A pH-responsive superwetting nanostructured copper mesh film for separating both water-in-oil and oil-in-water emulsions

Abstract

Recently, superwetting separating materials for emulsified oil/water mixtures have become a new research focus due to their special advantages such as high efficiency and high flux. So far, although lots of superhydrophobic/superoleophilic and superhydrophilic/underwater superoleophobic films have been prepared for the separation of water-in-oil and oil-in-water emulsions, respectively, smart films that can switch between the above two wetting states and be suitable for the separation of both the two types of emulsions are still rare. Herein, we advance a simple strategy by creating a nanostructure and attaching responsive molecules onto the copper mesh substrate, and report a novel pH-responsive nanostructured copper mesh film. Results indicate that the nanostructure can not only effectively adjust the substrate pore size from the microscale to the nanoscale to meet the requirements for emulsion separation, but can also enhance the surface wettabilities. Combined with the responsive molecules, the film wettability can be switched reversibly between the superhydrophobic/superoleophilic and the superhydrophilic/underwater superoleophobic states. As a result, both water-in-oil and oil-in-water emulsions can be separated on the film with high efficiency and high flux due to the synergy effect between the nanoscale pore structure and the switchable wettability. Given the as-prepared film has such a smart ability, it is believed to be potentially useful in many practical applications, such as sewage treatment and oil recovery.

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Introduction

In the past few years, oil/water separation has aroused much attention since lots of oil spill accidents have caused serious consequences to people’s health and the eco-environment.^1–6 In practice, most oil/water mixtures are emulsified, and are difficult to separate as they are prone to be present in multiple states under different conditions, for example, oil-in-water and water-in-oil emulsions differ by formulation, and surfactant-stabilized and surfactant-free emulsions vary due to their components.^7–9 Compared with the traditional filtration membranes based on the “size-sieving” effect, superwetting separating films have aroused much interest because they can partially overcome the shortcomings of the traditional filtration films, such as high energy consumption, easy fouling, low flux, and low efficiency.^10–12 Generally, these superwetting separating materials are fabricated on substrates with small sized porous structures and can be divided into two kinds: one kind is films with superhydrophobicity/superoleophilicity for the separation of water-in-oil emulsions.^13–26 For instance, by using a phase-inversion method, Zhang et al. reported such a superhydrophobic/superoleophilic poly(vinylidene fluoride) (PVDF) film, and realized the separation of water-in-oil emulsions.¹³ Li et al. reported a magnetic silicon sponge with superhydrophobicity/superoleophilicity, which can absorb oil from water-in-oil emulsions and realize the separation effect.¹⁵ Some other similar materials including cellular aerogels,¹⁶ hybrid organic/inorganic membranes¹⁷ and carbon nanotubes/aerogels were also reported.^18–20 Other kinds of separating materials are those with superhydrophilicity/underwater superoleophobicity that can be used for the separation of oil-in-water emulsions. For example, Gao et al. reported an ultrathin carbon nanotube/TiO₂ network film with such superhydrophilicity/underwater superoleophobicity,²⁷ which realized ultrafast separation of oil-in-water emulsions. Zhu et al. reported a zwitterionic polyelectrolyte grafted PVDF film,²⁸ which displayed an ultrahigh separation efficiency for oil-in-water emulsions. Beside these materials, many other separating films such as mussel inspired hybrid membranes,^29–31 chitosan-based aerogels,³² and hydrogel coated filter paper³³ that are suitable for the separation of oil-in-water emulsions were also prepared.^27–45 Noticeably, only one type of emulsion (water-in-oil or oil-in-water) can be separated on all these films; smart films that can separate both types of emulsion are more favourable, while related research is extremely rare and has only recently been reported for particular films^46–51 such as superhydrophilic/superoleophilic PVDF and ZnO/Co₃O₄ overlapped membranes with underwater superoleophobicity and underoil superhydrophobicity,^46,47 and hygro-responsive membranes with both superhydrophobicity and superoleophobicity in air and under water.⁴⁸ Unfortunately, the fabrication methods for these films are complex and the design strategy is difficult to extend, leading to a failure to summarize a general rule for designing smart separating films for both water-in-oil and oil-in-water emulsions.

Herein, we develop a simple strategy as shown in Scheme 1 and for the first time report a new pH-responsive film for separating both water-in-oil and oil-in-water emulsions. As displayed, the nanostructure was firstly created on the copper mesh substrate to adjust the pore size from the microscale to the nanoscale to meet the requirements for emulsion separation (Scheme 1a and b). Then, the nanostructured copper mesh substrate was modified with the pH-responsive molecules (HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH) (Scheme 1b and c), which show superhydrophobicity/superoleophilicity for non-basic water (Scheme 1c), and superhydrophilicity/underwater superoleophobicity for basic water (Scheme 1d). Different wetting performances allow the film to separate different types of emulsions (Scheme 1e and f). The film wettability can be tuned reversibly between the above two states by simply controlling the water pH, and therefore, the separation of both types of emulsion can be achieved on the film. Given the as-prepared film has such an excellent separating ability and smart wettability, we believe it can potentially be used in many other applications, such as microfluidic devices, sewage treatment, and controllable filtration. It should be stressed that the pH-responsivity just represents an example, the reported strategy of combining nanostructures and responsive molecules can provide a general rule for designing smart separating films for both water-in-oil and oil-in-water emulsions, which can easily be extended to other responsive systems by the modification of responsive molecules with a different stimulus, such as light, electricity, and temperature.

Scheme 1 Schematic illustration of the design principle for the pH-responsive separating film: (a to b) creating nanostructures on the copper mesh substrate to adjust the pore size. (b to c) Modification of responsive molecules to endow the film with different wettabilities: superhydrophobicity/superoleophilicity for non-basic water (c) and applications in separating water-in-oil emulsions (f); superhydrophilicity/underwater superoleophobicity (d) for basic water and applications in separating oil-in-water emulsions (e). A reversible transition between the two wetting states can be realized by alternately changing the water pH, and thus bidirectional separation of different types of emulsions can be realized.

Experimental

Materials

Copper mesh foils (99.9%) were purchased from a native hardware store, and are composed of a single layer of copper wires with square pores, where the thickness is approximately the diameter of the copper wire. (NH₄)₂S₂O₈, NaOH, HCl, ethane, toluene, hexane, chloroform, petroleum ether, Span80, and Tween80 were supplied by Beijing Fine Chemical Co., China. Gasoline was supplied by Sinopec group. HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH (Aldrich, Germany) were used as received. Double-distilled water (>1.82 MΩ cm, Milli-Q system) was used.

Fabrication of the nanostructured copper mesh film

The growth of Cu(OH)₂ nanowires on the copper mesh substrate was carried out through a solution immersion process. The copper mesh substrate was firstly cleaned with acetone and then ethanol under ultrasonic conditions (100 W) for about 10 min. Then the cleaned copper mesh substrate was placed into a water solution containing NaOH (2.5 M) and (NH₄)₂S₂O₈ (0.1 M) for a certain time (for more details see Fig. S1 in ESI†). After that, the substrate was taken out, washed with plenty of pure water and finally dried under N₂.

Modifying the substrate with responsive thiol molecules

After the growth of the nanostructures, the substrate was coated with a layer of Au (using a sputter-coater, Leica EM, SCD500) and then immersed into an ethanol solution containing HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH (the total concentration is 1 mmol l⁻¹ whereas the ratio between the two molecules is variable) for about 12 h. Afterward, the substrate was rinsed with plenty of ethanol and dried with N₂.

Preparation of emulsions

Surfactant-free water-in-oil emulsions were prepared by mixing 30 ml of oil (toluene, hexane, chloroform, gasoline, or petroleum ether) and 0.5 ml of water and then sonicating under a power of 600 W for about 2 h to obtain milk white solutions. Surfactant-free oil-in-water emulsions were prepared by the same process with 45 ml of water and 5 ml of oil. For surfactant-stabilized water-in-oil emulsions, 60 ml of oil and 0.5 ml of water were mixed together, and then Span80 (0.4 g for water-in-toluene, 0.2 g for water-in-hexane, 0.2 g for water-in-petroleum ether, 0.25 g for water-in-chloroform, and 0.2 g for water-in-gasoline) was added, and after stirring for about 3 h, the milk white emulsions were obtained. For surfactant-stabilized oil-in-water emulsions, 60 ml of water and 2 ml of oil were mixed together, then Tween80 (0.6 g for toluene-in-water, 0.2 g for hexane-in-water, 0.2 g for petroleum ether-in-water, 0.2 g for chloroform-in-water, and 0.2 g for gasoline-in-water) was added, and after stirring for about 3 h, the milk white emulsions were obtained. All the prepared surfactant-stabilized emulsions are stable for more than 1 month, and no de-emulsification or precipitation can be observed.

Separation of oil/water emulsions

The separating film was firstly fixed between two glass tubes. For the separation of water-in-oil and oil-in-water emulsions, the as-prepared/restored film (with superhydrophobicity/superoleophilicity) and the basic water (pH = 12) prewetted film (with superhydrophilicity/underwater superoleophobicity) were used, respectively. The emulsions were poured into the upper tube and the separation process was carried out under a certain pressure (5 kPa) with a vacuum driven filtration system.

Characterization

The morphology on the film was investigated using a scanning electron microscope (HITACHI, SU8000). The contact angles and sliding angles were investigated using a contact angle meter (JC 2000D5, Shanghai Zhongchen Digital Technology Apparatus Co., Ltd.). For water and oil contact angle measurements in air, liquid droplets were directly put on the film. For underwater oil wetting performance measurements, the substrates were firstly fixed in a quartzose container that was transparent and full of water. For 1,2-dichloroethane, with its density being higher than that of water, the oil droplet was directly put on the film. For oils with a lower density than that of water, such as petroleum ether, the oil droplet was released under the film through an inverted needle. The average values were achieved by examining five points on an identical film. The photographs in Fig. 4 were obtained using a camera (Canon HF M41). Optical microscopy images were taken using a Nikon AZ100 (Japan) by dropping emulsion solution onto a biological counting board. Water and oil concentrations in the original emulsions and the corresponding collected filtrates were measured using a Karl Fischer moisture titrator (831 KF Coulometer) and a total carbon analyzer (TOC, Aurora 1030W, America), respectively. The acidic droplet was the aqueous solution containing hydrochloric acid, and the basic droplet was the aqueous solution containing sodium hydroxide. The pH value of the solution was measured using a pH meter (PB-10 Sartorius).

Results and discussion

As previously reported, to obtain a filtration film for separating emulsions, small sized pore structures often are needed.^7,8 Herein, a copper mesh substrate was used because of its wide applications in industrial production. Through designing nanostructures on the copper mesh substrates, we realized effective control of the pore size on the substrates. As shown in Fig. 1a, the original copper mesh substrate has pores with an average diameter of about 30 μm. After the growth of Cu(OH)₂ nanowires for about 30 min (Fig. S1–S3 in ESI†), no apparent pores can be observed on the substrate (Fig. 1b). Amplified images show that lots of Cu(OH)₂ nanowires with diameters from 100 nm to 250 nm from adjacent copper lines interlace with each other and thus fill the pores (Fig. 1c and d). By using mercury porosimetry, the average pore sizes on the substrate were investigated (Fig. S4 in ESI†). The results show that as the reaction time for the production of Cu(OH)₂ nanowires is increased, the pore size on the substrate is decreased, and is about 416 nm on the film obtained after a 30 min reaction. These results indicate that the method used here is effective to obtain a film with nanoscale pores.

Fig. 1 SEM images of the copper film substrates: (a) the original substrate; (b–d) after production of Cu(OH)₂ nanowires with different amplifications.

To obtain pH-responsivity, the nanostructured copper mesh substrate was firstly coated with a layer of Au (Fig. S5 in ESI†), and then immersed into an ethanol solution containing mixed thiol molecules HS(CH₂)₉OH and HS(CH₂)₁₀COOH (the mole fraction of HS(CH₂)₁₀COOH in the solution is 0.6, Fig. S6–S8 in ESI†). It is expected that the introduction of carboxylic acid groups can endow the film with pH-responsivity.⁵² Fig. 2 shows the wetting performances on the as-prepared film. As shown in Fig. 2a, for a water droplet with pH = 7, the film shows superhydrophobicity and the droplet can stand on the film with a contact angle and sliding angle of about 154° and 3° (Fig. S9 in ESI†), respectively. In addition to neutral water, similar superhydrophobicity can also be seen for acid water on the film (Fig. S8c and S9 in ESI†). When an oil droplet (1,2-dichloroethane) is placed on the film, the oil droplet spreads quickly and the final contact angle is about 0° (Fig. 2b–d), demonstrating that the film is superoleophilic. These results indicate that the as-prepared film is superhydrophobic for non-basic water, and superoleophilic.

Fig. 2 (a) A water droplet (pH = 7) stands on the film with a contact angle of about 154°; (b–d) an oil droplet (1,2-dichloroethane) can spread on the film and the final contact angle is about 0°. These results indicate that the film is superhydrophobic/superoleophilic for neutral water. (e–g) Process of a basic water droplet (pH = 12) contacting the film with the final contact angle of about 0°; (h) shape of an oil droplet (1,2-dichloroethane) on the film in basic water, and the contact angle is about 162°. These results indicate that the film is superhydrophilic/underwater superoleophobic for basic water. Reversible variation of the water contact angle (i) and the oil contact angle (j) on the film as a function of water pH, demonstrating good recyclability.

When the water pH is increased, the film wetting properties are clearly changed (Fig. S8c in ESI†). Fig. 2e–g display the process of a water droplet with pH 12 contacting the film; it can be seen that upon contact, the droplet would spread and finally be absorbed into the film, which means that the film is superhydrophilic for basic water. Meanwhile, when the film is immersed into such basic water, the oil wetting performance is also changed. As shown in Fig. 2h and S10,† low adhesive underwater superoleophobicity can be observed, and the contact angle and sliding angle for the oil droplet (1,2-dichloroethane, 4 μl) are about 162° and 2°, respectively. These results indicate that for basic water, the film shows superhydrophilicity and underwater superoleophobicity.

Interestingly, after washing with pure water and drying under N₂, the superhydrophilic/underwater superoleophobic film returns to the original superhydrophobic/superoleophilic state. And as shown in Fig. 2i and j, such a transition can be repeated several times without any loss of the responsivity. In addition to 1,2-dichloroethane, some other oils including hexane, petroleum ether, gasoline, chloroform, and toluene were also used to investigate the film oil wettability (Fig. 3). It can be seen that for all these oils, pH-responsivity can also be observed, indicating that the responsive ability of the as-prepared film is universal and independent of oil type. Furthermore, such a pH-responsive transition between the superhydrophobic/superoleophilic and the superhydrophilic/underwater superoleophobic states can remain even after about one month without special protection, demonstrating a good chemical stability of the film.

Fig. 3 Oil contact angles and sliding angles on the film for various oils, indicating that the controllability of surface oil wettability is universal and independent of oil type.

As displayed in Fig. 1 and 2, the film integrates nano-sized pore structures and smart pH-responsive wetting performance and it is expected that such a film can be used to separate different types of emulsions. To test the separating ability, various emulsions including surfactant-free/surfactant-stabilized water-in-oil and oil-in-water emulsions with microscale droplet sizes were prepared (Table S1 in ESI†). The film was firstly fixed between two glass tubes, and then the emulsion was poured into the upper tube and separated under a certain pressure with a vacuum driven filtration system. It is found that for the separation of water-in-oil emulsions, the film with superhydrophobicity/superoleophilicity is suitable, while for the separation of oil-in-water emulsions, the superhydrophilic/underwater superoleophobic film is useful (the as-prepared film needed to be prewetted with basic water with pH = 12). Take the separation of Span80-stabilized water-in-petroleum ether and Tween80-stabilized petroleum ether-in-water emulsions as examples. As displayed in Fig. 4, a significant difference in phase composition between the feed and the corresponding filtrate can be observed using optical microscopy. Compared with the feed solution, after separation, no droplets can be observed in the image of the filtrate. Meanwhile, both types of emulsion changed from the original milk white state to the transparent state, indicating that water in the water-in-petroleum ether emulsion and petroleum ether in the petroleum ether-in-water emulsion has been successfully removed. Similar separating results can also be achieved for other emulsions, including hexane-in-water/water-in-hexane emulsions, chloroform-in-water/water-in-chloroform emulsions, toluene-in-water and water-in-toluene emulsions, and gasoline-in-water/water-in-gasoline emulsions, meaning that the separating ability of our film is universal and independent of oil type.

Fig. 4 Photographs of emulsions before and after separation: (a) Span80-stabilized water-in-petroleum ether emulsion, (b) Tween80-stabilized petroleum ether-in-water emulsion.

To prove the validity of the film for various emulsions, the separating efficiency and flux were further measured. Fig. 5a shows the oil purity of filtrates for water-in-oil emulsions. One can observe that the film has a remarkably high separating efficiency, and the oil purities are higher than 99.96% for all samples. The permeation flux for water-in-oil emulsions was also investigated and calculated from the valid area of the substrate under a pressure difference of about 5 kPa. As shown in Fig. 5b, one can see that all permeations exhibit a high flux, and surfactant-free emulsions have higher fluxes than surfactant-stabilized emulsions. For surfactant-stabilized water-in-toluene, water-in-hexane, water-in-chloroform, water-in-petroleum ether, and water-in-gasoline, the fluxes are 2826, 3805, 2929, 4357, and 2154 l m⁻² h⁻¹ bar⁻¹ respectively. The difference between the fluxes for these emulsions can be attributed to the difference in viscosity for various oils. Since oil is the continuous phase that passes through the film during the separation of water-in-oil emulsions, it is easy to understand that oil with a lower viscosity has a higher flux. The viscosities for toluene, chloroform, hexane, petroleum ether, and gasoline are 0.600, 0.560, 0.307, 0.300, and 0.760 mPa s, respectively, therefore, as shown in Fig. 5b, the water-in-petroleum ether emulsion has the highest flux while the water-in-gasoline emulsion has the lowest flux. As for the oil-in-water emulsions, the oil concentration in the collected filtrate was examined and the results are shown in Fig. 5c. One can observe that for all measured emulsions, the oil concentrations in the filtrates are extremely low, and are lower than 30 ppm for surfactant-stabilized emulsions and 20 ppm for surfactant-free emulsions, indicating that the film has a high separating efficiency. Similar to water-in-oil emulsions, the film also displays high fluxes for oil-in-water emulsions (Fig. 5d). However, the flux no longer depends on the viscosity and instead it heavily relies on the oil density. During the separation of oil-in-water emulsions, water is the continuous phase, and oil with a higher density than that of water can accumulate to form a barrier layer on the film and impede the permeation of water, which results in a relatively lower flux. Therefore, in this work, the chloroform-in-water emulsion has the lowest flux since chloroform has the highest density among the oils used. Furthermore, our film can be used to separate water-in-oil and oil-in-water emulsions alternately, and demonstrates a good recyclability. Herein, the toluene-in-water and water-in-toluene emulsions were used as model feeds to investigate the reusability of the film, and the separation efficiency was recorded. In every cycle after one filtration, the film was simply washed with ethanol and dried under N₂. Fig. 6 shows the separating efficiency for the film upon repeated separating processes. It can be seen that the film can retain a high separating efficiency even after ten times, indicating that the film has an excellent recyclability.

Fig. 5 Permeability of the membrane: (a) oil purity of the corresponding filtrates and (b) permeation flux of the membrane for a series of surfactant-free and surfactant-stabilized water-in-oil emulsions. (c) Oil concentration in the corresponding filtrates and (d) permeation flux of the membrane for various surfactant-free and surfactant-stabilized oil-in-water emulsions. These results demonstrate that the film has a high separating efficiency and high flux for different types of emulsions.

Fig. 6 The separating efficiency of the film for the separation of surfactant-stabilized water-in-toluene and toluene-in-water emulsions alternately. One can find that after several cycles, the film retains a high separating efficiency.

In addition to the separating efficiency and flux, the intrusion pressure, which indicates the maximum height of liquid the film can support, is another criterion to test the durability of the film and can be calculated according to the following equation:¹³

p = ρgh (1)

where p represents the intrusion pressure, and ρ, g, and h are the liquid density, acceleration of gravity, and the maximum height of liquid the film can support. It can be seen that the as-prepared film has a high water intrusion pressure (higher than 8.4 kPa, Fig. S11a in ESI†) and a relative low oil intrusion pressure (oil can permeate the film under gravity) due to the superhydrophobicity/superoleophilicity of the film. After being prewetted by basic water, the film becomes superhydrophilic and underwater superoleophobic. On such a film, the water intrusion pressure decreases (water can permeate the film under gravity), and the oil intrusion pressure is increased to higher than 5.8 kPa for hexane (see Fig. S11b and Table S2 in ESI†). From the above, it can be seen that high intrusion pressures for water and oil during the separation of water-in-oil and oil-in-water emulsions demonstrate that the separating device based on the as-prepared film has a good stability.

In practical applications, the environmental stability of the film is also very significant. During the separation of water-in-oil and oil-in-water emulsions, oil and water, respectively, contact the film intimately. Thus, the anti-corrosive properties of the superhydrophobic/superoleophilic film towards oils and those of the superhydrophilic/underwater superoleophobic film towards different water solutions (such as acidic, basic, and salty) are very important. Noticeably, in this work, the as-prepared film has such anti-corrosive properties. To examine the anti-corrosive properties, the film was firstly immersed into different solutions (acidic, basic and highly salty water solutions, and various oils) for about 24 h, and then the wettabilities and the separating properties (taking the separation of surfactant-stabilized water-in-toluene and toluene-in-water emulsions as examples) of the film were investigated (Fig. S12 and S13 in ESI†). Fig. 7 displays the anti-corrosive properties. It can be seen that both the wetting performances and the separating properties of the film are similar to those of the original film except those in strongly acidic solutions (Fig. S14 in ESI†),⁵¹ demonstrating that our film has good anti-corrosive properties.

Fig. 7 Water/oil (toluene) contact angles and separating efficiencies: (a) superhydrophobic/superoleophilic film after immersion into different oils for about 24 h; (b) superhydrophilic/underwater superoleophobic film after immersion into different water solutions for about 24 h. Results indicate that the film has good anti-corrosive properties.

From the above, it can be found that the special pH-responsivity can endow the film with controllable wettability between different states, and different wetting states allow the film to be used for the separation of different types of emulsions. To have a better understanding of the special separating performance, the formation of these wetting states and the effect of the wetting state on the separating properties are analyzed carefully. The pH-responsive wetting performance can be explained by the following two points: one is the transition between the protonation and deprotonation states of the surface carboxylic acids groups depending on the water pH,^52,53 and the other is the enhancement effect of surface nanostructures.^54,55 In this work, a layer of Au was firstly coated on the Cu(OH)₂ nanowires (Fig. S5 in ESI†), and then modified with mixed thiol molecules. During the modification process, the –SH groups would react with Au and –COOH would be used as the responsive group. For non-basic water, the protonated carboxylic acid groups are relatively hydrophobic (Fig. 8a), and together with the hydrophobic molecule HS(CH₂)₉CH₃ and the amplified effect of the nanostructures, the droplet would reside in the superhydrophobic Cassie state on the film (Fig. 8b). The high contact angle can be explained by the following equation:⁵⁶

cos θ_wr = f₁ cos θ_r − f₂ (2)

where θ_wr and θ_r are the water contact angles for the rough and flat surface modified by the same molecules, respectively. f₁ and f₂ are the fractions of solid and air under the water droplet, respectively (i.e., f₁ + f₂ = 1). In this work, θ_wr and θ_r are 154° and 93° (Fig. S15a in ESI†), respectively. According to eqn (2), f₂ can be calculated to be 0.893, indicating that the air trapped among the nanostructures is high enough to result in superhydrophobicity. Thus, a non-basic water droplet can stand like a sphere and roll on the film. Meanwhile, because both thiol molecules are intrinsically oleophilic (the oil contact angle on the flat surface prepared with X_COOH = 0.6 is less than 10°, Fig. S15c in ESI†), according to the Wenzel equation,⁵⁷ surface nanostructures can enhance the oleophilicity. Therefore, when an oil droplet contacts the film, superoleophilicity can be observed because oil can be absorbed into the nanostructures as a result of the capillary effect (Fig. 8c). From the above, it is easy to understand that the combined effect of nanostructures and the hydrophobic/oleophilic surface chemistry leads to the superhydrophobic and superoleophilic wetting properties of the film. It is well known that on such a superhydrophobic/superoleophilic film, oil can permeate the film while water cannot. Meanwhile, because the pore size on the as-prepared film is on the nanoscale (416 nm, Fig. 1 and S4†), which is clearly smaller than the microscale water droplets in the emulsions (Table S1 in ESI†), microscale water droplets cannot pass through these hydrophobic nanoscale pores. As a result, when the water-in-oil emulsion is in contact with the film, oil would pass through the film due to the superoleophilicity, while the water droplets are retained due to the superhydrophobicity and smaller pore size of the film. Therefore, as shown in Fig. 5, various water-in-oil emulsions can be separated on the film, which is similar to the previous report.¹³

Fig. 8 Schematic illustration of the wetting states of the film and the transition process. For non-basic water, the carboxylic acid groups are protonated (a), and show hydrophobicity/oleophilicity, so under the enhanced effect of nanostructures, the film shows superhydrophobicity (b) and superoleophilicity (c). When basic water is used to contact the film, the carboxylic acid groups are deprotonated (d), and show better hydrophilicity. The capillary effect induced by the nanostructures can result in the superhydrophilicity (e). When the superhydrophilic film is immersed into water, water can enter into the interspaces between the nanowires, the oil droplet would reside in the composite state and the film shows underwater superoleophobicity (f). Because surface carboxylic acid groups can be controlled reversibly between their protonated and deprotonated states, the film wetting states can be tuned reversibly between the superhydrophobic/superoleophilic and superhydrophilic/underwater superoleophobic states.

When basic water contacts the film, the deprotonated carboxylic acid groups with better hydrophilicity can be formed on the film (Fig. 8d and S15b in ESI†). Similar to the above oleophilicity, the hydrophilicity can also be intensified by the nanostructure due to the capillary effect and can result in the superhydrophilicity (Fig. 8e). Thus, as shown in Fig. 2, basic water can enter into the film quickly. Once the film becomes superhydrophilic, water can enter into the interspaces between the Cu(OH)₂ nanowires when the film is immersed in water. In this case, an oil droplet would reside in a composite state (Fig. 8f) and underwater superoleophobicity can be achieved, which can further be proved by the following modified equation:⁵⁸

cos θ′_ow = f cos θ_ow + f − 1 (3)

where θ′_ow and θ_ow are the underwater oil contact angles for the rough and flat surface, respectively. f represents the fraction of solid contacts with oil. Take 1,2-dichlorethane as an example: θ′_ow = 162°, θ_ow = 136° (Fig. S15d in ESI†), and f = 0.175, meaning that more than 82% of the contact area is oil/water contact area, and thus underwater superoleophobicity and easily rolling performances can be observed. The above explanation tells us that the synergistic effect of the hydrophilic deprotonated carboxylic acid groups and the nanostructures results in the superhydrophilicity and underwater superoleophobicity of the film. As is known, when the film shows such superhydrophilicity/underwater superoleophobicity, water can permeate the film while oil cannot. At the same time, because the pore size on the film (the average size is about 416 nm) is obviously smaller than the size of the oil droplets (the size is on the microscale, Table S1 in ESI†) in the oil-in-water emulsions, as the oil-in-water emulsion contacts the film, water can permeate the film due to the superhydrophilicity, while oil cannot because of the underwater superoleophobicity and smaller pore size of the film. Therefore, as shown in Fig. 5c, various oil-in-water emulsions can be separated on the film after prewetting by basic water. Furthermore, since the film wettability can be controlled reversibly between the superhydrophobic/superoleophilic and the superhydrophilic/underwater superoleophobic states several times without the loss of responsivity, as shown in Fig. 6, the alternate separation of oil-in-water and water-in-oil emulsions can be realized.

Conclusions

In conclusion, a simple strategy for the fabrication of a smart separating film was developed by creating nanostructures and attaching responsive molecules. As a demonstration, a pH-responsive superwetting separating film was prepared by simply attaching pH-responsive thiol molecules onto a nanostructured copper mesh substrate. On the as-prepared film, both water-in-oil and oil-in-water emulsions can be separated with high efficiency and high flux. The particular separating ability is ascribed to the combined effect between the small pore size and the pH-responsive wetting performances of the film. This paper reports a novel film with such an excellent separating ability, which is believed to be potentially useful in many other applications, for instance, sewage treatment and oil recovery. Furthermore, considering that the strategy reported here is so simple, it can easily be extended to other responsive systems by modifying molecules with a different stimulus, such as light,⁵⁹ solvent⁶⁰ and so on, which would open up new perspectives for the development of separating materials for oil/water mixtures.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC Grant No. 21304025), and the assisted project by Heilong Jiang Postdoctoral Funds for scientific research initiation (LBH-Q13063).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: SEM images of films with different immersion times for the production of Cu(OH)₂, XRD results of the film, variation of pore sizes with reaction time, SEM images of the film before and after coating with Au, XPS results, variation of water and oil contact angles with surface chemical composition, sliding angles of water and oil droplets on the film in air and in water, respectively, results and discussion for the intrusion pressure, SEM images of the film after immersion in different solutions, wettabilities on the flat film, and results for emulsion droplet size. See DOI: 10.1039/c6ra14454c

‡ Zhongjun Cheng and Chong Li contributed equally to the work.

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