Controlling Superwettability by Microstructure and Surface Energy

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Controlling Superwettability by Microstructure and Surface Energy Manipulation on Three Dimensional Substrates for Versatile Gravity Driven Oil/Water Separation Hao-Yang Mi, Xin Jing, Han-Xiong Huang, and Lih-Sheng Turng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10901 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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ACS Applied Materials & Interfaces

Controlling Superwettability by Microstructure and Surface Energy Manipulation on Three Dimensional Substrates for Versatile Gravity Driven Oil/Water Separation Hao-Yang Mi1,2,3, Xin Jing1,2,3*, Han-Xiong Huang1, and Lih-Sheng Turng2,3* 1

Department of Industrial Equipment and Control Engineering South China University of Technology, Guangzhou, 510640, China 2 Department of Mechanical Engineering, University of Wisconsin–Madison Madison, WI 53706, USA 3 Wisconsin Institutes for Discovery, University of Wisconsin–Madison Madison, WI 53715, USA

Corresponding Authors: Email: L.S. Turng [email protected] Email: X. Jing [email protected]

Keywords: Superhydrophobic, underwater superoleophobic, oil/water separation, microstructure, surface energy

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Abstract: Superwettable materials have gained tremendous attention due to their special wetting abilities. The key to obtaining and tuning superwettability is to precisely control the interfacial microstructures and surface energies of materials. Herein, we propose a novel approach to controlling the superwettability of three-dimensional (3D) foams. The surface microstructure was manipulated by the layer-by-layer covalent grafting of multidimensional nanoparticles (e.g. silica, carbon nanotubes, and graphene oxide), and the surface energy was tailored by grafting chemicals with different functional groups. This grafting approach improved the mechanical performance, reduced particle loading, and prevented particle disassociation, thereby increasing the absorption capacity and durability of the functionalized foams. More importantly, superhydrophobic/superoleophilic foams were obtained after heptanol grafting. They showed water contact angles of 153° in air and 158° in oil, an absorption capacity 113 times their weight gain, and a remarkable flux of 32.6 L m-2 s-1 when separating oil from water driven by gravity. Polydopamine grafting resulted in superhydrophilic/underwater superoleophobic foams which had an oil contact angle of 152° under water and a high flux of 19.3 L m-2 s-1 when separating water from oil. Thus, this study offers not only intelligent materials for versatile oil/water separation, but also a profound approach for engineering high-performance superwettable materials.


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Oil spills are one of the main causes of water pollution1. Developing materials and products for effective oil recycling and water remediation is an urgent need. Fabrication of high-performance superabsorbent and oil/water separation materials has been attracting much attention from both academic and industrial communities. Materials that have superwettability show promising performance in the selective separation of oil and water due to their special wetting phenomenon. To date, two types of superwettable materials have been developed for gravity-driven oil/water separation. Superhydrophobic/superoleophilic materials that allow oil penetration but repel water can separate heavy oil from water by gravity.2-3 Likewise, superhydrophilic/underwater superoleophobic materials that enable water transport but prevent oil flow can separate water from light oil by gravity.4-5 These peculiar interfacial selective wetting behaviors have been attracting tremendous attention in recent years due to their potential to be used in a variety of intelligent applications.6-7 The underlying mechanism for superhydrophobic surfaces has been elaborated on with the well-known Cassie–Baxter theory which suggests that the high surface roughness and low surface energy of a substrate are the keys for a liquid to achieve nonwetting contact with a substrate in air.8-9 However, other superwetting behaviors in more complicated interfaces, such as “solid–oil–water” systems, have not been fully elucidated, whereas, they are in the interest of developing multifunctional materials for particular applications including selective oil absorption, oil/water separation. So far, there are a vast number of studies on particulate-modified superhydrophobic materials since hierarchical nanostructures can be easily generated by loading nanoparticles.10 The nanoparticles are normally loaded onto substrates by dip coating,11 spraying,12 in situ deposition,13 or by using a commercial binder.14 However, a particular issue has arisen in practical applications. That is the particles tend to fall off and contaminate the recovered oil during usage due to the lack of strong chemical bonds. To resolve this problem, in this study, 3 ACS Paragon Plus Environment


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we proposed a layer-by-layer chemical grafting method to create strong covalent bonds between the substrate and the particles. Nanoparticles such as silica,12 titanium dioxide,15 graphene oxide (GO),16 carbon nanotubes (CNTs),17 and cobalt13 have been introduced to various 3D substrates like melamine,18 polyurethane,19 cotton,20 and copper21 foams to enhance their surface roughness. Recently, it was reported that combining multidimensional particles was an effective way to improve the surface roughness of the substrate.22 Therefore, in this study, 0D silica nanoparticles, 1D functionalized CNTs (fCNTs), and 2D GO were used to engineer the surface microstructure of melamine foam (MF). These three multidimensional particles are collectively called GSC particles for simplicity. The overall fabrication process of multifunctional foam is illustrated in Figure 1 and mainly includes two steps. The first step involves microstructure manipulation. MF was modified by hexamethylene diisocyanate (HDI) in dimethylformamide (DMF), followed by dipping into silica, fCNT, and a GO solution, and subsequent curing. Then the foam was sonicated in DMF to remove unreacted particles and vacuum dried. This procedure was repeated to achieve layer-by-layer grafting. The second step involves surface energy manipulation. Particle-grafted MF (GSCMF) was modified with isocyanate groups by reacting with HDI, then reacted with heptanol in DMF to graft long carbon chains, which yielded the superhydrophobic MF (HepGSCMF). On the other hand, GSCMF was also subjected to a dopamine (DA) coating in aqueous solution with pH = 8.5 to enhance the hydrophilicity. Dopamine was able to self-polymerize and coated GSCMF to result in superhydrophilic MF (DAGSCMF). In the process, since all reactions happened on the foam surface, the reactant solutions can be repetitively used until all particles were consumed, which significantly reduces cost compared with other batch-to-batch modification methods.


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Figure 1. Schematic illustration of MF surface microstructure and surface energy manipulation procedures to produce superhydrophobic HepGSCMF and superhydrophilic DAGSCMF.

The covalent bond was created not only between MF and nanoparticles, but also among GSC particles because of their abundant–OH and –COOH reactive groups. To reveal this, three particles were reacted in HDI and observed using transmission electron microscopy (TEM). Tight connections between silica and GO, GO and fCNT, fCNT and silica, and among GO, silica, and fCNT were observed (Figure 2a-c). The bridging among particles enabled multiple layer coating. In addition, Fourier transform infrared spectroscopy (FTIR) spectra confirmed the presence of C=O, N–H, and C–N bonds from urethane and amide bonds in GSCMF, thus indicating the successful grafting of particles via covalent bonds (Figure S1). After heptanol grafting, samples were characterized with X-ray photoelectron spectroscopy (XPS). Figure S2 showed a significant increase in the intensity of the C1s peak, and the 5 ACS Paragon Plus Environment


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carbon atom percentage increased from 65.7% to 76.5% after heptanol grafting (Table S1) because of the introduction of long carbon chains. Scanning electron microscopy (SEM) was used to investigate the morphology of hydrophobic foams. MF grafted with only heptanol showed a relatively smooth surface (Figure 2e). When grafted with multidimensional GSC particles, the surface became rougher with the increase of grafting layers (Figure 2f-j). The particle aggregates were able to be distinguished when there were less than four grafted layers, but it became indistinguishable after five layers of grafting (GSC-5L), which may be due to the simultaneous increase of HDI content. GSC-4L was scanned using energy-dispersive X-ray spectroscopy (EDS) to map the distribution of silicon (Si). The results (Figure S3) revealed a homogenous distribution of Si on the substrate surface. The photographs of corresponding foams (Figure 2k) clearly showed an increase of darkness as the GSC grafting layers increased. For comparison, a control MF made by the conventional dip coating method was also prepared. The color of the control MF with five coatings was completely black (Figure S 4a), while GSC-5L was dark grey. The difference in color indicates the different loading content of GSC particles. According to the measured data (Figure S 4b), the loading content was less than 30 wt.% with the layer-bylayer grafting method, while it was more than 90 wt.% for the alternative dip coating method. Although high particle loading can enhance surface roughness, it also causes absorption capacity reduction and potential particulate contamination to the recovered oil or water.


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Figure 2. TEM images of a) GO grafted with silica, b) GO grafted with fCNT, c) silica grafted with fCNT, d) GO and silica grafted with fCNT. Arrows indicate GO, silica, and fCNT particles. SEM images of e) HepMF, f) GSC-1L, g) GSC-2L, h) GSC-3L, i) GSC-4L, and j) GSC-5L. k) Photographs of corresponding functionalized foams. The density and surface area are important bulk characteristics of absorbent materials. According to Figure 3a, the bulk density increased with an increase in grafting layers, while the surface area showed a different trend where GSC-2L was the highest. This is because the surface area was increase by introducing nanoparticles initially, but it decreased when the weight of the loaded particles was too high. The water contact angle (WCA) of heptanolmodified MF (HepMF) was 130.2°, and it increased to 152.6° for GSC-4L indicating superhydrophobicity (Figure 3b). In addition, the MF grafted with multidimensional nanoparticles showed higher WCA than MF grafted with single kind of nanoparticles (Figure S5). In order to investigate the sustainability of the grafting, specimens were either thermally 7 ACS Paragon Plus Environment


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treated at 200 °C or immersed in liquid nitrogen for up to 4 h (Figure 3c and d). All specimens showed a reduction of WCA of less than 3°, suggesting that the modification was highly thermally stable. Moreover, GSC-4L and GSC-5L maintained their superhydrophobicity after being stored in air for 5 months. In the compression test, the compressive modulus and strength of modified MFs were significantly improved after GSC grafting (Figure 3f and Table S2). They also showed excellent recoverability (Figure 3e), which is an important property for absorbent materials to achieve effective oil recovery. When compressed to higher strains, the hysteresis loop of GSC-4L became larger along with a significant increase in strength (Figure 3g). Statistical data showed an increase of hysteresis loss from 23.4% to 54.5% when strain was increased from 10% to 60%, while GSC-4L still maintained a high recovery rate over 99.2% (Table S3). In 500 cycles of loading and unloading, GSC-4L showed a small reduction in strength and hysteresis loss, while a recovery rate of 96.3% was still achieved, even after 500 cycles (Figure 3h and Table S3). These results elaborate on the extraordinary mechanical and chemical stability of the modified foams.

Figure 3. a) Bulk density and surface area, and b) water contact angle of functionalized foams. Water contact angle change c) at 200 °C and d) in liquid N2 for up to 4 h. e) Photographs demonstrating high recoverability. f) Compression test stress vs. strain curves. Cyclical compression test g) at different strains and h) for 500 cycles. 8 ACS Paragon Plus Environment

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In practical applications, people often have to face the trade-off between absorption capacity and separation efficiency when choosing absorbent materials. As particle loading content increases, the WCA is increased but the absorption capacity is decreased due to the increase of bulk density as shown in Figure S6. Achieving high capacity and absorption efficiency with minimum particle loading is a major direction in superabsorbent materials research. Because GSC-4L is superhydrophobic (WCA > 150°) and has relatively high absorption capacity (11.3~46.3 times weight gain), it was used as absorbent and separation materials in the following study and is denominated as HepGSCMF in the following sections. To demonstrate the durability of HepGSCMF when used in various circumstances, it was subjected to repeated mechanical squeeze, ultrasonication, and sand paper abrasion. As shown in Figure 4a, HepGSCMF showed excellent tolerance to squeeze and ultrasonication with WCA still over 150° after 500 times squeezing and 60 min ultrasonication. After abrasion using sand paper for 10 m, the WCA was reduced to 146° which revealed pretty high tolerance of HepGSCMF to abrasion. In addition to neutral water (pH = 7.0), HepGSCMF showed a high contact angle (CA) to aqueous solutions, with a pH ranging from 1 to 13.5, and its tolerance to acidic solutions seemed slightly higher than to basic solution (Figure 4b). Figure 4c shows the relative absorption efficiency (RAE) results of HepGSCMF, which indicates the sustainability of the absorption ability in multiple cycles. The results showed that the decrease of RAE was less than 7% after 10 cycles of usage for both hexane and chloroform. In addition, the mass of the specimen remained almost the same, thus indicating negligible particle loss. On the contrary, the control MF showed a significant decrease in RAE, and about a 15% mass loss due to particle disassociation (Figure S7). The remarkable robustness of HepGSDMF emphasized the profound effects of the strong bonding between substrate and nanoparticles. As a demonstration of application of HepGSCMF as a superabsorbent material (Figure 4d), it was used to absorb heavy oil under water and light oil 9 ACS Paragon Plus Environment


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atop water. HepGSCMF was able to selectively absorb oils from water rapidly, and the absorbed oil can be simply recycled by squeezing or distillation. Figure 4e indicates that HepGSCMF was able to absorb various oils and organic solvents, and the absorption capacity of HepGSCMF was compared with other superabsorbent materials from the literature (Table S4), which suggests that the absorption capacity of HepGSCMF surpass all particulatemodified commercial foams, except the graphene coated foam.19, 23-24 Moreover, HepGSCMF showed less than a 10% decrease in absorption capacity in 50 cycles of usage for absorption of various oils (Figure S8). Taking the excellent durability into consideration, HepGSCMF is an excellent, durable superabsorbent material for high efficiency, selective oil recycling applications.

Figure 4. a) WCA of HepGSCMF after subjecting mechanical squeeze up to 500 times, ultrasonication up to 60 min, and sand paper abrasion up to 10 m. b) WCA of HepGSCMF to aqueous solutions with a pH ranging from 1 to 13.5. c) Relative absorption efficiency of HepGSCMF to hexane and chloroform over 10 cycles. e) Demonstration of absorption of heavy oil under water and light oil atop water. e) Absorption capacity of HepGSCMF to various oils.


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In addition to its superabsorption properties, HepGSCMF can be used for oil/water separation. As shown in Figure 5a and Movie 1, heavy oil (chloroform) was able to flow through thick water layers and be separated by HepGSCMF with high efficiency, as indicated by the microscope image of separated chloroform. In this study, superhydrophilic/underwater superoleophobic foams (DAGSCMF) was also fabricated by simply coating the GSC-grafted MF with dopamine. The characteristic peaks of polydopamine were confirmed by the FTIR spectrum of DAGSCMF (Figure S9). The coating of polydopamine was further verified by a high resolution C1s scan of XPS (Figure S10). A pre-wet DAGSCMF was used for water/oil separation as shown in Figure 5c and Movie 2. Similarly, water was able to flow through light oil (hexane) and high purity water was reclaimed. When control MF was used for the same separation experiment, the collected water was contaminated by the dip coated particles on the foam (Figure S11). HepGSCMF and DAGSCMF had the same surface microstructure but different surface chemistries which resulted in their distinctly different surface superwettabilities. Conventional composite boundaries for superhydrophobic surfaces involve solid–liquid–gas interfaces, which are governed by equation 1,25

cos θ * = ϕ ( cosθ + 1) − 1

(1)

where θ* is the CA to the microstructured surface (apparent CA), θ is the CA to the flat surface (intrinsic contact angle, ICA, affected by the surface energy), and φ is the area fraction of the solid that touches the liquid, which is typically reflected by the surface roughness.26 Obviously, θ* can be increased by reducing φ and increasing θ. The air trapped beneath the liquid (air pocket) is believed to be the reason preventing liquid infiltration, and more air pockets normally corresponds to smaller φ.6, 27 However, the composite boundary changes to the solid–oil–water interfaces in the case of separation. Thus, Equation 1 should still be applicable, but θ needs to be replaced with ICA in liquid (i.e. θo-w) instead of air. Because oil and water are generally immiscible, θo-w is typically a large value. For example, 11 ACS Paragon Plus Environment


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HepGSCMF has a WCA of 152.6°, but the WCA increases to 158.4° when it is submerged in hexane, while its CA to chloroform is 0° in air (superoleophilic) (Figure 5e). Since HepGSCMF is superhydrophobic to water, no pre-wetting is needed for separation. As illustrated in Figure 5b, the water layer is supported by air and oil pockets. The case is more complicated for DAGSCMF. DAGSCMF is both superhydrophilic and superoleophilic in air. In order to block the oil flow, the air in the foam needs to be replaced with water, which has a high θo-w. (Note: Sufficient surface roughness is still required to maintain a small φ according to Equation 1). Figure 5f and Movie 3 show that DAGSCMF was able to absorb water and showed a WCA of 0° in air, and its CA to chloroform was 151.3° when measured underwater indicating under water superoleophobicity. Therefore, the pre-wetted DAGSCMF was able to prevent the flow of oil thanks to the water pockets (Figure 5d). In addition to its versatile separation attributes, these modified foams showed excellent flux (32.6 L m-2 s-1 for HepGSCMF and over 19.3 L m-2 s-1 for DAGSCMF) and separation efficiency (over 99% initial efficiency). The flux was much higher than most recently developed separation materials as shown in Table S5.28-30 Moreover, the high flux and efficiency were able to be maintained very well over ten cycles of testing (Figure S12). The high flux was attributed to the low particle loading content, which led to a negligible diminution of pore size of the original foam. The high separation efficiency and property stability were attributed to the superwettability and strong bonding between substrate and nanoparticles.


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Figure 5. a) Demonstration of heavy oil separation from water using HepGSCMF. b) Mechanism of oil/water separation using superhydrophobic/superoleophilic material. c) Demonstration of water separation from light oil using DAGSCMF. d) Mechanism of water/oil separation using superhydrophilic/underwater superoleophobic material. e) Chloroform CA in air and WCA in hexane of HepGSCMF. f) WCA in air and chloroform CA in water of DAGSCMF.

Based on the fundamental Equation 1, perfluorodecyltriethoxysilane (PDTS) was grafted onto GSCMF in the second modification step via chemical vapor deposition (CVD) to further lower the surface energy (Figure S13). The functionalized foam was named FGSCMF. A strong F1s peak was detected using XPS (Figure S14), and the EDS areal mapping confirmed the uniform distribution of the F element on FGSCMF (Figure S15), thus suggesting uniform grafting. A further improvement of WCA to 160.4° was achieved for FGSCMF (Figure S16). More importantly, FGSCMF started to repel organic solvents. Contact angle tests of a series 13 ACS Paragon Plus Environment


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of chemicals revealed that FGSCMF possesses a CA of 147.3° to glycerol and 115.4° to hexane, and that the CA was lower for chemicals with smaller surface tensions (Figure S17). It is believed that superamphiphobicity can be achieved by grafting more GSC layers (by reducing φ). In summary, an ingenious approach is proposed to manipulate surface wettability of 3D substrates to produce superhydrophobic/superoleophilic materials and superhydrophilic/ underwater superoleophobic materials. The approach involves layer-by-layer covalent grafting of multidimensional particles that manipulate surface microstructure, and the grafting of chemicals with different functional groups. Covalent grafting of particles improved mechanical performance, reduced necessary particle loading content, and prevented particle disassociation, thereby increasing absorption capacity and property stability, and prevented contamination of recovered oil or water. Changing the material surface energy resulted in intelligent separation materials that were capable of separating heavy oil from water and water from light oil with unprecedented flux and separation efficiency. Grafting fluoro chains on the surface further enhanced superhydrophobicity and endowed amphiphobicity. Besides the outstanding performance of the materials, this study elucidates the fundamental theory in directing the design of advanced functional materials for a host of applications and providing guidelines for engineering robust surfaces with superwettability.

Experimental Section Preparation of GSCMF: MF was ethanol cleaned and dried and then immersed in DMF containing HDI at 80 °C for 3 h, then dipped into a DMF solution containing an equal mass of GO, silica, and fCNT nanoparticles. MF was cured at 80 °C for 3 h under vacuum then washed in DMF with ultrasonication to remove unreacted particles. This procedure was repeated to obtain GSCMF grafted with multiple layers of particles.


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Preparation of HepGSCMF: GSCMF with four layers of particles was immersed in DMF containing HDI at 80 °C for 3 h, then transferred to DMF containing heptanol and reacted at 70 °C for 3 h, followed by an ethanol wash and vacuum drying. Preparation of DAGSCMF: GSCMF with four layers of particles was immersed in an aqueous dopamine solution with a pH of 8.5 adjusted by tris(hydroxymethyl)aminomethane at room temperature for 16 h, followed by a 50% ethanol wash and vacuum drying.

Supporting Information Detailed experimental procedure, characterization, comparison with literature, and more results of chemical characterization (FTIR, XPS, EDS, WCA), absorption and separation performance are available in supporting information (PDF).

Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (51603075; 21604026), the Office of the Vice Chancellor for Research and Graduate Education, and the Wisconsin Institute for Discovery (WID) at the University of Wisconsin–Madison.

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Table of contents


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Controlling Superwettability by Microstructure and Surface Energy

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