Superoleophilic and Reinforced Ethyl Cellulose

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Superhydrophobic/Superoleophilic and Reinforced Ethyl Cellulose Sponges for Oil/Water Separation: Synergistic Strategies of Crosslinking, Carbon Nanotube Composite and Nanosilica Modification Yeqiang Lu, and Weizhong Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09160 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

Superhydrophobic/Superoleophilic and Reinforced Ethyl Cellulose Sponges for Oil/Water Separation: Synergistic Strategies of Crosslinking, Carbon Nanotube Composite and Nanosilica Modification

Yeqiang Lu, Weizhong Yuan∗ School of Materials Science and Engineering, Key Laboratory of Advanced Civil Materials of Ministry of Education, Tongji University, 201804, People’s Republic of China

ACS Paragon Plus Environment

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ABSTRACT Superhydrophobic/superoleophilic and reinforced ethyl cellulose (SEC) sponges were prepared by cross-linking EC with epichlorohydrin (ECH) and complexing with silanized carbon nanotubes (Si-CNTs) followed by coating nanosilica on the surface of porous sponges and subsequent modification with hexadecyltrimethoxysilane (HDTMS). These synergistic strategies endowed the SEC sponges with the superhydrophobic/superoleophilic properties (θwater=158.2°, θoil=0°, sliding angle=3°) and outstanding mechanical properties (could bear the pressure of 28.6 kPa without damage). The unique micro-nano structures and properties of the porous sponges were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and water contact angle measurements. The as prepared SEC sponges with high mechanical strength were able to collect a wide range of oils and organic solvents with absorption capacity up to 64 times of their own weight. Furthermore, the absorption capacity of the sponges decreased slightly to 86.4% of its initial value after 50 separation cycles, suggesting their excellent recyclable performance. The high efficiency and endurability of the sponges during oil/water separation made them ideal absorbent in oil spillage cleanup.

KEYWORDS Oil/water separation, ethyl cellulose, superhydrophobic/superoleophilic, cross-linking, carbon nanotube composite, reinforcement

1. INTRODUCTION Over the last three decades, the demand for fossil oils has increased in a dramatic manner.1, 2 The extraction of crude oil results in frequent occurrence of oil spills, such as the Gulf of Mexico


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oil spill accident in 2010—the largest accidental oil spill in the history.3 The huge quantity of oilcontaminated water is catastrophic for the marine environment and aquatic ecosystem.4 Besides, domestic and industrial waste oil may have a broad impact on city ecological environment and human health. Therefore, it is imperative, but also challenging, to explore high-performance absorbent materials possessing high absorption capacity, high selectivity, and high efficiency. Three-dimensional

(3D)

porous

materials

with

interconnected

structure

and

hydrophobic/oleophilic surface have been extensively investigated as the functional architectures obtained can be used as efficient absorbents for the removal of spilled oils or hazardous chemicals from water.5-9 A lot of effort has been devoted to developing porous absorbents with outstanding properties, such as low density, high porosity, desirable wettability, high oil absorption capacity, excellent oil/water selectivity, environmental friendliness, and good recyclability. Among the wide absorbent materials, polyurethane (PU),10,

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poly(melamine-

formaldehyde) (PMF),12, 13 poly(dimethylsiloxane) (PDMS),14 polyvinylidene fluoride (PVDF),15 and carbon/graphene-based sponges,16, 17 have been the most intensively investigated. Owing to the hydrophobic/oleophilic surfaces, these materials can only be wetted by organics and oils; and the 3D porous structures provide efficient channels and spaces for the diffusion and enrichment of organic liquid. However, some limitations still exist in these wide used absorbents, such as the use of expensive modifying agents, secondary pollution, poor selectivity and recyclability, complicated synthetic process, and high price of the equipments.18, 19 Hence, great effort should be dedicated to developing a robust, renewable, sustainable, and widely abundant absorbent material with excellent oil/water selective absorption ability. Recently, natural materials, like cellulose, starch, lignin and chitosan, are receiving increasing attention in the preparation of oil/water separation materials due to their low cost, good


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availability and nontoxicity to humans.20-23 Among them, cellulose is one of the most abundant, sustainable and environmental-friendly materials in nature. Zhang et al. reported an ultralightweight and flexible silylated nanocellulose sponge for selective removal of oil from water.24 Sai et al. reported hydrophobic bacterial cellulose aerogels (HBCAs) through a trimethylsilylation reaction in liquid phase followed by freeze-drying.25 Zhou et al. fabricated microfibrillated cellulose aerogels with superhydrophobicity, low density, and high porosity via a facile freeze-drying process followed by a silanization reaction.26 Although various novel cellulose based absorbents have been successfully prepared, practical applications of cellulose absorbents have been hampered by the laborious strategies, expensive precursors, complex equipment and poor recyclability. The dissolution processes of cellulose typically demand the use of large quantities of solvents and chemicals, such as N, N-dimethylacetamide (DMA)/LiCl,27 N-ethylmorpholine-N-oxide (NMMO),28 ionic liquids (ILs)29 and alkali/urea or thiouea.30 Moreover, almost all the reported cellulose absorbent materials showed relatively low mechanical strength and ease of deformation which could make negative influence on the retrieval of the oil-saturated sponge and restrict the reuse of sponge. For example, an absorbent material with relatively poor mechanical strength may suffer from deformation in the extraction process from oil/water mixture, leading to the drippage of oils and contamination of environment.31, 32 Different from cellulose, as one kind of important cellulose derivatives, ethyl cellulose (EC) is a non-toxic, stable, compressible, inert polymer and can be dissolved in common solvents (e.g., tetrahydrofuran, ethanol, chloroform, acetone, and toluene). Moreover, in contrast to the strong affinity to water of cellulose due to the preponderance of hydroxyl groups,33 EC is intrinsically hydrophobic and oleophilic which possesses good oil absorbency, water repellency, excellent


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acid and alkali resistance, and salt tolerance.34-37 Nevertheless, EC is known to be a brittle polymer, which is easily ruptured under stress. And the hydrophobicity of EC-based sponges can be further improved to achieve superhydrophobicity. Hence, EC-based sponges have great potential as a high-efficient and endurable absorbent for industrial applications and oil spill treatments. Herein, we attempted to construct a superhydrophobic and superoleophilic ethyl cellulose sponge with high mechanical strength and excellent oil absorption capacity to be employed as highly efficient and endurable oil/water separation absorbent. Generally, chemical cross-linking is known as an important strategy to increase the resistance to solvents and mechanical properties of polymers.38 Owing to the presence of residual hydroxyl groups in EC backbone, EC can be cross-linked by epichlorohydrin (ECH) in alkaline conditions. With outstanding mechanical properties, carbon nanotubes (CNTs) have emerged as a new material and attracted much interest in the field of CNTs/polymer nanocomposites.39, 40 But up to now, the CNTs/polymeric porous nanocomposites were rarely investigated and reported.41 Little work has been done on the 3D porous absorbent materials with excellent mechanical properties. In this work, synergistic strategies of cross-linking EC with ECH and complexing with Si-CNTs followed by superhydrophobic

surface

modification

were

employed

to

prepare

superhydrophobic/superoleophilic sponges with outstanding mechanical properties (as shown in Figure 1). The superhydrophobic sponge exhibited many remarkable properties, including very low density, high porosity, good mechanical strength and excellent oil absorption performance. Furthermore, the sponges showed excellent recyclability and durability. To the best of our knowledge, it is the first time to prepare an absorbent sponge which possesses excellent mechanical properties and at the same time exhibits good oil/water separation capacity and


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recyclability. The outstanding properties made the sponges an ideal candidate for absorbing oil and organic pollutants.

Figure 1. Schematic illustration of the preparation of SEC sponges: (a) The image and contact angle of cross-linked EC sponge; (b) The image and contact angle of CNTs reinforced EC sponge; (c) The load-bearing image, contact angle and oil absorption of SEC sponge.

2. EXPERIMENTAL SECTION 2.1 Materials Ethyl cellulose (EC) and the hydroxylated multi-walled carbon nanotubes (diameter: 8–15 nm) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). Epichlorohydrin (ECH), tetraethoxysilane (TEOS) and hexadecyltrimethoxysilane (HDTMS) were purchased from Sigma Aldrich. The common reagents including ammonia solution (NH3·H2O, 25%), chlorhydric acid (HCl, 35%), dioxane, sodium hydroxide, sodium chloride, sodium sulfate, methylene chloride, chloroform, n-hexane and ethanol were procured from Sinopharm Chemical Reagent Co., Ltd. Sudan I was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. All chemicals were analytical grade and were used without further purification. 2.2 Preparation of Cross-linked EC (CEC) sponges As shown in Figure S1a, 1.0 g of EC was added into a round-bottomed flask containing 5 mL of dioxane and the mixture was stirred for 1 h at room temperature, different amount (with


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respect to EC) of ECH were added dropwise with stirring into the resulting mixture (cf. Table S1). For simplicity, above compositions were referred to as C1, C2, C3, C4, and C5. The pH of the solution was maintained at 8.0 by adding 0.1 M NaOH. The reaction was conducted under stirring for 4 h at 40 ºC before neutralization with HCl solution. Subsequently, the desired amount of sodium sulfate was added into the solution. The mixture was poured into a beaker and stored in a -80 ºC refrigerator for two days. And then it was washed in water to remove the poreforming agent followed by freeze-drying at a condenser temperature of -50 °C under vacuum for 48 h to obtain the CEC sponges. For comparison, neat EC sponge, marked as C0, was prepared without adding cross-linker. 2.3 Preparation of Silanized CNTs (Si-CNTs) Reinforced Cross-linked EC (REC) Sponges Firstly, Si-CNTs were synthesized by the reaction of hydroxyl groups on the surface of CNTs with HDTMS (as shown in Figure S1b). Typically, 0.1 g of hydroxylated CNTs was added into a round-bottom flask with 20.0 mL of water and dispersed through ultrasonication for 30 min. Then 2.0 mL of 40.0 wt % methanol solution of HDTMS was added and stirred for 5 h at 65 °C for silanization. The product was obtained by filtration and washing with water and acetone sequentially. The Si-CNTs were dried in a vacuum oven at 50 °C for 24 h. Secondly, REC sponges were prepared by mixing the cross-linked EC with different amount of Si-CNTs (cf. Table S2). For simplicity, above compositions were referred to as R1, R2, R3, R4, R5 and R6. Subsequently, the desired amount of sodium sulfate was added into the solution. The mixture was poured into a beaker and stored in a -80 ºC refrigerator for two days. And then it was washed in water to remove the pore-forming agent followed by freeze-drying at a condenser temperature of -50 ºC under vacuum for 48 h to obtain the REC sponges. 2.4 Preparation of Superhydrophobic EC (SEC) Sponges


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Firstly, TEOS (10 mL) was added to 50 mL of water and stirred vigorously at 30 ºC for 90 min. Then 3 mL of NH3·H2O was added dropwise to the mixture solution. The reaction was continued at 30 ºC for 6 h under stirring. The solution was centrifuged, and the precipitate was dispersed in 50 mL of n-hexane. The SiO2 hydrosol was obtained. Subsequently, hydrolyzed HDTMS solution was prepared. 2 mL of HDTMS and 6 mL of water was gradually added to 45 mL of n-hexane. Then 1.5 mL of NH3·H2O was added dropwise to the mixture solution. The solution was stirred for 60 min at 60 ºC. Finally, REC sponge was dip-coated with the hydrolyzed HDTMS solution containing 12 mL of SiO2 hydrosol. The sponge was air dried and then cured at 120 ºC for 1 h in an oven. This obtained sample was designated as SEC sponge. 2.5 Characterizations Scanning Electron Microscopy (SEM). Surface morphologies of the sponges were investigated by a scanning electron microscopy (SEM) (FEI Nova SEM, USA) at accelerating voltage of 5 kV and a working distance of 5 mm. Prior to the SEM morphology investigation, samples were freeze-dried and sprayed with gold. Energy-dispersive X-ray (EDX) Spectra. The contents of the relative elements of the samples were determined by EDX spectra. The test equipment and conditions were the same as those used for SEM characterization. Thermogravimetric Analysis (TGA). TGA curves were carried out on a Netzsch STA 449 C thermogravimetric analyzer with a heating rate of 20 °C min-1 from 30°C to 700°C under the nitrogen atmosphere. Transmission electron microscopy (TEM). The TEM images were determined by an H-800 (Hitachi, Japan) TEM with an accelerating voltage of 120 kV. The TEM samples were prepared by dropping 8 µL of solutions on copper grids coated with thin films and carbon.


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Compression Test. The compression tests were conducted on a universal mechanical testing machine (10T, SANS Instruments) with two flat surface compression stages and 1000 N load cells at room temperature. The cylindrical samples measuring 30 mm in diameter and 12 mm in height were used. The compression speed was set at 2 mm/min. Five replicates were measured for each sample. Density. The densities ( ) of the sponges were calculated by measuring the weight and volume of each sample on the basis of the ISO 845:2006 standard. The weight ( ) of the sponge was measured using an analytical balance (readability 0.0001 g, Mettler Toledo). The volume ( ) of the sponge was measured with an electronic digital micrometer (MNTQFC02, MNT) at three different positions. Five blocks at least were used for density determination for each sample. The densities of the sponges were calculated according to eqn (1):

=

(1)

Porosity. The porosities ( ) of the sponges were calculated according to eqn (2):

% = 1 − × 100

(2)

where was the density of sponge and was the density of bulk EC (1.140 g/cm3), assuming the gas density was negligible. Wettability of Sponges. The water contact angles (WCA) measurements were measured on a Data Physics instrument (OCA 20, Filderstadt, Germany). 10 µL of water was dropped on the surface of the samples. At least five measurements were taken for each sample. Oil/water Absorption Capacity Measurements. To measure the absorption capacity of the sponges, various organic solvents and oils were used for the oil absorption tests. Sponges were first weighed with the value recorded and then were immersed into the organic liquids for about


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1 minute and weighed immediately with the value recorded. The mass-based absorption capacity ( ) of the sponges was calculated according to eqn (3): / = − /

(3)

where and were the weight of the sponges before and after absorption, respectively. The volume-based absorption capacity () was calculated according to eqn (4): / = !" / !" where and

!"

(4)

were the density of the sponges and oils, respectively.

Recyclability. To study the recyclability of the sponges, the oil-saturated sponges were taken out from the oil bath and dried to constant weight in a vacuum oven. The weights of sponges before absorption, after absorption and after removal of the oil were measured respectively. Four samples and 50 absorption cycles were tested for each experiment.

3. RESULTS AND DISCUSSION 3.1 Mechanical Properties of Sponges The mechanical properties of the sponges are important for endurability and structural stability of sponges during the oil/water separation processes. Traditional oil absorbents suffer from deformation in the extraction process from oil/water mixture, leading to the drippage of oils. Due to the presence of residual hydroxyl groups in EC backbone, EC can be cross-linked by ECH in alkaline conditions. Chemical cross-linking can increase the mechanical properties of the sponges.42 In addition, the exceptionally high aspect ratio in combination with a low density and a high strength and stiffness make CNTs potential candidate as reinforcing filler for polymeric materials.43 In order to improve the compatibility of CNTs with EC and increase the hydrophobicity, the surfaces of CNTs were silanized with HDTMS.


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Figure 2a shows the compressive stress-strain behavior of the pristine and cross-linked EC sponges, where two regions were apparent: a linear increasing stress response below 70% strain and an exponentially increasing stress response above 70% strain. In the linear increasing region, the stress increased slowly with the gradually shrinking of the sponges, while in the exponentially increasing region, the stress increased sharply due to the continuous reduction in the pore volume.44 At 80% strain, the compressive stress of C0, C1, C2, C3, C4, and C5 was 29.9, 43.5, 66.6, 54.1, 36.0, and 22.2 kPa, respectively. The ultimate compressive strength for the sponges at 80% strain increased at first and then decreased with the increase dosage of ECH. The reason for this change might be attributed to that at low cross-linker dosages the cross-linking between EC and ECH could be adequately carried out and the interlinkage of the EC skeleton was effectively formed. The strength of sponges declined at high ECH dosages due to the occurrence of excessive crosslink in EC chains which in turn produced a brittle material and thus reduced the compressive strength.45 The cross-linked C2 sponge which possessed highest compressive strength was selected for further investigation. The dosage of ECH (10 wt %) was employed for the further reinforcement using CNTs. Figure 2b shows the compressive stress-strain behavior of the CNTs reinforced EC sponges. The compressive stress increased slowly below 65% strain, but increased remarkably from 65% to 75% strain. At 75% strain, the compressive stress of R1, R2, R3, R4, R5, R5 and R6 was 118.1, 163.4, 222.2, 198.2, 179.1 and 139.8 kPa, respectively. These values were much higher than other reports, such as 3.7 kPa for CNC aerogels,46 12.1 kPa for carbon aerogels,47 and 15 kPa for PVA-CNF aerogels.48 The reinforced R3 sponge which showed highest compressive strength was selected for further modification with SiO2. And the obtained SEC sponge had the almost same ultimate compressive stress (225.4 kPa) with that of R3, indicating the surface coating of SiO2


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had no influence on the mechanical strength. The ultimate compressive strength for the sponges at 75% strain increased at first and then decreased with the increase amount of CNTs. The reason of change of the compressive strength with increasing CNTs concentration might be that at low CNTs concentrations CNTs could be well dispersed in the EC matrix and the mechanical properties were improved. As the CNTs concentration increased, the dispersibility of CNTs in EC declined, leading to the agglomeration of CNTs owing to the strong van der Waals attractive force. As a result, the agglomeration of CNTs exerted a pronounced weakening effect to the mechanical property of CNTs-reinforced sponges. As shown in Figure 2c and d, the SEC sponge of 7 cm2 could withstand the weight of 2000 g without damage, while the cross-linked EC sponge (C2) with the same area was out of shape under the same loading (Figure 2c and d). The enhancement of mechanical strength was mainly contributed to the mechanical reinforcing effect of CNTs mixed in/on the interconnected skeleton of the sponge.


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Figure 2. Mechanical properties of sponges: compressive stress-strain curves of (a) C0–C5, (b) R1–R6 and SEC; (c) Digital photographs of C2 and (d) SEC sponges bearing 2000 g. 3.2 Thermal Stability of the Sponges The thermal stability of the pristine EC, CEC and REC sponges was investigated by TGA in nitrogen atmosphere from 30 ºC to 700 ºC. The sponges showed excellent stability in a wide range of temperature (Figure 3). No weight loss was detected at temperatures below 330 ºC according to the TGA curve of the sponges. A sharp weight loss-occurring from around 330 to 400 ºC was observed for all eight sponge samples (a, b and e–j). The sharp weight loss was ascribed to the oxidative decomposition of EC chains.49 The TGA curves of the pristine EC sponge and the cross-linked sponge could be divided into three temperature ranges through which the weight losses appeared: 30–330, 330–400, and 400–700 ºC. It was worth noting that the decomposition temperature of the cross-linked EC sponge increased about 10 ºC compared to


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the pristine EC sponge. The better thermal stability exhibited by the cross-linked EC sponge might be attributed to the formation of extra carbon-carbon bonds as the result of EC skeleton network cross-linking. The residual weight difference between the pristine EC sponge and crosslinked sponge was attributed to the thermal decomposition of the cross-linker in cross-linked sponge. The thermal stability of the pristine CNTs and silane treatment CNTs were also examined. As shown in Figure 3, for sample c and d, there was no clear weight loss during the heating process in the case of pristine CNTs, while the weight loss was almost 68% for the SiCNTs during 390–540 ºC. This can be ascribed to the decomposition of Si-O bonds of HDTMS.50 The difference between the sponges containing CNTs (sample e–j) and the other two without CNTs was the relatively higher pyrolysis temperature. When the ratio of CNTs varied from 5 to 20 wt %, the complete pyrolysis temperature increased from 520 to 560 ºC, indicating that the presence of the CNTs could enhance the stability of the sponges.

Figure 3. TGA curves of (a) EC, (b) C2, (c) pristine CNTs, (d) Si-CNTs, and (e)–(j): R1–R6 sponges. 3.3 Structure and Morphologies Analyses of Sponges


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The surface morphology of CNTs before and after silanization was revealed by TEM (Figure S2). Compared to the unmodified CNTs (Figure S2a), the Si-CNTs showed larger diameters (Figure S2b). Some amorphous materials were seen to be attached to the surface of the Si-CNTs which were supposed to be derived from the silane molecules of HDTMS. The layered structure of the Si-CNTs remained largely intact (inset, Figure S2b), which indicated that there was no real damage to the CNTs during the silanization process. The morphologies of the cross-linked (C2), CNTs reinforced (R3), and SEC sponges were examined by SEM. The SEM images revealed that all the sponges exhibited similar randomly distributed micro-porous structures and highly 3D porous networks. As shown in Figure 4a, the C2 sponge exhibited an interconnected porous structure with the pore sizes ranging from several to one hundred microns. The 3D porous structures were formed by the mechanical sustention from pore-forming agent and following freeze-gelation procedure of ethyl cellulose. Nucleation and growth of large ice crystals occured within the porous network during the freeze-drying process, which pushed ethyl cellulose out from its original position. For the CNTs reinforced sponges, the pore sizes were relatively smaller (Figure 4c and d). This might be resulted from the denser and stronger EC networks in the systems that hinder the growth of ice crystals. As shown in Figure S3, the CNTs was randomly distributed on the surface of the sponge in high magnification SEM image. The most of the CNTs was tightly buried with the EC matrix. This might because the silanized CNTs was hydrophobic/oleophilic and it had good compatibility with the matrix. Under higher magnification, the skeletons of SEC sponge covered by numerous micro/nano-scale protrusions consisted of hydrophobic SiO2 nanoparticles coating layer could be clearly distinguished (Figure 4d). Compared with the smooth sponge skeletons (Figure 4a and b), the surface of SEC sponge exhibited a relatively much rougher structure with a range of pore sizes. The 3D porous structure


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in combination with the micro/nano-scale protrusions created a dually rough structure, which was very similar to the surface of lotus leaves. Furthermore, the EDX data were used to confirm the presence of C, O and Si elements of the sponges, as shown in Figure S4 and Table S3.

Figure 4. SEM images of the sponges at different magnifications: (a) C2 sponge, (b) R3 sponge, (c) SEC sponge, and (d) SEC sponge under higher magnification. The density and porosity of C2, R3 and SEC sponges were showed in Table S4. It can be seen that the three kinds of sponges possessed very low density (98%), which was beneficial for the use in oil/water separation. The density of R3 and SEC was somewhat higher than that of C2, which should be attributed to the presence of CNTs and SiO2 nanoparticles in these sponges. 3.4 Surface wettability of the sponges The surface wettability of the sponges was studied via a contact angle measurement. In consideration of the different usage conditions of the sponges, the contact angles in harsh


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environments like acid, alkali and salt solutions were measured respectively besides water. For the cross-linked sponge C2, the CAs of water, HCl, NaOH and NaCl solutions were 134.4°, 133.8°, 133.1° and 133.2° respectively (Figure 5a), indicating that the C2 sponge presented high hydrophobicity. After mixing with Si-CNTs, the CAs of R3 increased about eight degrees, namely, 143.6°, 143.2°, 142.7° and 142.4° for water, HCl, NaOH and NaCl solutions respectively, indicating that the hydrophobicity of the sponge was enhanced (Figure 5b). This could be attributed to the inherent hydrophobicity of Si-CNTs and the construction of a more compact structure into which water droplets could hardly penetrate. As for the SEC sponge (Figure 5c), the contact angles further increased (water: 158.2°, HCl: 154.6°, NaOH: 154.1° and NaCl: 154.8°), indicating that the sponge became superhydrophobic and could maintain its superhydrophobicity in acid, alkali and saline environments. The superhydrophobicity of the sponge could be attributed to the combination of the hierarchical micro/nanoscale rough structures and low surface energy of silanized SiO2 nanoparticles.

Figure 5. Contact angle measurements of (a) C2, (b) R3, and (c) SEC sponges using water, HCl (1 M), NaOH (1 M) and NaCl (10 wt %) solution droplets respectively.


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Combined with the intrinsic hydrophobicity of Si-CNTs and silanized SiO2 nanoparticles, the SEC sponge possessed micro/nanoscale hierarchical structure, which was very similar to the multiscale structure of the lotus leaves surface. The sponge displayed superhydrophobicity and could prevent the capillary permeability of water. As shown in Figure 6a, when the SEC sponge was immersed into water by an external force, it was reflective like a sliver mirror because of the existence of an air cushion between water and the sponge, which meant that water was in the Cassie–Baxter state, and the interaction between water and the SEC sponge was very weak. It was demonstrated in Figure 6b that the water droplets were hardly able to stick to the sponge surface and exhibited spherical shapes due to the property of super-repellency to water. A drop of n-hexane completely permeated through the surface with a contact angle of almost 0° (Figure 6c). Interestingly, a jet of water from an injector could bounce off the SEC sponge without leaving a trace, indicating excellent superhydrophobicity of the sponge (Figure 6d). When the water droplet was fell on the inclined surface, the water droplet rolled off the surface quickly within a very short time (Figure 6e). The sliding angle measured was only 3°. As shown in Figure 6f, in order to further illustrate the water-repellent behaviour of the SEC sponge, the contact process between substrate and water droplet was recorded. The water droplet (8 µL) suspending on the syringe could be hardly pulled down to the surface even when the droplet was squeezed, suggesting the superhydrophobicity of the sponge with a low water adhesion. Furthermore, the evolving contact process of a water droplet on the surface of SEC sponge was also demonstrated to characterize the self-cleaning property of the sponge. As shown in Figure 6g, the silica particles could be easily removed from the sponge surface without any contamination owing to the rolling motion of water on the surface.


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Figure 6. (a) Mirror-like phenomenon of the SEC sponge; (b) Spherical water droplets on the surface of the sponge; (c) n-hexane droplet with contact angle of 0°; (d) A jet of water bouncing off the sponge; (e) A water droplet (10 µL) easily slid off the SEC sponge surface with a sliding angle of 3°; (f) The substrate contact process using a water droplet (8 µL); and (g) Photographs of the self-cleaning process of the SEC sponge surface with a layer of SiO2 particles. 3.5 Oil/water separation performance of the sponges In order to compare the difference between the as prepared C2, R3 and SEC sponges in terms of oil/water separation, the absorption kinetic curves of these three sponges were plotted as illustrated in Figure S5. The mass-based absorption capacities (for n-hexane) of C2, R3 and SEC sponges were 30.31, 31.13 and 31.61 g/g, respectively. They almost took the same time (16s) to


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reach equilibrium and became saturated. The SEC sponge showed the best absorption capacity among them due to its superhydrophobicity achieved by complexing with Si-CNTs and surface modification with silanized SiO2 nanoparticles. The high porosity, good mechanical property, 3D porous structure and surface superhydrophobicity made the SEC sponge an ideal candidate for rapid removal of pollutants such as oils and organic solvents. The strong absorption capability of SEC sponge was demonstrated in Figure 7. When the sponge was brought into contact with a nhexane layer (dyed with Sudan I) on a water surface, it absorbed the n-hexane completely and rapidly (Figure 7a). In addition, the sponge could firmly absorb dichloromethane under water and be removed out of water essentially without any oil release (Figure 7b). Moreover, owing to its low density, mechanical integrity and superhydrophobicity, the SEC sponge floated on the water surface after absorption process and could still be used to further remove oils, indicating its potential use for the facile removal of oil spillage and chemical leakage.

Figure 7. Chronological photographs of removal of (a) n-hexane (dyed with Sudan I) from water surface and (b) dichloromethane (dyed with Sudan I) under water. The systematic absorption performance of the SEC sponge for different oils and organic solvents, including n-hexane, gasoline, toluene, dimethylsilicone oil, dichloromethane, trichloromethane, petroleum ether and soybean oil was investigated. For these oils and organics,


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the physical properties (density, viscosity and surface tension) were listed in Table S5. Figure 8ad demonstrated the oil absorption capacity, kinetic, and recyclability of the SEC sponge. As shown in Figure 8a, the mass-based absorption capacities for a wide range of oils and organic solvents ranged from 32 to 64 times its own weight. The absorption capacity of the sponge depended not only on the density but also viscosity and surface tension. For example, although n-hexane had a higher density than petroleum ether (0.69 vs. 0.64 g/cm3), its absorption by SEC sponge was poorer. This could be explained by the higher viscosity (0.30 vs. 0.29 mPa·s) and surface tension (18.43 vs. 17.5 mN/m) of n-hexane that of petroleum ether. The excellent oily substance absorption capability of the SEC sponge was attributed to the highly open porous surface and 3D-interconnected network, as well as the uniform superhydrophobic micro/nanoscale structure.51 Furthermore, the volume-based absorption capacities of the SEC sponges could reach up to around 90% (Figure 8b), indicating that almost the whole volume of the sponge was crammed by the oils and organics. For an ideal absorbent, the oil absorption kinetics was also an important parameter to identify the quality of the absorbent. The absorption kinetic curves of the SEC sponge in various organic media were shown in Figure 8c. The absorption capacity differed with different oils and organics for their different density, viscosity and surface tension.52-54 For theses oils with low viscosity such as n-hexane, gasoline, toluene, dichloromethane, trichloromethane and petroleum ether, the SEC sponge needed just about 16 s to reach the absorption equilibrium, while it took about 26 s in oils with higher viscosity, such as dimethylsilicone oil and soybean oil. Owing to the higher movement velocity of the low viscosity oil molecules in the pore channels of the SEC sponge, the solvation and swelling of the sponge network in the oil was relatively quicker compared with the high viscosity oil.55 The results indicated that the SEC sponge could absorb various oils and organics rapidly and


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efficiently. Furthermore, a moisture balance was used to investigate the water content in the SEC sponge. The results revealed that there was extremely little water (0.36–0.49% of its own weight), further verifying the strong water-repellency and high selectivity of the SEC sponge. As we all know, the recyclability of an absorbent played an important part in the practical environmental protection and pollution control. The durability of the silanized CNTs with the matrix was tested using TGA, as shown in Figure S6. There were no obvious changes in the TGA curves of R3 before and after twenty n-hexane absorption cycles, indicating the good durability of CNTs in the martix. To test the recyclability of SEC sponge, the repetitive absorption/distillation cycle method was employed. The n-hexane-absorbed SEC sponge was heated around the boiling point to release the absorbed liquid and the weights of sponges before and after absorption and after removal of n-hexane were measured respectively. The corresponding cyclic test was displayed in Figure 8d, after fifty absorption/distillation cycles, the absorption capacity of the sponge decreased slightly to 86.4% of its initial value and the WCA was still above 140°. The surface morphology of the SEC sponge after fifty absorption cycles was revealed by SEM, as shown in Figure S7. The surface roughness could be well maintained and most of the SiO2 nanoparticles were retained on the surface of the sponge. These results indicate a highly stable absorption and excellent recyclability. Therefore, the remarkable properties of high absorption capacity, good durability and recyclability of the SEC sponge made it an ideal absorbent for oil-water separation.


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Figure 8. (a) Mass-based and (b) volume-based absorption capacity of the SEC sponge; (c) Absorption kinetic curves of the SEC sponge (25 ºC); and (d) Recyclable performance of the SEC sponge for absorption of n-hexane with distillation method. To investigate the emulsified oil separation capability of the SEC sponge, we prepared Tween80-stabilized toluene-in-water emulsion (3/100 g/g) under vigorous stirring for 1 h.56 As shown in Figure 9a, the size of droplets of the emulsion ranged from several microns to almost one hundred microns. Figure 9b showed the comparison between the oil/water emulsion (left) and the separated water (right). Before separation, the feed of surfactant-stabilized toluene-inwater was a cloudy white liquid. After separation, it became transparent and there was no obvious droplet observed in the microscopy image (Figure 9c). Furthermore, the UV-visible spectroscopy was used to determine the specific amount of toluene present in the separated water. The separation efficiency was defined and calculated according to eqn (5):


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# % = 1 − / × 100%

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(5)

where and were oil concentration in the original oil-water emulsion and the separated water, respectively. As shown in Figure 9d, the separation efficiency was calculated at 99.87% according to the changes before and after separation on the characteristic peak at 261 nm for toluene, illustrating the high separation efficiency of SEC sponge.

Figure 9. (a) Image of the toluene-in-water emulsion before separation; (b) Images of the toluene-in-water emulsion before (left) and after (right) separation; (c) Image of the toluene-inwater emulsion after separation; and (d) UV-visible spectroscopy absorption data for the toluenein-water emulsion diluted 16 times, separated water and Tween80 aqueous solution diluted 10 times.

4. CONCLUSIONS


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A novel functional composite sponge was developed and its oil/water separation properties were demonstrated. Ethyl cellulose was firstly cross-linked by ECH and mixed with Si-CNTs. After a simple and green freeze-drying method, SiO2 nanoparticles were then coated on the surface of porous sponges. Finally, the superhydrophobic and superoleophilic ethyl cellulose sponges were obtained via modification with HDTMS. The sponges showed low density (98%) and good mechanical strength (could bear the pressure of 28.6 kPa without damage). Stable superhydrophobicity (θwater=158.2°, rolling contact angle=3°) was achieved in water, acid, alkali, and salt solutions. Furthermore, the sponges exhibited high mass absorption capacity for a wide variety of oils and organic solvents (32 to 64 times of their own weight). Benefitting from the good mechanical durability and robust superhydrophobicity of the sponges, the absorption capacity could maintain upon repeated use, indicating the excellent recyclability. The advantages made the sponges ideal absorbent for oil spillage cleaning. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Mass of EC and amount of ECH for the preparation of cross-linked CEC sponges (Table S1); mass of EC and CNTs for the preparation of REC sponges (Table S2); crosslinking reaction between ethyl cellulose and ECH, and silanization process of multi-walled carbon nanotubes (Figure S1); TEM images of multi-walled carbon nanotubes (Figure S2); SEM image of the surface of the CNTs reinforced sponge (Figure S3); EDAX spectra of the sponges (Figure S4); absorption kinetic curves and mass-based absorption capacity of C2, R3 and SEC sponge for nhexane (Figure S5); TGA curves of R3 before and after twenty n-hexane absorption cycles (Figure S6); SEM image of the surface of SEC sponge after fifty absorption cycles (Figure S7);


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the relative atomic percentage by element (Table S3); the physical properties of the C2, R3 and SEC sponges (Table S4); the physical properties of oils (Table S5). AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Tel.: +86 21 69580234. Fax: +86 21 69584723

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank the financial supports of the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the National Key Technology R&D Program (no. 2012BAI15B061).

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Graphical Abstracts


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Superoleophilic and Reinforced Ethyl Cellulose

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