Contents
Introduction
Materials and methods
Materials Fabrication of magnetically superhydrophilic/superoleophobic sponge Characterization
Results and discussion
Surface morphology and chemistry analysis Surface wettability analysis Durability of sponge Acting as a sorbent scaffold to collect water from bulk oils Acting as a separation membrane for oil–water separation
Conclusions
Disclosure of potential conflicts of interests
Acknowledgements
Appendix
References
Introduction
Incredibly large volumes of oil–water mixtures are produced worldwide in a wide variety of industries, which are particularly difficult and expensive to separate [1–4]. The large volumes of contaminated mixtures necessitate the development of durable and cost-effective means to separate oil–water mixtures selectively and quickly. Methods based on porous materials with selective wettability are now gaining increased attention due to their energy efficiency, versatility in treating a variety of industrial waste streams, and consistent performance [5–7]. To demonstrate an opposite wettability between water and oil, porous materials, such as textile, mesh and sponge, can be used to separate oil–water mixtures [5]. Various groups have developed hydrophobic/oleophilic materials for the oil–water separation by coating a hydrophobic material onto porous substrates [8–13]. Although many different materials with superhydrophobicity and superoleophilicity have now been developed, they are subject to oil fouling as oils can be absorbed into the porous texture of them [5,7]. The oil fouling decreases the oil–water separation performance and necessitates periodic washing of the materials, resulting in higher operating costs [7]. To overcome this disadvantage, some research groups have paid attention to design underwater superoleophobic surface [14–20]. Such underwater superoleophobic surfaces exhibited excellent oil fouling resistance when submerged in water. For these materials, however, they required to be prewetted with water prior to the separation process to avoid oil fouling, which was operation inefficiency [15,17–19]. Engineering porous materials that can repel oils while being wet by water would be an appealing way for oil–water separation while has been proven extremely challenging. The challenge resulted from the design principle that required an interface simultaneously exhibits surface energy higher than water and lower than oil, while superoleophobic surfaces generally exhibited superhydrophobicity in practice [21–22]. Recently, Yang et al. developed a superoleophobic surface by spray coating, and it became superhydrophilic/superoleophobic state after air-plasma treatment [23]. Xu et al. developed a superamphiphobic coating that turned the superhydrophilic/superoleophobic state upon ammonia exposure [24]. However, the above mentioned superhydrophilic/superoleophobic states required external stimulation such as plasma or vapor to achieve, and these indirect implementations restricted the practicability of the developed materials. Thus, more efforts should be made to address this challenge.
For practical applications, recent interest has also been paid to impart remote controllability to the oil–water separation materials [25–27]. The remote controllability function can help people avoid entering hazardous areas and health risks while collecting the water or oils freely. To achieve remote controllability, imparting magnetic pro-perty to the oil–water separation materials is a promising candidate [6]. For magnetic materials, their movements can be controlled remotely by an external magnetic field, allowing the oil–water separation to be performed from a distance without touching the materials directly. Herein, we develop a sponge with anti-fouling and remote control-lability through the deposition of Fe3O4 followed by coating with a polymer with hydrophilic and oleophobic components simultaneously. The sponge displays unusual superhydrophilic/superoleophobic property. Oil droplets such as hexadecane and crude oil display spherical shape on their surfaces, while water droplets wet them immediately and completely. Such superhydrophilicity/superoleophobicity does not need external stimulates to achieve, which simplifies the operation process. Owing to its superhydrophilic/superoleophobic property, the sponge can act as an adsorbing material to collect water from bulk oils selectively without any oil-absorbing. This selective water collection from bulk oils is completely different from conventional techniques based on removing oils from bulk water [8–13], which has seldom been reported. Interestingly, the sponge is controlled to move to water areas by a magnet without touching it directly, owing to its magnetic property. This remote controllability could potentially lower the cost of the water collection process. The absorbed water in the sponge is easily extracted by squeezing, and thus it can be reused for oil–water separation once more. Apart from acting as a water absorption material, the obtained sponge is also used as a filiation material to separate water from oil–water mixtures and oil in water emulsion efficiently, when placed in a cone funnel. Water is passing through the obtained sponge easily, whereas oils are accumulating above the sponge without any penetration. This work could provide a novel way to develop advanced oil–water materials.
Materials and methods
Materials
Perfluorooctanoic acid (CF3(CF2)6COOH) and bis(3-trimethoxysilylpropyl) amine (APS) were purchased from Sigma-Aldrich. Ferroferric oxide (Fe3O4) and sodium hydroxide (NaOH) were provided by Sinopharm Chemical Reagent Company. Sodium perfluorooctanoate (SPFO, 0.10 mol·L−1) was prepared by the reaction of CF3(CF2)6COOH with NaOH in ethanol. The commercially available sponge was purchased from a local store and cleaned with acetone and deionized water sequentially in an ultrasonic cleaner before use.
Fabrication of magnetically superhydrophilic/superoleophobic sponge
The fabrication process of superhydrophilic/superoleophobic sponge was shown in Fig. S1. The ultrasonic cleaned sponge was immersed into Fe3O4 ethanol solution (0.02 g·mL−1, 50 mL) for 5 min. Then, the Fe3O4 deposited sponge was immersed into APS ethanol (0.01 g·mL−1, 50 mL) solution for 5 min, followed by the immersion in PFO ethanol (0.1 mol·L−1) solution for 10 min. Finally, this Fe3O4@APS–PFO coated sponge was dried in an oven at 100 °C for 1 h.
Characterization
Contact angle (CA) and sliding angle (SA) measurements were performed using a Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at ambient temperature. The volume of probing liquids in the measurements was 5 μL. The average CA and SA values were determined by measuring the same sample at five different positions. Field-emission scanning electron microscopy (FESEM) was carried out by a JSM-6701F scanning electron microscope (JEOL, Japan). Energy dispersive X-ray spectrometry (EDX) was conducted on a NORAN System Six X-ray microanalysis system (THERMO) attached to FESEM. Optical images were obtained by a digital camera (Nikon).
Results and discussion
Surface morphology and chemistry analysis
The magnetic superhydrophilic/superoleophobic sponge was obtained by a dip-coating process. SEM results demonstrated that the Fe3O4@APS–PFO coating was uniformly coated onto the sponge skeleton without blocking sponge pores that were beneficial for oil–water separation (Figs. 1(a) and 1(b)). EDX imaging of iron (Fe), fluorine (F) and nitrogen (N) elements indicated their uniform distributions on the surface, as shown in Fig. 1(c). For APS–PFO, the PFO anions can coordinate to quaternary ammonium groups of APS by the electrostatic attraction, as shown in Fig. 1(d), and the high surface concentration of fluorinated groups (oleophobic constituents), together with carboxyl and quaternary ammonium groups (hydrophilic constituents), can give in the Fe3O4@APS–PFO coated sponge with oil repelling and water-attracting property simultaneously, based on recently published reports [23].
Surface wettability analysis
Figure 2(a) showed that the Fe3O4@APS–PFO coated sponge can be easily picked up by a magnet, demonstrating its magnetic property. The surface wetting property of the Fe3O4@APS–PFO coated sponge was shown in Figs. 2(b)–2(e) and Table S1, which demonstrated that the sponge behaved superhydrophilic and superoleophobic property simultaneously. Water droplet with a volume of 5 μL quickly spread on the sponge surface within 0.1 s and gave a water CA of 0°, as shown in Fig. 2(c), while oil droplets such as dodecane and rapeseed oil exhibited a spherical shape on the sponge surface and easily rolled off with the SA value lower than 5° (Figs. 2(b), 2(e), and Table S1). The sponge was also strongly repellent to other organic liquids including toluene, hexadecane, and dichloromethane (Fig. 2(b) and Table S1). Importantly, as shown in Fig. 2(f), the sponge can be cut and machined into desired shapes without sacrificing its superhydrophilic/superoleophobic property. This was because the Fe3O4@APS–PFO coating was extended throughout the sponge’s whole volume, forming a superhydrophilic/superoleophobic bulk material.
Fig.2 Magnetic and wetting property analysis: (a) magnetic property of the sponge; (b) optical image of different probing droplets on the sponge surface; (c)(d)(e) contact angle and sliding angle profiles of water (left), dodecane (middle), and rapeseed oil (left) on the sponge surface; (f) optical image of water and hexadecane droplets on the sponge cut by a knife. |
The total free energy of a solid (γs) is considered as the sum of contributions from dipole-hydrogen bonding (γsd) and dispersion forces component (γsp), as shown in Eq. (1):
As for this coating, the fluorinated groups (namely −CF3 and −CF2−) minimize the γsd constituent, resulting in high resistance against oils such as n-hexadecane and rapeseed oil. The hydrophilic components (namely carboxyl, quaternary ammonium groups, and sodium ions) increase the γsp constituent, leading to a strong affinity to polar water molecules. This hydrophilic/oleophobic wetting behavior of the coating was magnified when increasing the surface roughness by adding Fe3O4 nanoparticles into it. Therefore, this coating exhibited superhydrophilic and superoleophobic wetting properties simultaneously in the air.
Durability of sponge
For super-wettability materials to become practically relevant, their durability and robustness should be carefully tested under conditions required by the applications [31]. Herein, the sponge still retained its superhydrophilic/superoleophobic property after exposure in air for 90 d (from January to April 2018 in Yantai, China), indicating its good long-term stability. Moreover, the sponge was resistant to severe mechanical bending and twisting without sacrificing its original superoleophobicity, and oil droplets were moving readily on the bent and twisted surface (Figs. 3(a), 3(b), 3(e) and 3(f)). Also, oil droplets still maintained a spherical shape on the pressed and deeply cut surface without the change of the oil CA value, as shown in Figs. 3(c), 3(d), 3(g) and 3(h). Similarly, the obtained sponge retained its water-attracting and oil-repelling property after other tests including thermal treatments, water jetting, and water immersing tests (Table 1 and Fig. S2).
Tab.1 CA and SA values of water and dodecane on the sponge surface after different treatment |
| Treatment | Water CA/(° ) | Water SA/(° ) | Dodecane CA/(° ) | Dodecane SA/(° ) |
|---|---|---|---|---|
| Original sponge | 0 | – | 154.0 | 10.0 |
| Thermal treatment at 200°C for 24 h | 0 | – | 154.5 | 9.5 |
| Subzero treatment at −18°C for 24 h | 0 | – | 156.0 | 9 |
| UV irradiation at 254 nm for 24 h | 0 | – | 36.8 | 3.0 |
| Knife scratching | 0 | – | 154.5 | 9.5 |
| Water jetting | 0 | – | 152.5 | 12 |
| Water immersing | 0 | – | 154.1 | 9.8 |
Acting as a sorbent scaffold to collect water from bulk oils
The obtained sponge possessed superhydrophobicity and superoleophilicity simultaneously, allowing it to be an ideal candidate for removing water from bulk oils. With the remote control by a magnet, the sponge was driven to touch the water film spreading on the dichloromethane surface (Fig. 4(a)) and then sucked in the part of the water film in contact with it, resulting in a local white-color region around and behind the sponge where fresh dichloromethane was exposing. Finally, the sponge absorbing water was picked up by a magnet (see Appendix for the video). The sponge can also be applied to absorb water in oil, as shown in Fig. 4(b). The sponge was floating on the rapeseed oil surface, due to its superoleophobicity, and it was driven to touch the water in bulk rapeseed oil when a magnetic field was applied. Then, the sponge was controlled to move to the water area by a magnet, and in this way, water in bulk rapeseed oil was collected completely. After the collection, the water absorbed in the sponge was collected directly by a mechanical squeezing process, and the residual water was removed directly by heating in air. In this way, the sponge can be reused time and time again.
Herein, it should be emphasized that the obtained sponge selectively absorbs and collects water from bulk oils, which is completely different from conventional techniques that are based on removing oils from bulk water [8–13]. Also, the movement of sponge can be controlled by a magnet without touching it directly. Thus, this work here provides a good example that possesses great potential for applications in purification of water-containing oils.
The water collection stability of the sponge was further studied. The water absorption capacity of the sponge was 31.47, and it changed slightly even 20 cycles of water absorption (see Fig. 5(a)). No visible water existed in the oil phase after water collection, and the water collection efficiency for all oil–water mixtures was more than 97%, as shown in Fig. 5(b). Importantly, the sponge could keep its high water collection efficiency even after 20 cycles of water collection, indicating its good durability, as shown in Fig. 5(c). This enhanced water collection efficiency and reusability of the sponge enabled it to be a good candidate applying in industrial water-containing oils treatment, if it was scaled up adequately.
Acting as a separation membrane for oil–water separation
Besides acting as a water sorbent scaffold, the sponge was also applied as a filtration material to separate water from oil–water mixtures selectively. As shown in Figs. 6(a) and S3, the sponge was fixed into a cone-shaped funnel as a filter module to separate the oil–water mixture. When the mixture toluene and water was pouring onto the superhydrophilic/superoleophobic sponge, water was penetrating the sponge and flowing down the beaker underneath, whereas toluene was retaining on the sponge surface (see Figs. 6(a)–6(d)). As a result, the oil–water separation was easily achieved. No visible oil existed in the water phase after the separation process, and the separation efficiency was up to 99.1%. Other types of oil–water mixtures were also separated using this separation setup with high separation efficiency, as shown in Fig. 6(e). Moreover, the sponge maintained its excellent separation efficiency even after 20 cycles of oil–water separation, as shown in Fig. 6(f).
Fig.6 Oil–water separation: (a)(b)(c)(d) water was passing through the sponge, whereas toluene (colored with Oil Red) was accumulating above the sponge without any penetrating; (e) oil–water separation efficiency of the sponge for different types of mixtures; (f) oil–water separation efficiency of the sponge as a function of separation cycle number. |
The sponge was then applied to separate oil in water emulsion. As shown in Fig. 7, the filtrate became from milky to clear after one-step separation cycle by the sponge. Optical microscopy analysis showed that the diameter of the oil (namely toluene) droplets in the emulsion feed was mainly distributed in the range of 0.5–1.5 μm. After separation, no oil droplets in this range were observed in the filtrate (see Fig. 7(b)) and the separation efficiency was as high as 98.4%, indicating that almost all toluene droplets were removed after separation.
Conclusions
A sponge with unusual superhydrophilicity/superoleophobicity and magnetic property was developed through deposition with Fe3O4 followed by coating with APS–PFO. Water droplets spread over the sponge completely, while oil droplets such as rapeseed oil and dodecane rolled off the sponge surface easily without any penetration. The sponge retained its superhydrophilic/superoleophobic property after severe bending, twisting, pressing, and deep cutting, demonstrating its mechanical durability. Due to its superhydrophilic/superoleophobic property, the sponge was demonstrated a sorbent scaffold to absorb and collect water from bulk oils with enhanced efficiency and reusability. Interesting, the sponge was manipulated remotely by a magnet without touching it directly during the whole water collection process, due to its magnetic property. The obtained sponge was also applied as a filiation material to separate water from oil–water mixtures and oil in water emulsion efficiently. Our multifunctional sponge may contribute to the development of advanced oil–water separation materials for practical applications.