Fluorine-Free Superhydrophobic Coatings with pH-induced Wettability

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Fluorine-Free Superhydrophobic Coatings with pH-induced Wettability Transition for Controllable Oil−Water Separation Zhiguang Xu,† Yan Zhao,*,† Hongxia Wang,† Hua Zhou,† Chuanxiang Qin,†,‡ Xungai Wang,† and Tong Lin*,† †

Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

‡

S Supporting Information *

ABSTRACT: We present a simple, environmentally friendly approach to fabricating superhydrophobic coatings with pHinduced wettability transition. The coatings are prepared from a mixture of silica nanoparticles and decanoic acid-modified TiO2. When the coating is applied on cotton fabric, the fabric turns superhydrophobic in air but superoleophilic in neutral aqueous environment. It is permeable to oil fluids but impermeable to water. However, when the coated fabric is placed in basic aqueous solution or ammonia vapor, it turns hydrophilic but underwater superoleophobic, thus allowing water to penetrate through but blocking oil. Therefore, such a unique, selective water/oil permeation feature makes the treated fabric have capability to separate either oil or water from a water−oil mixture. It may be useful for development of smart oil−water separators, microfluidic valves, and lab-on-a-chip devices. KEYWORDS: superhydrophobic, superoleophobic, pH-responsive wettability, cotton, oil−water separation

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INTRODUCTION The wettability of a solid surface to oils in aqueous phase environment is an important property for a number of processes and applications including microfluidics,1−3 oil manipulation,4−6 control of protein adhesion on surfaces,7,8 and oil−water separation.9−11 In water, when a fully immersed solid substrate has a contact angle to oil droplet (also termed as “oil-in-water contact angle”12) greater than 150°, it is called underwater superoleophobic. Fish skin is a good example of underwater superoleophobicity, which enables fish to keep clean in oil-polluted water because of the micro/nanostructured hydrophilic scales.13 Inspired by fish scales, underwater superoleophobic surfaces have been developed to improve the antifouling and decontamination ability of aqueoussubmerged solid substrates.14 Underwater superoleophobicity was prepared typically by forming hierarchically rough hydrophilic surface structure on substrates. Apart from antifouling and underwater decontamination, underwater superoleophobicity shows great potential for separating oil from water.15−18 When a porous membrane has an underwater superoleophobic surface, it allows water permeation but blocks oil penetration. This “water-permeable” separation process is more suitable for separating light oils (i.e., density below 1.0 g/ cm3) from water. In contrast, underwater superoleophilic surfaces have an underwater−oil contact angle below 10°.19 They are typically prepared by hydrophobic surface chemistry. Normal super© XXXX American Chemical Society

hydrophobic, oleophilic surfaces often show underwater superoleophilicity. They are widely used for separation of oil from oil−water mixtures,20−29 though light oil separation could have a low separation efficiency because water tends to stay underneath the oil fluids and form a barrier film. Recently, stimuli-responsive underwater wettability has been reported. An external stimulus from the change of pH value30−35 or temperature,36,37 as well as light irradiation38 switches underwater wettability between superoleophilicity and superoleophobicity, and thus, a single material can show either water or oil permeability depending on the condition, which offers great flexibility for oil−water separation. Such a responsive feature also allows remote operation and “ondemand” automation of oil−water separation process.37−39 However, most of the stimuli-responsive underwater superoleophilic/superoleophobic surfaces were prepared using either precious materials such as gold30,33,34 and silver31 or a complex preparation process to attain hierarchical micro/nano structures and special surface chemistry.32,38 In the previous study, we have used coordination chemistry to prepare a superamphiphobic coating, and proved its potential for oil−water separation.40 By forming a complex between TiO2 and heptadecafluorononanoic acid (HFA), the Received: December 2, 2015 Accepted: February 3, 2016

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DOI: 10.1021/acsami.5b11720 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation procedure for the pH-responsive cotton fabric.

mixture of silica NPs and DA-TiO2 sol solution (silica NPs/ DA-TiO2), which was then used to treat a cotton fabric through dip-coating. Silica NPs were used here to increase the surface roughness of the coating. The root-mean-square (RMS) roughness of DA-TiO2 coating without and with silica NPs was 0.76 and 29.5 nm, respectively (Figure S4). The coating prepared from the silica NPs/DA-TiO2 sol solution was transparent. As revealed by the UV−visible transmission spectrum (Figure S5), the average transmittance of the coating on quartz glass (70 × 15 × 1 mm) in the region 400−800 nm was ∼90.2%. Figure 2 shows the SEM images of the pristine and treated cotton fabrics. For the pristine cotton fabric, native striations

coating showed a novel ammonia vapor-responsive property. As a result, it changed from superamphiphobic to superhydrophilic-superoleophobic when treated with ammonia vapor. The drawback of this coating is the use of a perfluoroalkyl acid. Although the family of perfluoroalkyl acids have been extensively used as surfactants and water/oilrepellent agents in industry for decades, environment and health concerns have been raised recently on the perfluoroalkyl acids because of their persistent and bioaccumulative characteristic.41 Most recently, attempts to use biobased materials for oil−water separation also indicated the demand for eco-friendly and sustainable strategies.42,43 So using environmentally friendly, fluorine-free chemicals to achieve a similar oil−water separation function is highly desirable. However, perfluoroalkyl acids often show much stronger acidity than their alkyl counterparts. It is unclear if alkyl acid can form complex with TiO2 to show similar responsive properties. In this work, we use decanoic acid (DA), a cheap, fluorine-free, nontoxic, and naturally occurring chemical, as a model to prepare a superhydrophobic coating that have both pH-responsive and ammonia vapor-induced wettability transition. The coating was prepared by a simple dip-coating method from a mixture of silica nanoparticles and TiO2 sol complexed with DA. At normal state, the coating was superhydrophobic and underwater superoleophilic. When the coating was treated by alkaline substance, it turned hydrophilic but underwater superoleophobic. We further proved that a cotton fabric treated by this coating showed two completely different oil−water separation performances: (1) “oil-permeable but water-impermeable” and (2) “oil-impermeable but water-permeable”. For on-demand operation, ammonia vapor was used to induce the wettability transition, which allows for a gas-triggered oil−water separation process.

Figure 2. SEM images of the (a, b) pristine and (c, d) silica NPs/DATiO2 coated cotton fabrics.

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along the fiber can be clearly observed (Figure 2a,b). After coating treatment, the fiber surface was covered by silica NPs and a thin layer of DA-TiO2 (Figure 2c). Figure 2d shows the enlarged view of the surface of a single fiber. Silica NPs were embedded within a thin layer of DA-TiO2, the thickness of which was less than the diameter of silica NPs (Figure S6). The presence of the silica NPs/DA-TiO2 coating on the cotton fabric was also verified by FTIR and XPS (Figure S7). In FTIR spectra, the absorption band at 3000−3600 cm−1, correspond-

RESULTS AND DISCUSSION The preparation of the superhydrophobic coating is illustrated in Figure 1. First, a DA-modified TiO2 sol (DA-TiO2) was prepared by mixing Ti(OBu)4 and DA, in which both titanium carboxylate coordination complexes and Ti−O−Ti networks were formed (Supporting Information S1). To the DA-TiO2 sol solution, silica NPs (diameter 152 ± 18 nm; see Supporting Information S2 for characterizations) were added to make a B

DOI: 10.1021/acsami.5b11720 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

an important role, which was revealed by the low water contact angle (pH 6.5, ∼ 104°) of a glass slide coated with silica NPs/ DA-TiO2 (Figure S11b). Figure 3b shows the oil wettability of the coated cotton fabric in air, where a hexadecane droplet was absorbed by the fabric within 66 ms. Besides measuring the wettability in air, we further tested the underwater−oil wettability of the coated cotton fabric using 1,2-dichloroethane (DCE) as probe oil. As shown in Figure 3c, for the coated fabric immersed in pH 6.5 water, a DCE droplet completely spread into the fabric within 1.2 s. This underwater superoleophilicity is useful for oil collection in aqueous media. As a demonstration, a DCE droplet was absorbed by the coated cotton fabric (Figure 3d). Because the coated fabric tends to be selectively wettable by oil in pH 6.5 environment (Figure 3e), the fabric is also expected to be useful for oil−water separation. To verify this, the fabric was mounted between two syringe tubes and a mixture of water (pH 6.5) and hexadecane was poured into the upper tube. Because of the superhydrophobicity and superoleophilicity at pH 6.5, the fabric was permeable to hexadecane but not to water (Figure 3f). Therefore, hexadecane passed through the fabric whereas water was retained in the upper tube. Driven by gravity only, the separation showed a high flux of ∼0.91 L m−2 s−1, with rejection rate above 99% (TGA; Figure S12). For acidic water, the coated fabric also kept superhydrophobic (Figure S13). However, it was wettable by alkaline water with a pH above 11 (Figure S13). The time required for a water droplet to completely spread (wetting time) depends on the pH of the water. A higher pH value resulted in less wetting time. When a pH 12 water droplet was placed on the coated cotton fabric, its initial contact angle was larger than 150°, but the droplet was gradually absorbed by the cotton fabric and completely spread within 180 s (Figure 4a). This hydrophobic to hydrophilic transition can be attributed to the cleavage of titanium-carboxylate coordination bonding under the effect of alkali.40,44,45 Once the bonding is cleaved, the ionic DA then migrates from the solid−air interface to the water−air interface, and even the water phase, driven by the attraction between ionic DA and the permanent dipole of water molecules. The migration of DA will result in the loss of low free energy DA molecules from the solid surface, thus increasing the solid-surface free energy. At the same time, when DA migrates to the water−air interface or the water phase, it will act like a surfactant to reduce water surface tension. Both the increase in solid-surface free energy and the reduction in water surface tension will result in the hydrophilicity.40 The migration of DA to the water phase was verified by examining the water phase using NMR (Figure S14). Because of the migration and loss of DA, the hydrophobic to hydrophilic transition is irreversible, indicating that high density of DA chains on the surface is important to keep the superhydrophobicity. Underwater superoleophobicity was observed for the coated cotton fabric immersed in pH 12 water. As shown in Figure 4b, a DCE droplet exhibits a spherical shape with a contact angle of 160° on the fabric in pH 12 water. The DCE droplet on such a surface could roll off easily with a sliding angle less than 10°, indicating the low oil adhesion of the surface. Figure 4c shows photographs of spherical DCE droplets sitting on the coated cotton fabric in pH 12 water. It is noted that the coated cotton fabric was wettable by DCE in air (Figure S15). Besides DCE, the fabric in pH 12 water also showed underwater superoleophobicity to other light oils such as hexadecane, paraffin oil,

ing to the hydroxyl groups of cellulose, was suppressed after surface coating. The bands around 1455 cm−1 were contributed to C−H asymmetric deformation and aliphatic carbon bonds of DA. The band at 1710 cm−1 was due to the carbonyl stretching. The absorption band at 792 cm−1 was contributed to Si−O−Si symmetric stretching vibrations. The increased absorption at 600−800 cm−1 was ascribed to the vibration of the Ti−O bonds. In XPS spectra, only peaks corresponding to C and O were observed for pristine cotton. After coating, distinctive peaks appeared at 151, 101, and 455 eV, which were attributed to Si 2s, Si 2p, and Ti 2p, respectively. The water and oil wettability of the coated cotton fabric was characterized by contact angle measurement. Figure 3a shows

Figure 3. Wettability of the coated cotton fabric in pH 6.5 aqueous environment. (a) Profiles of a pH 6.5 water droplet placed on the coated cotton fabric with a contact angle of 158°. (b) Profiles of a hexadecane droplet placed on the coated cotton fabric in air. (c) Profiles of a 1,2-dichloroethane (DCE) droplet placed on the coated cotton fabric in pH 6.5 water. (d) Snapshots showing that a DCE droplet (colored with oil red O) in pH 6.5 water was collected by a piece of coated cotton fabric. (e) Evolution of water contact angle in air and underwater−oil (DCE) contact angle with time for a pH 6.5 environment. (f) A mixture of water and hexadecane was poured into the upper tube. Hexadecane passed through the fabric while water could not. Water and hexadecane were colored with acid blue 25 and oil red O, respectively.

that a pH 6.5 water droplet on the coated cotton fabric has a contact angle larger than 150°. This superhydrophobicity was stable against pH 6.5 water. After being immersed in pH 6.5 water for up to 2 days, the coated fabric was still superhydrophobic (Figure S9). As a comparison, the pristine fabric was hydrophilic (Figure S10). It is believed that the superhydrophobicity of coated fabric arises from both the low surface energy of the long alkyl chain and the hierarchical roughness of microscale fabric weave structure and nanoscale silica nanoparticles. For the cotton fabric coated with DA-TiO2 only (without silica NPs), the water contact angle (pH 6.5) was tested to be ∼132° (Figure S11a), indicating that the nanoscale roughness provided by silica NPs plays a key role in generating the superhydrophobicity. The fabric weave structure also played C

DOI: 10.1021/acsami.5b11720 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

prediction is in agreement with our experimental result that the fabric is oleophilic when immersed in pH 6.5 water. For a hydrophobic and oleophobic surface, θow would be >90° when −γoacos θo is higher than γwacos θw. For a hydrophilic surface, θow would usually be >90°, because γwa is much larger than γoa. As described above, the fabric turned to be hydrophilic at pH 12, and θow is expected to be >90° based on eq 1. The experimental result showed a θow of ca. 160°. Herein, the extremely large θow can be ascribed to the hierarchical structure of the fabric. This amplification effect of surface roughness on the liquid wettability has been widely described based on the well-known Wenzel and Cassie−Baxter equations.47,48 Besides basic water, ammonia vapor can also induce the wettability transition. As shown in Figure 5a, when the spherical

Figure 4. Wettability of the coated cotton fabric in pH 12 aqueous environment. (a) Profiles of a pH 12 water droplet placed on the coated cotton fabric. (b) Profiles of a DCE droplet placed on the coated cotton fabric in pH 12 water, showing a contact angle of 160° and a sliding angle less than 10°. (c) Photographs of DCE droplets (colored with oil red O) sitting on the coated cotton fabric in pH 12 water. (d) Underwater superoleophobicity to oils with lower density than water: hexadecane, paraffin oil, and soybean oil (from left to right). (e) Evolution of water contact angle in air and underwater−oil (DCE) contact angle with time for a pH 12 environment. (f) The fabric was wetted with pH 12 water prior to the oil−water separation process. Water passed through the fabric, while hexadecane was retained in the upper tube. Water and hexadecane were colored with acid blue 25 and oil red O, respectively. Figure 5. (a) Superhydrophobic to superhydrophilic transition induced by ammonia vapor. (b) Oil−water separation triggered by ammonia vapor. Water and hexadecane were colored with acid blue 25 and oil red O, respectively.

and soybean oil (Figure 4d). Such an underwater superoleophobic property was maintained even after the fabric was immersed in pH 12 water for 48 h, indicating the good stability of the underwater superoleophobicity. Therefore, the oil and water wettability of the coated fabric in pH 12 environment (wettable by water, but not oil, Figure 4e) is completely contrast to that in pH 6.5 environment. When the coated cotton fabric was wetted with pH 12 water prior to the oil−water separation process, the opposite case happened, with water passing through the fabric while hexadecane being retained (Figure 4f). The water flux was ∼3.85 L m−2 s−1, and the hexadecane content in the filtrate water was ∼750 mg L−1. The underwater−oil wettability of the coated cotton fabric can be theoretically understood by considering the solid− water−oil interface that was formed when an oil droplet was placed on the fabric in water. For such a solid−water−oil interface, the underwater−oil contact angle, θow, can be expressed by the Bartell-Osterhof equation:46 cos θow =

water droplets sitting on the fabric are exposed to ammonia vapor, they completely spread and wet the fabric. On the basis of this phenomenon, the design of a gas-triggered oil−water separation process is feasible (Figure 5b). Because of the oleophilicity of the fabric, a small amount of water was poured into the upper tube prior to the pouring of oil−water mixture to form a thin water layer on the fabric, thus preventing oil from contacting with the fabric. The oil−water mixture kept stable in the upper tube and could not penetrate the fabric spontaneously due to the superhydrophobicity of the fabric. The separation process was triggered by introducing ammonia vapor into the lower tube through a syringe needle. Once exposed to ammonia vapor, the fabric became wettable to water and the water permeated through the fabric quickly, whereas the hexadecane was retained in the upper tube. The on-demand operation and controllable water/oil permeation make the coated fabric very promising for different applications.

γoa cos θo − γwa cos θw γow

(1)

where γoa, γwa, and γow are the surface tensions of the oil−air, water−air, and oil−water interfaces, respectively; and θo and θw represent the oil and water contact angle, respectively, of the solid surface in air. As predicted by this equation, θow would be always