Monoglyceride-Based Organogelator for Broad-Range Oil Uptake with

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Monoglyceride Based Organogelator for Broad-range Oil Uptake with High Capacity Dong Wang, Jian Niu, Zhenggong Wang, and Jian Jin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00053 • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 22, 2015

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Monoglyceride Based Organogelator for Broad-range Oil Uptake with High Capacity

Dong Wang,† Jian Niu,† Zhenggong Wang,†,‡ and Jian Jin†,* †

Nano-Bionics Division and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics,

Chinese Academy of Sciences, Suzhou 215123, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Oil/water separation has been a worldwide subject because of increasing release of oil-containing wastewater as well as several marine oil spills. The phase selective organogelators (PSOGs) are thought to offer a potential and effective implement for addressing this issue. An ideal PSOG for oil adsorption must fulfill some requirements involving effective gelation, easy synthesis, low cost, and recyclable for reuse. But beyond those, the ability of gelation for a broad-range oil phase without selectivity is also important. However, most of the reported PSOGs have limitation in this respect so far. In this paper, a new class of saturated 1-monoglyceride derived organogelators which can efficiently uptake almost all of the common fuel oils from water and gelate organic solvents with extremely low minimum gelation

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concentration (MGC). In addition, the oils in the gel and gelator molecules can be recovered quantitatively through simple vacuum distillation.

1. INTRODUCTION Organogels are formed via weak intermolecular interactions of molecules in a variety of organic liquids, among which low molecular weight organogelators (LMOGs) have attracted considerable interest since they provide not only a deep insight into the understanding of supramolecular assembly processes but also possess unique properties and a wide range of potential applications, such as oil recovery, toxicity remediation devices, electro-optical displays, drug delivery systems and as templates for the fabrication of nanomaterials.1–8 Oil/water separation has been a worldwide subject because of increasing release of oil-containing wastewater as well as several marine oil spills, which cause irrecoverable damage to the environment and ecosystem. The existing methods of uptaking oils by using dispersants, sorbents, or solidifiers still have some limitations to contain oil spills from a broad range of mixtures of water and oil and reclaim the oil.9–13 An effective and broadly applicable material for high-efficient oil adsorption is highly desired.

The phase selective organogelators (PSOGs) are thought to offer a potential and effective implement for addressing this issue.14 The first example of PSOG was demonstrated by Bhattacharya and Krishnan-Ghosh with amino acid amphiphiles.15 After that, various types of PSOGs have been reported.16–19 An ideal PSOGs for oil adsorption must fulfill some requirements involving effective gelation, easy synthesis,


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low cost, and recyclable for reuse. But beyond those, the ability of gelation for a broad-range oil phase without selectivity is also important since the oils spilled or discharged out are often species-diverse and have complicated features. However, most of the reported PSOGs have limitation in this respect so far. For instance, some of gelators can gelate only aliphatic hydrocarbons, but not aromatic hydrocarbons including benzene, toluene, and etc., and the gelation behavior is opposite for others.11, 17, 20–24 The recently reported sugar alcohol-derived gelators seem to be an all-rounder which can simultaneously gelate most of aliphatic and aromatic hydrocarbons, but are not effective to chloroform, tetrahydrofuran, and methanol.2 Herein, we present a new class of saturated 1-monoglyceride derived amphiphilic organogelators which can efficiently uptake almost all of the common fuel oils from water and gelate organic solvents with extremely low minimum gelation concentration (MGC) and high oil capacity. It shows a great potential to be used for remediation of spilled oil and cleansing toxic organic liquids from wastewater produced in industry and daily life.

2. EXPERIMENTAL SECTION 2.1 Materials. 1-Dodecanol (98%, Alfa), 1-hexadecanol (98%, Alfa), 1-octadecanol (98%, Alfa), itaconic anhydride (95%, Sigma-Aldrich), maleic anhydride (95%, Sigma-Aldrich), glycidol (96%, Sigma-Aldrich), ethyl alcohol absolute (≥99.7%, Sinopharm Chemical Reagent), n-hexane (AR, Sinopharm Chemical Reagent), acetic ether (AR, Sinopharm Chemical Reagent), acetone (AR, Sinopharm Chemical Reagent), chloroform (AR,


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Sinopharm Chemical Reagent), toluene (AR, Sinopharm Chemical Reagent), tetrahydrofuran (AR, Sinopharm Chemical Reagent), chlorobenzol (AR, Sinopharm Chemical Reagent),

ethylene

glycol

(AR,

Sinopharm Chemical Reagent),

cyclohexane (AR, Sinopharm Chemical Reagent), octane (AR, Sinopharm Chemical Reagent), undecane (AR, Sinopharm Chemical Reagent), dodecane (AR, Sinopharm Chemical Reagent), diesel (CP, Sinopharm Chemical Reagent), kerosine (AR, Aladdin), paraffin (CP, Sinopharm Chemical Reagent), olive oil (CP, Sinopharm Chemical Reagent), and deuterochloroform (CDCl3, Sinopharm Chemical Reagent) were used as received. Millipore deionized water was used in the experiments.

2.2 Synthesis. DGI were synthesized according to previous report as shown in Figure S1[1]. Typically, itaconic anhydride (25 g, 0.22 mol) was mixed and reacted with dodecanol (40 g, 0.22 mol) at 116 oC for 1 h to obtain dodecyl itaconate. After the reaction was completed, 100 ml of hexane was added to the above mixture at 80 oC under a vigorous mechanical stirring and the crude product of white crystal was precipitated. The crude sample of dodecyl itaconate was recrystallized from ethanol until the melting point of the sample was close to 76–78 oC. The obtained dodecyl itaconate (10 g, 0.034 mol) was dissolved in toluene (10 ml) and reacted with glycidol (7.5 g, 0.102 mol) at 105 oC with 20 mg (79.6 µmol) pyridinium p-toluenesulphonate as a catalyst. The mixture was refluxed for 5 h at 105 oC. When the reaction was finished, the crude product was applied to a silica-gel column and eluted with a hexane/ethyl acetate mixture (3/2 by volume). The collected DGI fraction was purified several times by recrystallization from an acetone/hexane mixture (1/1 by


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volume). The final pure product has a melting point of 76–78 oC. The synthesis procedures of HGI, OGI, and HGM were same as that of DGI. 2.3 Characterization. Scanning electron microscopy images were obtained on a fieldemission scanning electron microscope (Hitachi S4800, Japan). X-ray Diffraction was collected on a Bruke D8. Optical microscopy images were obtained on an OLYMPUS BX51. NMR analysis results were obtained on a Varian 400 MHz. Fourier transform infrared spectroscopy (FTIR) spectra were collected by using a Nicolet 6700 FTIR spectrometer. Rheological studies were performed on AR2000 rheometer.

3. RESULTS AND DISCUSSION 3.1 Formation and characterization of gels Three gelators with different alkyl chain lengths, namely dodecylglyceryl itaconate (DGI), hexadecylglyceryl itaconate (HGI), and octadecylglyceryl itaconate (OGI) were synthesized (Figure S1). During phase selective gelation process, the amphiphilic molecule is prone to self-assemble into nanofibers in liquid and sequentially form networks above the MGC and thus convert the liquid to opaque gel as shown in Figure 1a. In order to recognize the process of gelation, morphological feathers of the organogels (5% wt/v hexane gels) was investigated using optical polarized microscope (OM) and field emission scanning electron microscopy (FE-SEM) (HGI is taken as an example). The results as shown in Figure 1b and 1c indicate the formation of one-dimensional fibrous network structures in the organogel. The interwoven three-dimensional network structure could be clearly observed from


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the assembly of these nanofibers and the network structure is responsible for the entrapment of organic solvents. The individual fiber obtained from hexane has several hundreds of micrometers in length and about 20 µm in width. The strength of the organogels was studied by rheology test. Figure 1d shows the frequency response of a gel of 5% wt/v HGI in diesel. The elastic modulus G′ is much higher than the viscous modulus G′′ over the frequency range. This represents a nature of typical gels as it shows that the sample does not relax over long time scale. The value of G′ is a measure of the gel stiffness and its value of about 45 kPa indicates a gel with strong strength. Fig. 1e is the plots of moduli G′ and G′′ as a function of stress amplitude for the same sample. The stress amplitude at which the G′ and G′′ crosses each other is the yield stress of the gel, and its value of about 100 Pa is sufficiently high for the gel to support its own weight in an inverted vial. We then evaluated the ability of HGI to phase-selectively gelate oil in the presence of water. First, a high concentration of HGI was dissolved in hexane, and then an aliquot of the solution was added to a mixture of hexane and water in a vial. After a short time (~ 60 s), the gelation of oil phase was observed while the aqueous phase was left intact. In the photograph of an inverted vial in Fig. 1a, we can see that the oil gel is strong enough to hold not only its own weight but also the weight of the aqueous phase on the top, which is coincidence with the rheology test. Such efficient phase-selective gelation could be observed in the cases of many fuel oils such as diesel, kerosene and paraffin, and also mixtures of them, thereby indicating potential applicability to real oil-spill situation and treatment of refinery effluent.


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Figure 1. (a) Mechanism of gelation (The right is photograph of HGI organogel). (b) Polarized optical microscope image of self-assembled aggregates of HGI in hexane. (c) SEM image of the corresponding HGI gel in hexane. Dynamic rheology of frequency sweep (d) and oscillatory stress sweep (e) of HGI gel in diesel.

3.2 Evaluation of gelling ability of gelators The value of MGC is an essential indicator for evaluating gelling ability of gelators. The MGCs of DGI, HGI, and OGI molecules are summarized in Table 1. It can be seen that with the increase of alkyl chain length, their MGCs decrease accordingly, indicating longer alkyl chain gives better gelation performance. Among them, DGI can only gelate aliphatic hydrocarbons with ordinary MGCs. However, OGI could gelate almost all organic liquids including aliphatic and aromatic hydrocarbons with extremely low MGC except paraffin. The MGCs of OGI for diesel, n-hexane, undecane, toluene, ethanol, and chloroform are as low as 0.56, 0.54, 0.41, 1.2, 1.5, and 5.6% wt/v, respectively. These MGCs values rank in the top level of PSOGs


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reported so far and demonstrate powerful gelling ability of OGI for various organic liquids. HGI seems to be a real all-rounder, which can gelate all tested organic liquids such as paraffin, diesel, and kerosene with extremely low MGCs ranging from 0.4 to 0.7% wt/v. In other words, it could immobilize organic liquid more than 200 times its own dry weight. More importantly, besides common aliphatic and aromatic fuel oils, the gelators HGI and OGI could gelate even polar organic solvents such as chloroform, ethanol, and THF with similar low MGCs. Our gelators show outstanding versatility towards gelling organic liquids with high oil uptake capacity. In addition, all the gels are thermoreversible with Tgel of 38-55 °C corresponding to different organic solvents. The gels are stable and could be maintained for several months with no change. In addition, the MGCs of three representative organic liquids, diesel, toluene, ethyl acetate, from water solution containing 3.5 wt% NaCl was also measured in order to evaluate the gelation ability of our monoglyceride based organogelators under the condition of seawater. As shown in Table 1, the addition of NaCl has no effect on MGC values. The oil contents in water after gelation are measured by total organic carbon (TOC) analyser. The results show that for all the fuel oils and organic solvents tested in this work the oil contents in water after gelation are less than 50 ppm.


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MGCs (% wt/v, mg/100 μL) Liquid or oil n-Hexane Cyclohexane Octane Undecane Dodecane Diesel Diesel/brine Kerosine Paraffin Olive oil Chloroform Toluene Toluene/brine Ethyl acetate

DGI

HGI

OGI

2.0 1.8 3.0 1.3 1.5 2.3

0.65 0.51 0.73 0.52 0.51 0.70 0.70 0.60 0.40 0.70 7.5 1.6 1.6 2.1

0.54 0.43 0.60 0.41 0.43 0.56

Top level reported by others 1.0711, 0.715, 0.4120 2.52, 0.415, 0.6220 0.5111, 0.2415 0.3320 0.2715, 0.3520 2.52, 1.0311, 0.4120

0.54 I 0.58 5.6 1.2

1.5511, 0.3215, 0.520 2.02, 0.5115, 0.4320 1.02, 3.114 2.713 1.52, 0.23, 0.813

1.3

2.02, 0.913

5.0 1.0 1.5 2.0 1.4

1.013 1.413 3 0.2 , 0.913, 3.714

1.7 1.4 2.2 P 6.0 P

Ethyl acetate/brine Tetrahydrofuran Ethylene glycol Ethanol Acetone Chlorobenzol

2.1 P 4.0 P P P

7.8 1.1 2.0 2.6 2.0

Table 1. The MGCs of our monoglyceride based organogelators (DGI, HGI, and OGI) in various fuel oils and organic solvents. The given values are minimum gelation concentration of gelator (% wt/v, mg/100 µL). I = insoluble; P = precipitate. Brine is water solution containing 3.5 wt% NaCl.

3.3 Gelation-forming process of gelators The in-depth investigation of the intermolecular packing of the self-assembled 1-monoglyceride derived amphiphilic organogelators in the gel state was carried out by X-ray diffraction (XRD) diffraction and FT-IR spectra. Figure 2a shows the XRD pattern of the diesel organogel of HGI in wide angle and small angle (the inset) regions. In small angle region, it gives a distinct peak at 2θ = 2.11°, which corresponds to the d spacing value of 4.2 nm. Considering the molecular length of


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HGI is around 2.8 nm, the d spacing fits well a bilayer structure with a 41° tilt angle of alkyl chains (Figure 2c). The wide angle XRD pattern of the organogel shows periodical diffraction peaks, indicating that HGI molecules are self-assembled into a highly ordered structure. The periodic peaks at 7.51° (D/4), 9.39° (D/5), 11.29° (D/6), 13.19° (D/7), and 15.09° (D/8) indicate the existence of one-dimensional lamellar structure in the gel state.25 The peak at 2θ = 19.75° (d = 4.49 Å) corresponds to hydrogen bonding distance of gelator molecules in the gel state.11 The XRD results reveal that the HGI fibers are composed of repeating layered structural units of HGI bilayer. We therefore speculate that the HGI molecules firstly assemble into lamellar structure and the lamellae then stack each other under driven by van der Waals force to give a fibrous structure. Such an assembly is believed to be driven by intermolecular hydrogen bonding between the hydroxyl groups of adjacent molecules. The achievement of oil adsorption is ascribed to the migration of oil molecules into the interlamellar spaces through hydrophobic interaction between oil and alkyl chains of HGI. To further characterize the intermolecular interaction, FT-IR of HGI organosol and HGI organogel were recorded as shown in Figure 2b. The peak at 3413 cm-1 ascribed to the –OH stretching band for HGI in solution state shifts to 3280 cm-1 in the corresponding gel state, and the peak at 1716 cm-1 ascribed to the >C=O stretching band for HGI in solution state shifts to 1702 cm-1 in the corresponding gel state. These variations suggest the formation of strong hydrogen bond between HGI molecules after gelation. To further confirm the rationality of such packing, Forcite with the Dreiding force field and Gasteiger charges were used to calculate the packing


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of HGI molecules. As shown in Figure 2c, the result of theoretical calculation gives the optimized spacing of adjacent HGI molecules of 4.4 Å, which is in agreement with the XRD result. From the above discussions, we know that the transformation from lamellar phase units to the fibers is accompanied by the formation of intermolecular hydrogen bonds, and the hydrogen bonds network is proposed in Figure 2d. To further understand the relationship between the structure and the gel ability, a control experiment was carried out where hexadecylglyceryl maleate (HGM) was used to gelate oils (Figure S1 and S5). HGM has similar molecular structure to HGI but with a little different head group. Our results reveal that HGM cannot form stable organogel in common fuel oils and organic solvents. This is because that different from HGI, it is difficult to form hydrogen bonds between adjacent HGM head groups since the small volume of HGM head group is insufficient to form hydrogen bond in HGM bilayer (Figure S6), which has been confirmed by our previous reports.7 It is concluded that the strong hydrogen bonding network formed among HGI head groups contribute to the broad range gel-forming ability and to the low MGCs.


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Figure 2. (a) XRD pattern of HGI diesel gel in wide angle and low angle regions. (b) FT-IR spectra of HGI organosol and HGI organogel in diesel. (c) The simulation of packing structure of HGI molecules in organogel state. (d) Proposed hydrogen bonding network among HGI molecules correspondingly.

3.4 Recovery of gelators and oils The recovery of oils from the organogels and the recycling of the gelators are feasible in our system. Taking diesel as an example, the recovery of diesel and recycling of gelator from HGI gel is demonstrated in Figure 3. The gelation of 10 mL diesel in the presence of the same volume of water was firstly performed by adding a small amount of

HGI in ethanol to the above diesel/water mixture with the final concentration of

HGI 5% wt/v. The gelation of diesel occurred immediately, and the organogel was strong enough to keep stable state of diesel gel in inverted vial within one day. Afterwards, water was removed by syringe and diesel was collected almost


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quantitatively by vacuum distillation of the gel. The HGI as residue after vacuum distillation was then left at the bottom of the container and can be reused for oil uptake next time. Thin layer chromatography has proved that molecular structure of HGI was maintained during such a recycling process.

Figure 3. Phase selective gelation and recovery of diesel: (1) selective gelation of diesel after adding HGI gelator, (2) invert the flask, (3) remove water by syringe, (4) collect the distilled diesel, and (5) recycling HGI gelator.

4. CONCLUSIONS In summary, we have developed a new type of amphiphilic organogelators that is able to gel various fuel oils and organic liquids with high capacity. The gelators showed phase-selective gelation behavior and can separate the liquid oils or organic solvents by gelling the oil phase from the biphasic mixtures of oil-water at room temperature. The oil in the gel can be recovered quantitatively through simple vacuum distillation.


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Taking the advantages of easy preparation, environmental benign, versatility, and reusability into account, the molecular gelators holds future promise for remediation of oil spill and treatment of toxic organic liquid reagents.

ASSOCIATED CONTENT Supporting Information: Characterization data of gelators, preparation of gels, and analysis of gel-forming ability of gelators. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Key Project of National Natural Science Foundation (No.

21433012),

the

National

Basic

Research

Program

of

China

(No.

2013CB933000), the National Natural Science Foundation of China (Grant No. 21473239), the Key Development Project of Chinese Academy of Sciences (No. KJZD-EW-M01-3), and the Natural Science Foundation of Jiangsu Province (No. BK20130007).


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Langmuir

Table of Contents Graphic and Synopsis

Saturated 1-monoglyceride-based phase selective organogelators are successfully developed. These low molecular weight organogelators are able to efficiently uptake almost all of the common fuel oils and organic solvents from water with low minimum gelation concentration and high oil capacity.


Monoglyceride-Based Organogelator for Broad-Range Oil Uptake with

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