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Mussel adhesive-inspired Design of Superhy-drophobic Nanofibrillated Cellulose Aerogels for Oil/Water Separation Runan Gao, Shaoliang Xiao, Wentao Gan, Qi Liu, Hassan Amer, Thomas Rosenau, Jian Li, and Yun Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01397 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Sustainable Chemistry & Engineering

Mussel Adhesive-inspired Design of Superhydrophobic Nanofibrillated Cellulose Aerogels for Oil/Water Separation Runan Gao,† Shaoliang Xiao,† Wentao Gan,† Qi Liu,§ Hassan Amer,∥Thomas Rosenau,*,

∥

Jian Li,*,† Yun Lu,*,†,‡,∥ †

Key Laboratory of Bio-based Material Science and Technology Ministry of Education,

Materials Science and Engineering College, Northeast Forestry University, Harbin, 150040, P.R. China ‡

Research Institute of Wood Industry, Chinese Academy of Forestry, Yard 1, Dongxiaofu,

Xiangshan Road, Haidian District, Beijing 100091, P.R. China §

National Engineering Laboratory for Crop Efficient Water Use and Disaster Mitigation, Key

Laboratory of Dryland Agriculture, Ministry of Agriculture, and Key Laboratory for Prevention and Control of Residual Pollution in Agricultural Film, Ministry of Agriculture, Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, No. 12 Zhongguancun South Street, Beijing 100081, P.R. China ∥

Division of Chemistry of Renewables, Department of Chemistry, University of Natural

Resources and Life Sciences, Vienna (BOKU), Vienna A-1190, Austria

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS:

nanofibrillated

cellulose,

polydopamine,

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octadecylamine,

aerogel,

superhydrophobic, oil absorption

ABSTRACT：：Inspiration from biomimetics - the chemistry of mussel adhesives - was taken for imparting superhydrophobic properties to nanofibrillated cellulose (NFC) matrices. A polydopamine (PDA) surface coating was introduced, acting as an anchor between the NFC scaffold and octadecylamine (ODA): PDA is coated onto the NFC scaffolds by its adhesive properties and the ODA is successfully attached to the PDA by a Schiff base reaction. The ultralow density of 6.04 mg/cm3 combined with the high contact angle of 152.5° endows the composite aerogel with superb buoyancy and excellent oil/water separation selectivity. Oil can be rapidly absorbed from a mixture of oil and water. In addition, the modified aerogel can take up a wide range of organic solvents, the maximum absorption capacity reaching up to 176 g/g, depending on the density of the liquids. The novel superhydrophobic aerogel shows great potential as adsorber for oil and solvent spills and as oil-water separator.

INTRODUCTION

Oil spills have become a major concern with regard to both marine and terrestrial pollution, and have posed a huge burden to economy and ecology.1 The catastrophic oil spills of the last decades have called forth various strategies for oil leakage problem, such as filtration, mechanical extraction, chemical degradation, bioremediation and sorbents.2-3 Among these methods, the sorbent stands out due to their facile operability, significant capacity for oil capture, and high cost-efficiency.4-6 Such sorbents are not only important on a large scale,


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such as in supertanker accidents, but also in chemical labs, production sites and even households. Nanocellulose-based aerogels have attracted much attention for use in oil/water separation.7-10 Compared with synthetic polymer aerogels, the abundant and sustainable nanofibrillated cellulose (NFC) aerogels stick out because of their environmentally friendliness and biocompatibility. They have large specific area and high porosities, which renders them most promising for renewable absorbent materials.11-12 However, the intrinsic hydrophilicity of cellulose poses an obstacle for its application in oil/water separation.13 Nowadays, various methods have been developed to modify the surface of cellulose nanofibers in order to alter and tune the wettability of cellulose-based aerogels without significant intake of chemicals and without impairing density and pore characteristics. Chemical vapor deposition (CVD) technique is widely applied in hydrophobic modification of cellulose-based aerogels. For example, with the CVD method, the methyltrimethoxysilane (MTMS)-coated cellulose aerogel has a contact angle of 145° and an oil absorption capacity of 24.4 g.14 Atomic layer deposition (ALD),15 sol-gel16 and the cool plasma technique17 were also employed to alter the wettability of NFC aerogels. These methods imply use of toxic modifiers or precursors, specific equipment, complicated processes and result in non-uniform structures, making large-scale application of NFC aerogels difficult. A scalable strategy for facile and green modification of cellulose nanofibers, which renders NFC aerogels more favorable for widespread application, is still needed. Bionic – also called biomimetic – approaches provide an alternative tactics for material design and construction, trying to learn from nature and to transfer natural design into



technical principles. In nature, mussels can tightly attach to virtually all types of substrates, even on blank surfaces under wet conditions, on Teflon or glass or metals by secreting adhesive proteins.18 This robust adhesion is effected by 3,4-dihydroxy-L-phenylalanine (DOPA) and lysine amino acids.19-20 Further studies revealed that dopamine, a mimic of DOPA that contains both the catechol and the amine groups, can be oxidized and polymerized under very mild basic conditions to form to polydopamine (PDA) coatings on various solid surfaces.21 The different functional groups incorporated in PDA, such as phenolic hydroxyl, ortho-quinone (from catechol oxidation), amine and imine, can serve as links to surfaces and as covalent anchor groups to desired molecules.22-23 In addition, PDA exhibits excellent chemical reactivity toward various thiol- or amine-containing molecules.24-25 The mussel adhesive-inspired surface chemistry has opened new routes to diverse hybrid materials with application potential in a wide range of technology fields, such as chemical, biological, energy, sensing and environmental applications. Recently, mussel adhesive chemistry has also inspired fabrication of functional nanocellulose materials. For instance, Ag-PDA functionalized cellulose nanocrystals (CNC) were developed for antibacterial activity and reduction of 4-nitrophenol,26-27 and PDA-modified CNC was used for Fe3+ detection.28 In this work, we employ the PDA interlayer as a mediator to bridge the hydrophilic NFC and the desired hydrophobic octadecylamine (ODA) molecules, thus endowing the NFC surface with hydrophobicity. Simply by dipping NFC into a dopamine/ODA emulsion and subsequent freeze-drying, composite aerogels with favorable characteristics of superhydrophobicity, ultralow density and high porosity are obtained (Figure 1). They show great oil/water absorption selectivity and can absorb a wide range of


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organic solvents. While according to the CVD method, the hydrophobic reagent generally has an uneven distribution, the method presented here allow full contact between the NFC scaffolds and the modifiers, which leads to uniform structures of the composite aerogels and keeps its original porous structure intact. In addition, tedious procedures and specific equipment, which are for instance necessary in ALD or cool plasma techniques, are avoided in this work. This green and facile method may in fact provide an easy and scalable way for NFC-based oil absorbent preparations.

Figure 1. Schematic illustration of the facile fabrication of mussel adhesive-inspired, superhydrophobic NFC-based aerogels for highly efficient oil/water separation.

EXPERIMENTAL SECTION Materials Larix gemlini NFC suspension was prepared according to our previously reported method.29 Dopamine hydrochloride, octadecylamine, Tris (hydroxymethyl) aminomethane (Tris), oil red, and methylene blue were purchased from Sigma-Aldrich Co., Ltd. Lubricating oil was purchased from the Si Fang special oil factory (Beijing China). Other chemicals were used as received.



Fabrication of NFC and PDA-anchored NFC aerogel The NFC suspension (0.1 wt%) was poured into a dialysis bag and the solvent replaced with tert-butanol for 2 h. This replacement process was repeated twice, and the sample was placed into molds. NFC aerogels were obtained after the freeze-drying (-20 °C) for 12 h. PDA-anchored NFC aerogels (designated as PDA@NFC): 0.02 g dopamine was immersed in 0.1 wt% NFC-Tris suspension (pH=8.5, 100 mL) and stirred at room temperature for 24 h. The sample was subjected to solvent exchange with tert-butanol and freeze-dried (-20 °C). Preparation of the superhydrophobic aerogels ODA (0.5 g) was dispersed into Tris buffer (pH=8.5, 100 mL) and ultrasonicated for 3 min to obtain an emulsion. Dopamine hydrochloride (0.02 g) and the prepared ODA emulsion were added to the Larix gmelini NFC-Tris suspension (0.1 wt%, 100 mL, pH=8.5). The mixture was stirred at room temperature for 24 h. Formation of two phases showed completion of the process. The composite phase was rinsed with distilled water and then the water was replaced with tert-butanol, followed by freeze-drying (-20 °C), the superhydrophobic aerogels (designated as ODA-PDA@NFC) were obtained. ODA-admixed NFC aerogels for comparison were prepared by the above method just without adding dopamine hydrochloride.

Characterization Morphologies of NFC and composite aerogels were observed by a Hitachi JSM-7500F scanning electron microscope (SEM). Samples were sprayed with a layer of gold beforehand to improve conductivity. The Fourier transform infrared spectroscopy (FTIR) spectra of samples were examined on a Nicolet Nexus 670 FTIR instrument in the range of 4000–600 cm-1. The chemical composition on surface of NFC scaffolds and composite fiber were


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determined by a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (XPS). N2 adsorption–desorption isotherms were recorded on a BK112 specific surface area and pore size

analyzer.

The

specific

surface

area

was

determined

according

to

the

Brunauer–Emmett–Teller method. Pore size distribution was estimated according to the BJH approach. The bulk density of aerogels was calculated on the basis of the physical dimensions and weights of the samples. Porosity of aerogels was evaluated as Pa (%) =

(1)

where ρa is the bulk density of the NFC or the composite aerogel, and ρs is the bulk density of solid scaffold (NFC or composite NFC). Bulk densities of PDA@NFC and ODA-PDA@NFC were estimated as follow:

ρs =

(2)

Here wcellulose is the weight fraction of cellulose, wPDA is the weight fraction of PDA, and wODA is the weight fraction of ODA. The bulk density of cellulose (ρcellulose) is taken as 1600 mg/cm3, while the bulk density of PDA (ρPDA) is assumed to be 1400 mg/cm3, and that of ODA (ρODA) is taken as 862 mg/cm3. Water Contact angle (WCA) measurements were carried out with a Data Physics OCA 20 video contact angle measurement instrument (Data Physics). The stress–strain curves of aerogel materials were measured and recorded by a Shimadzu AG-A10T universal mechanical testing system. Cylinder aerogels with a size of 40 mm × 25 mm were compressed with a speed of 1 mm/min to 50% of its original length. Oil and organic solvents adsorption capacities of ODA-PDA@NFC



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Modified aerogels were immersed in 40 mL of oil or organic solvents. Sorbents were taken out after completely wetting and redundant solvents were extracted with filter paper. The practical absorption capacity (Cp) (w/w) of ODA-PDA@NFC was calculated by: Cp (w/w) =

(3)

where m0 and m1 are the weights of aerogel before and after absorption, respectively. The theoretical mass-based absorption capacity of aerogels was calculated by: Cm (w/w) = Porosity × ρliquid/ ρaerogel

(4)

The theoretical volume-based absorption capacity of aerogels was calculated by: Cv (v/w) = Porosity/ ρaerogel

(5)

RESULTS AND DISCUSSION

Figure 2. Behavior of NFC aerogel (a) and ODA-PDA@NFC aerogel (b) towards different liquids and in bulk water (c), SEM images of microstructure of NFC (d), PDA@NFC (e) and ODA-PDA@NFC (f).

“Switching” the medium compatibility of the aerogel by the ODA-PDA modifier is


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demonstrated in Figure 2(a)-(c). When a NFC aerogel encountered a water or oil droplet, both liquids are promptly taken up, causing partial collapse in the water case. After the modification, ODA-PDA@NFC still absorbed oil droplets rapidly, but blocked water and aqueous solutions (milk and tea as examples in the picture), which were staying outside the aerogel and not penetrating into inner structures. Upon immersing NFC aerogel into water, it was deformed immediately. Once saturated with water, it sank to the bottom of the vessel. Conversely, the ODA-PDA@NFC aerogel floated on water due to its light weight and superhydrophobicity. If submerged into water by external force, it was not wetted and immediately rose to the surface when the external force was withdrawn (Supporting Info, video 1). Its outstanding floatability renders ODA-PDA@NFC a promising absorbent for oil spills in bulk water. The morphology of the samples was illustrated by SEM images. Figure 2(d) shows the microstructure of a NFC aerogel. Intertwining of the fibrils, forming the porous three-dimensional (3D) network structure, can nicely be seen. Coating with PDA broadened the fibrils and roughened the surface. The 3D network became denser and the pore structure shrunk (Figure. 2(e)). It was obviously that ODA-PDA fully covered the NFC scaffolds, shaped the sheet-like fibrous structure and generated irregular roughness on NFC surfaces. When only ODA was mixed with NFC, it dispersed unregularly in the NFC network and jammed the pores. The characteristic pore structure of aerogel material was canceled out (see Figure. S1). The SEM result indicated that PDA can orient the ODA molecules on the NFC’s surface in a way that the 3D network is kept, and porous structure as well as low density are maintained.



Figure 3. Schematic illustration of the fabrication mechanism of the mussel adhesive-inspired, superhydrophobic NFC-based aerogels.

Generally, the PDA coating can easily adhere on various substrates through the oxidative self-polymerization of dopamine molecules under basic conditions. Dopamine is first transformed into intermediate products, such as 5,6-dihydroxyindole (DHI), then further oxidatively polymerized into the final PDA.15 The combination of multiple functional groups in PDA, such as hydroxyl groups, indole motifs, amino groups, catechol or quinone moieties, causes the strong adhesion capability of PDA to various substrates.30 When ODA molecules are involved in copolymerization (Figure 3), it is suggested that the amine moieties of ODA react with the intermediates and products of oxidative dopamine polymerization by Schiff base formation or/and Michael addition.31 In this work, the formation of C=N structures with simultaneous depletion of carbonyl (quinone) structures in the PDA-ODA product (Figure 4), as seen by FTIR and XPS, confirms the formation of Schiff bases (C=N).


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Figure 4. (a) FTIR spectra of NFC and composite aerogels, (b) survey XPS spectra of NFC and composite aerogels, (c-d) high-resolution XPS spectra (C1s and N1s, respectively) of NFC and composite aerogels.

Figure 4(a) shows the FTIR spectra of NFC and modified aerogels. The former showed the characteristic broad band in the region of 3600-3000 cm-1, originating from the stretching vibration of H–bonded –OH groups in cellulose molecules, and other characteristic peaks of cellulose. The peak at 2900 cm-1 was attributed to C–H stretching vibration,32 those at 1428 cm-1 and 1370 cm-1 to asymmetric CH2 wagging and bending,33 and the one at 897 cm-1 to anomeric carbon (C1) deformation. The prominent peak at 1050 cm-1 is associated with the C–O–C pyranose ring (anti-symmetric in-phase) stretching vibration.34 After coating with PDA, the band around 3300 cm-1 was broadened, indicating functional groups of PDA forming new hydrogen bonds with –OH groups of NFC. Moreover, the characteristic peak at



1508 cm-1 was attributed to the N–H shearing vibration of PDA,35 which was in agreement with the FTIR spectra of PDA and dopamine monomer (see Figure. S2). For ODA-PDA@NFC, the broad band assigned to –OH groups was weaker, and a peak at 3326 cm-1 attributed to the stretching vibration of N–H appeared. Two significant peaks at 2916 cm-1 and 2849 cm-1 were designated to stretching vibrations of –CH3 and –CH2– of ODA.36 The band at 1645 cm-1 was attributed to the C=N stretching vibrations of Schiff base reaction products between PDA and ODA; it cannot be found in the spectrum of pure ODA. The surface compositions of samples were further studied by XPS. Surface composition changes before and after the modification are clearly presented in survey spectrums (Figure. 4b). The characteristic peak of the N element appeared both after coating NFC with PDA and with the mixture of PDA and ODA. The N/C ratio of PDA@NFC is 0.1 (see the atomic composition listed in Table S1), slightly lower than the theoretic value of 0.125 of dopamine,37 indicating the NFC scaffolds to be largely covered with PDA. Moreover, after ODA was deposited on the surface of scaffolds, the intensities of the C1s peak remarkably increased while the intensity of O1s sharply decreased. The N/C ratio of ODA-PDA@NFC is 0.053, which is very close to the theoretical value of ODA (0.055).31 This indicates that most hydroxyl groups were hidden from the surface, which instead presents ODA molecules almost exclusively. To further prove this, high-resolution XPS C1s and N1s spectra were examined. Figure 4c presents the C1s core-level spectra of aerogel before and after the modification. For pure NFC aerogels, curve-fitted peaks at 284.6, 286.7, 288.3 and 289.2 eV were designated to C–C, C–O, C=O and O–C=O moieties, respectively.38 In the high-resolution C1s spectra of PDA@NFC, there were three characteristic contributions at


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284.5, 285.8 and 287.7 eV assigned to CHx/C–NH2, C–O and C=O species, respectively.39 The N1s spectra provided crucial evidence for the successful deposition of PDA on NFC. As shown in Figure 4d, the N1s region was composed of two peaks from primary (R–NH2, 401.6 eV) and secondary (R–NH–R, 399.6 eV) amine functionalities.40. The primary amine was associated with self-assembled (dopamine)2/DHI trimer and the secondary amine was associated with both intermediate species (DHI) and polydopamine. In the case of ODA-PDA@NFC, peaks from carbon-oxygen bonds almost disappeared and the peak attributed to C–H and C–N at 284.6 and 285.5 eV, respectively, intensified. In the N1s region, a new intense signal at 398.8 eV appeared, assigned to tertiary (–N=) imine moieties, implying the formation of Schiff bases (see above). This result was consistent with that from FTIR characterization and further supported the successful anchoring of ODA on the NFC-PDA scaffolds. The specific surface area and pore structure of the different aerogel specimens were examined by N2 adsorption-desorption (Figure 5). The presence of H3 type hysteresis demonstrated their mesoporous structure.41 The NFC aerogel had a large specific surface area of 186.5 m2/g and a pore volume of 0.76 cm3/g, respectively. As the functional molecules deposited on the NFC, the density of aerogel material increased. Inversely, the specific surface area and pore volume of composite aerogels decreased. However, compared with other reported hydrophobic modified NFC aerogels, ODA-PDA@NFC possessed relatively low density (6.0 mg/cm3) and high specific surface area (93.1 m2/g).13, 16 Using tert-butanol as the solvent helped to maintain the high specific specific area. The fast crystallization of tert-butanol during freezing leads to small pores and fine interconnected structure.42 When use water as



the solvent, large ice crystals formed during freezing will press the network flat and generate large pores with honeycomb structures, leading to a low specific surface area and large pore structure of the composite aerogel. As for ODA mixed NFC aerogel, the specific surface area and the pore volume was only 22.07 m2/g and 0.07 cm3/g, respectively. As revealed in Figure S1, in the absence of the mediating PDA interlayer the ODA molecules were disordered and irregularly dispersed between NFC scaffolds, jamming its pore structure and this way causing a sharp decline in both specific surface area and pore volume. Detailed information about density, specific surface area and pore volume is listed in Table 1.

Figure 5. N2 adsorption-desorption isotherms of the aerogel samples: (a) NFC, (b) PDA@NFC, (c) ODA-PDA@NFC and (d) ODA mixed NFC.


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Table 1. Bulk density, specific surface area and pore volume of the studied NFC and composite aerogels.

Sample

BET specific Pore Pore ρaerogel surface diameter volume (mg/cm3) area (nm) (cm3/g) (m2/g)

NFC

2.57

186.50

10.92

0.76

PDA@NFC 3.04 ODA-PDA@NFC 6.04 ODA mixed NFC 7.37

178.80 93.08 22.07

10.34 8.04 5.73

0.70 0.37 0.07

Upon water contact angle (WCA) measurements, it was hard to place a drop of water on the surface of ODA-PDA@NFC. When the microsyringe was withdrawn, the water droplet lifted with it (Supporting Video 2), a nice visualization of the water repellency. In Figure 6b it is shown that water droplets rolled off the support very easily as soon as it was tilted to a certain angle (Supporting Video 3). The ceraceous (wax-like) ODA molecules anchored on the NFC surface evidently decreased the surface energy and sealed off the hydrophilic hydroxyl groups quite efficiently. Moreover, the porous and rough structure of aerogel traps air below the water drops to form air pockets, i.e., a Cassie-Baxter wetting regime. Since the drops “sit” partially on the air, hydrophobicity is strengthened.43 The ODA-PDA@NFC showed superhydrophobic characteristics and possessed a high contact angle of 152.5° (see Supporting Figure S4). Such superhydrophobic aerogels exhibit neither water-wetting nor water-absorbing characteristics, however, they still absorb oil and some organic solvents.



Figure 6. Water repellency (a) and self-cleaning characteristic (b) of ODA-PDA@NFC.

As shown in Figure 7, the composite aerogel can rapidly collect oil from the surface of water. To separate organic solvents with higher density than the aqueous phase - taking chloroform as an example - a great amount of “air pockets” on the surface of the aerogel formed a water barrier surrounding the aerogel, blocking the water, but enabling the solvent to be absorbed. The mechanical integrity of the aerogel remained unchanged. The NFC aerogel, by contrast, was thoroughly wetted and saturated with water, which incapacitated it for further interaction with the chloroform. Compared with ODA-PDA@NFC, the NFC aerogel was more fragile and friable and very easy to be disintegrated after saturation with water (Figure 7c). This can be verified by the mechanical testing. The mechanical performance of aerogels was proved after the modification (Figure S3). No apparent linear elastic deformation was observed for both NFC and composite aerogels. The compressive stress of aerogels at 50% strain dramatically increased from 0.43 kPa (NFC) to 2.6 kPa (ODA-PDA@NFC). The shape recovery of ODA-PDA@NFC increased by 12% compared to that of NFC. ODA covered on the scaffolds weakened hydrogen bonds inside the interconnected network, which allows the


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composite aerogel to recover.44 The improvement in mechanical performance rendered the aerogel more favorable as an absorbent.

Figure 7. Separation the lubricating oil (a) and chloroform (b) from the water phase with ODA-PDA@NFC, (c) unsuccessful attempt to collect chloroform with neat NFC aerogel, for comparison.

Absorption performance of different composite samples for some organic solvents was investigated. The mass-based absorption capacity, to a large extent, depended on the solvent density (Figure 8(a) and (c)). The maximum absorption capacities of ODA-PDA@NFC for different oil and organic solvents were 83 to 176 g/g. Compared with cellulosic aerogels modified according to the CVD method, the value reported in our work is higher than that of



sorbents with large densities (24-95 g/g), and lower than that with low densities (88-356 g/g) (Table S2). Furthermore, ODA-PDA@NFC has a significantly better absorption performance than synthetic polymer materials, such as nickel foam (3.5 g), polydimethysiloxane sponges (4.3 g), polyurethane sponges (25-87 g/g), or melamine sponges (60-150 g/g). The volume-based absorption capacities (Figure 8(b)) reveal the absorption performance of the material itself. Approximately 72% of the pore volume was enabled to withhold the organic solvents. However, there is an exception when taking up the dichloromethane and only 64% of the pore volume was filled up. This may be due to high volatility of dichloromethane, which makes it difficult for ODA-PDA@NFC to hold the solvent. The average 28% of pores that remained inaccessible to the solvent are thought to be occupied with air bubble or failed to keep the solvent when the composite aerogel was removed from the liquid phase due to unfavorable geometries. The pore volume of aerogels plays a crucial role in absorbing process as revealed by Arie et al. In this work, pore size and density of materials were shown to have also an apparent effect on the practical absorption capacities. PDA@NFC with much higher pore volume, which is supposed to have a double volume-based absorption capacity (328.3 mL/g) compared to ODA-PDA@NFC (164.7 mL/g), is only 0.47 times higher in absorption performance. In fact, only 54% of the pores of PDA@NFC were occupied by solvent. This may be caused by the lower density and larger pores of PDA@NFC as depicted in Figure 5 and Table 1. The result indicates that denser aerogels with smaller pores are more capable of holding the absorbed solvents, which is consistent with previous work13, 46 using the NFC as the matrix. For ODA-ad mixed NFC foam aerogel, the mass-based absorption capacities (27-45.6 g/g)


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were significantly smaller. In addition, there was no predictable pattern in volume-based absorption performance (Figure S5). This may be not only due to the low pore volume but also the chaotic pore structure. Without adding PDA, random dispersion of ODA may seal off pores from both the outside and inside and cut off the chance for liquids to get through. The comparison of the different aerogels confirmed the orientation feature of PDA and proved that PDA rendered the composite aerogels more favorable for absorbing purposes.

Figure 8. Absorption capacities of ODA-PDA@NFC for various organic solvents: (a) mass-based, (b) volume-based, (c) absorption capacity contrast between various composite samples, (d) kinetics of absorption of organic solvents to ODA-PDA@NFC



The curves of the absorption kinetics of different organic solvents to ODA-PDA@NFC are shown in Figure 8d. In addition to above mentioned factors, the absorption capacity was also related to the viscosity. For high-viscosity lubricating oil, it took 80 s to reach the absorption equilibrium, while for low-viscosity solvents, such as n-hexane, ether and acetone, the absorption equilibrium was reached within 25 s. The open pore structure is more accessible to organic solvents with low viscosity, thereby faster establishing the absorption equilibrium. It was evident that ODA-PDA@NFC efficiently absorbed a variety of organic solvents. An absorption-squeezing test was preformed to evaluate the reusability of the composite aerogels (Figure S6). Taking the lubricating oil as example, the absorption capacity was reduced to half after the first test cycle and remained constant until the structure collapsed at the forth squeezing process. However, this performance of ODA-PDA@NFC is less satisfactory compared with 30 usage cycles of microfibrillated cellulose aerogels reported by Wang et al.

CONCLUSIONS We have developed a facile and reproducible method to fabricate novel superhydrophobic NFC aerogels. The mussel adhesive-inspired polydopamine (PDA) coating was evenly spun on the NFC scaffolds and worked as a mediator or compatibilizer between cellulose and the hydrophobizer, which in our case was octadecylamine (ODA) grafted on the scaffolds by Schiff base reaction. The obtained ODA-PDA@NFC aerogel had a high contact angle of 152.5° and can collect oil and a wide range of organic solvents from aqueous phases. The maximum absorption capacity reached as much as 176 g/g, depending on density and viscosity of the target liquids. We currently work on the reusability of ODA-PDA@NFC, but


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already at the present stage we can state that the ODA-PDA@NFC aerogels are promising as absorbent materials for handling oil and solvent spill challenges.

ASSOCIATED CONTENT

Supporting Information The supporting Information was listed in an independent word file. This file includes the the method of fabrication of NFC, characterizations of samples and figures.

AUTHOR INFORMATION Corresponding Author *Email:

[email protected]

*E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare they have no competing interests.

ACKNOWLEDGEMENT This study was supported by the Key Laboratory of Bio-based Material Science & Technology

(Northeast

Forestry

University),

Ministry

of

Education

(grant

no.

SWZCL2016-01), the National Natural Foundation of China (grant no. 31500468 and 31470584), and Overseas Expertise Introduction Project for Discipline Innovation, 111 Project (No. B08016).



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Synopsis: Bio-inspired superhydrophobic nanofibrillated nanocellulose aeroegls show high oil/water separation efficiency. TOC:


Mussel Adhesive-Inspired Design of ... - ACS Publications

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