL, 0.663 mmol) and 2,3,4-tri-O-acetyl-1-azido-β-D-arabinopyranoside (0.200 g, 0.663 mmol) according to the general procedure followed by column chromatography afforded 3g (198 mg, 80%) as a colorless solid. Mp 195–197 °C. [α]³⁰_D = 13.63 (c = 1 in CHCl₃). ¹H NMR (400 MHz, CDCl₃); δ (ppm) 8.39 (s, 1H), 5.80 (d, J = 9.1 Hz, 1H), 5.52 (dd, J = 9.6 and 9.6 Hz, 1H), 5.45–5.44 (m, 1H), 5.25 (dd, J = 10.2 and 3.4 Hz, 1H), 4.21 (dd, J = 13.5 and 2.1 Hz, 1H), 3.98–3.95 (m, 4H), 2.22 (s, 3H), 2.03 (s, 3H), 1.90 (s, 3H). ¹³C NMR (100 MHz, CDCl₃); δ (ppm) 170.1, 169.8, 169.2, 160.7, 140.5, 126.2, 86.9, 70.2, 68.3, 67.5, 52.2, 20.9, 20.6, 20.2. HRMS calcd for C₁₅H₁₉N₃O₉Na⁺ (M + Na)⁺ 408.1019; found 408.1017.

Rhelogical experiments

Rheological properties of gel sample were determined by using a Bohlin Gemini-2 Malvern reheometer using parallel plates (25 mm, stainless steel). The gap between the parallel plates was 500 micron. The experiments were carried out for gels of compounds 3a or 3g at 1% (w/v) concentration except for the concentration dependant experiments, in which cases the gels at respective concentrations were used. The gel samples were placed on parallel plates by spatula in such a way that it covered the surface of the parallel plates. The three different test Dynamic Strain Sweep (DSS), Dynamic Frequency Sweep (DFS) and Dymanic Time Sweep (DTS) were performed to determine the viscoelastic nature of the gels. DSS experiment were carried out at a constant frequency of 1 Hz at temperature 25 °C. The strain value was determined at a point where tan δ = 1 (G′′ = G′). Variation of storage modulus (G′) and loss modulus (G′′) was tested within frequency range of 1 Hz to 100 Hz at strain range 0.001% by DFS experiments. DTS experiments were carried out at constant frequencies of 1 Hz and strain (0.001%).

Phase selective organogelation (PSOG) of petrol/diesel

In a 15 mL sample vial, 4 mL of saline water was taken. 1.5 mL of petrol/diesel was poured to the 4 mL of saline/tap water. 0.5 mL hot and concentrated solution of 3a or 3g in petrol/diesel was added to the biphasic mixture so that the overall concentration of the gelator in the oil phase was 0.5% (w/v). The resulting biphasic mixture was set aside upon which the petrol/diesel layer formed a gel having sufficient strength to hold the weight of 4 mL water during the inversion test. The congealed oil layer was then scooped out with a spatula, transferred into a distillation flask and subjected to distillation on an oil-bath. The distilled oil was then measured to calculate the efficiency of the recovery process. The residue was subjected to a filtering column chromatographic process to demonstrate the recovery of the gelator.

Phase selective organogelation (PSOG) of crude-oil

Over a sample of saline/tap water (4 mL) in a vial, 2 mL of crude oil was poured to form a biphasic system. A hot solution of 3a or 3g in meta-xylene (0.5 mL) was added to the floating crude-oil layer. The resulting biphasic mixture was set aside for 10 min after which the crude-oil layer formed gel. Inversion of the vial proved the gelation of crude-oil. The congealed crude-oil layer was then scooped out with a spatula.

Conclusions

In conclusion, we have reported two new triazolylarabinoside gelators that are capable of gelling crude-oil and other petroleum fractions in addition to several hydrocarbon based organic solvents as well as chlorobenzene. The structural and mechanical properties of the gels have been studied. The gels have been demonstrated to be thermally and mechanically reversible through rheological experiments. The mechanical strength of the gels have also been demonstrated to be tunable by variation of the concentration of the gelator as demonstrated by the concentration dependant rheological studies. The gelators reported herein are completely acid stable and insoluble in water making them suitable all types of crude-oils. The gelation of the crude-oil has also been confirmed and established by rheological experiments. The PSOG of crude-oil using these gelators provides a practical and simple means for the remediation of marine as well as terrestrial oil-spills. Finally, with respect to the choice between gelators 3a and 3g for practical applications of crude-oil spill remediation 3g may offer minor advantages due to better gelation ability as can be concluded from the lower MGCs and better rheological properties.

Acknowledgements

SY is grateful to Department of Science and Technology, India for financial support (SR/FT/CS-130/2011). The authors thank Dr T. K. Naiya, Department of Petroleum Engineering, IIT (ISM), Dhanbad for generous supplies of crude-oil procured from ONGC. The authors also thank SAIF, Panjab University for the NMR analyses, Central Scientific Services, IACS for the HRMS, Dr D. Sarkar (NIT, Rourkela) for help with the optical rotation experiments and Dr B. Kuila (CUJ, Ranchi) for help with the WXRD experiments.

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Footnote

† Electronic supplementary information (ESI) available: Images related to gelation experiments and gelation data, additional FESEM, AFM, optical microscopy images, graphs of rheological experiments, photos of PSOG experiments and NMR spectra are available. See DOI: 10.1039/c6ra21109g

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