Ionic liquid-infused nanostructures as repellent surfaces

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Ionic liquid-infused nanostructures as repellent surfaces Yaraset Galvan, Katherine R. Phillips, Marco Haumann, Peter Wasserscheid, Ramon Zarraga, and Nicolas Vogel Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03993 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Ionic liquid-infused nanostructures as repellent surfaces Yaraset Galvan,1,2 Katherine R. Phillips,3 Marco Haumann,4 Peter Wasserscheid,4 Ramon Zarraga,2 and Nicolas Vogel1*

1. Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany 2. Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Norial Alta s/n, 36050 Guanajuato, Mexico 3. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, USA 4. Institute of Chemical Reaction Engineering, Friedrich-Alexander University ErlangenNürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany

ABSTRACT. We investigate conditions to prepare lubricant-infused repellent coatings on silica nanostructures using ionic liquids as lubricants. We study the wetting behavior of a set of imidazolium-based ionic liquids with different alkyl side chains as a function of the applied

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surface functionalities, which we introduce via silane chemistry onto silica substrates. We take advantage of the structural color of inverse opals prepared from a colloidal co-assembly technique to study the infiltration of ionic liquids into these nanoporous structures. We find that the more hydrophobic ionic liquids with butyl and hexyl side chains can completely infiltrate inverse opals functionalized with mixed self-assembled monolayers composed of imidazole groups and aliphatic hydrocarbon chains. These molecular species reflect the chemical nature of the ionic liquid, thereby increasing the affinity between liquid and solid surface. The mixed surface chemistry provides sufficiently small contact angles with the ionic liquid to infiltrate the nanopores while maximizing the contact angle with water. As a result, the mixed monolayers enable the design of a stable ionic liquid/solid interface that is able to repel water as a test liquid. Our results underline the importance of matching chemical affinities to predict and control the wetting behavior in complex, multi-phase systems.

INTRODUCTION

The uncontrolled adhesion of contaminants onto surfaces can drastically decrease the performance of a material in a wide range of technological applications.1–3 The consequences of fouling range from loss of vision in optical and medical technologies4,5, decreased performance of solar cells6,7, and increased drag in pipelines8,9 to severe hazards to health and safety by bacterial adhesion onto surfaces in the healthcare sector10–12 and even ice adhesion to airplanes or infrastructure causing energy-efficiency and safety concerns13,14.


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The traditional approach to prevent fouling is to create repellent surfaces that mimic the selfcleaning and strongly water-repellent nature of the Lotus leaf15,16 by minimizing the contact area between water (or other contaminating liquids) droplets and the surface.1,17,18 A combination of hydrophobic surface chemistry at the molecular level and topographic features at the micro- and nanometer level prevents a water droplet from contacting the entire surface topography, forming an air/solid/liquid composite interface in a Cassie-Baxter wetting state.17 This strategy has been tremendously successful in creating surface coatings able to repel water and, by introduction of hierarchical features with re-entrant curvature, even low surface tension liquids.19,20 However, several drawbacks arise from this design of repellent surface coatings. As the air pockets in the air/solid/liquid composite interface can be compressed, the repellency properties can be compromised at elevated pressure or droplet impact.21 Direct contact between the solid surface and the contaminating liquid is minimized but not prevented, so fouling by complex fluids via adhesion of contaminants such as proteins22 or bacteria23 cannot be excluded. Finally, the coating is unable to self-heal and will suffer from pinning of liquid droplets at damaged parts of the surface.24

An alternative approach to impart liquid repellency employs a fluid lubricant layer separating the surface from the contaminating liquid.

25,26

In such lubricant-infused surfaces, this lubricant

layer is confined to the solid substrate by a combination of chemical affinity induced by surface chemistry and surface roughness. If the interfacial energy between the solid surface and the lubricant is lower than the interfacial energy of the solid with the contaminating liquid, the latter will not replace the lubricant layer.25,27,28 The resulting formation of a liquid/liquid interface prevents direct contact and pinning of the contaminating liquid and leads to a facilitated removal


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of the liquid from the surface. This strategy combines superior repellency of water and organic liquids with several additional attractive characteristics. The prevention of direct contact between the contaminating liquid and the solid surface efficiently prevents the adsorption of proteins,22 bacteria,23,29 or algae,30 enabling applications as non-fouling healthcare5 or marine31 materials. Similarly, the adhesion of ice is drastically reduced with potential benefits in aviation and infrastructure.14,32–34 Coatings with efficient repellency properties can be created from extremely small nanostructures, enabling the design of transparent, repellent coatings with applications as solar cell coatings

32,35

or in medical endoscopy.5 Finally, the fluid nature of the lubricant film

leads to pressure tolerance of the coating and self-healing characteristics, as the lubricant can flow back into damaged parts of the surface to regenerate repellency.25,36

These attractive repellency characteristics result from the liquid nature of the lubricant, which, however, also provides two fundamental degradation mechanisms. Firstly, the lubricant can flow off the surface under the influence of gravity, shear forces or by the formation of a wrapping layer around droplets or bubbles.28,37 These lubricant losses can be mitigated by using polymerand other organo-gels that can be swollen with a lubricant to form a solid/liquid material. Such organogels provide a liquid lubricant interface while strongly enhancing the molecular interactions of the lubricant with the supporting solid, increasing the stability of the coating. 38–42 The design of solid, self-replenishing surface layers by the incorporation of longer chain alkane molecules within the lubricant can further add to stability of such coatings.43 Secondly, liquids evaporate, thus degrading the coating over time or at elevated temperatures. Even though the versatility in the choice of lubricant can delay this process to enable performance at different


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temperatures, a strategy to prevent losses by evaporation is highly desired in the design of repellent coatings.

Ionic liquids (ILs) offer a potential solution to this challenge. ILs consists of ions that, due to their bulkiness or asymmetric structure, remain liquids over a wide temperature range.44,45 Additionally, ionic liquids are characterized by ultra-low vapor pressures (approximately 10-12 mmHg),46 non-flammability and high thermal stability, 47 all of which are attractive features for a lubricant-infused repellent coating. Most importantly, the extremely low vapor pressure essentially eliminates the possibility of lubricant evaporation, thus providing an avenue towards coatings with increased long-term stability and robustness. This potential has been recently realized by Ding et al., who used ionic liquid infused polymer gels as repellent materials and demonstrated increased long-term stability since evaporation was effectively prevented.40

To further develop and systematically explore the concept of using ionic liquids as lubricants in repellent surface coatings, it is crucial to understand and optimize the interactions of ionic liquids with the underlying porous surface structure on a molecular level. To create a stable lubricant-infused coating, the solid-lubricant and solid-contaminating liquid interfacial energies need to be carefully adjusted. Generally, the lubricant needs to infiltrate the porous surface, a pre-requirement that is generally easily fulfilled with typical low surface tension liquids used as lubricant. 5,22,25,48 Secondly, the lubricant must not be replaced by the contaminating liquid. This requires maximizing the difference in interfacial energy between the lubricant in contact with the solid and the contaminating liquid in contact with the solid.

25

In other words, an optimized


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system requires a maximum contrast in wettability of the surface with the two different fluids, i.e. the lubricant and the liquid to be repelled.

This optimization requires a careful examination of the chemical affinities in the system. Typically, the solid surface is modified using a surface functionalization matching the chemical identity of the lubricant. Ideally, this surface functionality can be decoupled from the preparation of the surface topography and can be flexibly adjusted to the desired characteristics of the lubricant, such as silanization reactions on oxidic surface nanostructures.22,49 A fluorinated lubricant is held in place by a fluorinated surface, while employing organic lubricants or silicone oils necessitates a hydrocarbon-based surface chemistry. However, the proper surface functionalization to minimize the interfacial energy for an ionic liquid used as a lubricant is less obvious since the ionic liquid itself features several different functional groups within its molecular structure. We hypothesize that multiple surface functionalities, replicating the chemical moieties present in the ionic liquid, may enhance the IL-surface interaction. Here, we investigate in detail the wetting characteristics of three different imidazole-based ionic liquids with surfaces of aliphatic, imidazole, and mixed surface chemistries, which we introduce via silane chemistry onto a model porous substrate. We then describe their use in the stable operation of a lubricant-infused coating based on surface-functionalized nanostructures. 22

EXPERIMENTAL SECTION

All reagents were obtained from Sigma-Aldrich. MilliQ water was used for all experiments and nitrogen to dry all substrates. Silicon wafers and glass microscope slides were used as substrates.


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Inverse opal fabrication Inverse opals were prepared following a protocol from literature.50 In brief, a solution of tetraethyl ortosilicate (TEOS), hydrochloric acid and ethanol (1:1:1.5 by weight) was prepared and stirred for 1 h in order to pre-hydrolyze the TEOS. 1.25 mL of 1.16 % v/v suspension of monodispersed colloidal particles of polystyrene (232 nm diameter) was added to 18.75 mL of milliQ water to prepare a 0.1 % w/v polystyrene solution, to which 140 µL of pre-hydrolyzed TEOS solution was also added. Substrates were cleaned in base piranha (1:1:5 ratio of ammonia solution (25% in water) to hydrogen peroxide (33% in water) to milliQ water) at 80°C for 0.5 h and then rinsed in milliQ water. Cleaned and dried substrates were vertically suspended in a vial containing the colloid/TEOS suspension. The solvent content was evaporated slowly in an oven at 65°C to form a thin film on the suspended substrate. The substrates were subsequently heated at 500°C for 2 h with a 5 h ramp time to remove the polystyrene particles and sinter the SiO2 structure to form the inverse opal structure.

Fabrication of layer by layer-based surface nanostructures Layer by layer samples were prepared following a protocol from literature.22 In brief, substrates were cleaned by base piranha treatment as described above and subjected to oxygenplasma for 5 min to activate the surface. The layer by layer deposition was performed by complete immersion of the substrates in a 0.1% w/w solution of poly(diallyldimethylammonium) chloride (PDADMAC) for 5 min, followed by thorough rinsing in DI water by immersion into three consecutive water-filled vials. The next layer was deposited by immersion of the substrate


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into a solution of 0.1 % w/w Ludox silica colloids for 5 min followed by rinsing in water. This cycle was repeated 10 times to deposit a multilayer. Afterwards the polyelectrolyte PDADMAC was removed by combustion at 500ºC for 2 h with a 5 h ramp time.

Surface functionalization of inverse opals The substrates were cleaned in an oxygen-plasma for 10 min at 10 sccm oxygen flow and 100 W plasma power to activate the surface. Silanization of the surfaces with trichlorododecylsilane and butyl(chloro)dimethylsilane were carried out by vapor phase deposition for 24 h at reduced pressure (approximately 1 mbar) and room temperature, following a protocol from literature.36 Dry substrates were placed in a desiccator adjacent to a vial with 1 mL of silane. Then, the desiccator was closed and a vacuum pump was used to reduce the pressure for 10 min. The desiccator was then sealed and the substrates were incubated for 24 h and washed with ethanol afterwards to remove unbound silane. Silanization of the surfaces with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol was performed by liquid phase deposition, in which the substrates were placed in a 1 % v/v solution of the silane in ethanol for 24 h at room temperature, followed by thorough rinsing with ethanol to remove unbound silane. Mixed monolayer substrates were prepared by a sequential approach. First, liquid phase silanization was performed with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol as described above. The substrates were then dried and subjected to a vapor phase deposition of trichlorododecylsilane or butyl(chloro)dimethylsilane for 24 h as described above. The number ratio between the functional groups could be altered by changing the immersion time in the liquid


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imidazole-silane solution. For all experiments shown in the manuscript, an incubation for 6h was used. All substrates were heated to 95 °C on a hotplate for 1 h to complete the condensation reaction and to covalently bind the silane molecules to the surface.

Infusion of ionic liquids and measurement of contact angles Static contact angles were measured using a goniometer (Data Physics model OCA 30) under ambient conditions at five different areas per substrate and averaged. For water contact angles on lubricated inverse opal samples, 10 µL/cm2 of each ionic liquid was added to each type of inverse opal substrate, and uniform coverage was achieved by tilting. All samples were placed vertically for 10 min in order to drain the excess lubricant film by gravity. Sliding angles were measured using a tilted stage and the angle necessary to induce sliding of a 5 µl droplet was recorded as the sliding angle. Wettability of inverse opals with ionic liquids was determined by optical microscopy.

Scanning electron microscopy (SEM) A Zeiss Gemini instrument was used and samples deposited onto a silicon wafer substrate were used for the investigations. Cross-sectional samples were fabricated by carefully breaking the wafer along its crystal plane using a diamond cutter.

X-ray photoelectron spectroscopy (XPS) XPS measurements were performed using a Thermo Scientific K-Alpha XPS system. Low resolution survey scans were taken followed by high resolution scans with a dwell time of 50 ms


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and a resolution of 0.1 eV for the individual elements. The high resolution N 1s and C 1s scans were used for quantification.

RESULTS AND DISCUSSION

Ionic liquid-infused inverse opals as repellent surfaces We investigate conditions to achieve stable repellency properties of ionic liquid-based lubricant-infused coatings on nanostructured glass surfaces. We chose inverse opals as the porous surface, and use silane chemistry to modify their surface properties in order to control the wetting behavior with both water, the test liquid to be repelled, and ionic liquids, the lubricant. Inverse opals are three-dimensional arrays of ordered, interconnected nanopores that can be prepared via colloidal templating.51,52 The periodic structure at the nanoscale leads to macroscopically observable structural coloration.52 If a liquid infiltrates the nanopores, the refractive index contrast is drastically decreased and the structural color is lost. Inverse opals can thus be used as colorimetric sensors to detect liquid wetting, which, in turn, can be controlled via the surface chemistry.53,54 We take advantage of this simple visualization of the wetting state to characterize the wetting and repellency characteristics and identify suitable surface functionalization to create a lubricant-infused coating based on ionic liquid- infiltrated surface nanostructures. We prepared the inverse opals by an evaporative co-assembly technique of tetraethyl orthosilicate (TEOS) and polystyrene colloidal particles.50 This co-assembly technique leads to defect-free inverse opals with a periodic and interconnected nanoporous structure (Figure 1a).


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With respect to IL selection, we focused on established imidazolium-based ionic liquids,55,56 and varied the length of the alkyl side chain in the 1-position of the imidazolium ring in presence of a highly-hydrophobic anion. The following ionic liquid structures were investigated: 1-ethyl3-methylimidazolium imidazolium

bis(trifluoromethylsulfonyl)imide,

bis(trifluoromethylsulfonyl)imide,

[EMIM][NTf2],

[BMIM][NTf2],

1-butyl-3-methyland

1-hexyl-3-

methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMIM][NTf2]. These are shown in Figure 1b.

We investigated the wetting properties of these ILs as a function of the applied surface chemistries on both plain glass substrates and within the inverse opal nanoporous coating. For a functional lubricant-infused system, two wetting conditions need to be fulfilled. First, the lubricant needs to wick into the structures, providing a thin, wetted film covering the nanostructures. Second, this lubricant film must not be replaced by the liquid to be repelled (i.e. the interfacial energy of the lubricant/solid interface needs to be smaller than the interfacial energy of the water/solid interface).25 Unlike for fluorocarbon or hydrocarbon liquid lubricants, where the surface chemistry can match the chemical identity (fluorosilanization for fluorinated lubricants, hydrocarbons for hydrocarbon oils),22,25 no such trivial choice of surface chemistry exists for ionic liquids. Their molecular structures include different functional moieties, which lead to multiple, competing interaction forces with a surface. Ionic groups induce coulombic interactions, while hydrophobic interactions arise from the alkyl side chains and the aromatic imidazolium group can provide pi-pi interactions. We chose different silanes with functional groups that match important functional groups found in the molecular structures of the ionic liquids,

focusing

on

surfaces

functionalized

with

trichloro-dodecylsilane

(Dod),


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butyl(chloro)dimethylsilane (But) and N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol (Imi) (Figure 1c), mimicking the alkyl side chain and the imidazole group of the ionic liquid, respectively. We use water as a contaminating test liquid, therefore we exclude hydrophilic surface components such as amine- or ionic groups. Would the surface carry such polar termination groups, high surface affinity to water would inherently result and the probability of IL replacement from the surface by water would increase.

As mentioned above, we took advantage of the change in optical properties of inverse opals upon IL wetting (as schematically shown in Figure 1d).52,53 52 In the dry state, the high refractive index contrast between matrix and air-filled pores leads to strong interference conditions of reflected light and a macroscopically observable structural color (Figure 1d, i). When a liquid is added onto the inverse opal structure, it can either be prevented from wetting the pores, infiltrate the pores completely or partially wet the first layer (Figure 1d, ii). The wetting behavior is determined by the pore opening angle 57 and the contact angle of the liquid with the solid surface, which, in turn, depends on the applied surface chemistry.

53

Low contact angles favor wetting

while high contact angles prevent pore infiltration. 53 For intermediate contact angles, the partial wetting of the first layer is a consequence of the change in pore opening angle between the top layer and the underlying three-dimensional pore network. In the topmost layer, the pore opening angle is close to 90° (i.e. pores corresponding to colloidal particles embedded up to their equator in the inorganic matrix), while the pore opening angle is given by the necks in between the individual pores and generally much smaller (around 23°).

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The much larger pore opening

angle of the topmost layer allows much easier wettability of these pores as compared to the more closed pores in the interior of the inverse opal.

57

Similarly, this colorimetric assessment of the


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wetting state also enables the investigation of the stability of the lubricant layer upon exposure to a different, contaminating liquid (Figure 1d,iii). We chose water as this contaminant test liquid since its refractive index is much lower than that of the ionic liquids.

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If the ionic liquid is

replaced by water, which subsequently fills the pores, a slight change in coloration is expected as a consequence of the different refractive index contrast.

Figure 1. Fabrication of an ionic-liquid infused repellent coating from nanoporous coatings. a) SEM images of an inverse opal showing the long range periodicity of the pores in front-view (left) and side-view (right), scale bars = 2µm; b) Chemical composition of the imidazoliumbased ionic liquids tested in this study: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][NTf2], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMIM][NTf2], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMIM][NTf2]; c) Chemical composition of the tested silanes used to modify the surface chemistry: trichloro-dodecylsilane (Dod), butyl(chloro)dimethylsilane (But) and N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Imi); d) Schematic representation of the possible wetting states of an inverse opal in a lubricantinfused coating. Depending on the contact angle of the surface with the ionic liquid (dark blue), the ionic liquid cannot infiltrate the pores of the inverse opal or can infiltrate them totally or


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partially (ii). Addition of a second, contaminating liquid (light blue) can either create a stable coating in which the ionic liquid is not replaced by the second liquid or failure of the coating when the IL is replaced by the second liquid. The refractive index-dependent structural coloration of inverse opals enable the observation of the different wetting scenarios directly from the change in macroscopic coloration.

Surface functionalization and characterization on flat substrates We started by investigating the wetting characteristics of the different liquids (three imidazolium [NTf2]--ILs with varying alkyl side chains to change the hydrophobicity, Figure 1b, and water as the liquid to be repelled) as a function of the applied surface chemistry. We used flat silicon wafers as substrates and silane chemistry for surface functionalization as shown in Fig. 1c. We chose an imidazole-based silane (Imi) as well as two hydrocarbon silanes with different chain lengths (dodecyl (Dod) and butyl (But)) as well as mixed monolayers of imidazole and hydrocarbon to match the different chemical moieties found within the molecular structure of the ionic liquids. We characterized the wetting behavior by static contact angle measurements (Fig. 2a, c) and the surface functionalization by X-ray photoelectron spectroscopy (XPS) (Figure 2b, d).

All three ionic liquids showed low contact angles with the Imi-functionalized surfaces with values below 20° (Figure 2a and 2c). However, Imi-functionalized surfaces also showed a moderately hydrophilic character with a water contact angle of 57±4°. The contact angle of water with the dodecyl-functionalized surface exceeded 110°, indicating a successful surface functionalisation with the hydrophobic silane. The contact angle of the ionic liquids with the dodecyl-functionalized surface was significantly lower. As expected, the more hydrophilic ionic


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liquid [EMIM][NTf2] showed the highest contact angle (78±1°) while the more hydrophobic ionic liquids with butyl ([BMIM][NTf2]) and hexyl ([HMIM][NTf2]) side chains showed slightly lower contact angles (68±2°). Interestingly, the contact angle values for the mixed monolayer were similar to the completely hydrophobic monolayer with pure dodecyl functionalization. We interpret these results as a shielding of the imidazole groups by the larger dodecyl chains that thus dominate the surface properties (Figure 1c). The ionic liquids with longer alkyl chains also had a slight decrease in contact angle for the mixed surface compared to the Dod-only surface, as a result of the increased affinity. We used X-ray photoelectron spectroscopy (XPS) to analyze the surface composition of the three samples (Figure 2b) and used the N1s peak to confirm the presence of the nitrogen atoms in the imidazole ring. For pure imidazole surface functionalization, a clear nitrogen signal was detected corresponding to an overlap of the two N peaks expected for imidazole.58 For the dodecyl-functionalized sample, no nitrogen was detected. The mixed sample with dodecyl and imidazole groups only showed a very weak N1s peak, indicating little presence of imidazole groups on the surface, even though the imidazole-silane functionalization step occurred prior to treatment with the dodecyl silane. This corroborates our results from the contact angle measurements.

We then prepared new samples, exchanging the long dodecyl chain with a shorter butyl chain that cannot be expected to shield the imidazole groups sterically (Figure 1c). Subsequently, we measured the wetting behavior of all test liquids on these butyl-functionalized samples (Figure 2c). As expected for a shorter hydrocarbon chain, the water contact angle of the butylfunctionalized surface was smaller compared to the dodecyl-functionalized sample. However, the hydrophobic character of the surface, with a contact angle of 101±1°, still indicated a successful


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functionalization step. Similarly, the ionic liquids showed lower contact angles on the butylfunctionalized surface compared to the dodecyl surface with the expected trend of higher contact angles for the more hydrophilic ionic liquid. Importantly, for the mixed But/Imi self-assembled monolayer, a clear decrease in contact angle for all test liquids was observed, indicating the presence of the imidazole functional group at the interface. This interpretation was corroborated by our XPS measurements that showed a clear N1s signal for both the pure imidazole surface functionalization as well as the mixed butyl/imidazole self-assembled monolayer (Figure 2d).

Figure 2. Surface functionalization and wetting properties. a) Contact angle of water and ionic liquids on a flat silicon wafer surface with self-assembled monolayers of Imi, Dod, and mixed composition (Imi and Dod). b) XPS spectra in the N1s region of Imi-, Dod- and mixed monolayers. c) Contact angle of water and ionic liquids on a flat silicon wafer surface with selfassembled monolayers of Imi, But, and mixed composition (Imi and But). d) XPS spectra in the N1s region of Imi-, But- and mixed monolayers. All contact angles were averaged over five separate measurements.


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Wetting states of ionic liquids on inverse opals with different surface functionalities Next, we examined the wetting properties of the different ILs in contact with the inverse opal nanoporous coating as a function of the applied surface chemistry (Figure 3). We used the structural color of the inverse opals to directly visualize the wetting state.

53

Loss of color

indicates pore infiltration of the ionic liquids which eliminates the refractive index contrast. We assign this infiltrated wetting state as state 1 in Figure 3. Partial infiltration of the ionic liquid in the topmost pore layer, caused by the substantially larger pore opening angle as discussed above,53,57 can be identified by the presence of a liquid film on the surface while the structural color is retained (wetting state 2 in Figure 3). Finally, complete prevention of pore infiltration, leading to a liquid droplet sitting on the substrate that retains its structural color, is assigned to as wetting state 3. Inhomogeneities in the inverse opals, for example cracks or different layer thicknesses (leading to variations in the observed color), do not affect the wetting behavior, which is governed by the wicking of liquids into the interconnected nanopores.

Generally, the wetting states of the inverse opal reflects the contact angles of the ILs on flat surfaces. Low contact angles, for example for imidazole-functionalized surfaces, lead to complete pore infiltration (wetting state 1) for all ionic liquids (Figure 3, left). This type of surface functionality therefore fulfils the first criterion to create a repellent coating based on lubricant infiltration. Dodecyl-functionalized substrates, showing the highest ionic liquid contact angles of all tested surface chemistries do not enable liquid infiltration (wetting state 3) and can therefore not be used for an IL-infused coating (Figure 3, second row). Butyl-functionalized inverse opals did not lead to complete wetting of the inverse opal pores. However, while the


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most hydrophilic ionic liquid ([EMIM][NTf2]) was not able to enter the pores, an intermediate wetting state (“2”) was observed for the more hydrophobic ionic liquids (Figure 3, right). The decreased contact angle enabled the infiltration of the topmost layer but was too high to infiltrate the underlying pores.

Mixed self-assembled monolayers of imidazole and both butyl and dodecyl groups modified the wetting behavior of the ionic liquids and showed a remarkable sensitivity to the applied ionic liquid. The most hydrophilic ionic liquid [EMIM][NTf2] did not infiltrate the pores with mixed functionalization, while the intermediate hydrophilic [BMIM][NTf2] showed a partial wetting state for both types of hydrophobic silanes in combination with imidazole groups (Figure 3). The most hydrophobic ionic liquid [HMIM][NTf2] completely infiltrated the inverse opal structures for both types of mixed monolayers.

While a change in wetting behavior was expected from the reduced contact angles for the butyl/imidazole surface functionalization, the clear change in wetting behavior between dodecyland dodecyl/imidazole mixed monolayers was unexpected as the contact angles of the different ionic liquids were similar for both cases. We tentatively attribute this clear difference in wetting properties to effects caused by surface roughness. While steric shielding of the large dodecyl chains may be efficient on a planar substrate, the sharp edges and curved geometry in the inverse opals could enable the imidazole groups to influence the surface properties more efficiently. As the contact angle on imidazole-functionalized surface was very low for all ionic liquids, this increased presence translates into an increased tendency for pore infiltration.


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Langmuir

Figure 3. Visual identification of the wetting state of the different ionic liquids on inverse opals with different surface functionality, observed via the structural coloration of the inverse opal. Complete liquid infiltration eliminates the refractive index difference and thus the structural color (assigned as “1”); partial infiltration, caused by a different pore opening angle between topmost layer and underlying pores (“2”) leads to a wetted substrate with retained structural color. Complete prevention of wetting leads to a droplet sitting on the nanoporous structure (3). The scale bar is 5mm.

Repellency of water in ionic-liquid infused inverse opals Finally, we employed the control of pore wetting characteristics of the tested ILs to investigate their performance as repellent surface coatings. We infused ILs into inverse opals with surface chemistries that enable wetting (Figure 3), then added a droplet of water as the test liquid to be repelled. We investigated if water replaced the ionic liquid (leading to pinning and failure of the infused coating) or if the coating enabled water-repellency (occurring when the water droplet did not replace the ionic liquid and instead caused the droplet to slide off the surface). As before, we


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used structural coloration to monitor the nanoscale wetting properties of the surface coating. Any appearance of color therefore indicates a change in the infiltration medium, such as the replacement of ionic liquid with water (which has a lower refractive index) (Figure 4a). We found that imidazole-functionalized substrates failed as infused coatings: the water droplet clearly displaced the ionic liquid and left a colored area on the substrate. Mixed monolayers, made from butyl and imidazole silanes, enable a stable repellency for the two hydrophobic ionic liquids [BMIM][NTf2] and [HMIM][NTf2]; the water droplet slid off the substrate without replacing the ionic liquid (indicated by a red arrow in the figure). Importantly, we found that this repellency can even be induced without a completely wetted inverse opal: the middle column of Figure 4a shows a sample that is only wetted in the topmost layer by the ionic liquid [BMIM][NTf2] (as indicated by the presence of structural color). Yet, the water droplet slid off the surface without replacing the lubricant layer.

We complemented our wetting studies by measurements of the water sliding angle as a simple measure of the repellency properties22,25,36 (Figure 4b). Table 1 summarizes the measured sliding angles. The repellency properties are in complete agreement with the predictions of the wetting experiments of Figures 3 and 4a. Stable ionic liquid films that enable a facile sliding of the water droplets can be realized with the two more hydrophobic ILs as lubricants on the butylfunctionalized or mixed self-assembled monolayers.

To demonstrate a more universal applicability of these surface-chemistry/IL combinations, we prepared transparent, nanoporous coatings consisting of disordered silica nanoparticles,22 and then surface-functionalized the coating similarly to the inverse opal with an imidazole/butyl


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mixed self-assembled monolayer. These films do not show structural color, so we instead used an aqueous dye droplet to monitor repellency. As shown in Figure 4c, the ionic liquid-infused substrate was not stained by a Rhodamine-containing water solution, indicating a stable repellency and intact lubricant layer separating the surface from the contaminated water droplet.

Figure 4. Water-repellency properties of ionic-liquid infiltrated surface nanostructures. a) Time-lapse images of a water droplet on an inverse opal surface nanostructure with different surface functionalities infiltrated with different ILs. The wetting state of the IL and the water droplet can be followed by changes in structural color: with an imidazole functionalization (Imi), the ionic liquid is replaced by water, while a stable film can result from mixed self-assembled monolayers (Imi + But). The scale bar is 5mm. b) Time-lapse image of a water droplet show sliding under an angle of 2° on an ionic liquid ([HMIM][NTf2])-lubricated substrate with a mixed surface chemistry (Imi + Dod); c) Transparent ionic liquid-infused nanoporous coating based on silica nanoparticles enables efficient water repellency: A droplet of Rhodamine B colored water stains an untreated glass slide (left) but slides down without staining the substrate for the ionic liquid-infused sample (right).

Table 1. Sliding angle of water droplet on inverse opal substrates with different surface chemistries infiltrated with different ionic liquid lubricants.


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Lubricant

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Water sliding angle/° Imi

(Imi + But)

But

(Imi + Dod)

Dod

[EMIM][NTf2]

pinned

non-infused

non-infused

non-infused

non-infused

[BMIM][NTf2]

pinned

Ionic liquid-infused nanostructures as repellent surfaces

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