Tunable Wettability of Polyimide Films Based on Electrostatic Self

Mar 11, 2008 - We have demonstrated that the surface wettability of negatively charged polyimide films could be tuned by electrostatic self-assembly o...
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Langmuir 2008, 24, 3937-3943

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Tunable Wettability of Polyimide Films Based on Electrostatic Self-Assembly of Ionic Liquids Yan Zhao, Mei Li, and Qinghua Lu* School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, China ReceiVed NoVember 25, 2007. In Final Form: January 21, 2008 We have demonstrated that the surface wettability of negatively charged polyimide films could be tuned by electrostatic self-assembly of ionic liquids. The water contact angles of the polyimide films varied in the range 27-80° for 13 different ionic liquids based on imidazolium and ammonium salts. The surface morphology of the resulting surfaces was characterized using atomic force microscopy. The results revealed that the assembly of longer-substituent cations was characterized by the formation of spherical nanoparticles that were formed due to sequent aggregation of cations on those electrostatically assembled ones via hydrophobic interaction. In this case, the counteranions are present in the assembled layers and the wettability is accordingly affected. Whereas for shorter-substituent cations, no aggregates were formed due to the less hydrophobic interaction than the electrostatic repulsive interaction between the cations, and the counteranions were absent from the assembled layers. This method can also be utilized to quantify the hydrophobicity of various ionic liquids.

Introduction Ionic liquids, generally defined as salts composed of organic cations and inorganic anions that melt at or below 100 °C, are often referred to as “designer liquids” since their physicochemical properties can be tailored by the choice of cation, anion, and substituent.1-5 At present, the most common ionic liquids used are those with alkylammonium, alkylphosphonium, alkylpyridinium, and 1,3-dialkylimidazolium cations. They exhibit many unique properties such as negligible vapor pressure, relatively high ionic conductivity, good chemical and thermal stability, nonflammability, outstanding catalytic property, and a wide electrochemical potential window. Therefore, ionic liquids have been extensively explored as novel media for many practical applications, such as chemical synthesis,1 organometallic catalysis,2 liquid-liquid extractions,6,7 analytical techniques,8,9 and electrochemistry.10,11 In recent years, much research effort has been devoted to studying the interfacial and aggregation behavior of ionic liquids in aqueous solution because of their inherent amphiphilic characteristics.12-18 For example, Bradley et al.19 reported that * To whom correspondence should be addressed. E-mail: qhlu@sjtu. edu.cn. Phone & Fax: +86-21-54747535. (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (3) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (4) Zhao, D.; Fei, Z.; Ohlin, A.; Laurenczy, G.; Dyson, P. J. Chem. Commun. 2004, 2500. (5) Fei, Z.; Geldbach, T. J.; Zhao, D.; Dyson, P. J. Chem.sEur. J. 2006, 12, 2122. (6) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (7) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494. (8) Armstrong, D. W.; Zhang, L.-K.; He, L.; Gross, M. L. Anal. Chem. 2001, 73, 3679. (9) Yanes, E. G.; Gratz, S. R.; Baldwin, M. J.; Robison, S. E.; Stalcup, A. M. Anal. Chem. 2001, 73, 3838. (10) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem. 2004, 5, 1106. (11) Endres, F.; Abedin, S. Z. E. Phys. Chem. Chem. Phys. 2006, 8, 2101. (12) Goodchild, I.; Collier, L.; Millar, S. L.; Prokesˇ, I.; Lord, J. C. D.; Butts, C. P.; Bowers, J.; Webster, J. R. P.; Heenan, R. K. J. Colloid Interface Sci. 2007, 307, 455. (13) Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. J. Phys. Chem. B 2007, 111, 6181. (14) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T. Langmuir 2007, 23, 4178.

longer-chain 1-alkyl-3-methylimidazolium salts ([Cnmim]X, n ) 16, 18) can self-assemble to display a thermotropic liquid crystalline mesophase. Spontaneous formation of lamellar lyotropic liquid-crystalline gel in concentrated aqueous solution of 1-decyl-3-methylimidazolium bromide ([C10mim]Br) was reported by Firestone et al.20 On the basis of measurements of the surface tension, electrical conductivity, turbidity, or smallangle neutron scattering, recent studies have shown that [Cnmim]X (n g 4) can form aggregates in aqueous solution above a critical aggregation concentration (CAC).12-18 The understanding and control of this aggregation behavior driven by amphiphilicity, both in aqueous solution and at solid-liquid interface, are of great importance to the application of ionic liquids in the field of colloid and interface science. However, to our knowledge, the self-assembly of ionic liquids on charged solid surfaces via electrostatic force and/or hydrophobic interaction has not yet been studied. It is known that interfacial properties of a solid surface, such as wetting behavior, are determined by its outermost molecular level structure.21,22 As a result, controlling the wettability of a surface has been demonstrated by various means of surface modification. Recently, there appeared a route to tailor surface properties of charged solid substrates via surface-based supramolecular chemistry.23-28 Huck et al.23,24 showed that the (15) Modaressi, A.; Sifaoui, H.; Mielcarz, M.; Doman´ska, U.; Rogalski, M. Colloids Surf., A 2007, 302, 181. (16) Dorbritz, S.; Ruth, W.; Kragl, U. AdV. Synth. Catal. 2005, 347, 1273. (17) Miskolczy, Z.; Sebo¨k-Nagy, K.; Biczo´k, L.; Go¨ktu¨rk, S. Chem. Phys. Lett. 2004, 400, 296. (18) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 2191. (19) Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McMath, S. E. J.; Nieuwenhuyzen, M. Chem. Mater. 2002, 14, 629. (20) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258. (21) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (22) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5898. (23) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. AdV. Mater. 2007, 19, 151. (24) Azzaroni, O.; Moya, S.; Farhan, T.; Brown, A. A.; Huck, W. T. S. Macromolecules 2005, 38, 10192. (25) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S. J. Am. Chem. Soc. 2004, 126, 480. (26) Chi, Y. S.; Lee, J. K.; Lee, S.; Choi, I. S. Langmuir 2004, 20, 3024. (27) Lee, B. S.; Lee, S. Bull. Korean Chem. Soc. 2004, 25, 1531. (28) Shen, Y.; Zhang, Y.; Zhang, Q.; Niu, L.; You, T.; Ivaska, A. Chem. Commun. 2005, 4193.

10.1021/la703673s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/11/2008

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wetting properties of substrates modified with cationic polyelectrolyte brushes, poly(2-(methacryloyloxy)ethyl-trimethylammonium chloride), were extremely sensitive to the nature of the counteranion. In this case, the advancing contact angles of the polyelectrolyte brushes varied from 19° to 90° upon coordination with PO43- or [N(SO2CF3)2]-, respectively. Choi et al.25,26 prepared self-assembled monolayers (SAMs) presenting methyl-imidazolium cations at the tail ends on Au or Si/SiO2 surface via covalent bonding of alkanethiols or siloxanes. They found that the water wettability of the SAMs was strongly dependent on the counteranion of the bound imidazolium cations; the water contact angles varied from 23° (Br-) to 65° ([N(SO2CF3)2]-). All the above-reported work focused on using different anions to change the surface wettability of positively charged substrates. However, compared to using anions as wettability controlling agents, using the cations of ionic liquids to tailor the surface properties has the advantage that their physical and chemical properties can be easily changed by structural modifications.4 Our goal here is to explore how well the surface wettability can be tuned by varying the substituents of cations and to report preliminary results on how the ionic liquids aggregate on charged solid surfaces via supramolecular chemistry based on electrostatic self-assembly rather than by a covalent atom-by-atom approach. To do this, we have used a negatively charged polyimide film, whose surface was chemically treated with KOH, as substrate. The surface morphology of the resulting surfaces was characterized using atomic force microscopy, and the wettability was monitored by using water contact angle measurements. Experimental Section Materials. 1,3-Bis(9-anthracenylmethyl)imidazolium chloride ([Amim]Cl) was kindly provided by Dr. Zhaofu Fei (Prof. Paul Dyson’s group, Swiss Federal Institute of Technology, Lausanne, Switzerland). 1-Hexyl-3-methylimidazolium bromide ([C6mim]Br) was purchased from Chemer Chemicals Ltd., Hangzhou, China. Other ionic liquids used in this study, including 1-ethyl-3methylimidazolium bromide ([C2mim]Br), 1-butyl-3-methylimidazolium bromide ([C4mim]Br), 1-benzyl-3-methylimidazolium bromide ([Bmim]Br), 1-benzyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][N(SO2CF3)2]), 1-octyl-3-methylimidazolium bromide ([C8mim]Br), 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][N(SO2CF3)2]), as well as N-substituent-N,N-dimethyl-N-(2-hydroxyethyl)ammonium ([C2ham]+, [C4ham]+, [Bham]+, and [C8ham]+, where the substituent is ethyl, butyl, benzyl, and octyl, respectively), salts having the counteranion [N(SO2CF3)2]- were synthesized in our laboratory by Dr. C. Andre´ Ohlin via the quarternization of methylimidazole and N,N-dimethyl-N-(2-hydroxyethyl)amine with alkyl bromides, followed by anion metathesis reactions where applicable.29-32 The purity of all ionic liquids was confirmed by 1H NMR. Methylimidazole, N,N-dimethyl-N-(2-hydroxyethyl)amine, ethylbromide, butylbromide, benzylbromide, octylbromide, sodium tetrafluoroborate, and lithium bis(trifluoromethylsulfonyl)imide were obtained from commercial sources and used as received. Electrostatic Self-Assembly of Ionic Liquids. Commercially available 40 µm thick pyromellitic dianhydride-oxydianiline (PMDA-ODA)-type polyimide films were used as substrates. The polyimide films were hydrolyzed with 1 mol L-1 KOH aqueous solution at room temperature for 40 min to give the corresponding (29) Ohlin, C. A.; Dyson, P. J.; Laurenczy, G. Chem. Commun. 2004, 1070. (30) Vidisˇ, A.; Ohlin, C. A.; Laurenczy, G.; Ku¨sters, E.; Sedelmeier, G.; Dyson, P. J. AdV. Synth. Catal. 2005, 347, 266. (31) Sun, J.; MacFarlane, D. R.; Forsyth, M. Ionics 1997, 3, 356. (32) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168.

Zhao et al. Scheme 1. Ring-Opening Reaction of (PMDA-ODA)-type Polyimide

potassium polyamate, followed by rinsing extensively with deionized water. For the electrostatic assembly of ionic liquids, the hydrolyzed polyimide films were immersed in 2 mmol L-1 solutions of ionic liquids dissolved in a mixed solvent of deionized water and acetone (1:1, v/v) for 2 h at room temperature, and afterward, the films were thoroughly rinsed with the mixed solvent and dried with a stream of nitrogen gas. Characterization. Contact angle measurements were performed using the sessile drop method on a Contact Angle System OCA 20 (DataPhysics Instruments GmbH, Germany) at room temperature. Contact angle values were calculated from dynamic video files that captured 25 frames/s using the software provided by the manufacturer. We recorded the contact angles of 2 µL water drops over several minutes and took the values corresponding to the time of 1 min for all samples. Advancing contact angles were measured by placing the needle in the water drop and continuously increasing the drop volume. The contact angles reported here are the average values of at least five independent measurements. The contact angle variability was within 3°. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra DLD instrument with a focused monochromatic Al KR X-ray source (hν ) 1486.6 eV) operated at 15 kV. The test chamber pressure was about 2 × 10-9 Torr during spectral acquisition. XPS spectra were recorded at a takeoff angle of 90° to the surface. Survey spectra were acquired using a pass energy of 160 eV at a resolution of 1 eV, while highresolution spectra were obtained with a pass energy of 80 eV at a resolution of 0.05 eV. All binding energies in XPS spectra were internally referenced to C 1s peak, which was assigned a value of 284.6 eV. Atomic force microscopy (AFM) imaging was performed on a Nanoscope IIIa multimode scanning probe microscope (Digital Instruments, Santa Barbara) operating in the tapping mode in air. Root-mean-square (rms) roughness was analyzed from AFM height images using the Nanoscope software version 5.30b4.

Results and Discussion Polyimide films are known as high-performance polymeric materials because of their high thermal stability and good mechanical properties. As shown in Scheme 1, the imide rings of polyimide can be opened with KOH to yield potassium polyamate (metal-carboxylate salt). This reaction is a modified “Ing-Manske” reaction and can be confined to the polyimide film surface without altering the bulk properties by adjusting the reaction conditions.33-37 Figure 1 shows how the advancing contact angles vary as a function of the hydrolysis time for the polyimide films hydrolyzed in 1 mol L-1 KOH at room temperature. The advancing contact angles decreased dramatically at the beginning and then reached a plateau at about 20° for the polyimide films hydrolyzed for more than 30 min, indicating complete conversion of polyimide to polyamate salt on the polyimide film surface. The evolution of sessile water drops (33) Ing, H. R.; Manske, R. H. F. J. Chem. Soc. 1926, 2348. (34) Lee, K.-W.; Kowalczyk, S. P.; Shaw, J. M. Macromolecules 1990, 23, 2097. (35) Lee, K.-W.; Kowalczyk, S. P.; Shaw, J. M. Langmuir 1991, 7, 2450. (36) Thomas, R. R.; Buchwalter, S. L.; Buchwalter, L. P.; Chao, T. H. Macromolecules 1992, 25, 4559. (37) Thomas, R. R. Langmuir 1996, 12, 5247.

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Figure 1. Advancing contact angles as a function of hydrolysis time for the polyimide films hydrolyzed in 1 M KOH at room temperature. The inset shows the evolution of contact angle with time for the polyimide film which was hydrolyzed for 40 min.

with time on these films was examined by using a video contact angle system operating at a capture speed of 25 frames/s (time interval between frames 40 ms). The inset in Figure 1 displays the evolution of contact angle with time for the polyimide film which was hydrolyzed for 40 min. The water contact angle on this highly wettable surface decreased to less than 10° within several seconds. The polyimide films served as substrates in this study were all hydrolyzed in 1 M KOH for 40 min to ensure the uniformity of the effect of substrate on the electrostatic assembly of ionic liquids. The electrostatic self-assembly of ionic liquids was accomplished by immersing the hydrolyzed polyimide films in 2 mmol L-1 solutions of ionic liquids for 2 h. Unless indicated otherwise, all ionic liquids were dissolved using a mixed solvent of deionized water and acetone (1:1, v/v). Here two types of ionic liquids, imidazolium- and ammonium-based ionic liquids, were examined. Figure 2 shows the changes in water wettability of polyimide films assembled with different imidazolium salts. The counteranion of these imidazolium-based cations was Clor Br-. We observed a trend of increasing hydrophobicity with increasing chain length of the substituent. The water contact angles shown in Figure 2a are the values corresponding to the time of 1 min after the placement of water drops. The evolution of contact angles with time for polyimide films assembled with imidazolium cations is shown in Figure 2b. It can be seen that water contact angle initially decreased dramatically and then remained almost stable for the imidazolium cations with longer substituents ([Amim]+, [C8mim]+, [C6mim]+, and [Bmim]+). However, in the case of the imidazolium cations with shorter substituents ([C4mim]+ and [C2mim]+), a decrease of the water contact angle over time was observed, indicating strong interaction between water and the hydrophilic surfaces.38,39 A similar trend of increased hydrophobicity with increasing chain length of the substituent was observed for polyimide films assembled with ammonium-based ionic liquids (Figure 3). This result is quite consistent with that obtained in evaluation of the solubility of ionic liquids in water; ionic liquids change from water miscible (hydrophilic) to immiscible (hydrophobic) with increasing alkyl-chain length.40 Indeed, the only proven method currently available to quantify the hydrophobicity of ionic liquids is the comparison of 1-octanol/water partition coefficient.6,25 (38) Miyama, M.; Yang, Y.; Yasuda, T.; Okuno, T.; Yasuda, H. K. Langmuir 1997, 13, 5494. (39) Nicolas, M.; Guittard, F.; Ge´ribaldi, S. Langmuir 2006, 22, 3081. (40) Dullius, J. E. L.; Suarez, P. A. Z.; Einloft, S.; de Souza, R. F.; Dupont, J. Organometallics 1998, 17, 815.

Figure 2. (a) Molecular structures of imidazolium cations and changes in water wettability, and (b) evolution of contact angles with time for polyimide films assembled with the corresponding imidazolium salts. The counteranion of these imidazolium-based cations is Cl- or Br-.

Herein, the simple method of immobilizing ionic liquids on a charged solid surface via electrostatic interaction can be used to directly quantify the hydrophobicity of ionic liquids. Moreover, it is important to note that the tunable wettability in this study was realized by self-assembly of cations with different substituents predominately via electrostatic interaction. Whereas in the method developed by Choi et al.,25,26 the imidazolium cations of ionic liquids were covalently linked onto substrate surfaces and the wettability was controlled by changing the counteranions. In our study, in order to evaluate the effect of the counteranions, we measured the water contact angles of polyimide films assembled with 1-benzyl-3-methylimidazolium and 1-octyl-3methylimidazolium salts having different counteranions. The results are shown in Table 1. It is interesting that we observed no changes in the contact angles of polyimide films assembled with 1-benzyl-3-methylimidazolium salts having the counteranion Br-, [BF4]-, or [N(SO2CF3)2]-, and the water contact angle was about 56°. However, in the case of 1-octyl-3-methylimidazolium salts, the water contact angles were about 68°, 72°, and 75° for Br-, [BF4]-, and [N(SO2CF3)2]-, respectively, which clearly shows that water wettability of the polyimide films assembled with 1-octyl-3-methylimidazolium salts was influenced by counteranions. To verify the presence (or absence) of the counteranion in the assembled layer, the elemental composition of the surface was

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Figure 3. (a) Molecular structures of ammonium cations and changes in water wettability, and (b) evolution of contact angles with time for polyimide films assembled with the corresponding ammonium salts. The ammonium-based cations have the same counteranion [N(SO2CF3)2]-. Table 1. Water Contact Angles of Polyimide Films Assembled with 1-Benzyl-3-methylimidazolium and 1-Octyl-3-methylimidazolium Salts Having Different Counteranionsa contact angles varying with anions, deg cations

Br-

[BF4]-

[N(SO2CF3)2]-

[Bmim]+

56.0 ( 2.6 68.2 ( 0.9

56.5 ( 2.4 71.5 ( 0.7

56.2 ( 2.7 74.8 ( 1.6

[C8mim]+

a The error values represent the standard deviation obtained from five measurements.

determined using XPS, which is a surface-sensitive analytic technique. Figure 4 shows the high-resolution XPS spectra for the polyimide films assembled with 1-octyl-3-methylimidazolium salts having the counteranion Br-, [BF4]-, or [N(SO2CF3)2]-. The appearance of the peaks of Br 3d3/2 at 70.9 eV, Br 3d5/2 at 70.0 eV, F 1s at 687.3 eV, and S 2p at 168.5 eV confirmed the presence of the counteranion in the assembled layer. In contrast, for the polyimide films assembled with 1-benzyl-3-methylimidazolium salts having different counteranions, these characteristic peaks of the three anions were not observed (not shown). The XPS data suggested that the presence or absence of the counteranions accounted for the contact angle results shown in Table 1. To further explore why the counteranions in the assembled layers were present for 1-octyl-3-methylimidazolium salts and not for 1-benzyl-3-methylimidazolium salts, we examined the surface morphology before and after electrostatic assembly of ionic liquids by using a tapping mode AFM. The hydrolyzed polyimide surface was flat and featureless within the degree of resolution (Figure 5a, rms roughness of 0.37 nm). After electrostatic assembly of [C2mim]Br, [C4mim]Br, [Bmim]Br, [Bmim][BF4], or [Bmim][N(SO2CF3)2], no significant difference

Figure 4. High-resolution XPS spectra in the binding energy range of (a) Br 3d, (b) F 1s, and (c) S 2p for polyimide films assembled with 1-octyl-3-methylimidazolium salts having the counteranion (A) Br-, (B) [BF4]-, or (C) [N(SO2CF3)2]-.

was found on the surfaces when compared with the bare substrate (see Figure 5b and Supporting Information Figure S1a-d). The rms roughness values of these surfaces were in the range from 0.28 to 0.63 nm. However, closely packed spherical nanoparticles with an average diameter of 25 nm were observed for the polyimide film assembled with [C6mim]Br or [C8mim]Br, and the rms roughness increased to 2.03 and 3.84 nm, respectively (Figure 5c,d). When the counteranion of [C8mim]+ was changed to [BF4]- or [N(SO2CF3)2]-, spherical nanoparticles were also found on the polyimide films (see Supporting Information Figure S1e,f). For [Amim]Cl assembled on polyimide film, the diameter of the spherical nanoparticles increased to about 35 nm (rms roughness of 7.38 nm) (Supporting Information Figure S1g). Figure 6 shows the tapping mode AFM images of polyimide films assembled with ammonium-based ionic liquids. The polyimide surface assembled with [C2ham]+ or [C4ham]+ was relatively smooth and featureless, with similar rms roughness to the initial substrate (Figure 6a,b). When the substituent changed from butyl to benzyl, spherical nanoparticles with a diameter of about 10 nm were found on the surface (Figure 6c). For the polyimide surface assembled with [C8ham]+, the nanoparticle diameter increased to 15 nm (Figure 6d). It is known that amphiphilic compounds have the ability to form self-assembled structures in solution. The cations of ionic liquids possess an inherent amphiphilicity and have been found to form aggregates in water above their CACs.13-18 For example, Bowers et al.18 have reported that [C4mim][BF4], [C8mim]Cl,

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Figure 5. Tapping mode AFM images of (a) hydrolyzed polyimide film as a control and polyimide films assembled with ionic liquids of (b) [Bmim]Br, (c) [C6mim]Br, and (d) [C8mim]Br.

and [C8mim]I formed aggregates in aqueous solutions when their concentrations were higher than 800, 100, and 100 mmol L-1, respectively. In this study, we found that for water soluble ionic liquids, such as [C2mim]Br, [C4mim]Br, [Bmim]Br, [C6mim]Br, and [C8mim]Br, almost the same morphology and contact angle values were obtained when the mixed solvent was replaced by water. However, the concentration of ionic liquids was 2 mmol L-1, which was rather lower than their CACs. Therefore, the spherical nanoparticles for ionic liquids with longer substituents were not previously formed in the solutions but were formed by the aggregation of cations on the polyimide surface, since amphiphilic molecules form aggregates on solid sufaces beyond a critical concentration below the CAC.41 We proposed the following two-stage mechanism for the surface aggregation of cations with longer substituents on polyimide film. Initially, free cations assemble as a result of electrostatic interaction with the anionic polyimide film. Afterward, additional free cations in solution could further aggregate with the electrostatically assembled cations, driven by the hydrophobic interaction between the hydrocarbon chains of the cations. Here the additional cations will preferentially adsorb from the bulk solution on those electrostatically assembled ones, that is, the already assembled cations act as anchors or nucleation (41) Paria, S.; Khilar, K. C. AdV. Colloid Interface Sci. 2004, 110, 75.

sites for the formation of aggregates. A cartoon of the corresponding structural organization is presented in Figure 7a. Once the assembly of cations via hydrophobic interaction occurs, the polyimide surface charge is reversed, whereupon the counteranions could adsorb. Therefore, the counteranions were present in the assembled layers with aggregated nanoparticles for longersubstituent ionic liquids, such as 1-octyl-3-methylimidazolium bromide, and affected the water wettability. However, for shorter-substituent ionic liquids, such as 1-benzyl3-methylimidazolium bromide, the hydrophobic interaction between the hydrocarbon chains of the initially assembled cations via electrostatic attraction and the free cations in bulk solution is less than the electrostatic repulsive interaction between the cations, and hence no spherical nanoparticles are formed on the polyimide films (Figure 7b). Consequently, there was no counteranion present in the assembled layers, and the water wettability did not depend on the counteranion for the same cations. It should be pointed out that the two types of cations with the same main substituent, benzyl, presented different assembly behavior (Figure 5b vs Figure 6b). For [Bmim]Br, no aggregates were found on the surface. However, for [Bham][N(SO2CF3)2], nanometer-scale aggregates (about 10 nm) were formed. To evaluate the effect of counteranion, the electrostatic assembly of [Bham]Br was also studied, and spherical aggregates were found

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Figure 6. Tapping mode AFM images of polyimide films assembled with ionic liquids having the cations (a) [C2ham]+, (b) [C4ham]+, (c) [Bham]+, and (d) [C8ham]+. The four ionic liquids have the same anion [N(SO2CF3)2]-.

Figure 7. Schematic representation of surface aggregation on polyimide film for cations with (a) longer and (b) shorter substituents.

on the polyimide film (Supporting Information Figure S1h). This suggested that the assembly process was closely related to the chemical structure of the cation, which still requires further investigation. Finally, we examined the stability of the assembled layer and the possibility of polyimide recovery from the polyamate on the suface. The assembled layer was stable on repeatedly washing with water, as indicated by the negligible change of the contact angle. It is known that silver or copper polyamate can be simultaneously reduced to metal nanoparticles and converted

back to polyimide upon heat treatment above 200 °C.42,43 Here, the hydrolyzed polyimide films assembled with ionic liquids were treated at 350 °C for 2 h to complete the decomposition of ionic liquids on the surface and re-imidization of the hydrolyzed polyamate layer, which was evidenced by the contact angle measurement. The contact angle of the recycled polyimide films was found to regain its original value of 74°. (42) Zhao, Y.; Lu, Q.; Chen, D.; Wei, Y. J. Mater. Chem. 2006, 16, 4504. (43) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Deki, S. Chem. Mater. 2003, 15, 2488.

Tunable Wettability Via Assembly of Ionic Liquids

Conclusions The work reported here has demonstrated that ionic liquids based on imidazolium and ammonium salts can self-assemble on negatively charged polyimide substrates via electrostatic force. The water contact angles of the resulting surfaces varied in the range 27-80° for different substituents of the cations. Further investigations revealed that the assembly of longer-substituent cations was characterized by the formation of spherical nanoparticles due to sequent aggregation of cations on those electrostatically assembled ones via hydrophobic interaction. In this case, the counteranions were present in the assembled layers and affected the wettability. Whereas for shorter-substituent cations, no aggregates were formed due to the less hydrophobic interaction than the electrostatic repulsive interaction between the cations and the counteranions were absent from the

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assembledlayers and did not influence the wettability. Since the ionic liquids can be functionalized via the design of cation and anion, we believe that diverse functions of ionic liquids could be translated onto solid surfaces via this strategy. The work described herein may provide insight in integrating various surface properties into one single substrate that may find useful application in microfluidics and biomedical areas. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 60577049) and Shanghai Leading Academic Discipline Project (B202). Supporting Information Available: Surface morphologies of some polyimide films assembled with ionic liquids. This material is available free of charge via the Internet at http://pubs.acs.org. LA703673S