pH-responsive smart fabrics with controllable wettability in different surroundings

Abstract

Carboxyl-terminated polymers are known as good candidates for realizing the surfaces pH-responsive wettability. In the present work, we have successfully fabricated smart fabrics with both pH-responsive water wettability in air and pH-responsive oil wettability underwater via in situ growth of Ag nanocrystals on the fabric surface followed by surface modification with a mixture of methyl-terminated thiol and carboxyl-terminated thiol. In air, the resultant fabric shows superhydrophobic property to neutral and acidic water and superhydrophilic property to basic water. In aqueous environment, the fabric shows superoleophilic property in acidic/neutral aqueous environment and superoleophobic property in basic aqueous environment. The resultant fabric is expected to be used in many practical applications, such as the design of functional interface materials for either in-air or underwater surrounding and dual water/oil on–off switch.

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Introduction

Wettability is known as one of the most important aspects of a solid surface. As a special wettability, superhydrophobicity, an extreme water resistant state that generally possesses a water contact angle (CA) typically greater than 150° and sliding angle (SA) no more than 10°, has attracted most attentions both in theoretical investigation and industrial applications in various fields, including self-cleaning surfaces,^1,2 drug delivery system,^3,4 microfluid channels,^5,6 and oil/water separation.^7–14 Superhydrophobic surfaces are commonly synthesized by either modifying chemicals with low surface energy on rough structured surfaces or constructing high surface roughness on low-surface-energy surfaces.^15–19 Recently, particular attention has been paid to the smart surfaces that can show switchable and reversible water/oil wettability under external stimuli such as pH,^20–23 temperature,²⁴ light^25–28 and electric field.^29–31 Zhang and co-workers²¹ successfully fabricated the Au nanoparticles electrodeposited surfaces with reversible switching between superhydrophobic and superhydrophilic properties triggered by pH variation. Wong and co-workers²⁵ demonstrated an UV-responsive ZnO nanorod composite film that can change its wettability between superhydrophobic and superhydrophilic via UV irradiation. Thomy and co-workers³⁰ reported a reversible electrowetting of liquid droplets on superhydrophobic silicon nanowires. Therein, pH-responsive wetting material is an important branch and has many advantages when compared to the other stimuli-responsiveness, such as quick responsiveness, easy operation and facile superhydrophobicity recovery without extra modification.

While the vast majority of the reported pH-responsive materials were aiming at investigating the pH-induced switchable water wettability in air,^{20–23,32–34} the studies of the oil wettability under different water environments, as well as the emerging investigations of both water wettability in air and oil wettability underwater were seldom reported.^12,35–39 Wang and co-workers³⁵ used a block copolymer comprising pH-responsive poly(2-vinylpyridine) and oleophilic/hydrophobic polydimethylsiloxane blocks to functionalize the non-woven textiles and polyurethane sponges and prepare surfaces with switchable superoleophilicity and superoleophobicity in aqueous media. Gao and co-workers³⁹ fabricated a temperature controlled water/oil wettability surface by solution casting of a block copolymer. The surface presented two states of wettability (hydrophilicity/oleophobicity and hydrophobicity/oleophilicity) at different temperatures, thus realizing a temperature controllable dual water/oil on–off property, i.e. open to water and closed to oil below the low critical solution temperature (LCST), and open to oil, closed to water above the LCST. The researcher commonly applied a layer of self-assembled monolayer which is formed by spontaneous and strong chemisorption of a long-chain polymer at the substrate surfaces. However, as-reported approaches either used costly Au (due to its corrosion resistant property) to construct surface rough structure or involved complex surface fabrication process and wettability recovery.

As is known, in addition to Au, Ag also possesses good corrosion resistant property to the normal acid and base solutions with a wide range of pH and Ag is much cheaper than Au. Therefore, Ag is a good candidate and has more advantages than Au to prepare acid and base resistant interfacial materials. In this paper, we reported a single-step fabrication method to obtain smart Ag nanocrystals coated fabrics with pH-responsive water wettability in air, as well as pH-responsive oil wettability in an aqueous environment. By assembling methyl-terminated thiol and carboxyl-terminated thiol on the fabric with Ag nanoparticles in situ growth on it, the fabric shows pH-responsive wettability to water and oil under certain circumstance. Not only the water wettability of the resultant fabrics in air, but also various oils wettabilities (such as hexane, chloroform and methylbenzene) underwater were investigated. This is necessary for developing smart interfacial materials and devices with underwater applications.

Experimental section

Materials

The fabrics (containing 65% polyester and 35% cotton) were purchased from a local store in Lanzhou, China. The raw fabrics were sequentially cleaned with deionized water, anhydrous ethanol, and deionized water in an ultrasonic cleaner to remove possible impurities. Silver nitrate (AgNO₃) was obtained from Sinopharm Chemical Reagent Co., Ltd., P. R. China. n-Decyl thiol (HS(CH₂)₉CH₃) and 11-mercaptoundecanoic acid (HS(CH₂)₁₀COOH) were purchased from Sigma-Aldrich. All of the chemicals were analytical-grade reagents and used as received. Aqueous solutions of hydrochloric acid and sodium hydroxide were applied to regulate the pH of water. The acidic aqueous phase at pH 1–6 was prepared by dissolving hydrochloric acid at preassigned concentrations. The basic aqueous phases at pH 8–14 were prepared by dissolving NaOH in deionized water at preassigned concentrations.

Preparation of Ag nanocrystal coated fabrics

The original fabric was immersed into 0.04 M AgNO₃ aqueous solution (50 mL) at room temperature for 3 min. In this process, the aqueous solution of AgNO₃ was absorbed into the fabric adequately and penetrated into the fabric fibers because of its hydrophilicity. Then, 0.03 M ascorbic acid solution (50 mL) was added dropwise into the AgNO₃ solution at room temperature under constant magnetic stirring (a homemade porous polyethylene baffle was used to separate the fabric from the magneton, for the purpose of preventing the nonuniform growth of nanoparticles aroused by the magnetic stirring). Next, the fabric with Ag nanocrystals coated on it was removed and washed with deionized water and ethanol, followed by drying in a drying oven. After fabrication of Ag, the fabric was immersed into the mixed thiol ethanol solution of HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH for 14 h. The concentration ratio of HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH was varied while maintaining the total concentration of the thiol in ethanol solution constant at 1 mM. Finally, the fabric was thoroughly cleaned with anhydrous ethanol to remove any residual thiol and dried in a drying oven.

Characterization

The liquid CA was measured by sessile droplet method. The water CA in air was measured with a 5 μL droplet of distilled water at ambient temperature with a DSA100 contact angle meter (Kruss Company, Germany). The oil CA underwater was measured with a 5 μL oil droplet at ambient temperature with a JC2000D contact angle meter (Shanghai Zhongchen, China). Therein, a cuboid-shaped glass is filled with water and the samples were fixed in the water phase. The CAs were measured after half a minute's deposit on the fabric surface to observe whether the droplet would spread and penetrate the fabric. The spreading processes were recorded as a video. All of average CA values were obtained by measuring the sample at five different positions, and the images were captured with a traditional digital camera. Scanning electron microscopy (SEM) images were obtained on JSM-5600LV SEM microscopes with Au-sputtered specimens. X-ray energy dispersive spectrometer (EDS) attached to the scanning electron microscope (JEOL JSM-7100F) was used for the examination of the chemical composition of the as-prepared fabric.

Results and discussion

It is well-known that the wettability of a surface is determined by the combined effect of surface chemical composition and surface morphology.^15,16 Fig. 1 shows the FESEM images of the morphologies of as-prepared fabric, from integral view (Fig. 1a) to magnified views (Fig. 1b–d), with Ag nanocrystals coated on it by a facile chemical in situ growth process. It can be observed that the fabric was woven by a lot of interlaced fibers and every single fiber was compactly covered with a layer of Ag nanocrystals. The average size of the Ag nanocrystals is of about 500 nm. It reveals the typical hierarchical structured fabric surface (fibers provide the micro-scale roughness and Ag nanocrystals offer the nano-scale roughness). Fig. 1d provides a high-resolution image of the Ag nanocrystals on a fiber. The Ag nanocrystals mainly existed in hard edged forms such as regular pentagon, trapezoid and triangle and other irregular polygons. As we know, such micro- and nano-scale hierarchical structures can magnify the wettability of the fabric to the extremes, i.e. superhydrophobicity and superhydrophilicity.^40–42 To detect the element composition of the nanocrystals, the EDS spectrum was applied to demonstrate the existence of Ag on the resultant fabric as shown in Fig. 1e. In addition, the precursor concentration is the key influence factor that can influence not only the weight of nanoparticles coated on the fabric, but also the wettability of both water and oil in a certain range. The amount of Ag nanocrystals increases with the increase of the concentration of AgNO₃. An insufficient amount of Ag nanocrystals will induce a small surface roughness and the wettability thus varies with the concentration of AgNO₃. However, while the coating capacity reaches its upper extreme it is difficult to recognize the fibers that compose the fabric. Moreover, with the increase of the coating capacity, the nanoparticles will obviously grow. This oversized nanoparticles will no longer contribute to the surface roughness. Additionally, the fabric may lose its original functions and utility as well. The variation is particularly investigated in our previous work⁹ and here we applied a proper precursor concentration to conduct this experiment.

Fig. 1 FESEM images of Ag nanocrystal coated fabric via in situ growth process at different magnifications. (a) ×100, (b) ×3000, (c) ×5000 and (d) ×10 000. (e) EDS spectrum of the Ag nanoparticle coated fabric.

It is notable that, instead of adding the AgNO₃ solution into the ascorbic acid, here the opposite adding order was applied (ascorbic acid was added dropwise into the AgNO₃ solution) since it is more beneficial for the heterogeneous nucleation on the fabric. In situ growth of Ag nanocrystals on the fabric allows the fabric to participate in the entire reaction process. As is known, in the reaction thus exist two kinds of nucleation, i.e. homogenous nucleation (occurred in solution) and heterogeneous nucleation (occurred on the fabric). The pre-immersing procedure allows the AgNO₃ to be absorbed on the fibers and thus makes the crystal critical nucleus easily formed on the fabric, which is crucial to the heterogeneous nucleation. If the AgNO₃ solution was added dropwise into the ascorbic acid, the fabric is pre-immersed in the ascorbic acid solution and there is no Ag⁺ absorbed on the fibers. Therefore, the critical nucleus is formed with more difficulty on the fabric and the drops of AgNO₃ solution are much more likely to nucleate in the solution (homogenous nucleation).

As is reported,^43,44 a carboxyl-terminated polymer is a good candidate for realizing the surfaces pH-responsive wettability. Surfaces with methyl-terminated self-assembled monolayers show hydrophobic property whereas surfaces with carboxyl-terminated ones provide hydrophilic property.⁴³ To realize the pH-responsive wettability on the fabric, the fabrics coated with Ag nanocrystals were immersed into a mixed solution of HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH for 14 h. Transition metals commonly possess strong interactions with thiols, which is essentially the interaction between the transition metal atoms and the S atoms and can be found from Lange's Chemistry Handbook, 15th Edition. As a transition metal in Group IB, Ag, possesses an ultrahigh pK_sp (pK_sp = 49.2) compared to other metals such as Fe, Co, Ni and Cu, making it readily coordinated with thiol ligand. A layer of mixed thiol of HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH was thus assembled on the surface of Ag nanocrystals because of the strong interaction between Ag and S ligand. The layer of mixed thiols is decisive to lower the surface free energy of the fabric surface. Moreover, the surface composition can be turned by altering the composition of the modified solution, resulting in the changing of the surface free energy and surface wettability. The wettabilities of the as-prepared fabrics for deionized water were examined by the CA measurements. It was observed that the concentration ratio of HS(CH₂)₁₀COOH (here we used Cont(COOH) to represent the proportion of HS(CH₂)₁₀COOH to the mixed thiol solution) in the thiol solution is crucial to the surface wetting property as schematically shown in Fig. 2. The CA generally decreased with the increase of the Cont(COOH). The CAs at lower values of Cont(COOH) are larger than 150°, denoting the superhydrophobic property. While the Cont(COOH) arrived at 0.7, the CA acutely declined from obtuse angle to the minimum value (0°), denoting a transition from hydrophobicity to superhydrophilicity. Moreover, as for the fabrics with a Cont(COOH) ≥0.7, a larger Cont(COOH) will induce a faster spreading and penetrating of water droplets as illustrated under the curve in Fig. 2. Here, the as-prepared fabric modified with Cont(COOH) = 0.3 is proper to realize a pH-responsivity since the as-prepared fabric is superhydrophobic while it simultaneous possesses the highest amount of HS(CH₂)₁₀COOH possible.

Fig. 2 CA of as-prepared fabric surface with varying Cont(COOH). The inset shows the still images of the CA at different Cont(COOH)s: 0, 0.6 and 1. The successive images under the curve show the dynamic contact behaviors of water droplets at high Cont(COOH)s.

The fabric surface modified with mixed thiol of HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH at Cont(COOH) = 0.3 is specifically studied. The fabric coated with Ag nanocrystals without surface modification of the mixed thiol shows a superhydrophilic property (Fig. 3a). After being modified with the mixed thiol solution (Cont(COOH) = 0.3), it turns into superhydrophobic in addition to a little high adhesion, with a water CA of 149.7° and a water SA of 26° (Fig. 3b, d and f). Furthermore, the wettability for both acid water droplet (Fig. 3c) and basic water droplet (Fig. 3e) were investigated. The result showed that the as-prepared fabric provides pH-responsive effect, with opposite wettability to acid and basic water. While an acid water droplet resided on the mesh film, the superhydrophobicity remains almost unaltered and stable, indicating the acid resistant property of the resultant fabric. As is known, silver nanoparticles are acid-resistant which makes the resultant fabrics superhydrophobic in a wide range of pH. Besides, Ag nanoparticles are well-known for the antibacterial ability that can prevent the resultant fabrics from contamination by the microorganism. While a basic water droplet resided on the mesh film, the droplet was initially hydrophobic. Unfortunately, the droplet was unstable and eventually collapsed and spread over the surface, indicating a transition from hydrophobicity to superhydrophilicity.

Fig. 3 Photographs showing the water wettability of Ag nanocrystals coated fabrics before (a) and after thiol modification (Cont(COOH) = 0.3) (b). Wettability of as-prepared fabric (Cont(COOH) = 0.3) in air. Still images of the CAs for an acid (pH = 2) (c), a neutral (pH = 7) (d) and a basic water droplet (pH = 12) (e) residing on the as-prepared fabric. (f) Successive images of the dynamic movement of a neutral water droplet on the fabric which reflect the SA of water droplets (26°). (g) Line graph showing the reversible transition between superhydrophobicity and superhydrophilicity. (h) Still image of the oil CA of the fabric in air, indicating a superoleophilic property.

The comprehensive investigation of the relationship between the water CA and pH is shown in Fig. 4. The wettability is determined by the pH of a water droplet. While the superhydrophobic property is nearly invariable for an acid and neutral water droplet, the wettability is easily changed by basic water droplets. Moreover, the successive image of the absorption processes with different pH values below the curve indicate that the higher pH value will induce a quicker transition from hydrophobic to superhydrophilic, denoting the faster wetting behavior of fabric. This phenomenon is attributed to the better hydrophilicity of the deprotonated carboxylic acid groups compared with the protonated carboxylic acid groups.²¹ In addition, the recyclability of the fabric to the acid and basic water was studied. Remarkably, the basic-water wetted fabric easily recovered its superhydrophobicity while it was washed with deionized water and dried with nitrogen stream, indicating the reversible transition between superhydrophobicity and superhydrophilicity. Fig. 3g shows the reversible transition between the superhydrophobicity and superhydrophilicity of the as-prepared fabric by altering the acid–base property of water. The fabric remains well superhydrophobic property after five-time repeats, indicating a stable superhydrophobicity and sensitive switch to pH. Moreover, the oil wettability of the fabric in air is also studied as shown in Fig. 3h. The hexane droplet was instantly absorbed by the fabric as soon as it was dropped on the fabric, denoting the superoleophilic property in air.

Fig. 4 Relationship between water CA and pH of the resultant fabric (Cont(COOH) = 0.3). The successive images under the curve show the dynamic variation of the unstable CA of dot 1 (pH = 11), dot 2 (pH = 12), dot 3 (pH = 14).

While the wettability of water droplets in air was investigated, next, the oil wettability in an underwater environment was studied. As we know, most of the investigations of stimuli-responsive materials are focused on the water wettability in air. However, for the underwater smart interfacial materials, such as underwater micro/nanofluidic devices, the oil wettability in certain aqueous environment also needs to be considered. When the resultant fabric (Cont(COOH) = 0.3) was immersed in water environment with pH equalling to 2, the fabric was water repellent and a layer of silvery white mirror can be found (Fig. 5a). The underwater wettabilities of oils in this case, including light oil (hexane) and heavy oil (chloroform), were examined by the CA meter as shown in Fig. 5e. Both the hexane and chloroform droplets spread and were absorbed by the fabric within 480 ms, suggesting its superoleophilic property in the non-alkali water environment. Interestingly, when the resultant fabric was immersed in basic water environment (pH = 12), the fabric was wettable (Fig. 5b) and showed oil resistant property to both the light and heavy oil droplets (Fig. 5c and d). Moreover, the underwater oil CAs of various oils and organic solvents were measured and recorded as shown in Fig. 5g. Also, the shapes of these oil droplets were presented as the insets. The fabric in the basic water environment showed superoleophobic property to the selected oils with few exceptions.

Fig. 5 Photographs showing the appearance of the resultant fabric (Cont(COOH) = 0.3) in acid water environment (pH = 2) (a) and alkali water environment (pH = 12) (b). Optical images of the hexane (light oil) (c) and chloroform droplets (heavy oil) (d) reside on the resultant fabric under alkali water environment. Successive images of the CA for hexane droplet (e) and chloroform droplet (f) at neutral aqueous environment (pH = 6.8). (g) CAs of a series of typical oil droplets (n-pentane, n-hexane, n-heptane, methylbenzene, dichloromethane, chloroform, 1,2-dichloroethane and 1,1,2,2-tetrachloroethane) on the fabric under alkali water environment. Insets in (g) show the CA images of oil droplets on the fabric.

As concluded above, the resultant fabrics (Cont(COOH) = 0.3) possessed switchable water wettability in air and switchable oil wettability underwater by adjusting the pH of water. The pH-responsive wettability of the resultant fabric is essentially attributed to the combined effect of surface methyl-terminated thiol and carboxyl-terminated thiol. Moreover, the micro-scale fibers on the fabric and the nano-scale Ag nanocrystals on the fibers presented the typically hierarchical structure. The high surface roughness of hierarchical structure can magnify the hydrophobicity and hydrophilicity to the extremes, as well as enhance the wetting transition.^{15,16,45–47} As is known, the relationship between the wettability and the surface roughness/chemical composition in air is commonly expressed by the combined form of Wenzel⁴⁸ and Cassie equations⁴⁹

cos θ_L = r_ff cos θ₀ + f − 1

where θ_L represents the apparent CA of the liquid in air, r_f is defined as the roughness ratio of the wet part of the solid surface, θ₀ is the Young's CA on the solid surface and f is the solid–liquid fraction under the contact area. θ_L is the increasing function of r_f when θ₀ > 90° and decreasing function of r_f when θ₀ < 90°. The previous report illustrated that the self-assembled monolayers terminated with –CH₃ produced hydrophobic surfaces (advanced CA θ_a > 90°) whereas those with –COOH formed moderately hydrophilic surfaces (θ_a = 48°).⁴³ By adjusting the amount of the surface HS(CH₂)₉CH₃ and HS(CH₂)₁₀COOH, the θ₀ in the equation can be controlled in a wide range (from hydrophilic to hydrophobic) and the apparent CA can realize its extrema (both superhydrophilic and superhydrophobic, Fig. 4) because of the magnified effect of surface roughness (r_f).

When an acid or a neutral water droplet is deposited on the resultant fabric (Cont(COOH) = 0.3), the water droplet and the solid surface are all Lewis acid (at least the solid surface is acidic due to the modifier, HS(CH₂)₁₀COOH) and thus the water droplets deposited on the solid surface will not interact with each other. The carboxylic acid group will be protonated and thus shows relative hydrophobic property (Fig. 6 left column). Accordingly, the integral water wettability in air is superhydrophobic and the oil wettability underwater is superoleophilic since a layer of air exists between the fabric and water phase. When a basic water droplet was deposited on the fabric surface, the droplet will tend to increase their contact area with the surface due to the reaction of the base in the droplet with the surface carboxylic acid group. In this case, the carboxylic acid groups are deprotonated and show hydrophilic property (Fig. 6 right column). Therefore, the fabric shows superhydrophilicity in air due to the magnified effect of the hierarchical rough structure. The larger the pH is, the easier and faster the carboxylic acid groups will be deprotonated. Accordingly, the CA keeps almost unaltered while pH ≤ 7, and decreases steeply thereafter (Fig. 4).

Fig. 6 Schematic illustration of the wettability of the acid and alkali water droplets on the resultant fabrics in air and oil droplets under acid and alkali water environments.

As is experimentally demonstrated, for a hydrophilic surface, the surface is simultaneously oleophilic (in air) due to the lower surface tension of oil (γ_OA) in air than that of water (γ_WA). Thus, the values of cos θ_O and cos θ_W are all positive. Therein, θ_W and θ_O represent the water CA and oil CA in air. As the surface tension of oil/organic liquids is lower than that of water (γ_OA < γ_WA), the value of γ_OAcos θ_O − γ_WAcos θ_W is commonly negative and thus it can be concluded most hydrophilic surfaces in the air show oleophobic property underwater at the solid–water–oil interfacial by using the underwater Young's equation⁵⁰

where θ_OW represents the apparent oil CA in an aqueous environment and γ_OW is the interface tension oil–water interfaces.

Conclusions

In conclusion, smart fabrics were obtained through a simple in situ growth process and followed by assembling a layer of mixed alkanethiols of n-decyl thiol and 11-mercaptoundecanoic acid. By adjusting the amount ratio of n-decyl thiol and 11-mercaptoundecanoic acid, the wettability can be controlled, varying from superhydrophilic to superhydrophobic. The integral superhydrophobic fabric (Cont(COOH) = 0.3) was selected and showed superhydrophobic property to acidic/neutral water droplets and superhydrophilic property to basic water droplets in air because of the protonated and deprotonated process. Accordingly, the as-prepared fabric shows superoleophilic property in an acidic/neutral aqueous environment and superoleophobic property in a basic aqueous environment. The excellent pH-responsive wettability to water/oil is attributed to the combined effect of the chemical component variation (methyl-terminated thiol and carboxyl-terminated thiol) and the surface rough structure on the fabric. It is expected that the fabric with such excellent controllable in-air water wettability and underwater oil wettability could potentially be used in a wide range of applications, and help people design and prepare smart functional materials in the field of micro/nanofluidic devices, lab-on-a-chip devices and controllable separation materials for oil-and-water mixture.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (no. 31070155, 11172301 and 21203217), the “Funds for Distinguished Young Scientists” of Hubei Province (2012FFA002), the “Western Light Talent Culture” Project, the Co-joint Project of Chinese Academy of Sciences and the “Top Hundred Talents” Program of Chinese Academy of Sciences and the National 973 Project (2013CB632300) for financial support.

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