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Counterion Exchange to Achieve Reversibly Switchable Hydrophobicity and Oleophobicity on Fabrics Jin Yang,†,‡ Zhaozhu Zhang,*,† Xuehu Men,† Xianghui Xu,†,‡ Xiaotao Zhu,†,‡ and Xiaoyan Zhou†,‡ †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China ‡ Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China ABSTRACT: We describe a simple layer-by-layer (LbL) technology and counterion exchange procedure to tune the liquid wettability of commercially available cotton fabrics. A polyelectrolyte multilayer is deposited on the fabric surface by the LbL technology, and counterion exchange is used to control the surface composition and thereby to modulate the solid surface energy. The tunability of the solid surface energy, along with the inherent re-entrant texture of the cotton fabric, results in simultaneously switchable wettability between a nonwetting state and a fully wetted state for water and hexadecane. This switchable hydrophobicity and oleophobicity can be explained within a robustness factor, which is a quantitative criterion for the transition between the two states. The counterion exchange can be confirmed by X-ray photoelectron spectroscopy analysis.
’ INTRODUCTION Smart surfaces with dynamically tunable wettability have recently generated extensive interest due to their wide applicability in various fields, including the development of self-cleaning surfaces, antifogging films, microfluidics, tunable optical lenses, and so forth.15 Various research groups have developed surfaces that can effectively switch their wetting properties in response to changes in their surrounding environment.6 There are numerous reports of stimuli-responsive surfaces that switch their wettability between superhydrophobicity and superhydrophilicity in response to changes in temperature,7 solvent,810 UV light,1113 electrical voltage,14 and mechanical strain.15 Because of the difficulty in fabricating oleophobic surfaces, previous work on tunable wettability has focused on studies with water droplets. Until recently, switchable wettability for low surface tension liquids using mechanical strain or thermal annealing as stimuli was realized on fabrics.16,17 However, on these fabric surfaces, the realizable range of contact angle values for water droplets is rather limited. Indeed, there are no reports of surfaces that can simultaneously control the contact angles of water and low surface tension liquids in a wide range. In general, the wettability of a solid surface is strongly influenced both by its surface energy and by its surface roughness.18 While previous work has shown that oleophobic surfaces can be realized by coating overhang or re-entrant structures such as microfabricated surfaces and commercial fabrics with fluorinated materials to lower surface energy,1925 altering their surface composition, and hence their wettability, is still a great challenge. In this study, counterion exchange is used to control the surface composition and thereby to modulate the wettability of a polyelectrolyte multilayer deposited on a commercially available r 2011 American Chemical Society
cotton fabric. The layer-by-layer (LbL) technology is used to deposit polyelectrolytes and provides a versatile and general route to immobilize charges onto fabric. The exchange of counterions coordinated to quaternary ammonium (QAþ) groups easily emerges to tune the solid surface energy and the resulting apparent contact angles. This surface tunability, combined with the inherent re-entrant structure of the cotton fabric, leads to a switchable liquid wetting surface for water (surface tension γlv = 72.1 mN/m) and hexadecane (γlv = 27.5 mN/m) at the same time.
’ EXPERIMENTAL SECTION Materials. A pure cotton fabric was purchased from a local fabric store. Poly(diallydimethylammonium chloride) (PDDA, Mw = 200 000 350 000), poly(sodium 4-styrene sulfonate) (PSS, Mw = 70 000), and perfluorooctanoic acid (CF3(CF2)6COOH) were all purchased from Sigma-Aldrich. Sodium chloride (NaCl) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Sodium perfluorooctanoate (PFO) (0.10 M) was prepared by the reaction of perfluorooctanoic acid with NaOH in water. Sample Preparation and Counterion Exchange. Cotton fabrics were cleaned with ethanol and deionized water to remove possible impurities. Glass sides were cleaned in a hot piranha solution (H2SO4/H2O2, 7:3 mixture) at 80 °C for 30 min and then washed sequentially with copious amounts of deionized water. The polyelectrolyte multilayer was fabricated following a literature procedure.26 The substrate was immersed in PDDA (1.0 mg/mL, with 1.0 M NaCl Received: March 25, 2011 Revised: April 27, 2011 Published: May 17, 2011 7357
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Figure 1. Switchable liquid wettability for water and hexadecane via counterion exchange between Cl and PFO anions on flat surfaces. The insets show the shapes of the liquid droplets on the surfaces. present) aqueous solutions for 30 min to obtain a positively charged surface. Then, the PDDA-modified substrate was immersed in PSS (1.0 mg/mL, with 1.0 M NaCl present) aqueous solutions for 30 min, followed by rinsing with water. This cycle of PDDA treatment followed by PSS treatment was repeated 3.5 times to obtain the (PDDA/ PSS)3PDDA multilayer. Counterion exchange was carried out by immersing the deposited substrates in an aqueous solution (0.10 M) of the required anion, followed by rinsing with deionized water and drying in air at 100 °C for 2 h to complete evaporation of the water (the heating has no influence on the properties of fabric according to control experiments such as drying the fabrics by flowing nitrogen or in a vacuum oven at ambient temperature). In the repeated experiments, hexadecane wetted on the cotton fabrics can be removed by rinsing with ethanol and deionized water. Surface Characterization. Contact angle measurements were performed using a Kr€uss DSA 100 (Kr€uss Company, Ltd., Germany) apparatus at ambient temperature. The volumes of probing liquids in the measurements were approximately 5 μL. Scanning electron microscopy measurements were carried out using a JSM-5600LV scanning electron microscope (SEM JEOL, Japan). X-ray photoelectron spectroscopy (XPS) characterization was performed on a PHI-5702 electron spectrometer using an Al KR line excitation source with the reference of C 1s at 285.0 eV.
’ RESULTS AND DISCUSSION Contact angle measurements were performed on polyelectrolyte-deposited glass sides after consecutive counterion exchange between Cl and PFO anions. The contact angles for water and hexadecane displayed oscillatory behaviors for four cycles of the two different anion treatments (Figure 1). Higher contact angles were consistently observed for the polyelectrolyte multilayer bearing PFO anions, in comparison to the corresponding sample bearing Cl. The contact angle with water varied from 30 ( 2° for Cl to 110 ( 1° for PFO anions (Figure 1 inset). The corresponding hexadecane contact angles were 5 ( 1° and 70 ( 2° (Figure 1 inset), respectively. Using the measured values of the contact angles with water and hexadecane, the surface energy of the polyelectrolyte-deposited flat surface carrying Cl and PFO anions was estimated using OwensWendt analysis.27 For the surface coordinated with Cl, the polar component of the solid surface energy was 36.7 mN/m, with a dispersive component of 27.4 mN/m, giving
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Figure 2. SEM images of the cotton fabric before (a,b) and after (c,d) the LbL deposition of polyelectrolytes.
Figure 3. Schematic illustration of the procedure for the LbL deposition and counterion exchange on the fabric surface.
a total surface energy of 64.1 mN/m. Similarly, the polar component for the surface coordinated with PFO anions was 1.1 mN/m, and the dispersive component was 12.4 mN/m, leading to a total surface energy of 13.5 mN/m. The data illustrates that the surface energy of the surface carrying Cl is considerably higher than the surface energy carrying PFO anions, and the surface energy can be reversibly modulated via counterion exchange in the polyelectrolyte multilayer. This variation in the contact angle on a flat surface, as highlighted in Figure 1, can be amplified by combining the counterion exchange with a re-entrant textured surface such as that of a commercially available fabric. It is known that the LbL deposition of polyelectrolytes can be performed on virtually any kind of substrate without the need for aggressive chemical or physical pretreatment of the substrate.28 The absorption of positively charged polyelectrolyte such as PDDA easily emerges due to the net negative charges of the surfaces in water. A cotton fabric was sequentially dipped in the PDDA and PSS aqueous solutions to generate a re-entrant textured surface with a polyelectrolyte multilayer. Figure 2 shows the surface morphology of a cotton fabric before and after the LbL deposition of polyelectrolytes. The polyelectrolyte multilayer is so thin that the morphology before and after deposition shows no apparent difference. As a consequence, the hydrophobicity and oleophobicity of the surface can be precisely tuned via direct exchange of 7358
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Figure 4. (a,b) Water and hexadecane droplets on the polyelectrolytedeposited surfaces coordinated with PFO anions and Cl, respectively. (c) Switchable hydrophobicity and oleophobicity of the polyelectrolytedeposited fabric with water and hexadecane via consecutive counterion exchange.
the anions coordinated to the QAþ groups. Figure 3 outlines the procedure for the LbL deposition and counterion exchange on the fabric surface. The contact angle measurements on the polyelectrolytedeposited fabric surface with water and hexadecane are presented in Figure 4. The polyelectrolyte coordinated with Cl counterion is superhydrophilic and superoleophilic with the contact angles of 0° (Figure 4b). This is a result of the 3D capillary effect: the liquids completely fill all the surface asperities to form a fully wetted interface, and the wetting regime is in the Wenzel state.29 However, when the Cl counterions are replaced with PFO anions, the solid surface energy on a flat surface reduces from 64.1 to 13.5 mN/m. The reduction in the surface energy leads to an increase of surface liquid-repellency; consequently, the contact angles with water and hexadecane increase to 151 ( 3° and 140 ( 4°, respectively (Figure 4a). Additionally, the contact angle hysteresis for water and hexadecane is 20 ( 3° and 41 ( 4°. A water droplet can be rolled off the fabric surface by tilting to about 20°. The bright, reflective surface visible underneath the liquid droplets in Figure 4a is a signature of trapped air and the establishment of a composite solidliquidair interface. The corresponding wetting regime is in the CassieBaxter state.30 As a result, switchable liquid wettability for water and hexadecane can be simultaneously achieved between the two states, corresponding to the Wenzel state and the CassieBaxter state, by using simple counterion exchange. To our knowledge, this is the first demonstration of tunable hydrophobicity and oleophobicity at the same time on any surface. Moreover, this tunability in the wetting behaviors is repeatable, and the reversibility is demonstrated over five cycles of counterion exchange between Cl and PFO anions (Figure 4c). The polyelectrolyte-deposited fabric can keep its switchable wettability for at least 4 months at atmosphere conditions and the contact angles for water and hexadecane are unchanged, which show its long-term stability. According to the previous report,31 the mechanical stability of the polyelectrolyte multilayer is also investigated. The methodology illustrated in Figure 5 is invoked; stainless steel served as an abrasion surface, with the polyelectrolyte-deposited fabric to be tested facing this abrasion material. While pressure (∼1.2 MPa) was applied to the cotton fabric, the surface was moved back and forth. After the abrasion test (abrasion length ∼ 200 cm), switchable wettability can also be observed on the cotton fabric and the contact angles for water
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Figure 5. Schematic illustration of the abrasion test employed to evaluate mechanical stability of the polyelectrolyte multilayer deposited on the cotton fabric.
and hexadecane were nearly unchanged. After a longer abrasion test (abrasion length ∼ 400 cm), the water contact angle decreased slightly from 151° to 147°, while the sliding angle increased significantly from 20° to 70°. These results indicate that the polyelectrolyte multilayer deposited on the fabric is mechanically robust in this abrasion test. To further understand the remarkable wettability transition in response to counterion exchange, we used the robustness factor A* developed by Cohen and McKinley to quantify the robustness of the composite solidliquidair interface based on properties of the contacting liquid, surface texture parameters, and the interfacial properties of the triple phase line.16 Large values of the robustness factor (A* > 1) indicate the formation of robust composite interface and the corresponding high apparent contact angles. For a cotton fabric surface, the robustness factor A* is given as Rlcap ð1 cos θÞ A ¼ 2 D ð1 þ 2ðR=DÞsin θÞ where θ is the equilibrium contact angle on a smooth surface, R is the bundle radius, D is the half the interbundle gap, lcap = (γlv/Fg)1/2 is the capillary length of the liquid, F is the liquid density, and g is the acceleration due to gravity.17 For the polyelectrolyte multilayer bearing Cl on the fabric surface, the robustness factors are found to be smaller than unity (A*water ∼ 0.9, A* hexadecane ∼ 0.1). Therefore, water and hexadecane easily wet the fabric surface to form the fully wetted interfaces. When the Cl counterions are replaced with PFO anions, the robustness factors for water and hexadecane increase to 5.2 and 2.7, respectively; consequently, the fully wetted interface on the fabric transitions to the composite solidliquidair interface. Droplets of water and hexadecane can maintain spherical shapes on the fabric surface (Figure 4a). In order to confirm the exchange of counterions, XPS was used to probe the local composition of the surface. The Cl 2p peak (at 197.0 eV) is observed in Figure 6a for the as-prepared polyelectrolyte multilayer, but this peak disappears as the fabric surface was exposed to a solution of PFO anions, and the F 1s peaks at 688.8 eV can be observed (Figure 6b). These results indicate the Cl counterions were completely replaced with PFO anions. When the surface is treated with NaCl solution, the peaks of Cl 2p and F 1s can recover to the original states. Therefore, it can be concluded that the exchange of counterions indeed generates in the polyelectrolyte multilayer. This subtle counterion exchange in the local chemical composition, when combined 7359
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Figure 6. XPS spectra of the polyelectrolyte multilayer on the fabric surface with different anions in the regions of (a) Cl 2p and (b) F 1s.
with a re-entrant structure, gives rise to fabric surfaces with tunable hydrophobicity and oleophobicity simultaneously.
’ CONCLUSIONS In the present work, we have demonstrated a novel technology for obtaining surfaces with switchable hydrophobicity and oleophobicity. A polyelectrolyte multilayer was deposited on a commercially available cotton fabric via the LbL technology. The combined effect of the re-entrant surface texture of the cotton fabric coupled with counterion exchange in the polyelectrolyte multilayer led to a surface that displays switchable wettability with water and hexadecane at the same time. Owens Wendt analysis on a flat surface indicated the tunability of the solid surface energy by counterion exchange between Cl and PFO anions. A robustness factor provided a quantitative criterion for the wettability transition with water and hexadecane on the fabric surface. This simple and novel technology for switchable hydrophobicity and oleophobicity could be applied to other reentrant structures on metal or inorganic substrates. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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’ ACKNOWLEDGMENT The authors acknowledge the financial support of the National Science Foundation of China (Grant Nos. 50835009 and 51002162) and the National 973 Project of China (Grant No. 2007CB607601). ’ REFERENCES (1) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (2) Verplanck, N.; Coffinier, Y.; Thomy, V.; Boukherroub, R. Nanoscale Res. Lett. 2007, 2, 577. (3) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842. (4) Lee, S. G.; Lee, D. Y.; Lim, H. S.; Lee, D. H.; Lee, S.; Cho, K. Adv. Mater. 2010, 22, 5013. (5) Qing, G. Y.; Wang, X.; Fuchs, H.; Sun, T. L. J. Am. Chem. Soc. 2009, 131, 8370. (6) Xin, B. W.; Hao, J. C. Chem. Soc. Rev. 2010, 39, 769. (7) Sun, T. L.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (8) Zhu, Y.; Shi, M. H.; Wu, X. D.; Yang, S. R. J. Colloid Interface Sci. 2007, 315, 580. (9) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Minko, S. Adv. Mater. 2008, 20, 200. 7360
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