Novel Class of Isoxazole-Based Gelators for the Separation of Bisphenol A from Water and Cleanup of Oil Spills.

A series of low-molecular-weight gelators based on an isoxazole backbone were synthesized, which showed robust and phase-selective gelation of a series of oils. Due to their excellent phase-selective and cogelation properties, they were employed for the separation of bisphenol and the recovery of oil spills from water. The driving force and morphology of these gels were characterized by spectroscopic and microscopic studies.

Introduction

Low-molecular-weight gelators have started to evoke considerable interest in recent years on account of their application in cosmetics,[1] food industry,[1] controlling/triggering drug release,[2,3] tissue engineering,[3,4] sensors,[5] template materials, dye-sensitized solar cells,[6] and so on. The one-dimensional self-assembly of the gelling agent with a fiberlike structure eventually entangles to produce a three-dimensional network followed by immobilization of solvent molecules via capillary force, leading to gelation in a particular solvent. Low-molecular-weight gelators based on carbohydrates,[7 ,8] amino acids,[9] and several other orn class="Chemical">ganic building blocks[10] and their applications are well known. We have designed and synthesized a series of isoxazole-based low-molecular-weight gelators and shown their applications in the separation of bisphenol from water and the recovery of oil spills. Bisphenol is one of the highest volume chemicals produced in the industry as a starting material.[11] It often leaches from plastic and food containers as well as plastic water pipes and has an adverse and pervasive effect on humans and wildlife. Bisphenol as an endocrine disruptor that binds with some bisphenol-binding proteins[12−14] and hormone receptors[15−17] and shields their normal functionality in cells.[18] This could cause an abrupt and dramatic alteration of cell functionality,[19−21] human reproductivity,[22] brain adipose tissue,[23] and so on. With growing demand and an increase in the production of plastic, human exposure to bisphenol has increased significantly over the years. There exist certain methods for the separation of bisphenol from water. They are mostly based on chromatographic techniques[24−26] or reverse osmosis.[27] However, the practical implementation of these methods is questionable as they fail to quantitate separation of bisphenol from water and are largely uneconomic. Thus, there is a constant demand for an effective, alternative, and an economical way to separate bisphenol from water.

We have now synthesized a series of isoxazole-based gelators by a systematic alten class="Species">ration of the hydrocarbon chain length (from C8H16 to C16H31) in the lipophilic part of the molecule and reveal their potential for efficient and quantitative separation of bisphenol from water. Besides that, these classes of isoxazole-based gelators were used for the cleanup of oil spills owing to the phase-selective gelation of these gelators. Marine oil spills were caused by leakage and release of more than 5 million tons of crude oil and petroleum products (refined fuel oils) into the ocean over the period of 1965–2010 in the Gulf of Mexico.[28,29] It has become a major threat to marine life and ocean ecosystems. There is an immediate and earnest need for the effective and efficient development of smart materials and technologies for oil spill control and recovery for combating oil spills.[30,31] Certain methods like bioremediation,[32] use of dispersants,[33] adsorption,[34] and use of solidifiers[8] and sorbents[35,36] have also been reported for the cleanup of oil spills. However, the problems associated with these existing methods are the release of toxic residues. Also, they are noneconomic, time-consuming, and allow only poor recovery. We have thus used isoxazole-based low-molecular-weight gelators for the removal of oil spills from water.

Results and Discussion

Syntheses of Gelators

Scheme shows the syntheses of gelators Ga–c. For this, n class="Chemical">3,4-dihydroxybenzaldehyde was treated with alkyl bromide (compounds 2a–c) containing different hydrocarbon chains under basic conditions to obtain compounds 3a–c. 3a–c were reacted with hydroxylamine hydrochloride in ethanol for 2 h to afford compounds 4a–c, and these were treated further with compound 1 in the presence of NaOCl in dry DCM at room temperature for an hour to obtain gelators Ga–c (for details, see Scheme S1 in the Supporting Information, SI).

Scheme 1

Syntheses of Isoxazole-Based Low-Molecular-Weight Gelators

Reaction Conditions: (ii) K2CO3, dry acetone, reflux, 12 h. (iii) H2NOH·HCl, ethanol, 25 °C, 2 h, (iv) 1: NaOCl, dry DCM, 25 °C, 1 h.

Reaction Conditions: (ii) K2CO3, dry n class="Chemical">acetone, reflux, 12 h. (iii) H2NOH·HCl, ethanol, 25 °C, 2 h, (iv) 1: NaOCl, dry DCM, 25 °C, 1 h.

Gelation Properties

The gelation abilities of these compounds were determined by the simple method of being stable to the inversion of the container,[8] and gelation is considered to have occurred while the gel is stable once the vial is turned upside down after cooling. The gelation abilities of all of these compounds in various solvents are depicted in Table S1 in the Supporting Information. All gelators are versatile organic gelators as they induce gelation in not only n class="Chemical">ethanol but also a series of oils like castor oil, olive oil, etc., and ambidextrous gelation properties of these gelators could be useful for the preparation of hybrid material in different solvents and also for sensing and tissue engineering application purposes.[3,37] It should be noted that all of these gels are thermoreversible since they turn into liquid state upon heating and revert to the gel state upon cooling (see Figure A). Figure A depicts the thermoreversible nature of the gel obtained from compound Ga in ethanol as it undergoes a change to the solution state while heating and reverts to the gel state again after cooling.

Figure 1

(A) Photographic image of gel during the heating–cooling cycle; rheology data for isoxazole-based gel in (B) castor oil and (C) ethanol. Red: Gc, blue: Gb, and magenta: Ga.

(A) Photographic image of gel during the heating–cooling cycle; rheology data for isoxazole-based gel in (B) n class="Chemical">castor oil and (C) ethanol. Red: Gc, blue: Gb, and magenta: Ga.

Rheology Study of the Gel

The rigidity, viscoelastic property, and mechanical stability of the gel were determined by rheological property measurements of the gels derived from 1% (w/v) gelators of compounds n class="Chemical">Ga–c in castor oil and ethanol. All gels were cured for a period of 12 h before the rheology measurements (Figure B,C). Rheology data of the gels in castor oil (Figure B) and ethanol (Figure C) demonstrate that for all of these gels, storage modulus (G′) is higher than loss modules (G″), implying the viscoelastic nature of these gels.[38] Notably, for all of these gels derived from castor oil, both G′ and G″ are almost independent of the measured range of frequency of oscillation at a constant strain. The dependence of the mechanical strength of the gel on the alkyl chain length is illustrated.[39] The variation of the mechanical strength of the gel with the hydrocarbon chain can be illustrated by considering its tangent of the phase angle value (tan δ),[40] which is the ratio of (G′) over (G″). In the case of an isoxazole-based gel derived from castor oil with C16 hydrocarbon chain length, the value of tan δ is much lower than for C10 (Gb) and for C8 (Ga) (Table S2, Supporting Information). This clearly indicates that the gel derived from the C16 hydrocarbon chain gel is more mechanically stable than gels with shorter chains. The variation of tan δ value with the hydrocarbon chain length is reversed in the case of a gel derived from ethanol compared to the gel derived from castor oil. This signifies different gelation mechanisms and different orientations of the gel fibers in the three-dimensional (3D) structure of the gel network in these two organic solvents.[41]

Morphology of the Gels

To visualize the morphology of the organogel, field emission scanning electron microscopy (FESEM) experiments were carried out using xerogel of compounds n class="Chemical">Ga, Gb, and Gc obtained from organogels of compounds Ga, Gb, and Gc in ethanol, respectively. Figure A–C depicts the FESEM images of xerogels derived from gels Gc, Gb, and Ga in ethanol, respectively. The FESEM image of the xerogel of Gc (Figure A) revealed that the organogel of Gc consists of numerous fibrous networks with a length of several nanometers and a diameter of 100–300 nm, which are entangled with each other to eventually give a self-assembled woven structure, whereas the FESEM images of xerogels of Ga and Gb (Figure B,C) reveal a thin, flat, and ribbon-like structure, implying that there might be some relation among gel strength, morphology, and the 3D structure of the gel with the hydrocarbon chain length of the lipophilic part of the molecule.

Figure 2

FESEM image of xerogel made from ethanol gel of (A) compound Gc, scale bar 500 nm; (B) compound Gb, scale bar 2 μM; and (C) compound Ga, scale bar 2 μM; (D) variable-temperature 1H NMR spectra of methanol-d4 gel of compound Gb; (E) extended region of Fourier transform infrared (FTIR) spectra of Gb chloroform solution (red curve) and in the ethanol gel (black curve).

FESEM image of xerogel made from ethanol gel of (A) compound n class="Chemical">Gc, scale bar 500 nm; (B) compound Gb, scale bar 2 μM; and (C) compound Ga, scale bar 2 μM; (D) variable-temperature 1H NMR spectra of methanol-d4 gel of compound Gb; (E) extended region of Fourier transform infrared (FTIR) spectra of Gb chloroform solution (red curve) and in the ethanol gel (black curve).

Investigation of Driving Forces Leading to the Formation of the Gel

To investigate the nature of intramolecular forces constituting the self-assembly processes leading to the formation of gel in orn class="Chemical">ganic solvents, temperature-dependent 1H spectroscopy measurements were carried out by using 0.5% of organogel of Gb in methanol-d4. Figure D depicts variable-temperature 1H spectra of the gel obtained from Gb. At low temperatures, due to the formation of a rigid supramolecular gel network, signals for protons are broad and unresolved due to high relaxation time owing to the strong gel network.[42] With a gradual increase in temperature, the proton signals become sharper compared to the signals at 15 °C due to the disordering of the self-assembled rigid gel network.[43] Temperature-dependent NMR spectra clearly reveal that the proton at the isoxazole ring plays a vital role in intermolecular H bonding, which is one of the key parameters in inducing the gelation process to make a self-assembled structure in the organic solvent, as the proton of the isoxazole ring suffers an enormous upfield shift (shown by the red arrow in Figure D) as the temperature increases, leading to a disruption of intermolecular H bonding of the gel network.

The fact that H bonding plays a vital role during the gelation process in the organic solvent was further confirmed by FTIR spectroscopic studies on Gb. Figure E represents the extended region of the FTIR spectra of the gelator Gb in n class="Chemical">chloroform solution (red curve) and ethanol gel (black curve). In the case of a chloroform solution of Gb, the band appearing at 1665 cm–1 can be assigned to the stretching vibration of the C=N bond[44] of the isoxazole backbone, which is shifted to 1672 cm–1 in ethanol gel, clearly suggesting the existence of H bonding on the gel network in the self-assembled gel state.[45]

Separation of Bisphenol Using Isoxazole-Based Gel

The organogel in n class="Chemical">castor oil was used for the separation of bisphenol from water (Figure A). A specific volume of castor oil was added into a standard stock solution of bisphenol in water, followed by the addition of the gelator to the mixture under mechanical shaking, heating, and bath sonication for a short period. The gelator exclusively indulges in gelation with the upper oil layer. The water layer is untouched and can be discarded. Notably, during the process of gelation as well as the formation of entangled fibrous gel network under mechanical shaking and bath sonication, the gel fibers engross, entrap, and immobilize all of the biphenolic substances due to supramolecular interaction (probably due to π–π* stacking). This causes a quantitative removal of the biphenolic substances from water into the upper oil gel, which was confirmed by the UV–visible spectroscopic study. Figure C depicts the UV–visible spectra of bisphenol in water before and after separation. The black curve shows the corresponding absorption spectrum of bisphenol in water before separation with an absorption maximum at 276 nm, and the red curve reflects the remaining bisphenol concentration in water after the addition of castor oil and gelator. The separation of the upper gel by simple tweezers allows a quantitative removal of bisphenol from water. To the best of our knowledge, this is the first example of the separation of a biphenolic substance using such a supramolecular gel system.

Figure 3

(A) Schematic representation of the separation of bisphenol from water; (B) calibration curve showing the concentration-dependent absorption of bisphenol in water; (C) concentration of bisphenol in water before (black curve) and after separation using gelator (red curve).

(A) Schematic representation of the separation of n class="Chemical">bisphenol from water; (B) calibration curve showing the concentration-dependent absorption of bisphenol in water; (C) concentration of bisphenol in water before (black curve) and after separation using gelator (red curve).

Phase-Selective Gelation and Recovery of Oil from Oil–Water Mixture

Ga–c were also used for the sepan class="Species">ration of oil from an oil–water mixture (Figure A–D) due to their excellent ability to allow robust cogelation in a wide variety of oils, including crude mineral oil from an oil–water mixture, which is an essential requirement for their recovery from oil spills.[29] The rheology data in Figure B reflects that the value of G′ in the castor oil gel of Gc is around 3500 Pa, which signifies high gel strength (stiffness) of the gel in castor oil and therefore Gc was used for oil separation and recovery from the oil–water mixture. For the recovery of oil spills, 10 mg of gelator Gc was scattered over the upper oil layer of the binary mixture of tap water and diesel in a 20:1 ratio (v/v) and heated at 40 °C for a few minutes. While cooling to room temperature, the upper diesel layer stopped swirling but not the lower water layer, which confirms selective gelation of the top diesel layer (Figure B). The as-formed gel was strong enough and was scooped out by a spatula and placed in a round-bottom flask (Figure C), followed by distillation to recover the diesel in another round-bottom flask (Figure D). Heating a binary mixture of seawater in the presence of a gelator is not economical. Thus, in a modified approach, a tetrahydrofuran (THF) solution of the gelator Gc was sprayed over the binary mixture of tap water and diesel in a 20:1 ratio (v/v), leading to instant gelation within a few seconds, making this approach more useful and practical for oil spill recovery. To make this approach closer to a real scenario, the above oil spill recovery procedure was carried out using artificial seawater, prepared according to the literature procedure,[46] and the same result was obtained, implying that the phase-selective gelation properties of these isoxazole-based gelators were not hampered in the presence of a high salt concentration in seawater. To investigate the maximum volume of water that can be used for the recovery of oil spills in a binary mixture of oil and water, a recovery experiment was carried out using a water and diesel mixture in a 1000:1 ratio (v/v) in the presence of THF solution of gelator Gc. Gelator Gc was still able to carry out phase-selective gelation of upper diesel layer. This further supports the utility of this method for real applications.

Figure 4

Separation of diesel from the water–oil binary mixture. Water–diesel mixture (A) before the addition of gelator and (B) after spraying the gelator to the water–diesel mixture, (C) scooped-out gel placed in a round-bottom flask under a distillation setup, and (D) recovered diesel through distillation under vacuum.

Separation of n class="Chemical">diesel from the water–oil binary mixture. Water–diesel mixture (A) before the addition of gelator and (B) after spraying the gelator to the water–diesel mixture, (C) scooped-out gel placed in a round-bottom flask under a distillation setup, and (D) recovered diesel through distillation under vacuum.

Conclusions

In summary, we have synthesized a series of isoxazole-based low-molecular-weight gelators for the efficient sepan class="Species">ration of bisphenol A from water, owing to their robust gelation properties with excellent mechanical strength. The phase-selective cogelation allows their use for a wide range of applications, including the cleanup of oil spills. These classes of gelators can be easily prepared and offer fast and efficient gelation.

Experimental Section

Materials

1-bromohexadecane, n class="Chemical">1-bromodecane, 1-bromoctane, 3,4-dihydroxybezaldehyde, 3-nitrophenol, and hydroxylamine hydrochloride were purchased from Sigma-Aldrich and used without further purification. Structure determinations were carried out by a Brucker AscendTM 400 MHz spectroscope. Field emission scanning electron microscopy images were recorded using Supra 55 (Carl Zeiss). Xerogels for FESEM were prepared by slow evaporation of the gel samples, which were drop-casted on a 1 × 1 cm2 glass plate and dried overnight in the air inside a desiccator. The xerogel samples were then sputter-coated with Au and subjected to a FESEM study. UV–visible spectroscopic experiments of the bisphenol solutions were conducted using a PerkinElmer Lambda 365 spectrophotometer.

Infrared (IR) Spectroscopy

IR spectroscopic analysis of the gel was performed using a Cary 660 FTIR spectrophotometer in ATR mode. Samples were prepared using 0.5% (w/v) of compound Gb in n class="Chemical">ethanol or chloroform and cured overnight. The experiments were carried out by placing a small amount of the gel sample on the crystal of ATR, and the data was recorded. For a solution sample, a small drop of a solution of compound Gb was placed on the crystal of ATR.

Gelation Method

Typically, gelation tests were done by adding the gelator (10 mg) to the required solvent (1 mL) in a sealed 3 mL vial and heated until the solid was dissolved entirely. Subsequently, the solution was slowly allowed to cool to room temperature, and the gelation was visually observed. The gel sample was produced, which did not show any gravitational flow in the inverted tube. All gels found were thermally reversible.

Rheological Studies

Rheological studies of gel samples were carried out by a Bohlin Gemini-2 Malvern rheometer using parallel plates (25 mm, stainless steel). The gap between the parallel plates was 500 μm. Gel samples were prepared by taking 1% of the corresponding gelators (w/v) in the desired solvents and cured for 12 h before measurements. A small portion of the gel was placed on the smooth plate of the rheometer by a spatula, and the gel was allowed to equilibn class="Species">rate for 10 min before starting the experiment. Measurements were performed in frequency sweep (0.01–50 Hz) mode. All experiments were repeated twice.

Oil Spill Recovery

10 mg of compound Gc was dissolved in 100 μL of n class="Chemical">THF, and the resultant solution was sprayed onto the test tube containing a binary mixture of 1 mL of diesel and 20 mL of water. The upper diesel phase of this biphasic mixture was immediately converted to gel within a minute. The aqueous phase remained. There was no movement of the gel layer in the presence of the aqueous layer observed in an inverted test tube, which indicated that the gel in the test tube was robust enough and supported its utility on a large scale for real use. The presence of a small amount of THF does not change the gelation ability of gelator in diesel, petrol, kerosene, and crude mineral oil. Phase-selective gelation was not affected in the presence of different salts (NaCl, KCl, MgSO4), acid, and base in the water medium. This property is strongly inspiring for the practical application of these gelators, forming a gel even in the oil phase in the presence of an oil–saltwater mixture such as seawater.

After the formation of diesel gel, it was successfully collected from the n class="Chemical">diesel–water mixture using tweezers/spatula. After that, we recovered the gelator Gc using vacuum distillation at above 125 °C. Gelation studies have found that the gelation ability of recovered Gc from the gel phase by vacuum distillation is the same as in the original state.

Separation of Bisphenol from Water

A stock solution of bisphenol was prepared by adding 3 mg of n class="Chemical">bisphenol to 25 mL of water, forming a series of standard stock bisphenol solutions, with the concentrations 0.12, 0.06, 0.03, and 0.015 mg/mL prepared by dilution. The corresponding absorbances of these stock solutions were measured and plotted against concentrations. 1 mL of 0.12 mg/mL bisphenol solution was taken in a vial, and to this, 1 mL of castor oil was added, followed by simultaneous sonication, heating, and the addition of 10 mg of gelator Gc. The mixture was cooled to room temperature and kept overnight in a refrigerator, causing a clear separation between the top oil gel and the bottom water layer. The top gel layer was scooped out by a spatula to get a clear water layer, which was then subjected to UV–visible spectroscopic studies to check the remaining bisphenol concentration in water.