Porous amorphous powder form phase-selective organogelator for rapid recovery of leaked aromatics and spilled oils

Journal Pre-proof Porous amorphous powder form phase-selective organogelator for rapid recovery of leaked aromatics and spilled oils Baohao Zhang, Shipeng Chen, Hao Luo, Bao Zhang, Fumin Wang, Jian Song

PII:

S0304-3894(19)31414-1

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121460

Reference:

HAZMAT 121460

To appear in:

Journal of Hazardous Materials

Received Date:

14 August 2019

Revised Date:

10 October 2019

Accepted Date:

10 October 2019

Please cite this article as: Zhang B, Chen S, Luo H, Zhang B, Wang F, Song J, Porous amorphous powder form phase-selective organogelator for rapid recovery of leaked aromatics and spilled oils, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121460

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Porous amorphous powder form phase-selective organogelator for rapid recovery of leaked aromatics and spilled oils Baohao Zhanga,b, Shipeng Chena,b, Hao Luoa,b, Bao Zhanga*, Fumin Wanga*, and Jian Songa,b* a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China.

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Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China.

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Highlights

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Graphical abstract

Porous amorphous powder form gelator for fast oil spill treatment

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Powder form gelator for instantaneous gelation of aromatics Powder form gelator for rapid gelation of crude oils with broad viscosity The alkyl chain structures of gelator affect room temperature gel property

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Abstract Phase-selective organogelators (PSOGs) have drawn wide attention due to their potential applications in recovery of leaked aromatics and spilled oils. However, powder form PSOGs with fast gelling abilities and broad applicabilities are still limited. Herein, we developed three D-gluconic acetal-based gelators with different alkyl chains, all of which show excellent gel properties for hydrocarbon solvents. The spectroscopic and X-ray results revealed that the gel formation was the synergy of hydrogen bonding, π-π stacking and van der Waals forces. Surprisingly, the powder form gelator A with a cis double bond in the alkyl chain could instantly and selectively gel aromatic hydrocarbons, and also rapidly solidify crude oils with widely ranging viscosities from seawater at room temperature within minutes. Further research revealed that A powder exhibited porous amorphous morphology because the cis double bonds broke

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the crystalline chain-chain interdigitation between the assemblies. Therefore, the fast dispersion and recombination of fibers under the action of oil molecules lead to the fast room temperature gel process. Overall, a non-toxic and low-cost powder form PSOG with rapid room temperature phase selective gelation ability for a wide range of oils makes it promising for the emergency treatment of oil spill and aromatics leakage.

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Keywords: Supramolecular gel, Phase-selective organogelators, Environmental remediation, Oil spill recovery, Aromatics leakage

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1. Introduction

Water pollution caused by frequent marine oil spills and release of oily wastewater (especially

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aromatics) not only cause devastating negative effects on aquatic ecosystems but also lead to great economic losses.[1, 2] The conventional treatments for oil spill pollution involve in-situ burning, use of booms and skimmers[3], dispersants[4, 5], biodegradation[6], sorbents[7-9] and solidifiers.

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However, their practical application is limited by certain defects such as time-consuming, low recovery, slow degradation, secondary pollution or poor adsorption selectivity, etc. Therefore, developing attention.

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efficient and environmentally friendly technique for spilled oil treatment attracted increasing research Recently, the utilization of supramolecular phase-selective organogelators (PSOGs) which could

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selectively solidify the oil phase from their aqueous mixtures has recently gained significant interest as an emerging “smart” material to mitigate or partially solve oil contaminants on the surface of water.[10, 11] Since Bhattacharya and coworkers reported the first PSOG based on amino acid derivatives in 2001,[12] many different kinds of PSOGs have been developed including carbohydrates,[13-18] amino acids,[19-24] organic salts[25] and others[26-30]. However, the majority of hitherto developed PSOGs fail to achieve room temperature phase-selective gelation in powdered form because of the slow diffusion of the gelator in the oil. Heating-cooling process or carrier solvents as alternative aids were usually required to uniformly distribute PSOGs in oil phase to solidify the oil 2

layer. Such technical drawbacks make the solution-based method not feasible in real-life application. Thus, the use of powder form PSOGs directly at room-temperature conditions is the superior strategy for oil spill recovery. Zhang et al. reported a glucose-based PSOG which could gel aniline or nitrobenzene in powder form from the contaminated water within 1 min after a simple shaking at room temperature.[31] Sureshan and Zeng respectively reported powder form PSOGs for recovery of crude oil and these groundbreaking works demonstrated the potential application value of powder form PSOGs.[32-34] Despite of these great progresses in this filed, it is still of a great challenge to design a gelator with powder form gelation abilities at room temperature. As far as we know, the powder form PSOG with rapid gelling ability and broad applicability for aromatics, petroleum products and crude

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oil has not been reported yet. Therefore, there is a great need to develop new environmental-friendly powder form PSOGs with excellent room gelation properties for a wide range of oils. Moreover, the mechanism of the room temperature gelation process still remains obscure and further investigation on the relationship between PSOG structure and room temperature gelation property is still necessary. Herein, we developed a series of D-gluconic acetal-based gelators (Scheme 1) with different

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lipophilic alkyl chains, all of which show excellent gel properties for hydrocarbon solvents by the synergy of hydrogen bonding, π-π stacking and VDW forces between the alkyl chains. Excitingly, the

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pristine powder of gelator A could instantly and selectively gel aromatics, petroleum products, and also rapidly solidify crude oils with widely ranging viscosities from seawater at room temperature

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within minutes. SEM, FT-IR, and XRD studies revealed that gelators with irregular alkyl chains were prone to form porous amorphous powder, which was favorable for adsorbing the oil via the strong affinity between the oil molecules and alkyl chains on the surface of the powder. Thus, the fast

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diffusion and recombination of the powder fibers at room temperature achieved because the weak VDW force at the junction point of the intertwined fibers was easily broken by the oil molecules, and the rapid room temperature gelation properties were achieved. Our detailed structure-performance

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studies, as well as the subsequent studies on the room temperature gelation mechanism for D-gluconic acetal-based PSOGs, may provide some insights for better understanding of the room temperature

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gelation phenomenon.

Scheme 1. Chemical structures of gelator A, B and C.

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2. Experimental 2.1 Synthesis. All the compounds were synthesized by a simple two-step process based on our previous report.[17] Synthetic routes of gelators are shown in Scheme S1. The details of synthetic procedures and characterization data of every gelator are recorded in Supporting Information. 2.2 Gelation tests Gelation tests were investigated by a typical “stable to inversion” method. Briefly, the quantitative

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gelators were added to a tube with an inner diameter of 10 mm containing the tested solvent by heating to obtain a homogeneous solution and then cooled to room temperature. Formation of a gel was confirmed by a quasi-solid state without liquid flowing upon inversion the test tube. Gel-sol phase transition temperature (Tgel) measurements were determined by a conventional “ball-drop method”. A 0.15g glass ball with a diameter of 5 mm was placed on the top of the gel in a test tube which was in a

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thermostated oil bath and was heated at ca. 1.5 °C/min. The temperature corresponding to submersion of the glass ball in the solution was regarded as the Tgel of the gel. The measured experiments were

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carried out in duplicate. Critical gelator concentrations (CGCs) were determined at 25°C by the inverted tube method using a series of gels in which the gelator concentrations were changed in 0.01%

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increments. The room temperature gelation ability was formed by adding a quantitative gelator to the solvent with slightly agitation at 25°C, and then determining whether the gel was formed by an inverted

2.3 Characterization

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tube method.

All the NMR studies were carried out on a Bruker DPX 400MHz spectrometer using cryo probe.

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The mass spectra were recorded using a TOF-QII high-resolution mass spectrometer. Field Emission Scanning Electron Microscope (FESEM) were performed by a Hitachi S-4800 SEM instrument operating at 3-5 kV. The samples were prepared by dropping the diluted solution of gels on the thin

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aluminum sheets and then dried under vacuum for 24 h. The samples were coated with a thin layer of Au before test. Fourier transform infrared (FT-IR) spectra measurements were collected by a FTS3000 spectrometer with KBr pellets. Powder X-ray diffraction (PXRD) diagrams were obtained by using a Bruker D8-S4 (CuKα radiation, λ=1.546 Å), operating at 40 KV and 100 mA. The d spacing values were calculated by Bragg’s law (nλ=2d sin θ). Nitrogen adsorption-desorption experiments were carried out using Autosorb-iQ2-MP surface area and porosity analyzer at 77K. Pore size distribution and specific surface area were calculated by Barret-Joyner-Halenda (BJH) method and BrunauerEmmett-Teller (BET) method, respectively.[35] Porosity and pore volume were determined by 4

mercury porosimetry using a AutoPore Iv 9510 device. The pressure range was from 0.1 to 60000 psia.[36] Rheological measurements were carried out using a strain-controlled rheometer (Anton Paar Physica MCR 301) equipped with steel-coated parallel-plate geometry with a diameter of 15 mm. The gap distance was fixed at 0.5 mm. A solvent trapping device was placed above the plate and measurement was set at 25°C in order to avoid solvent evaporation. The frequency sweep was conducted at a constant strain of 0.1%. Strain sweep was performed at a constant frequency (1 Hz). The contact angle (CA) measurements were measured by a JC2000D1 contact angle measuring instrument (POWEREACH, Shanghai, China). The samples were pressed into sheet before test. Theoretical calculation was realized by Gaussian 09 program and geometry optimizations for all

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structures were performed using the semiempirical method AM1. 3. Results and discussion 3.1 Gelation ability tests

The gelation abilities of the as-prepared gelators were first investigated by the heating-cooling

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method in 9 solvents including aromatics, alkanes and refined petroleum products (gasoline, diesel and kerosene) and the results were summarized in Table S1. The three gelators exhibited excellent gelation

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abilities in above tested solvents, suggesting that the fine-tuning of the side chain did not compromise their overall gelation abilities. The CGC values (%w/v, g/mL) in above solvents ranged from 0.07 to

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0.28%w/v for A, from 0.12 to 0.36%w/v for B, and from 0.05 to 0.18%w/v for C, respectively (Table S1). In general, the CGCs of A and B were slightly higher than that of C (Fig. S1a), indicating that the presence of irregular alkyl chains enhanced the solubility of the gelator in hydrocarbon solvents.

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Remarkably, gelator A and C could induce gelation of fuel oils (e.g. diesel and kerosene) with CGC values below 0.1% w/v, falling into the category of “super-gelators”. In addition, the Tgel values of gelator A and B with irregular alkyl chains were generally lower than those of C (Fig. S1b), confirming

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that the structures of the alkyl chains had a remarkable effect on the thermostability of the gels. Even so, the Tgel values of the fuel oil (gasoline, diesel and kerosene) based gels were all more than 100°C,

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confirming their excellent thermal stabilities. The gelation abilities of the three gelators remained unaltered even after several sol-gel interconversion cycles, suggesting the thermoreversible nature of the gels. All the gels were found stable at room temperature with no noticeable change after storage in closed containers for several months, further demonstrating their stabilities. It is worth noting that these three gelators were insoluble in water even under heating conditions (Table S1), which was mainly due to the hydrophobicity of the structures, which makes them promising for oil-water separation as phaseselective gelators. Then, the room temperature gelation abilities of A, B and C were examined both in aforementioned 5

pure hydrocarbon solvents by simple dispersing small amounts of gelator powder to the above solvent system. A “stable to inversion” method employed to confirm the formation of the gel, and the results summarized in Table S2. Excitingly, gelators A and B (with a cis double bond or branched structure in the alkyl chain) both showed room temperature gelation properties for the tested solvents. By contrast, the C powder failed to gel these solvents under the same conditions. Furthermore, gelator A induced gelation of all tested solvents with powder critical gelation concentration (PCGC) values ranging from 1.4 to 2.3%. Whilst the PCGCs of B were relatively higher than A, ranging from 2.0 to 3.7%. These results suggested that gelator A with a cis double bond in the alkyl chain could achieve a better balance between the dispersion and recombination of the fibers in the hydrocarbon solvents. Overall, the fine-

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tuning of the alkyl chain via the introduction of a middle cis double bond or branched structure could significantly enhance room temperature gelation abilities of the D-gluconic acetal-based gelators in hydrocarbon solvents.

3.2 Insight into the self-assembly mechanism and room temperature gelation process

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In order to investigate the self-assembly information of these gels, the morphology of the xerogels obtained from toluene and gasoline gels of three gelators were examined by SEM. As shown in Fig.

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S2, gelator A and B were primarily self-assembled into one-dimensional (1D) fibers (with diameter of 80 and170nm, respectively) and the fibers further entangled into a three-dimensional (3D) network

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morphology (Fig. S2a-d), which might be caused by the irregular arrangement of the alkyl chains on the fiber surface. However, the fibers of C xerogels were closely packed in same direction (Fig. S2e, f) because of the strong VDW force between alkyl chains on the surface of adjacent fibers, leading to

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the higher thermostability of C gels than A and B (Fig. S1b).

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Fig. 1. (a) FT-IT spectra of chloroform solution and toluene xerogel of gelator A. (b) Partial 1H NMR spectra of 20

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mg gelator A in 0.6 mL mixed solvent of C6D6 and DMSO-d6 (20:1) as the temperature increases from 25 to 70°C. (c) Powder XRD spectrum of A toluene xerogel. (d) Possible self-assembly mechanism of gelator A provided by

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Gaussian 09 AM1 method.

To investigate the driving forces for the self-assembly of gelators, FT-IR and 1H NMR experiments

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were performed. Taking gelator A as an example (Fig. 1a), the typical OH (or NH), amide C=O and CH2 stretching frequencies that appeared at 3534, 1678, 2932 and 2863 cm-1 in the chloroform solution state shifted to 3382, 1637, 2925 and 2854cm-1 in the toluene xerogel, respectively. These lower

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wavenumber shifts indicated that hydroxyl and amide groups might be involved in hydrogen bonding and the interaction between the alkyl chains via the VDW forces in the gel state. The similar trends were also observed for gelator B and C (Fig. S3, 4). The hydrogen bonding in gel formation was also

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confirmed by the upshifted signals of NHa and OHb, OHc, OHd protons from 6.67, 5.08, 4.74, and 4.12 ppm to 6.60, 4.51, 4.10, and 3.52 ppm as the temperature increased from 25 to 70°C in 1H NMR

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spectroscopic analysis (Fig. 1b). It is notable that the CONH tended to be in a trans conformation in the optimized monomer. The NH group formed intramolecular hydrogen bonds with oxygen atom on the acetal, so that the signal of NH proton did not change significantly with the increasing of the temperature.[37] Additionally, the protons of the benzene ring also moved slightly shift to upfield as the temperature increased, indicating that the benzene ring participated in assembly via π-π stacking.[29] Based above finding, the gelation process of three gelators was mainly dominated by the synergy of hydrogen bonding, π-π stacking, and VDW forces between alkyl chains. To further investigate the detailed structural information of the self-assembly, powder XRDs have 7

been performed. The XRD diffraction patterns of the toluene xerogels showed that all three gelators adopted a typical lamellar organization in the gel state. Typically, xerogels of A displayed four ordered reflection peaks at 3.26, 1.64, 0.86 and 0.43 nm (Fig. 1c), indicating the xerogels maintained a lamellar organization with periodicity of 3.26 nm. This value is larger than the calculated geometry-optimized single molecular length of A (2.68 nm), but smaller than twice molecular length, strongly confirming an interdigitated layered morphology between adjacent assemblies through the VDW force interaction.[19, 38] In addition, simulated assembly pattern also showed that layer spacing between the 1D assemblies was 3.26 nm and the distance between the neighboring benzene rings was 0.45 nm (Fig. 1d), which were consistent with the XRD pattern (3.26 and 0.43nm). Therefore, based on the

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above experimental analysis combined with the theoretical calculation, a feasible self-assembly mechanism of gels was proposed in Fig. 1d. The gelators self-assembled via intermolecular hydrogen bonding between C=O group and the hydroxyl groups to form a two-molecule adduct, which aggregated to form 1D assembly through π-π stacking between the adjacent benzene rings. The assemblies further aggregated to form fiber bundles by VDW forces between the alkyl chains packing,

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which then intertwined to form a gel network with solvents trapped.

On the other hand, it should be noted that the packing in the aliphatic region are different for three

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gelators depending on whether the alkyl chain is saturated, unsaturated or branched. The saturated analogue C showed a longer d-spacing of 4.09 nm. Gelator A and B exhibited much shorter spacing

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(3.26 and 2.63 nm, respectively) due to the bending or branching effect of the alkyl chain (Fig. S9-11). Furthermore, the absence of sharp signals in the wide-angle area of A and B as observed in XRD patterns (Fig. 1c, S7-8), possibly due to the poor crystallization in the aliphatic region. It is reasonable

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to argue that the alkyl chains of gelator A and B cannot densely pack or crystallize like the saturated alkyl chain analogue in C, and the side chains of A and B has a fluid-like nature.[39] Therefore, the strong VDW force between the fiber bundles endows the xerogels of C have a more tightly packed

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morphology than those of A and B (Fig. S2).

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Fig. 2. (a) Powder XRD spectra of A, B and C powder. (b) FT-IT spectra of A, B and C powder. SEM images of (c, c’) A pristine powder, (d, d’) B pristine powder and (e, e’) C pristine powder.

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It is well known that the formation low molecular weight organogels (LMOGs) are usually assisted by external energy (such as heating, sonication etc.) leading to a 3D network structures trapping the

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solvents by surface tension and capillary force.[40, 41] Hence, the fast dissolution of powder form gelators in solvent at room temperature may contradict with the self-assembly tendency of gelators.[34] For a long period, room temperature gelation by powder form gelator has been considered as a “special”

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gel process, and there is no consensual understanding of the formation mechanism of room temperature gel.[32, 34] In order to understand the mechanism of this room temperature gelation system, it is

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necessary to conduct further investigation on the pristine powder of gelators. All the pristine powders of three gelators were obtained in the purification procedure by precipitating the gelator-containing methanol solution and further drying under vacuum at room temperature without further operation.

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Firstly, through comparing the XRD pattern of the three pristine powders with their corresponding toluene xerogels (Fig. 2a, 1c, S7-8), it was clearly found that the powder and its corresponding xerogel

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both adopted basically same self-assembly patterns as discussed above. The sharp diffraction peaks in the aliphatic region of XRD pattern of C powder indicated a much more ordered molecular arrangement, while the absence of sharp signals in the aliphatic region for A and B powder strongly confirmed the inadequate crystallization of the alkyl chains.[38, 39] These results were further supported by FT-IR spectra (Fig. 2b). The CH2 stretching vibrations of C powder appeared at 2921 and 2850 cm-1, confirming all carbon atoms were in a zigzag pattern and the alkyl chains packed in wellordered form.[42] However, the corresponding bands of A and B both appeared at 2925 and 2854 cm1

, indicating the alkyl chains in A and B could not regularly pack or crystallize due to the bending or 9

branching effect of alkyl chains.[39] Besides, the contact angle values of dodecane and water with A powder were 27.17 and 94.34° respectively, further illustrating that the surface of the powder assembly was still covered by the hydrophobic alkyl chains which enhanced the affinity with oil (Fig. S12). Furthermore, the morphology of pristine powder obtained by SEM (Fig. 2) also supported the results obtained by XRD and FT-IR. The powder particles of C showed a planar structure composed of closely packed 1D fibers, while A and B had a loose porous 3D network structure comprising of interconnected fibers due to the VDW force. Furthermore, the porous structures of A and B powder were characterized by nitrogen adsorption-desorption experiments and mercury intrusion porosimetry in Fig S13. The porous structure and the lipophilicity of the surface made it easier to adsorb oil solvents. In addition,

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these gelators have very low CGC in oils, such the poor solubility seems to prove that the gel formation at room temperature is the dispersion and recombination of the powder fibers under the action of solvents rather than the dissolution and reassembly process of gelator molecules. By comparing the morphology of their corresponding xerogels, we found that the fibers of powder and xerogel have the substantially same diameter. Thus, it is reasonable to preliminarily speculate that the junctions between

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the fibers were broken upon contact with oil while the interior of powder fiber kept intact in the room

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temperature gelation process.

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Fig. 3. Possible hierarchical self-assembly mechanism revealed the difference between room temperature gelator A and non-room temperature gelator C. Both of A and C are self-assembled to form a 1D layered assembly by hydrogen bonding and π-π stacking and then inter-columnar aggregate to form 1D fiber structure. The strong VDW force between adjacent fibers of C resulted in close packing structure, which was fail to de-packing by oil molecules. Conversely, weak VDW force between the fibers of gelator A given a relatively loose porous amorphous structure and the junction zones were easily to be destruct by oil at room temperature, thus room temperature gelation was achieved.

Based on the above analysis, a reasonable diagram of hierarchical room temperature self-assembly mechanism was proposed in Fig. 3. For this D-gluconic acetal-based gelators obtained by precipitation 10

from methanol, a 1D layered assembly was formed by alternating hydrogen bonding and π-π stacking between molecules and then further aggregated to form 1D fibers via VDW force. For gelator C, the strong VDW forces between of alkyl chains on the surface of the fibers resulted in further aggregating to form close packing structure. Understandably, the C powder had poor dispersibility in oil at room temperature because the intermolecular crystalline chain-chain packing was too strong to be destructed by the oil molecules at room temperature. Extremely large enthalpies were thus required to destruct the intermolecular crystalline interdigitation of alkyl chains[43, 44] and the gelation could be realized by heating-cooling process as an alternative way. Whereas the presence of cis double bond in the alkyl chains led to a relatively weak VDW force between the fibers of gelator A, giving a relatively loose

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porous amorphous structure. Therefore, the porous structure of the A powder and the lipophilicity of the fiber surface could show strong affinity with the oil. Hence, the weak VDW forces in junction zones between fibers were easily to be destructed by oil solvents at room temperature. When a certain concentration was reached, the dispersed fibers spontaneously intertwined again by the interaction of properties of A in hydrocarbon solvents were obtained.

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the VDW force in the surface of the fibers, and eventually, the excellent room temperature gelation

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3.3 Excellent room-temperature phase-selective gelation for aromatics and crude oils

Fig. 4. Recovery process of toluene from oil-water mixture: (a) Biphasic mixture of 15 mL toluene (dyed with

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Disperse Red 152) and 200mL water. (b) Sprinkling A powder over the layer of toluene (2%w/v). (c) The toluene layer was gelled instantly. (d) The toluene gel was removed with a spoon net. (e) The removed toluene gel. (f) Recovered toluene (13mL, the recovery rate was 86.67%) and powders by vacuum distillation. (All processes were

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operated in a fume hood and the experimenter was equipped with a gas mask.)

The aromatics leakage on the surface of waters seriously threat the aquatic ecosystem due to the toxic, mutagenic, and carcinogenic characteristics of aromatic hydrocarbons.[45, 46] Given the excellent room temperature gel properties of gelator A both for aromatics and alkane solvents compared with B and C, the phase-selective gelation ability of powder A for above hydrocarbon solvents in the presence of seawater were tested. For instance, 19 mg gelator A was added to the biphasic system containing 1 mL toluene and 2 mL seawater. After slight shaking to imitate the waves 11

action, the oil phase was solidified instantly and the water phase remained intact. Surprisingly, A could also instantly gel other aromatics such as benzene and o-xylene from their biphasic system in powder form. As shown in Table S2, the biphasic critical gelation concentrations (BCGCs) of A were consistent with its PCGC values, because the natural hydrophobicity of the gelator made itself to preferentially interact with the oil rather than the water phase. The aromatic solvent recovery in large scale was then simulated by sprinkling 300mg of A powder into the 15 mL toluene-200 mL water mixture (Fig. 4). It was observed that the toluene layer was gelled instantly without any agitation, and the water phase kept intact. The solidified toluene was easily skimmed off with a net spoon to achieve separation. Furthermore, toluene and gelator A could be separated by simple distillation (Fig. S13), and the

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recovered gelator could be reused after grinding. The instant phase-selective gelation abilities could efficiently slow down the spreading of the aromatics on the water surface, indicating their potential application for emergency treatment of aromatics leakage.[18] As far as we know, this is the fastest powder form room temperature PSOG for aromatic solvents.

The powerful room temperature phase-selective gelation ability of powder gelator A in

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aromatic/water mixtures then prompted its further evaluation for the treatment of marine oil spills. Herein, room temperature phase selective gelation abilities of A powder in the biphasic systems

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involving gasoline, diesel and seven types of crude oils with widely viscosities ranging from 0.4 to 8384.7mPs·s were investigated. The physical properties of these crude oils (viscosity, density and API,

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see Table S3) basically covered all the crude oils employed by the currently reported PSOGs and should represent the most of traded crude oils.[30, 33] In a typical procedure, the quantitative amount of powder of gelator A was added to biphasic systems containing 1 mL of oil and 3 mL of seawater.

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After slight shaking to imitate the wave action, the oil phase was fully gelled with the aqueous layer untouched and the gels were stiff enough to hold the weight of themselves and the water (Fig. 5c). The corresponding powder biphasic CGC (BCGC, %w/v) values were summarized in Table S4. For light

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oils, the BCGC values were ranging from 2.0 to 3.6 %w/v. For heavy crude oils, the values ranged from 3.9 to 7.0% w/v. In general, crude oils with higher viscosities required more gelators to achieve

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full gelation. More importantly, the obtained BCGC values remained unchanged in presence of seawater or acidic/basic water media (as shown in Figure S15-16), making it suitable for use under even harsher conditions. The gelling times of powder A at room temperature during oil spill recovery was thus examined (Table S4). For light oils, such as gasoline, diesel, QingHai Light, XinJiang Light with viscosity ranging from 0.4 to 10.6 mPs·s, powder A could gel the oil phase within 1 minute at 5% w/v. For heavy crude oils, the gelling times were slightly longer, ranging from 2.4 to 7.5 minutes at 10 %w/v. The increased viscosity of crude oils retarded the rapid dispersion of the gelator in oils. Furthermore, by 12

comparing the weights of recovered oil with the original weight, the recovery rates of oils were found to be all above 96.4%, indicating that the majority of the spilled oil has been recovered. The recovered gasoline and diesel gels could be distilled to recover the oil and gelator. For the crude oil, only the light components could be recovered by distillation, and the remaining high-boiling components and gelators could be further refined in the refinery to realize resource utilization. Besides, the gelling times in cold water were further investigated (Table S4). The gelling times for the heavy crude oils increased to more than 15 min at 15°C due to the increased viscosities of the crude oils at low temperature, whilst gelator A could still instantly solidify light oils such as gasoline, diesel and Qinghai light at 15°C. However, XinJiang Light, ZhongJie Heavy, Napo Heavy oils have self-

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solidified without adding gelator at 15°C. When the temperature decreased to 5°C, all the crude oils self-solidified (Fig. S17). Interestingly, the light oils such as gasoline could still be gelled instantly in cold seawater even at 0°C in the presence of floating ice (Video S2†). Hence, gelator A could be considered as an efficient gelator employed as powder at room temperature with fast gelling abilities

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and exceptional universality towards crude oils with different viscosities.

Fig. 5. Dynamic rheology of Napo heavy gel formed by A powder(10%w/v): (a) frequency sweep and (b) stress sweep. (c) Phase-selective gelation of A powder in different oils in the presence of seawater at room temperature at their BCGC, from left to right: gasoline, diesel, QingHai Light, Xinjiang Light, ZhongJie Heavy, WuHan Heavy, Arab & Napo Heavy, BoHai Heavy, and Napo Heavy. The phase selective gelation of QingHai Light oil by pristine 13

A powder: (d) biphasic mixture of crude oil and seawater (35 mL/300mL), (e) spreading A powders (1.75g, 5%w/v) over the crude oil layer, (f) gelation of the crude oil layer, and (g) crude oil gel removed with a net spoon.

For practical applications, the floating gels should be strong enough for easy collection and separation. Accordingly, oscillatory rheology analyses were performed to examine the strength of recovered oil gels formed by A powder (Fig. S18). For all gel samples, the storage modulus G’ was found to be independent of frequency and considerably higher than the loss modulus G’’ in the entire range of frequency sweep, confirming the gel nature. Besides, the G’ values were recorded between 2×103-7.5×103 Pa for light crude oil samples with viscosities of 0.4-73.7 mPs·s, and between 1.5×1045.5×104 Pa for heavy crude oils with viscosities of 83.2-285.5 mPs·s. For the most viscous Napo heavy

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crude oil, the G’ value reached 3×105Pa, indicating the excellent mechanical strength of the gels (Fig. 5a, b). The yield stress values of all crude oil gels were found above 100 Pa, suggesting that the gels could withstand high pressures. These mechanical properties made the gels suitable for simple salvage during oil recovery processes.

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More importantly, since no toxic carrier solvents were required for practical applications of gelator A in oil spill treatments, the recovery process would be environmentally benign. Besides, the acute toxicology study of the gelator A with a group of adult zebrafish further confirmed its non-toxic

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characteristic (Fig. S19). In this regard, the recovery of crude oil from marine oil spills was then simulated in large-scale platforms (Fig. 5d-g). An oil film was prepared by mixing crude oil (35 mL,

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QingHai crude oil) with seawater (300mL). When the A powder (1.75g, 5%w/v) was sprinkled into the prepared biphasic system, the oil phase gelled instantly after slight agitation. The floating crude oil gels could be scooped off easily with a net spoon. As an environmentally friendly powder form gelator

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with fast phase-selective gelation abilities, broad applicabilities, gelator A could be considered as a promising material for potential used in oil spill recovery.

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4. Conclusion

In this current work, by introducing irregular structures (cis double bond or branched structure) into

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the alkyl chain of D-gluconic acetal derivatives, we developed efficient porous amorphous powder form phase-selective gelators for aromatics, refined petroleum products and crude oils with widely ranging viscosities at room temperature. Light oils could be gelled instantly by A powder at room temperature. Even the viscosity of crude oils reached as high as 8384.7 mPs·s (Napo Heavy oil), with gelation completed within 7.5 min. The recorded BCGC values ranged from 3.0 to 7.0% w/v. The nontoxic, fast gelation ability, excellent mechanical properties, high recovery rates and broad applicability of this PSOG made it as a promising material for the emergency recovery treatment of oil spill and aromatics leakage. Furthermore, it is proven that the introduction of irregular structures (such as cis 14

double bond) in the alkyl chain is the key to the room temperature gelation ability. These findings might shed a light for the future molecular design of more efficient powder form PSOGs in the treatment of oil spills and aromatics leakage. Conflict of interest The authors declare no competing financial interest. Acknowledgments We thank to the financial support of the National Natural Science Foundation of China (Grant Nos.

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21676185) and Tianjin science and technology innovation platform program (No. 14TXGCCX00017). Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version.

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