One-Step Preparation of Highly Hydrophobic and Oleophilic Melamine

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One-step Preparation of Highly Hydrophobic and Oleophilic Melamine Sponges via Metal-ion Induced Wettability Transition Yichun Ding, Wenhui Xu, Ying Yu, Haoqing Hou, and Zhengtao Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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ACS Applied Materials & Interfaces

One-step Preparation of Highly Hydrophobic and Oleophilic Melamine Sponges via Metal-ion Induced Wettability Transition Yichun Ding,† Wenhui Xu,‡ Ying Yu,‡ Haoqing Hou,‡ and Zhengtao Zhu*† †

Biomedical Engineering Program, Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA

‡

Department of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, China

KEYWORDS. Melamine sponge; Hydrophobic effect; Metal complex; Oil absorbent; Surface chemistry

ABSTRACT. Hydrophobic and oleophilic absorbent materials have received wide attentions in recent years for potential applications in pollutant removal from accidental spills of oil or organic chemicals.

In this work, we report a Metal-Ion Induced Hydrophobic Melamine

Sponge (MII-HMS) prepared by a one-step solution immersion process.

The commercial

melamine sponge (intrinsically superhydrophilic with a water contact angle of ~ 0°) is immersed in an aqueous solution of transition metal ions (e.g., FeCl3, Fe(NO3)3, Zn(NO3)2, Ni(NO3)2, and

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Co(NO3)2) for a short period, followed by drying. This simple process renders the transition of the superhydrophilic melamine sponge to become highly hydrophobic (a water contact angle of ~130°). Results from the X-ray photoelectron spectroscopy (XPS) and the infrared spectroscopy suggest that the unprecedented transition is likely due to the formation of metal complexes during immersion.

The MII-HMS is also oleophilic, exhibiting excellent oil absorption

capabilities, ~ 71 to 157 times of its weight for a wide range of oils and organic solvents. Our work offers a simple, scalable, and economical approach to fabricate highly-efficient absorbent materials for potential applications in oil spill recovery and environmental remediation.

1. INTRODUCTION Accidental oil and organic solvent spillages may cause severe environmental and ecological damage.1, 2 In particular, the oil spill accidents in waters (such as the Gulf oil spill in 2010) are the worst; the spills not only contaminate water and kill a large number of aquatic organisms,3, 4 but also have significant long-lasting effects. Cleanup of the spilled oil in water has been challenging. At present, mechanical absorption by the porous materials is considered to be the most effective and economical method.5, 6 Conventional absorbent materials such as natural wood sawdust, zeolite and wool fibers, and nonwoven polypropylene/polyester fabrics typically have the drawbacks of low absorption capacities, poor selectivity, and poor recyclability.6 In order to separate oil from a water medium, the absorbent material should be hydrophobic (water contact angle> 90°) and oleophilic.7, 8 The wettability of a solid surface is determined by its chemical composition and the surface roughness.8

Lowering the surface energy and

increasing the surface roughness are two common strategies for preparation of hydrophobic


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surface.

For examples, a hydrophilic surface can become hydrophobic by coating with

perfluorosilanes;9-11 methods such as deposition of nanoparticles, surface etching, and electrospinning can be used to increase the surface roughness to make a hydrophobic material superhydrophobic (with a water contact angle > 150°).12-16 The melamine sponge (melamineformaldehyde resin sponge) is a commercially available sponge with a highly porous open-cell structure.9,

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It is commonly used as an insulating material for pipes and ductwork, a

soundproofing material, and a kitchen cleaning sponge (e.g., Mr. Clean® Magic Eraser). Melamine sponge is intrinsically both superhydrophilic and oleophilic. Many studies have explored strategies of surface modification and roughing to convert the melamine sponge to a hydrophobic material. Ruan et al.9 prepared a superhydrophobic melamine sponge by depositing a thin layer of polydopamine, followed by grafting with a low surface energy molecule, 1H,1H,2H,2H-perfluorodecanethiol. Pham et al.18 and Chen et al.10 hydrophobilized melamine sponges using octadecyltrichlorosilane (CH3(CH2)17SiCl3) and polydimethylsiloxane (PDMS), respectively. Melamine sponge can also be carbonized to form a hydrophobic carbon sponge.17, 19-21

Despite many significant progresses in preparation of hydrophobic and oleophilic materials

in recent years, application of the hydrophobic materials as oil absorbents has been limited due to complicated manufacture processes, high cost, and difficulty to scale up. Herein, we report a simple, economical, and scalable method for preparation of a hydrophobic sponge for oil and organic solvent removal. Metal-ion Induced Hydrophobic Melamine Sponge (MII-HMS) is prepared by immersion of a melamine sponge in a salt solution (e.g., FeCl3, Fe(NO3)3, Zn(NO3)2, Ni(NO3)2, and Co(NO3)2) for a short period of time, followed by drying. To the best of our knowledge, the ion-induced wettability reversion in melamine sponge has not been reported previously.

The metal-ion induced hydrophilic to hydrophobic transition of


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melamine sponge is characterized by x-ray photoelectron spectroscopy (XPS) and infrared spectroscopy. The as-prepared MII-HMS sponge exhibits excellent oil absorption capabilities that is 71 to 157 times of its own weight for a wide variety of oils and organic solvents. Furthermore, the MII-HMS sponge can be used to absorb oils/organic solvents both under and on water surface. 2. EXPERIMENTAL SECTION Preparation of hydrophobic melamine sponge: Melamine sponge (melamine-formaldehyde resin sponge) was purchased from SINOYQX (Sichuan, China) and used as received. Other chemicals were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA).

For

preparation of a hydrophobic melamine sponge (i.e., MII-HMS), a melamine sponge was immersed in a salt solution. After a set period of time, the sponge was removed from the salt solution; the solution absorbed in the sponge was squeezed out and further sucked out using towel paper. Thereafter, the sponge was dried in an oven at a setting temperature. Oil/organic solvents absorption capacity measurement: A cubic MII-HMS sample with the size of 2.00 × 2.00× 2.00 cm3 was used for testing the absorption capacity of the sponge to various oils and organic solvents. The absorption capacity was calculated based on the following formula. .⁄ . =

−

where m0 is the mass of the prepared MII-HMS sample, m1 is the mass of the MII-HMS sample fully absorbed with organic solvent/oil. For each experiment, 5 samples were measured, and the average value was reported.


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Density and porosity measurement: A cubic pristine melamine sponge or MII-HMS with the size of 2.00 × 2.00× 2.00 cm3 was used for measuring the density (ρ) and porosity (P). The density and porosity were calculated using the following formulas. ⁄ =

and = 1 −

!"

# × 100%

where ms is the mass of the sample, Vs is the volume (i.e., 8.00 cm3) of the sample, ρ is the density of the sample, and ρbulk is the density of the bulk melamine resin (ρbulk=1.51 g/cm3). Five samples were measured, and the average value was reported.

The density of the pristine

melamine sponge is 9.87±0.10 mg/cm3, and the density of the MII-HMS (prepared with a 0.1 M FeCl3 solution) is 10.16±0.06 mg/cm3; the porosity of MII-HMS (prepared with a 0.1 M FeCl3 solution) is 99.3%. Stability test: The stability of the hydrophobicity of MII-HMS was evaluated. The MII-HMS sample was treated at different conditions, including sonication in DI water, steaming in boiling water, dipping in ethanol, and dipping in acid (0.1 M HCl & 0.1 M H2SO4) and alkali solutions (0.1 M NaOH), and then dried in an oven at 100 °C. The water contact angles of the samples treated in these harsh conditions were measured. Since the MII-HMS samples were hydrophobic, they were immersed in the above hydrophilic solutions forcibly during the experiment. Characterization: The morphology and structure of the sponges were characterized by Field Emission Scanning Electron Microscopy (FE-SEM, Zeiss Supra 40 VP) operated at an accelerating voltage of 2 kV. The chemical mapping using energy-dispersive x-ray spectroscopy


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(EDS) was conducted in the same SEM microscope operated at an accelerating voltage of 8 kV. Fourier transform infrared spectrometer (FT-IR, Tensor 27, Bruker, Germany) was used to characterize the chemical structure of the pristine melamine and MII-HMS sponges. X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi, USA) was used to analyze the surface elemental information. Water contact angles were measured by Contact Angle specific surface area analyzer (OCA 15EC, Dataphysics instruments GmbH, Germany) from 2 µL droplets of deionized (DI) water. For each sample, the average value of the 5 to 8 measured water contact angles at different locations of the sample was reported; one standard deviation was reported as the error bar. Note that the water droplets for the contact angle measurement were not dyed; in the case that the water or oil droplets were on the surface of the sponges for photo and video taking, oil or water was colored by common food dyes for better visual observation, and was referred as dyed oil or water.

3. RESULTS AND DISCUSSION Figure 1a and Movie S1 (Supporting Information) demonstrate the simplicity of the metal-ion induced wettability transition in melamine sponge. The pristine melamine sponge with a water contact angle ≈ 0° is intrinsically superhydrophilic and oleophilic (Figure 1b). A piece of melamine sponge is immersed in a FeCl3 solution (0.1 M) for less than 10 minutes followed by drying; this simple treatment results in a hydrophobic melamine sponge (denoted as MII-HMS). As shown in Figure 1c and Movie S2, the water droplets (dyed red) bead up on the surface of the MII-HMS, indicating that the melamine sponge becomes hydrophobic. Interestingly, MII-HMS remains oleophilic; the vegetable oil droplets are immediately absorbed into the sponge (as shown in Figure 1c and Movie S2).

The hydrophobic behavior of MII-HMS is further


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demonstrated by photos in Figure 1d-f. A piece of MII-HMS floats on the water surface (Figure 1d), and can be used to seal water in an Erlenmeyer flask (Figure 1e); the water droplet on the surface of the MII-HMS sample is readily removed by a piece of tissue paper (Figure 1f & Movie S3). When a monolith of MII-HMS is cut into several small pieces, these pieces remain hydrophobic (Movie S4); therefore, hydrophobilization occurs not only on the surface but also inside the sponge, demonstrating a clear advantage of the highly porous spongy materials.

Figure 1. a) Preparation of the Metal-ion Induced Hydrophobic Melamine Sponge (MII-HMS) by immersion of the melamine sponge in a salt solution (e.g., FeCl3) followed by drying. b) Photograph of the water (dyed red with red ink) and vegetable oil droplets on the pristine melamine sponge. (c) Photograph of the water (dyed red with red ink) and vegetable oil droplets on a MII-HMS sample. d) A floating MII-HMS sample, and a sunk pristine melamine sponge sample in water (dyed red). e) A MII-HMS sample used as a cap sealing water in an Erlenmeyer flask. f) A droplet of dyed water on the MII-HMS sample removed by a piece of tissue paper.


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Figure 2a shows the water contact angle of MII-HMS prepared by immersion in a FeCl3 solution (0.1 M) for different periods followed by drying at 100 °C. After immersed in the salt solution for just a few seconds and then dried, the obtained MII-HMS sample becomes hydrophobic with a water contact angle of 122.5±2.6°; the contact angle raises to 127.4±4.4° after 2 minutes and remains at ~130° afterwards. The effect of the drying temperature on the water contact angle of MII-HMS is shown in Figure 2b; when dried at above 40 °C, MII-HMS has a water contact angle higher than 130°; even when dried at ambient temperature (~20 °C) after immersed in a FeCl3 solution, MII-HMS becomes hydrophobic with a water contact angle of 115.3±3.0°.

Figure 2. a) Water contact angle of MII-HMS prepared by immersion in a FeCl3 solution (0.1 M) for different duration, followed by drying at 100 °C. b) Water contact angle of MII-HMS prepared by immersion in a FeCl3 solution (0.1 M) for 10 minutes, followed by drying at different temperatures. c) Water contact angle of MII-HMS prepared by immersion in the FeCl3


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solutions of different concentrations. d) Water contact angle of the melamine sponge treated by immersion in DI water (blank experiment) and different salt solutions (0.1 M).

The effect of the FeCl3 concentration is subsequently investigated. As shown in Figure 2c and Figure S1a (Supporting Information), after treated by the FeCl3 solutions with concentrations of 0.0005, 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, and 0.1 M, the resultant MII-HMS sponges have uniform and stable hydrophobic surface with the water contact angles of 130.1±5.1°, 125±6.4°, 123.2±7.9°, 121.3±8.1°, 126.9±9.6°, 127.3±8.6°, 129±2.2°, and 130.7±0.9°, respectively. The results indicate that the water contact angle of MII-HMS does not depend on the concentration of the FeCl3 solution in the range of 0.0005 to 0.1 M. It is quite remarkable that the melamine sponge becomes hydrophobic (water contact angle of 130.1±5.1°) even when treated by a FeCl3 solution with concentration as low as 0.0005 M (i.e. 0.5 mM). When the melamine sponges are treated with the FeCl3 solutions with concentrations of 0.00005 M and 0.0001 M (i.e., 0.05 mM and 0.1mM), the resultant sponges show unstable/nonuniform hydrophobic behavior; some areas of the sponge surface are hydrophobic with the contact angles higher than 120° and the water droplets beading up, whereas some areas are hydrophilic with the contact angles of 0~20° and the water droplets sinking in the sponge (as shown in Figure 2c and Figure S1a). When the FeCl3 solution of 0.00001 M (i.e., 0.01 mM) is used, the treated melamine sponge remains completely hydrophilic and no change of wettability is observed. As we will discuss later, the hydrophilic to hydrophobic transition is attributed to the metal complex formation between Fe3+ and N of the melamine sponge. Since the melamine sponge is highly porous with a porosity of 99.3%, the weak FeCl3 concentration dependence during preparation of MII-HMS suggests that the Fe3+ and N complexes are formed uniformly through the sponge when the concentration of a


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FeCl3 solution is higher than 0.0005 M (i.e. 0.5 mM); when the Fe3+ concentration is too low (e.g. 0.1 mM) to form complexes uniformly in the sponge, the treated sponge shows nonuniform surface properties with patches of hydrophobic and hydrophilic areas. Such nonuniform surface behavior suggests that there are not enough Fe3+ ions to interact with the melamine sponge uniformly at very low concentrations; some areas with sufficient complexes formation would render the hydrophilic to hydrophilic transition, whereas some patches could not form sufficient complexes to induce the wettability transition. On the other hand, when the concentration of FeCl3 for preparation of MII-HMS is too high, the water contact angle of MII-HMS decreases, as shown for the samples treated by 0.2 and 0.5 M FeCl3 solutions in Figure 2c. One possible explanation of the result is that the excessive FeCl3 salt at high concentration may deposit on the surface of the melamine sponge, which weakens the hydrophobicity of MII-HMS. Interestingly, the metal-ion induced hydrophilic to hydrophobic transition of the melamine sponge is not limited to the FeCl3 solution. As shown in Figure 2d and Figure S1b, similar hydrophilic to hydrophobic transition of the melamine sponge is observed when it is treated in the Fe(NO3)3, Zn(NO3)2, Ni(NO3)2, and Co(NO3)2 solutions.

As a control experiment, the

melamine sponge immersed in the DI water and then dried at 100 °C remains hydrophilic. When the melamine sponge is treated by the NaCl and CaCl2 solutions, no hydrophilic to hydrophobic transition is observed. Figure S1c shows that all melamine sponges treated by the NaCl solutions with different concentrations are hydrophilic. Furthermore, when the melamine sponge is treated in a NaCl solution with a trace amount of FeCl3, the sponge becomes hydrophobic. As shown in Figure S1d, when the melamine sponge is treated by as dilute as 0.0001 M FeCl3 in the 0.1 M NaCl solution (i.e., by adding 1.6 mg FeCl3 salt in 100 mL 0.1 M NaCl solution), the hydrophilic to hydrophobic transition is observed in the melamine sponge. These results suggest that the


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transition metal ions are required for preparation of MII-HMS and the main group metal ions do not induce the wettability change in the melamine sponge. It is worth noting that the metal-ion induced hydrophilic to hydrophobic transition is unique to the melamine sponge. A control experiment shows that no hydrophilic to hydrophobic transition is observed in a cellulose sponge when it is treated with the salt solutions (Figure S2). In addition, the hydrophobic melamine sponge (i.e, MII-HMS) prepared by the metal-ion induced wettability transition is very robust. As shown in Figure S3; the MII-HMS sponge remains highly hydrophobic after it is sonicated in DI water or dipped in boiling water, ethanol, and acid/base solutions and then dried at 100 °C. Scanning electron microscopy (SEM) images in Figure 3a-a’ and 3b-b’ show the morphology of the pristine melamine sponge and MII-HMS. The melamine sponge has a typical open cell spongy structure; and the MII-HMS prepared with a FeCl3 solution (0.1 M) shows no structural and morphological changes. Additionally, the surface of the melamine sponge prepared with different FeCl3 concentration remains smooth (Figure S4), except some degrees of roughening at the sporadic locations for the samples prepared with high FeCl3 concentrations (0.2 and 0.5 M), which may support our explanation that the deposition of excessive FeCl3 salt on the sponge surface results in the decreased contact angles in these samples (Figure 2c). Therefore, the SEM results indicate that the hydrophilic to hydrophobic transition in melamine sponge is not caused by change of the surface roughness. The composition of the MII-HMS sample prepared with the FeCl3 solution (0.1 M) is mapped by SEM Energy Dispersive Spectroscopy (EDS) analysis (Figure 3c-h); C, N, O, Fe, and Cl elements are identified on the surface, confirming that the metal ions are incorporated into the MII-HMS.


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Figure 3. a-a’) SEM images of the pristine melamine sponge. b-b’) SEM images of the MIIHMS prepared with a FeCl3 solution (0.1 M).

c-h) SEM-EDS elemental mapping images

acquired from a representative MII-HMS prepared with a FeCl3 solution (0.1 M).

To better understand the mechanism of the hydrophilic to hydrophobic transition in MII-HMS, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are used to analyze the chemical constituents and the chemical bonds in the melamine and MII-HMS sponges. Figure 4 shows the FTIR spectra of the pristine melamine sponge and MII-HMS prepared with the FeCl3 solutions of different concentrations. The IR spectrum of the pristine melamine sponge shows the characteristic stretching vibration modes of C=N and C-N on triazine ring at 1542 cm-1 (C=N),13 and 1329 cm-1 (C-N),22, 23 respectively. For the spectrum of MII-HMS, the vibration modes at 1542 and 1329 cm-1 shift to the lower wavenumbers of


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1539~1541 cm-1 and 1321~1327 cm-1, respectively, suggesting that the triazine N atom forms the coordination bonds,24, 25 presumably with the metal ion Fe3+ . The vibration mode of the amino C-NH shifts from 1468 cm-1 in the pristine melamine sponge to the higher wavenumbers of 1471~1478 cm-1 in MII-HMS, supporting the formation of the coordination bonds between the amino N atom and the metal ion.26

Figure 4. FTIR spectra of the melamine sponge and the MII-HMS samples prepared with the FeCl3 solutions of different concentrations. XPS analysis further confirms the interaction between the metal ion and the melamine sponge. The XPS spectrum of the pristine melamine sponge shows the peaks of C, N and O, whereas the peaks of Fe and Cl are observed in the spectrum of MII-HMS (Figure 5a, 5b, and Figure S5). Compared to the XPS spectrum of the pristine melamine sponge, the C 1s and O 1s peaks in the spectra of the MII-HMS samples do not change significantly (Figure S6), whereas the N 1s peak shifts significantly to the higher binding energy (Figure 5c). In the melamine sponge, there are two types of the N atoms: N of the triazine ring (=N-) and N of the amino group (–NH-). As shown in Figure 5d, the N 1s peak of the pristine melamine sponge is de-convoluted to two peaks at 397.98 eV and 399.06 eV, which are assigned to the =N- and –NH- groups, respectively.27, 28


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The N 1s peak of MII-HMS prepared with the FeCl3 solution (0.1 M) is de-convoluted to three peaks at 397.98 eV, 399.06 eV and 399.53 eV, among which the new peak at 399.53 eV would be attributed to the formation of the coordination bonds between the N atoms and the metal ions.28 The peaks of the corresponding metal elements are also observed from the XPS spectra of the MII-HMS samples prepared with other salt solutions (as shown in Figure 5e); the N 1s peak shows a significant shift to a higher binding energy (Figure 5f), whereas the C 1s and O 1s peaks show no changes (Figure S7).

These results clearly indicate the formation of the

coordination interactions between the metal ions and the N atoms during preparation of MIIHMS, resulting in the hydrophilic to hydrophobic transition.

Figure 5. a) XPS spectra of the pristine melamine sponge and the MII-HMS sample prepared with a FeCl3 solution (0.1 M). b) XPS spectra of the pristine melamine sponge and the MII-


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HMS sample prepared with a FeCl3 solution (0.1 M) in the energy region of Fe 2p. c) N 1s peak of the pristine melamine sponge and the MII-HMS sample prepared with the FeCl3 solutions of different concentrations. d) Fitting of the N 1s peak of the pristine melamine sponge and the MII-HMS sample prepared with the FeCl3 solution (0.1 M). e) XPS spectra of the pristine melamine sponge and the MII-HMS sample prepared with different salt solutions (0.1 M). f) N 1s peak of the pristine melamine sponge and the MII-HMS sample prepared with different salt solutions (0.1 M).

Based on the results of the FTIR and XPS analysis, a mechanism of the metal-ion induced hydrophilic to hydrophobic transition is proposed. Figure 6a shows a representative metal ion-N coordination structure, in which the Fe3+ ion forms a six-coordinated complex. It is known that the surface energy is proportional to the polarity of the chemical bonds on the surface.12 The repeating unit of the melamine sponge is a nitrogen-containing benzoheterocyclic skeleton of 2,4,6-triamino-s-triazine, in which the N atoms of the triazine are sp2 hybridized and the N atoms of the amino groups are sp3 hybridized;29 both types of the N atoms have the lone-pair electrons. The abundance of the lone-pair electrons results in a high polar surface and accordingly makes the melamine sponge intrinsically hydrophilic. The d-block transition metal ions can coordinate with the lone-pair electrons of N in the melamine sponge to form the metal complexes through the coordinate covalent bonds.24, 25, 28, 30-35 The nitrogen-metal ion complexes, together with the counter ions, may re-organize the chemical structure on the surface of the melamine sponge, which results in the reduced surface polarity of the melamine sponge and makes it hydrophobic. Similar hydrophilic to hydrophobic transition was also observed in a mesh coated with the


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hydrophilic poly(acrylic acid) after the poly(acrylic acid) chelated with the metal ions (e.g., Hg2+) in solution.36

Figure 6. Schematic illustrations of (a) the coordination interactions between the metal ions and the N atoms in MII-HMS and (b) the corresponding hydrophilic to hydrophobic transition of the melamine sponge. It is worth noting that the spongy morphology may play an important role in amplification of the hydrophilic to hydrophobic transition (Figure 6b). After the formation of complexes, which reduces the surface polarity of the melamine sponge and makes it hydrophobic, the highly porous/rough surface might substantially improve the hydrophobicity because of the trapped air cushion underneath the water droplet, as suggested by the Cassie model.37 This also explains that the types of salts have no significant effects on wettability transition; as long as the ions could form complexes with the melamine sponge to initiate the wettability transition, the contact angles of the hydrophobic sponge would be amplified by the porous structure. To investigate the effects of the morphology on the metal-ion induced wettability change in melamine polymer, the melamine resin films are prepared by cross-linking melamine with formaldehyde in a basic aqueous solution.38 The preliminary results of the wettability of the


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melamine resin films treated by salt solutions (0.1 M FeCl3, Fe(NO3)3, Zn(NO3)2, Ni(NO3)2, Co(NO3)2) are presented and discussed in the Supporting Information.

Based on these

preliminary results, the metal ions’ influence on the wettability of flat melamine film is not conclusive. To further understand the metal-ion induced wettability reversion in the melamine materials, the melamine materials with different composition (e.g., composites of melamine and other polymers cross-linked by formaldehyde) or morphology (e.g., flat melamine film, melamine sponge with different porosity/roughness, or melamine nanofibers), the strength of the metal-nitrogen complex, and different ionic surfactants besides the simple transition metal ions may be interesting. Research along these directions is currently under way. MII-HMS is evaluated as an absorbent for removal of the various oils and organic solvents from a water medium, given its hydrophobic and oleophilic characteristics and other highly attractive physical properties, including high porosity (>99%), light weight (ρ=10.16±0.06 mg/cm3), opencell structure, and good elasticity. Figure 7a and Movie S5 shows that a piece of MII-HMS can extract a puddle of heavy oil/solvent such as chloroform (dyed red) underwater quickly without uptake of water. In Figure 7b, a cube of MII-HMS is used to absorb the vegetable oil which floats on the water surface; the absorbed oil can be readily squeezed out from the cube. The absorption capacities of MII-HMS for the various oils and organic solvents are evaluated. As shown in Figure 7c, MII-HMS exhibits the excellent absorption capabilities of 71 times (for hexane) to 157 times (for chloroform) its own weight, where the absorption capability is correlated with the density of the solvents/oils. The absorption capabilities of MII-HMS are comparable or exceed those of the sponge materials prepared by more complicated methods, as shown in Table S1. Furthermore, the melamine sponge is flame retardant due to the high nitrogen content,9 and MII-HMS maintains this property. As shown in Figure 7d, MII-HMS


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withstands the combustion of the absorbed ethanol, and no heavy smoke is released during the fire (Movie S6). Besides the excellent oil absorption capability, the total material cost of preparation of MII-HMS is estimated to be only ~ ¢0.53 per 1 inch3 (Table S2). Therefore, the method for preparation of the hydrophobic melamine sponge reported here is simple, facile, and economical.

Figure 7. a-b) Photographs showing the absorption of (a) chloroform (dyed red) under water and (b) vegetable oil on the water surface using the MII-HMS sample; c) Absorption capacity of the MII-HMS prepared with the FeCl3 solution (0.1 M) for various organic solvents and oils; d) Photographs showing the absorption and burning of ethanol using MII-HMS.

4. CONCLUSION In summary, a highly hydrophobic and oleophilic melamine sponge (MII-HMS) is prepared by one-step immersion of an intrinsically hydrophilic melamine sponge in a salt solution (e.g., FeCl3, Fe(NO3)3, Ni(NO3)2, Zn(NO3)2, and Co(NO3)2) for a short period (as short as a few seconds). FTIR and XPS results suggest that the nitrogen atoms of melamine may coordinate with the transition metal ions, leading to the wettability change from superhydrophilicity to high


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hydrophobicity in the melamine sponge. The obtained hydrophobic MII-HMS exhibits excellent oil absorption capacity (up to 157 times of its own weight), and can remove both light and heavy oils on the water surface or under water, respectively. The preparation process of MII-HMS is simple, fast, economic, environmentally friendly, and readily scalable. We envision that MIIHMS may have great potentials for oil spill recovery and environmental remediation. ASSOCIATED CONTENT Supporting Information. Photos of the water droplets on the melamine sponge prepared with various salt solutions and concentrations; Cellulose sponge treated with different salt solutions (control experiment); Stability of hydrophobicity of MII-HMS; XPS spectra of MII-HMS prepared with different salt solutions; Wettability of the melamine resin films treated by salt solutions (0.1 M FeCl3, Fe(NO3)3, Zn(NO3)2, Ni(NO3)2, Co(NO3)2); Comparison of MII-HMS and other hydrophobic spongy materials for oil/organic solvent absorption; Cost estimation of the materials for preparation of MII-HMS. (PDF) Supporting videos showing various properties of MII-HMS. (AVI) AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors.

All authors have given

approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge financial support from the Biomedical Engineering Program of South Dakota School of Mines and Technology.

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Table of Contents Graphic


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One-Step Preparation of Highly Hydrophobic and Oleophilic Melamine

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