Superhydrophobic self-assembled monolayers of long-chain fluorinated imidazolium ionic liquids

Abstract

Self-assembled monolayers (SAMs) of 1-long-chain fluorinated-alkyl-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazolium ionic liquids (C_nFtespim⁺X⁻) were fabricated via –Si–O– covalent bonds on Al substrates and the wettability of Al substrates with SAMs was studied. The water contact angles (CAs) of Al substrates coating with SAMs increase with the increase of the chain length of C_nFtespim⁺X⁻ and X⁻ can be tuned by the exchange of counteranions. It was found that the effect of counteranions on the wettability of SAMs was dependent on the cations in some degree. Combining with micro- and nano-scale hierarchical structures via being treated with 4 mol L⁻¹ HCl aqueous solution for 12.5 min, superhydrophobic Al surfaces having CA above 160° at n ≥ 6 can be obtained. The unique chemical and thermal stabilities of ionic liquids could be transferred to the Al substrate surfaces through anchoring, which should satisfy the superhydrophobic requirements of especial environments.

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1. Introduction

Superhydrophobic surfaces, referring to the surfaces with a CA larger than 150°, have drawn great attention because of their importance in both fundamental research and practical applications,^1,2 especially in the fields of engineering materials. Basically, the hydrophobic properties are governed by surface chemical components and morphologies. Chemical composition determines the surface free energy and a lower surface energy leads to higher hydrophobicity. The maximum CA can only reach about 120° for smooth surfaces^3,4 and the surface roughness is a crucial factor for fabricating a superhydrophobic surface. Combining the decorating hydrophobic materials with moderated morphologies of the surface, several research groups have successfully fabricated superhydrophobic surfaces with some specific metallic substrates,⁵ such as superhydrophobic Al surfaces.^6,7 Generally, in their studies, fluoropolymers, organisilane materials, and inorganic oxides (TiO₂, ZnO, etc.) are conventional chemical modification agents.⁸

Room temperature ionic liquids (RTILs), particularly imidazolium ions have been widely used in many fields such as chemical synthesis,⁹ catalysis,^10,11 separation,¹² and electrochemistry,¹³ due to their low volatility, nonflammability, high chemical and thermal stabilities.^14,15 Their physicochemical properties, especially the hydrophilicity and hydrophobicity, can easily be tuned by varying alkyl appendages of the imidazolium cation and/or counteranions. Therefore, ILs have widely been used in surface science to modulate excellent surface behavior such as wettability,¹⁶ lubricants,¹⁷ optical properties,¹⁸ and so on. In recent years, their intrinsic low surface tension has induced many studies on the wetting behavior of ILs.¹⁹ The self-assembly technique,^16,20 layer-by-layer method,²¹ electrospinning,²² and spin-coating²³ have been used to fabricate IL films including simple ionic liquids, polyelectrolyte-functionalized ILs (PFILs), and poly(ionic liquid) brushes. Among them, the self-assembly technique is an effective and simple method to prepare monolayers on substrates by introducing covalent bonds or electrostatic interactions between ionic liquids and substrate surfaces.²⁴ In 2004, Choi^16,25 and Itol¹⁸ used independent SAMs of imidazolium ion-terminated ILs to tailor the wettability of gold and Si–SiO₂ surfaces through exchanging anions. The tunable wetting property of ionic liquids can be extended to modulate the wettability of CNT-based materials,^26–31 negatively charged polyimide films,³² and so on. However, the wettability of SAMs of ionic liquids has been limited from 21° to 90° until now.

It is known that the increase of chain length of the substituent in ionic liquids can result in the increase of hydrophobicity.^19,33 One believes that long-chain ILs should be the promising hydrophobic materials. In the meantime, inherently hydrophobic and oleophobic nature of the F element will make ILs having perfluoroalkyl chains, i.e., fluorinated ionic liquids possess high hydrophobicity. Ragogna³⁴et al. has obtained superhydrophobic films of the highly fluorinated phosphonium ionic liquids (HFPILs) on Ag coated Cu substrates. Herein we prepared the SAMs of C_nFtespim⁺X⁻ (naming followed Ref. 25) on rough Al surfaces, and their wetting behavior and effect factors were discussed. The surface composition and counteranion exchange were confirmed by XPS. The surface morphology was characterized by using SEM observations. The wettability was measured as water contact angle (CA) by contact angle goniometry. Increasing chain length of C_nFtespim⁺X⁻ can improve the CA values of SAMs of fluorinated ILs. Combining with ideal micro- and nano-scale hierarchical structures, superhydrophobic Al surfaces can be obtained having CA above 160° at n ≥ 6. It was found that the effect of counteranions on the wettability of SAMs was dependent on the cations in some degree. The fabrication of superhydrophobic SAMs of ILs could not only widen the applications of fluorinated ILs but also provide a new kind of superhydrophobic materials. The high chemical and thermal stabilities of ionic liquids can be transferred to the substrate surfaces and satisfy the superhydrophobic requirements under especial environment. These results would undoubtedly open up a new research field on superhydrophobic engineering materials.

2. Experimental

2.1 Chemicals

Aluminum was used as the substrate. N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol was purchased from Gelest Inc., Shanghai, China. 1-iodo-1H,1H,2H,2H-perfluorobutane, 1-iodo-1H,1H,2H,2H-perfluorohexane, 1-iodo-1H,1H,2H,2H-perfluorooctane, 1-iodo-1H,1H,2H,2H-perfluorodecane, 1-iodo-1H,1H,2H,2H-perfluorododecane, 1-bromobutane, 1-bromohexane, 1-bromooctane, 1-bromodecane, and 1-bromododecane were purchased from Alfa Aesar and were used without further purification. KPF₆ and LiTf₂N (Tf₂N: (CF₃SO₂)₂N) were purchased from Shanghai Jinlu Chemical Co. Ltd, China. All aqueous solutions were prepared with deionized water.

2.2 Cleaning Al substrates

Al substrates were cut into 1 cm × 1 cm pieces and were sonicated in soapy water for 30 min, rinsed with deionized water, and then sonicated with ethanol and acetone for 15 min, respectively.

2.3 Roughening Al substrates

The freshly cleaned Al substrates were immersed into the HCl solution for 11, 11.5, 12, 12.5, 13 min, respectively, rinsed with deionized water, and sonicated with deionized water for 30 min.

2.4 Grafting C_nFtespim⁺I⁻ or C_nHtespim⁺Br⁻ on Al surfaces (synthetic procedure of f-1 and f-2, as shown in Scheme 1)

In a 50 mL conical flask, an amount of N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol was added into 20 mL toluene, and 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, and 2.0% toluene solutions were obtained, respectively. The treated Al substrates were immersed into the solution and reacted for 12 h at 80 °C. After the formation of the films of N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol, equal molar alkyl halide (CF₃(CF₂)_mCH₂CH₂I or CH₃(CH₂)_mCH₂CH₂Br (m = 1, 3, 5, 7, 9) was added to toluene solution. The reaction was undertaken for 12 h at 80 °C, the films of C_nFtespim⁺I⁻ or C_nHtespim⁺Br⁻ formed on the substrates directly. Finally, the substrates were rinsed with toluene, ethanol, and deionized water several times and then dried under a stream of N₂.

Scheme 1 Synthetic route of films of C_nFtespim⁺X⁻.

2.5 Exchanging the counteranions of ionic liquids (synthetic procedure of f-3, as shown in Scheme 1)

In a 50 mL conical flask, aluminum substrates coated with the SAMs of C_nFtespim⁺I⁻ or C_nHtespim⁺Br⁻ were immersed in an aqueous 10 mmol L⁻¹ solution of KPF₆ or LiTf₂N at room temperature for 24 h, and the resulting Al substrates were thoroughly washed with deionized water and ethanol and dried with a stream of N₂.

2.6 Characterization

In order to confirm the composition of the films, the XPS analyses were performed on a PHI-5702 multifunctional X-ray photoelectron spectroscope (America), using Al K_α radiation (photon energy 1476.6 eV) as the excitation source.

The surface morphology of the resulting surfaces was characterized by using JEOL JSM6701F scanning electron microscope (SEM, Japan).

The static contact angles (CA) were measured by the sessile drop method with a deionized water droplet (~3.5 μL) being placed on the surface by using TRACKER (France) at room temperature. All measurements were repeated three times and calculated the averaged values.

3. Results and discussion

3.1 Fabrication and characterization of C_nFtespim⁺X⁻ SAMs on Al surfaces

Chemical erosion is a simple and effective method to make metal substrates rough, and HCl solution as an erosion agent was used to roughen Al surfaces in our study. To optimize the ideal concentration of HCl solution, 1, 2, 3, 4, and 5 mol L⁻¹ HCl were evaluated, respectively. According to the reaction rate, we found that 1, 2 or 3 mol L⁻¹ HCl solution was too dilute for reaction with the Al substrates and it took a long time, whereas 5 mol L⁻¹ HCl was too fast for the reaction and was difficult to control. Therefore, 4 mol L⁻¹ HCl solution was chosen as an optimum erosion solution, in which Al substrates were immersed for eroding to successfully accomplish the process of roughening Al.

As we know, the purification of ILs is very difficult, and a small quantity of impurities will produce disordering and defective films and result in poor reproducibility. Thus grafting ILs on the substrates can be fabricated by two synthetic routes according to literature:^26–28 i) ILs are prepared first and then self-assembled on the substrate surface directly;^26,27 ii) an imidazol moiety is first self-assembled on the substrate surface via covalent bonds, and was followed by the preparation of ILs in situ on the substrate surface.²⁸ Our results showed that the second route avoided the purification of ILs, and reproducibility in wettability was better than that of the first one.

For the synthetic procedure of ILs films, C₈Ftespim⁺X⁻ was chosen as the model and a detailed experimental procedure was illustrated in Scheme 1. First, we considered the optimization of synthetic conditions such as the effect of solvents and concentrations of the imidazol moiety on the quality of the films. Toluene was found to be the ideal solvent because no methylene groups were interfered with the packing of the alkyl chains of C₈Ftespim⁺X⁻, which allowed the formation of close-packed, well-ordered monolayers.^20,35 Secondly, it is well known that water contained in solvents would affect the formation of high-quality SAMs.^24,35 The absence of water will cause the incomplete monolayers while excess water will result in the partial hydrolysis of the triethoxysilyl groups of N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol, and the hydrogen-bonded silanol network will lead to the formation of covalent siloxane (–Si–O–Si–) bonds.³⁶ In our study, toluene was used without dehydration, in which a small quantity of water would induce to plentiful hydroxyl groups on the Al surfaces.⁷ The presence of hydroxyl groups on Al surfaces could lead N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol molecules to self-assemble on surfaces through the reaction between triethoxysilyl groups and hydroxyl, as shown in Scheme 1, and f-1 can form via –Si–O– covalent bonds. Subsequently, 1-iodo-1H,1H,2H,2H- perfluorooctane was added to the reaction system, and the films of C₈Ftespim⁺I⁻ (f-2 in Scheme 1) formed directly on the substrate surfaces after reacting 12 h at 80 °C. To obtain the ideal SAMs, the concentration of the imidazol moiety in toluene for the synthesis of f-1 was tested for 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, and 2.0 wt%, respectively. Through measuring CAs of f-2, it showed that concentration of the imidazol moiety in toluene between 0.5 and 1.0 wt% was optimum, while much higher concentration was a disadvantage for the hydrophobicity. This phenomenon can be explained by the processes of the formation of SAMs. Generally, molecules are first adsorbed to the substrate surface from solution and then functional groups of molecules react with Al substrate to bind the surface.³⁷ Exorbitant concentration may result in the excess adsorption and restrain further reaction. Therefore, the concentration of the imidazol species in toluene will affect the reaction rate and the quality of SAMs. For the exchange of anions, the substrates coated with the SAMs of C₈Ftespim⁺I⁻ were immersed into an aqueous solution of 10 mmol L⁻¹ KPF₆ or LiTf₂N at room temperature for 24 h, and SAMs of C₈Ftespim⁺PF₆⁻ or C₈Ftespim⁺Tf₂N⁻ were synthesized, respectively.

The chemical composition of films was detected by X-ray photoelectron spectroscopy measurements. From Fig. 1a for f-2 monolayers, the characteristic signals of six elements can be determined, which were assigned to F1s, O1s, N1s, C1s, Si (2s, 2p), and I (3d3, 3d5, 4d3), respectively. This demonstrated that C₈Ftespim⁺I⁻ had been anchored to Al surfaces, and the signals of I3d3, I3d5, and 4d3 at 628.00 eV, 621.00 eV, and 54.00 eV revealed that the anion was I element.

Fig. 1 XPS images of C₈Ftespim⁺X⁻: C₈Ftespim⁺I⁻ (a) and C₈Ftespim⁺Tf₂N⁻ (b).

The anionic exchange of ILs on the surfaces can be confirmed by XPS. After exchanging by LiTf₂N, as shown in Fig. 1b, the peaks of I element disappeared and peaks of S: 2s at 169.00 eV; 2p at 232.16 eV for Tf₂N⁻ were observed, indicating that I⁻ was completely exchanged with Tf₂N⁻.

3.2 Effect of Al surface roughness on the wettability of films

It is well known that the surface roughness is a crucial factor to superhydrophobic surfaces as well as hydrophilic surfaces.⁸ Morphologies of aluminum substrates were observed by SEM, as shown in Fig. 2. It was clear that commercial Al surfaces were uniform and smooth with many slim grooves before reacting with HCl (Fig. 2a). However, after being treated with 4 mol L⁻¹ HCl aqueous solution for 12.5 min, Al surfaces formed 0.5–1.0 μm branched microscopic structures, on which 100–300 nm nanostructures were remarkably grown on the microscopic structures (Fig. 2(b and 2c)).

Fig. 2 SEM images and water contact angles of aluminum surfaces. SEM image before erosion with inserting CA of Al surface grafted C₈Ftespim⁺I⁻ (a), SEM images at different magnifications (b) and (c) after treatment with 4 mol L⁻¹ HCl aqueous solution for 12.5 min with the inserts showing the CA of Al surface grafted C₈Ftespim⁺I⁻.

Measurements of static water contact angles could provide quantitative information of the effect of roughness on the wettability of the films. For originally untreated Al substrates, grafting of C₈Ftespim⁺I⁻ could improve the value of CA to be 122 ± 2° from a completely hydrophilic state (Fig. 2a), which approached the maximum value about 120° on smooth surfaces.⁴ However, the hydrophobicity of Al substrates coated SAMs of C₈Ftespim⁺I⁻ remarkably increased after treatment with 4 mol L⁻¹ HCl solution. The different values of CA were exhibited under different erosion times. As shown in Table 1, when treated for 11 min and grafted to the C₈tespim⁺I⁻ films, the hydrophobicity increased to be 154 ± 2° from 122 ± 2°. When the erosion time was prolonged to 12.5 min, the CA went up to 165 ± 2° (Fig. 2b). Obviously, extending the reaction time can deepen the erosion degree, and the ideal films of micro- and nano-scale geometrical structure with the superhydrophobicity was obtained at erosion for 12.5 min with 4 mol L⁻¹ HCl solution. It was shown that the ideal roughness of surfaces could increase the hydrophobicity. However, erosion for 13 min led to a negative effect on the wettability of Al surfaces due to the excess roughness.

Table 1 Effect of erosion time using 4 mol L⁻¹ HCl on the values of CA

Erosion time (min)	11	11.5	12	12.5	13
CA (°)	154 ± 2	155 ± 3	157 ± 3	165 ± 2	156 ± 3

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3.3 Effect of chemical composition of ILs on the wettability of films

The effect of chain length of substituent and counteranions of ionic liquids on the wettability has been studied by many research groups.^8,18,22 It was found that the facile change of chain length and composition of ionic liquids has a distinct advantage in tailoring the surface wettability. In our study, we grafted two series of SAMs of ILs, C_nFtespim⁺I⁻ and C_nHtespim⁺Br⁻ (n = 4, 6, 8, 10, 12) on the rough Al surfaces. One kind of ILs possessed F element as the substituent of the imidazolium ion-terminated ILs, whereas another IL had an alkyl appendage without F element. As shown in Fig. 3a, a trend of increasing hydrophobicity with the increase of the chain length of the substituent was observed in C_nHtespim⁺Br⁻. The value of CA increased from 107° to 150° with the chain length varying from 4 to 12, in which the superhydrophobic surface was obtained at n = 12. This result was quite consistent with that obtained in evaluation of the solubility of ionic liquids in water, in which the extension of the appendage would lower the surface tension.^11,33 In the series of C_nFtespim⁺I⁻, however, the increasing trend had slight differences from that of C_nHtespim⁺Br⁻. From n = 4 to 8, hydrophobicity presents a similar increase to the case of C_nHtespim⁺Br⁻. One can see that thr CA of C_nFtespim⁺I⁻ was 140 ± 2° at n = 4, and a superhydrophobic surface was obtained at n = 6. The value of CA was up to 165 ± 2° in the case of C₈Ftespim⁺I⁻, however, C₁₀Ftespim⁺I⁻ and C₁₂Ftespim⁺I⁻ gave similar wetting behaviors, remaining at 164 ± 2°. On the other hand, comparing the wetting property of two series of SAMs of ILs, C_nFtespim⁺I⁻ and C_nHtespim⁺Br⁻, one could find that the SAMs of C_nFtespim⁺I⁻ possessed more hydrophobic ability than that of the corresponding C_nHtespim⁺Br⁻ SAMs, as shown in Fig. 3a. For instance, the water contact angle of C₈Htespim⁺Br⁻ SAMs was only 142 ± 2°, while the CA of C₈Ftespim⁺I⁻ SAMs was 165 ± 2°, suggesting that introduction of F element to the substituent of ILs could impart the inherent hydrophobic character of the perfluorinated chain to C_nFtespim⁺I⁻. Therefore, long-chain fluorinated imidazolium-based ionic liquids could be considered to be excellent hydrophobic materials. So, superhydrophobic aluminum surfaces can be obtained combining the hierarchical structures with the SAMs of C_nFtespim⁺I⁻.

Fig. 3 Effect of chemical composition of ILs on the wettability. Cations (a) and counteranions (b).

It is known that the nature of the imidazolium counteranions will influence the wettability of the surfaces. In order to evaluate the effect of counteranions on the wettability, PF₆⁻ and Tf₂N⁻ were chosen to replace Br⁻ or I⁻ to form C₈Ftespim⁺PF₆⁻, C₈Ftespim⁺Tf₂N⁻, C₈Htespim⁺PF₆⁻ and C₈Htespim⁺Tf₂N⁻. Fig. 3b revealed that the CAs of the SAMs of C₈Htespim⁺Br⁻, C₈Htespim⁺PF₆⁻, and C₈Htespim⁺Tf₂N⁻ were 142 ± 2°, 151 ± 2°, and 157 ± 2° on rough Al surfaces, respectively, indicating that the wettability of the Al substrates could be altered through the exchange of counteranions of ILs. However, one can find that almost stable CA values were observed for SAMs of C₈Ftespim⁺Br⁻, C₈Ftespim⁺PF₆⁻, and C₈Ftespim⁺Tf₂N⁻ remaining at about 165 ± 2°. These results indicate that counteranions could alter the CAs in a limited range and the effect of anions was dependent on the cations of imidazolium-based ionic liquids in some cases.³²

4. Conclusions

In summary, a new class of superhydrophobic SAMs of imidazolium ion-terminated fluorinated ionic liquids were synthesized on rough Al substrate surfaces, in which a triethoxysilyl group was used for immobilization via –Si–O– covalent bonds. Water contact angles of ILs films were improved with the increase of the chain length of C_nFtespim⁺X⁻, having a CA above 160° at n ≥ 6 with micro- and nano-scale hierarchical structures. The exchange of counteranions can be tuned to the wetting behavior of ILs depending on the cations of imidazolium-based ionic liquids. There are some advantages, (i) the self-assembled technique and roughening method are simple; (ii) the introduction of –Si–O– covalent bonds to the substrate offers durability; (iii) retaining the unique stability properties of ionic liquids and their superhydrophobic films can satisfy the demands of engineering materials.

Acknowledgements

This work was financially supported by the NSFC (Grant No. 21033005) and the National Basic Research Program of China (“973” Program, 2009CB930103), the NSF of Shandong Province (Grant No. ZR2009BM042), the Chinese Postdoctoral Science Foundations (No. 201003634 and 20090461211), and the Shandong province Postdoctoral Foundation (No. 201002033).

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