A UV-driven superhydrophilic/superoleophobic polyelectrolyte multilayer film on fabric and its application in oil/water separation

Abstract

Superhydrophilic/superoleophobic materials have more obvious advantages in anti-pollution and oil/water separation, but the preparation is very challenging due to the higher surface tension of water than that of oils. Reported herein is a new superhydrophilic/superoleophobic surface that is prepared through UV irradiation of a polyelectrolyte multilayer film (PMF) with perfluorooctanoate (PFO) as the counterions on a commercially available cotton fabric. Hydrophilic defect domains along with PFO counterions on the PMF-coated fabric result in simultaneous superhydrophilicity and superoleophobicity both in air and underwater. Water can pass through the fabric by the driving force of gravity, while oils remain above the fabric without any pollution and permeation. The peculiar wetting behavior of the fabric is useful for the gravity-driven oil/water separation with excellent anti-fouling capacity and high efficiency. The recyclability and stability of the superhydrophilic/superoleophobic PMF-coated fabric were also investigated. We anticipate that this novel method could promote the application of superhydrophilic/superoleophobic materials in the field of oil/water separation.

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1. Introduction

Extreme wetting behaviors of solid surfaces by liquids (water or oil), namely superhydrophobicity, superoleophobicity, superhydrophilicity, and superoleophilicity, have recently attracted great attention due to a variety of practical applications.¹ Superhydrophobic/superoleophobic surfaces can be used in the fields of self-cleaning, anti-icing, and anti-corrosion;^2–4 superhydrophilic/superoleophilic surfaces possess excellent anti-fogging function;⁵ and superhydrophobic/superoleophilic surfaces or superhydrophilic/superoleophobic surfaces are very attractive for separating oil/water mixtures.^6–9 In comparison, superhydrophilic/superoleophobic materials have more obvious advantages in anti-pollution and oil/water separation, because water can permeate through the surface only by the driving force of gravity and oil-based contaminants can be retained above the surface.¹⁰ However, due to the higher surface tension of water than that of oils, on a surface the water contact angle normally is higher than the oil contact angle, and so surfaces that simultaneously have superhydrophilicity and superoleophobicity seem impossible.

Recently, the problem has been solved by designing novel interface materials with the hydrophilic groups and fluorinated alkyl chains.¹¹ Water can penetrate through hydrophilic defects in the fluorinated outermost layer, while oil is hindered by this top layer. Tuteja et al. fabricated oil/water separation materials with superhydrophilicity and superoleophobicity using a blend of fluorodecyl polyhedral oligomeric silsesquioxane and cross-linked poly(ethylene glycol) diacrylate.¹² Zhao et al. prepared a superamphiphobic coating with an ammonia-triggered transition to superhydrophilic and superoleophobic for oil/water separation using silica nanoparticles and heptadecafluorononanoic acid-modified TiO₂ sol.¹³ In our previous work, we fabricated a serial of polyelectrolyte–fluorosurfactant complex-based meshes with superhydrophilicity and superoleophobicity, which can selectively separate water from oil/water mixtures with the advantages of anti-fouling and easy recycling in contrast to traditional oil/water separation materials.^14,15 However, the poor solubility and film-forming performances of materials with the hydrophilic groups and fluorinated alkyl chains limit the application in the fabrication of superhydrophilic/superoleophobic surfaces.

It is well-known that layer-by-layer (LbL) deposition of the polyelectrolyte multilayer film (PMF) can be performed on virtually any textured rough surfaces in water as a simple and green surface modification method. The PMF assembled on the basis of electrostatic interactions has a strong bonding strength with the substrate, and its surface exist excess charges and associated counterions that can be capitalized to modulate the wettability of PMF.¹⁶ Recently, superoleophobicity has been achieved on the rough substrate after PMF ionpaired with fluoro-containing anions,¹⁷ and the surface of PMF is heterogeneous due to interpenetration of constituent layers and desorption of assembled molecules. Some hydrophilic defect domains are always concomitant with PMF, which can cause increased water contact angle hysteresis according to the previous report.^18,19 Moreover, the amount of hydrophilic defect domains on PMF can be tuned through the external stimulus. Therefore, it is possible to obtain superhydrophilicity/superoleophobicity on the PMF via the appropriate increase of the hydrophilic defect domains.

In the present work, we report a novel method to fabricate a superhydrophilic/superoleophobic surface through UV irradiation of PMF on a commercially available cotton fabric. Hydrophilic defect domains, combined with fluoro-containing anions on the UV-irradiated PMF-coated fabric leads to a water droplet spread completely within seconds and oil droplets remain the spherical shapes with the contact angles greater than 150°. The peculiar wetting behavior of the fabric is useful for the gravity-driven oil/water separation via water permeation and oil retaining, which is completely different to the conventional techniques that selectively remove oils from bulk water.^20–22 More importantly, the fabricated surface is superoleophobic both in air and underwater. No matter water or oil first contact the surface, there is no pollution and permeation of oil. This feature makes it have the obvious advantages in the field of oil/water separation in contrast to the widely reported underwater superoleophobic materials.^23–25

2. Experimental

2.1. Materials

Poly(diallyldimethylammonium chloride) (PDDA, average M_w ∼200 000–350 000, 20 wt% in water) and poly(sodium 4-styrene sulfonate) (PSS, powder, average M_w ∼70 000) were purchased from Sigma-Aldrich. Sodium chloride (NaCl), sodium perfluorooctanoate (PFO, C₈F₁₅O₂Na), methylene blue (MB), oil red O (C₂₆H₂₄N₄O), and hexadecane (CH₃(CH₂)₁₄CH₃) were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of PMF

Glass slides and cotton fabrics were used as the substrates and cleaned with ethanol and deionized water via ultrasonication to remove possible impurities. PDDA (1 mg mL⁻¹) and PSS (1 mg mL⁻¹) aqueous solutions with 1 M NaCl were prepared for LbL assembly. The PMF was assembled on the substrates by sequential dipping of the substrate in PDDA and PSS aqueous solutions for 20 min. The substrates were rinsed with deionized water for 1 min between each deposition step. This deposition cycle was repeated 2.5 times to obtain PMF on the substrate. Finally, the PMF-coated substrates were immersed in a sodium perfluorooctanoate aqueous solution (0.1 M) for 1 min to absorb PFO anions, followed by rinsing with deionized water and drying with nitrogen.

2.3. UV-driven conversion of wettability

The PMF-coated substrates were placed under a UV lamp, which emits UV light in the wavelength range around 185 and 254 nm. The light intensity was maintained at 2 μW cm⁻². The UV lamp was directly illuminated on the substrates at a working distance of 10 cm in air at room temperature. Relative humidity was kept in all experiments at 50%.

2.4. Characterization

Scanning electron microscopy (SEM) was conducted on a JSM-6701F operated at an acceleration voltage of 5.0 kV. X-ray photoelectron spectra (XPS) were obtained on an ESCALAB250xi spectrometer equipped with a focused monochromatic Al X-ray source (1486.6 eV). The surface roughness (RMS) was measured using an atomic force microscope (MFP-3D-SA, AR, USA). The water and oil static contact angles were measured using a Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at ambient temperature. Underwater oil static contact angle was measured by placing an oil droplet (chloroform) onto the underwater PMF-coated fabric. The volume of the individual water and oil droplets in all measurements was 5 μL. The average water and oil contact angle values were obtained by measuring the same sample in five different positions. Sliding angle was measured by tilting the sample until a 5 μL of liquid droplet rolled off. The water concentration in oil was determined using a Karl Fischer moisture titrator. The images of liquid droplets on the fabric and oil/water separation process were captured with a traditional digital camera (Canon SX50 HS).

3. Results and discussion

The absorption of positively charged polyelectrolyte such as PDDA easily emerges due to the net negative charges of the substrates in water. Subsequently, the multilayers assemble on the basis of electrostatic interactions because of excess charges and associated counterions. By being dipped into the PDDA and PSS aqueous solutions alternately, the substrate surface can form a PMF with PDDA as the cap layer.^26,27 For the glass slide, its surface is very smooth (Fig. 1 inset) and the surface roughness is 8.4 nm after the assembly of the PMF. Its wettability is dependent on the type of anion coordinated to the quaternary ammonium (QA⁺) groups in the PDDA cap layer. After the PFO counterions attached to the QA⁺ groups on the PDDA in the outermost layer, the contact angles for water and hexadecane are 110 ± 1° and 74 ± 1° (Fig. 1), respectively. However, the most distinctive characteristic of PMF is its tunable wettability in response to UV light. When the PMF-coated glass slide is exposed to UV light, the contact angle for water gradually decreases to 28 ± 1° within 5 h, while the contact angle for hexadecane maintains about 75 ± 2° all the time (Fig. 1).

Fig. 1 UV irradiation time dependence of contact angles for water and hexadecane on the PMF-coated glass slide. The inset shows a SEM image for the PMF-coated glass slide.

After UV irradiation, the surface roughness of the PMF-coated glass slide is 10.1 nm and the smooth morphology shows no observable change, indicating that the UV-driven wettability transition is caused by the change of surface chemical composition. XPS analysis was performed on the PMF-coated glass slide to probe the local surface composition, and the survey spectra are shown in Fig. 2. The surface elements of the PMF are all C, F, O, and N before and after UV irradiation. The F 1s peak at 688.8 eV is assigned to the PFO counterions, and the N 1s peak at 402.8 eV is assigned to the positively charged nitrogen atoms in the QA⁺ groups of PDDA. However, after UV irradiation for 5 h, the content of fluorine element gradually decreases from 50.16% to 33.01%, while the content of oxygen element gradually increases from 13.90% to 21.58% (Fig. 2 inset), indicating that UV irradiation results in oxygen enrichment and simultaneous defluorination of the PMF surface. The reason may be that the PFO counterions attached to the QA⁺ groups could be excited under UV irradiation, and then the C–C bond between –C₇F₁₅ and –COO– group is cleaved. The perfluoroalky radical may undergo alcoholization, HF elimination, and hydrolysis to form C₆F₁₃COO–, which can be further degraded through a similar process.²⁸

Fig. 2 XPS survey spectra of the PMF-coated glass slide before (a) and after UV irradiation for 1, 3, and 5 h (b)–(d).

Additionally, high-resolution C 1s spectra for sample before and after UV irradiation are shown in Fig. 3a and b to further clarify the changes of the surface chemical composition. The C 1s peak can be deconvoluted into five peaks assigned to the –CF₃, –CF₂–, –COO–, –CO–, and –CH₂– groups, respectively. Compared to the original sample (Fig. 3a), the spectrum for the UV-irradiated sample (Fig. 3b) has larger peaks associated with the –COO– and –CO– moieties, which once again indicates the generation of oxygen enrichment via UV irradiation. On the other hand, the great intensity of the –CF₃ and –CF₂– peaks confirms the presence of PFO counterions at the UV-irradiated sample surface still (Fig. 3b). With the increase of oxygen-containing groups, more hydrophilic defect domains may be formed around the PFO counterions, which as the intermolecular “hole” to lead the fast penetration of water molecules.²⁹ Meanwhile, the PFO counterions on PMF surface still hinder the penetration of oil molecules. Therefore, the UV-irradiated PMF shows the peculiar wetting behavior that is more wettable to water than to oil.

Fig. 3 High-resolution C 1s spectrum of the PMF-coated glass slide before (a) and after (b) UV irradiation for 5 h.

This peculiar wetting behavior on the smooth glass slide can be amplified by combining the UV-irradiated PMF with the rough surface. The fabrics as the typical materials with re-entrant textured structures have been used to fabricate the superoleophobic surfaces previously.^30–32 A cotton fabric was assembled PDDA/PSS and absorbed PFO counterions to obtain a re-entrant textured surface with a conformal PMF. Fig. 4 shows SEM micrographs for a piece of the cotton fabric before and after the deposition of PMF. There is no apparent difference due to the individual fibers of the fabric conformally coated by the thin PMF. The contact angle values on the original and PMF-coated fabric surfaces with water and hexadecane are presented in Fig. 5. The original cotton fabric is superhydrophilic and superoleophilic with the contact angles of 0°. For the PMF-coated fabric, the contact angles for water and hexadecane increase to 156 ± 1° and 150 ± 1° respectively. The corresponding sliding angles are 16 ± 3° and 13 ± 2° (Fig. 5 inset). After being exposed to UV light, the contact angle for water gradually reduced to 0°, whereas the hexadecane contact angle has no obvious decrease and the sliding angle maintains at 12 ± 3° (Fig. 5). Moreover, for the original cotton fabric and the fabric modified with PDDA/PSS or PFO counterions, the contact angles for water and hexadecane are 0° under UV light all the time. Therefore, superhydrophilicity and superoleophobicity can be simultaneously achieved only on the UV-irradiated cotton fabric with PDDA/PSS and PFO counterions.

Fig. 4 SEM images of the cotton fabric before (a and b) and after (c and d) the LbL deposition of PMF.

Fig. 5 UV irradiation time dependence of contact angles for water and hexadecane on the original and PMF-coated fabric. The insets show the images of the contact angles and sliding angles for water and hexadecane.

The surface wettability of the PMF-coated fabric is highlighted in Fig. 6. Droplets of water (colored with MB), vegetable oil, paraffin oil, and hexadecane (colored with oil red O) exhibit spherical shapes as they are placed onto the original PMF-coated fabric (Fig. 6a). The bright, reflective surface visible underneath the liquid droplets indicates the establishment of composite solid–liquid–air interfaces.³³ For the UV-irradiated fabric (Fig. 6b), a water droplet spread over the surface within several seconds to form a fully wetted interface, which is due to the combination of hydrophilic defect domains and 3D capillary effect. Meanwhile, other oil droplets can keep the spherical shapes for 24 h without any permeation. Moreover, as shown in Fig. 6c, the oil droplets also exhibited spherical shapes on the underwater fabric with the contact angle of 165 ± 1° and sliding angle of 5 ± 2°. When we dropped liquid droplets onto the tilted fabric, hexadecane droplets rolled off the fabric easily (Fig. 6d) and water droplets spread over it rapidly (Fig. 6e). For the water-wetted fabric, droplets of hexadecane (Fig. 6f) and chloroform (Fig. 6g) can still slide easily. In other words, the UV-irradiated PMF-coated fabric exhibits superhydrophilicity and superoleophobicity both in air and underwater with excellent anti oil-fouling capacity.

Fig. 6 Photographs of droplets of water (colored with MB), vegetable oil, paraffin oil, and hexadecane (colored with oil red O) on the PMF-coated fabric before (a) and after (b) UV irradiation. (c) Photograph of the oil droplets on the UV-irradiated PMF-coated fabric underwater. Photographs of liquid droplets continuously dropped onto the tilted fabric: (d) the rolling hexadecane droplet; (e) the spreading water droplet; the sliding (f) hexadecane droplets and (g) chloroform droplets on the water-wetted fabric.

The most important application for the superhydrophilic/superoleophobic materials is the separation of oil/water mixtures. As shown in Fig. 7a, a mixture of water (colored with MB) and hexadecane (colored with oil red O) was injected onto the UV-irradiated PMF-coated fabric that was fixed between two glass tubes. Water with higher density than oil can pass through the fabric by the driving force of gravity within a few minutes (Fig. 7b and c), while hexadecane was remained above the fabric without any permeation (Fig. 7d). Importantly, no matter water or hexadecane first contact the fabric in the separation process, there is no pollution and permeation of hexadecane due to its superoleophobicity both in air and underwater. The purity of hexadecane after onetime separation examined using a Karl Fischer analyzer is above 99.9%. The separation efficiency, calculated by the ratio between the mass of hexadecane before and after the separation process, was above 99.0%. The flux of separation, measured by calculating the flow volume per unit time from the valid area of the fabric, was 38 mL (cm⁻² min⁻¹). Other oils, such as vegetable oil, paraffin oil, petroleum ether, toluene, and hexane, can also be separated from oil/water mixtures efficiently, as shown in Fig. 7e. Moreover, the used fabric can be easily recovered to its superhydrophilic/superoleophobic state by being washed with water and dried, leading to that the separation for hexadecane/water mixtures could be repeated for several times with the efficiency of around 99.0% (Fig. 7f).

Fig. 7 (a–d) Photographs of the separation of hexadecane (colored with oil red O) and water (colored with MB) using the PMF-coated fabric by the driving force of gravity. (e) The separation efficiency of the PMF-coated fabric for various oil/water mixtures. (f) The separation efficiency versus the recycle numbers by taking the hexadecane/water mixture as an example.

Additionally, mechanical and chemical stabilities of the superhydrophilic/superoleophobic PMF-coated fabric were investigated. As shown in Fig. 8 inset, a friction test was performed by dragging back and forth of 500 g weights on the fabric. The line in Fig. 8 show that the water contact angle was 0° all the time and the hexadecane contact angle gradually decreased with the sliding distance. When the sliding distance increased to 1000 cm, the contact angle of hexadecane decreased to 140 ± 1°. But the separation efficiency for the hexadecane/water mixture still maintained above 99.0% (Fig. 8 inset). For the chemical stability, the superhydrophilic/superoleophobic PMF-coated fabrics were placed in an oven at 100 °C and immersed into acetone, toluene, 5 wt% HCl, 5 wt% NaOH, and 5 wt% NaCl for 24 h, respectively. It can be seen from Table 1 that the wetting and separation properties of the fabric are hardly affect by high temperature, acetone, toluene, and HCl. But when the PMF-coated fabrics were exposed to NaOH and NaCl, the contact angle of hexadecane decreased to 0° due to the destroy of the PFO counterions attached to the QA⁺ groups on the PDDA.

Fig. 8 Variation of the contact angles of water and hexadecane for the PMF-coated fabric versus the sliding distance in the friction test. The insets show the changes of separation efficiency and photographs of droplets of water and hexadecane on the friction tested fabric.

Table 1 The wettability and separation efficiency of the superhydrophilic/superoleophobic fabric after treated under various conditions

Treatments	Contact angle (water) [°]	Contact angle (hexadecane) [°]	Separation efficiency [%]
After preparation	0	150 ± 1	99.4 ± 0.5
100 °C (24 h)	0	153 ± 1	99.7 ± 0.1
Acetone (24 h)	0	151 ± 1	99.5 ± 0.3
Toluene (24 h)	0	148 ± 2	99.2 ± 0.4
5 wt% HCl (24 h)	0	154 ± 1	99.7 ± 0.1
5 wt% NaOH (24 h)	0	0	—
5 wt% NaCl (24 h)	0	0	—

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4. Conclusions

In summary, we have demonstrated a novel methodology for obtaining surfaces with superhydrophilicity/superoleophobicity. A commercially available cotton fabric was deposited a PMF with PFO as the counterions. After UV irradiation, the combined effect of hydrophilic defect domains coupled with PFO counterions on the PMF-coated fabric led to its surface that exhibits superhydrophilicity/superoleophobicity with the contact angles of 0° and 150 ± 1° for water and hexadecane, respectively. The superhydrophilic/superoleophobic fabric can selectively separate water from oil/water mixtures with separation efficiency above 99.0%. Water passed through the fabric by the driving force of gravity within a few minutes, while oils were remained above the fabric without any pollution and permeation due to superoleophobicity both in air and underwater. So the separation efficiency can still keep up to 99.0% after 30 separations. Moreover, the superhydrophilicity/superoleophobicity of the fabric has excellent stability facing to the mechanical friction, high temperature, organic solvent, and acid solution. We expect that this novel method will be helpful in the popularization and application of superhydrophilic/superoleophobic materials in the field of oil/water separation.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (51602132) and the Natural Science Foundation of Jiangsu Province, China (BK20140562).

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