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May 21, 2015 - Effects of Water on Solvation Layers of Imidazolium-Type Room. Temperature Ionic Liquids on Silica and Mica. Kenichi Sakai,*. ,†,‡...
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Effects of Water on Solvation Layers of Imidazolium-Type Room Temperature Ionic Liquids on Silica and Mica Kenichi Sakai,*,†,‡ Kohei Okada,† Akihito Uka,† Takeshi Misono,‡ Takeshi Endo,†,‡ Shinya Sasaki,‡,§ Masahiko Abe,‡ and Hideki Sakai†,‡ †

Department of Pure and Applied Chemistry, Faculty of Science and Technology, and ‡Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan § Department of Mechanical Engineering, Faculty of Engineering, Tokyo University of Science, 6-3-1 Niijuku, Katsushika, Tokyo 125-8585, Japan S Supporting Information *

ABSTRACT: Effects of the addition of water on solvation layers of imidazolium-type room temperature ionic liquids (RT-ILs) have been studied through force curve measurements of atomic force microscopy (AFM). Two kinds of RT-ILs were employed in this study; one is a hydrophilic RT-IL (1-butyl-3-methylimidazolium tetrafluoroborate, BmimBF4), and the other is a hydrophobic one (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EmimTFSI). These RTILs form solvation layers on hydrophilic solid substances (i.e., silica and mica) in the absence of added water. The addition of water into BmimBF4 resulted in the disruption of the solvation layers and then the formation of an interfacial water phase on silica. In contrast, the formation of the interfacial water phase was not evidenced on mica because of the absence of hydrogen-bonding sites on the mica surface. Interestingly, the addition of water into EmimTFSI induced the formation of the interfacial water phase on the two solid surfaces. In the EmimTFSI system, importantly, significantly greater adhesion forces were observed on silica than on mica. This reflects the different formation mechanisms of the interfacial water phase on the two solid surfaces. We conclude that the hydrogen bonding is a key factor in determining whether water molecules can be adsorbed on the solid surfaces, but it is also necessary to take into account the hydrophilic/hydrophobic nature of the RTILs.

1. INTRODUCTION Characterizing physicochemical properties of solid/liquid interfaces has been an important subject not only in academia but also in industry. One of the particular interests in this research field lies in the adsorption or self-assembly of surfactants (or amphiphilic materials) that occurs in aqueous and nonaqueous media on various solid substances. In the past two decades, room temperature ionic liquids (RT-ILs) have attracted much attention as a new type of nonaqueous solvents to promote self-assembly of surfactants.1−3 The nature of solid/ RT-IL interfaces has also been characterized systematically with and without surfactants.4−12 The key finding is that in the absence of surfactants solvation layers consisting of ion pairs (and in some cases cations in the innermost solvation layer) are generally formed on hydrophilic surfaces (such as silica and mica).4−7 The soft-contact and amplitude-modulation atomic force microscopy (AFM) has demonstrated that a protic RT-IL (ethylammonium nitrate, EAN) forms an interfacial innermost layer on mica in a slightly disordered wormlike morphology, and an upper layer formed on the innermost layer gives less ordered and more undulating appearance.9 In the case of an aprotic RT-IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EmimTFSI), the imidazolium cations are adsorbed on mica in a more isolated fashion.9 This © 2015 American Chemical Society

difference is attributed to the hydrogen-bonding network of the protic RT-IL to promote self-assembly at the solid/liquid interface as well as in the bulk liquid.9 Because of the unique physicochemical properties of RT-ILs such as low volatility, nonflammability, and high ionic conductivity, RT-ILs are expected to be useful in various industrial applications. One of the possible application fields can be found in developments of electrochemical devices.13−15 In this field, it is necessarily important to understand phenomena that occur at electrode (solid)/solution interfaces. Here RT-ILs are more or less hygroscopic, and hence water dissolved in electrolyte solutions (i.e., RT-ILs) may have a significant impact on the electrochemical reactions. RT-ILs have also attracted attention as a new type of reaction solvents to prepare nano- or micro-ordered inorganic materials.16,17 A typical example is preparation of metal oxide particles such as silica or titania.17 Here the addition of water is essentially required to promote the hydrolysis and polycondensation of metal oxide precursors, and the morphologies and crystalline structures of the metal oxide particles prepared are strongly Received: March 31, 2015 Revised: May 20, 2015 Published: May 21, 2015 6085

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Figure 1. Inward force curves measured in BmimBF4 without added water. The solid substances employed here are (a) silica and (b) mica. system and filtered with a Millipore membrane filter (pore size 0.22 μm). 2.2. Solid Substrates. Flat silica plates used in this study were either a Q-Sense QCM-D sensor with a silica coating (QSX303) or a chemically oxidized silicon wafer. In the latter case, the silicon wafer purchased from Nilaco was immersed in a mixed solution of H2O:NH3:H2O2 = 5:1:1 (in volume) for 15 min at 80 °C, followed by a copious rinsing with deionized water to give a hydroxylated silica surface. Before each measurement these silica substrates were cleaned in an aqueous DCN90 detergent solution using an ultrasonication water bath, rinsed by deionized water thoroughly, cleaned in a UV/O3 cleaner for 20 min, and finally again rinsed by deionized water. Muscovite mica substrates were purchased from Nilaco and freshly cleaved before each experiment. Spherical silica particles (1.0 μm in diameter) were purchased from Polysciences and used as received without further cleaning. 2.3. Instrumentation. In situ force curve measurements were performed using a Seiko (Hitachi High-Technologies) SPI3800 AFM. V-shaped cantilevers with a silicon nitride tip (Olympus OMCLTR800PSA, the nominal spring constant = 0.15 N m−1) were used for all AFM experiments. These cantilevers were immersed in ethanol to remove organic contaminant and then cleaned in the UV/O3 cleaner for 10 min. The solid substrate (mica or silica) was assembled in the AFM instrument, and the RT-IL sample in the presence or absence of added water was injected into the AFM fluid cell. After equilibration, force curve measurements were performed at a scan rate of 0.1 Hz at ca. 25 °C (room temperature).

dependent on the preparation conditions including the water concentration.18 These examples indicate key effects of water dissolved in RT-ILs on the observed solid/RT-IL interfacial phenomena. The interfacial properties regarding the binary mixtures of RT-IL with water have been studied for the liquid/liquid19 and solid/liquid20−24 interfaces. The most relevant study to our current work has been carried out by Atkin et al.22 They demonstrated that the addition of water in the protic RT-IL (EAN) reduces both the number and resilience of EAN solvation layers originally formed on silica, leading to the instability of colloidal silica suspensions in the EAN/water mixtures. Again this finding indicates significant effects of water added to such RT-IL systems, and we believe that further studies are required in order to understand solid/RT-IL interfacial phenomena that occur in the presence of water. Hereafter, we focus on effects of the addition of water on the solvation layers formed on the two solid surfaces, i.e., silica and mica. These two solids are typically used as anionic and hydrophilic substrates, but a key difference in surface chemistry between the two substrates lies in a fact that hydrogen-bonding sites are present on silica (i.e., surface silanol groups) but not on mica. The RT-ILs employed in this study were aprotic 1butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and EmimTFSI. The former RT-IL is completely miscible with water in a whole range of concentrations whereas the latter one exhibits a limited miscibility with water.25 In other words, BmimBF4 is a hydrophilic RT-IL whereas EmimTFSI is a hydrophobic one. We expect, through this study, that it is possible to demonstrate the adsorption or condensation mechanism of water molecules from the binary solutions of RT-IL with water if this occurs onto the two solid surfaces.

3. RESULTS AND DISCUSSION 3.1. BmimBF4 (Hydrophilic RT-IL) + Water System. Figure 1 shows the inward force curves measured in BmimBF4 on (a) silica and (b) mica without added water. Before each measurement, the BmimBF4 sample was dried in the vacuum oven set at 80 °C under reduced pressure. The Karl Fischer titration measurements demonstrated that the water concentration remaining in the BmimBF4 sample was ca. 0.03 wt % under the experimental conditions. Oscillating solvation forces are clearly observed in Figure 1 on the two solid substances. Under the normal pressure applied from the cantilever, the thickness of the innermost layer (i.e., the layer nearest the solid surfaces) was measured as less than 0.5 nm, whereas that of the upper layers was estimated to be slightly larger than the thickness of the innermost layer. Here we note that the sizes of Bmim+ ion and BmimBF4 ion pair are 0.35 and 0.68 nm, respectively.8,27 When taking the finding reported for a wide variety of IL-solid systems into consideration,7 it seems likely that Bmim+ ions primarily form the innermost solvation layer on the two solid surfaces whereas the upper solvation layers are composed of BmimBF4 ion pairs. Our current force curve results are consistent with this general behavior.

2. EXPERIMENTAL SECTION 2.1. Materials. Both of the two aprotic RT-ILs (BmimBF4 and EmimTFSI) used in this study were purchased from Aldrich. On the basis of the information provided by the supplier, the purity of BmimBF4 is more than 97% and that of EmimTFSI is more than 98%. Before use, these samples were dried in a vacuum oven set at 80 °C for 8 h, and then the concentration of water remaining in the samples was determined using a Karl Fischer moisture titrator (Kyoto Electronics Manufacturing, MKC-610). We note that hydrolysis of BF4− is reported in the literature,26 but within the resolution of our 19F-NMR measurements (JEOL JNM-AL300), no significant peak suggesting the presence of any hydrolyzed products was detected at least within 1−2 h after sample preparation (corresponding to AFM measurement time) under the current experimental conditions. The resultant spectra are given in Supporting Information Figure S1. The water used in this study was deionized with a Barnstead NANO Pure Diamond UV 6086

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Figure 2. Inward force curves measured in BmimBF4 with added water. The solid substance employed here is silica and the water concentrations are (a) 1, (b) 7, and (c) 15 wt %.

occurs in these force curve measurements. At the water concentration of 1 wt %, we can still see oscillating solvation forces, but the force magnitude is much smaller than that observed in Figure 1a (without added water). Indeed, the number of detectable layers is also decreased when compared with Figure 1a. A further increase in the water concentration results in the disappearance of the oscillating solvation forces, and instead an attractive jump-to-contact appeared. This suggests that the BmimBF4 solvation layers are disrupted as a result of the addition of water and finally exchanged by a water phase localized on the silica surface. Here, under the resolution of our experiments, it is not clear whether the Bmim+ ions forming the innermost layer are completely exchanged by water molecules or not. We assume that the attractive interaction observed here results from two possibilities. That is, one is the van der Waals interaction,28 and the other is the attractive interaction caused by the interfacial energy between the surface layer (or phase) and the bulk liquid.29 The latter interaction has been observed, e.g., in hydrogen-bonding-induced “surface molecular macrocluster” formation of ethanol in a nonpolar liquid (cyclohexane) on silica, and detected when two facing surface layers are contacted with each other.29 Dispersion stability of colloidal silica suspension was also influenced by the addition of water. Figure 3 shows the visual appearance of colloidal silica particles dispersed in BmimBF4 with and without added water. These samples were prepared as follows. The silica particles (0.01 g) were mixed with the liquid samples (1 cm3) in a plastic vial, and then the mixtures were homogenized in an ultrasonication water bath for 30 min. The pictures shown in this figure were taken after 6 h from the preparation. We found that the dispersion stability of the waterfree sample was greater than that of the water-added samples. Clearly, this reflects the force curve data shown in Figures 1 and 2. That is, the oscillating solvation force observed in the waterfree system prevents flocculation of the silica particles, whereas the attractive interaction results in rapid flocculation in the

It is important to discuss significant differences in the force curve data obtained for the two solid substances. That is, (i) the number of detectable layers is greater on mica (at least six in this case) than on silica (at least four), and (ii) the magnitude of repulsive forces requiring the removal of the innermost Bmim+ layer is much larger on mica than on silica. Again, these results are consistent with the finding reported in the previous paper,4 where both the number of detectable layers and the force magnitude are suggested to be strongly dependent on the nature of the solid materials (such as surface charge density and surface roughness) as well as the physicochemical properties of RT-ILs (such as their protic−aprotic nature and molecular flexibility). As is well-known, mica is a layered crystalline aluminosilicate material with anionic basal planes. These negative sites are compensated by potassium ions in the solid state. Once the mica is immersed in BmimBF4, the potassium ions are probably exchanged by Bmim+ ions, although the high ionic strength in the RT-IL considerably screens the electrostatic interaction.9 Similarly, Bmim+ ions can be adsorbed on amorphous silica due to the presence of surface silanol groups. Here we hypothesize that the Bmim+ ions adsorbed are fastened more strongly on mica than on silica because of the following reasons: (i) the mica surface is atomically smooth, (ii) the anionic surface sites on mica are highly ordered, and (iii) the number density of the anionic surface sites is generally greater on mica than on silica. These lead to the formation of well-structured solvation layers on mica, and hence we see the differences in the force curve data observed for the two solid substances. The addition of water in the BmimBF4−silica system dramatically changed the force curve data, as shown in Figure 2. Here the concentration of added water was varied from 1 to 15 wt %. These concentrations correspond to the mole fractions of water in the range of 0.11−0.76. It should be recalled here that BmimBF4 can be miscible with water in a whole range of water concentrations, and no phase separation 6087

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between the mica and silica systems suggests that the formation of the interfacial water phase does take place on silica but not on mica in the range of water concentrations investigated. It seems likely that hydrogen bonding drives the adsorption of water molecules on the silica surface. In contrast, we assume that the absence of hydrogen-bonding sites on mica inhibits the adsorption of water molecules at least under the current experimental conditions. We will discuss these results later again with the force curve data obtained in the more hydrophobic EmimTFSI system. 3.2. EmimTFSI (Hydrophobic RT-IL) + Water System. As mentioned in the Introduction, EmimTFSI is a more hydrophobic RT-IL, and the solubility of water in EmimTFSI is very limited.25 The concentration of water saturated in EmimTFSI was estimated to be ca. 2.0 wt % on the basis of our Karl Fischer titration data. In the following force curve measurements, we always set the water concentrations below the solubility limit. Figure 5 shows the inward force curves measured on silica at various water concentrations. In the absence of added water (the remaining water concentration was measured as 3 × 10−4 wt % after the vacuum drying), we observed oscillating forces originated from the solvation layers of EmimTFSI ion pairs. Qualitatively, this result is consistent with that obtained in the BmimBF4−silica system and has already been discussed in our previous paper.12 The increased water concentration results in the disruption of the oscillating solvation forces and then in the appearance of attractive interaction. Again, these results are qualitatively similar to those obtained in the BmimBF4−silica system and suggest the formation of a water phase from the mixed EmimTFSI−water solutions. When compared with the BmimBF4 system, the EmimTFSI system gives longer jump-to-contact distance and hence greater attractive interaction. If the attractive interaction results from the interfacial energy between the surface water phase and the bulk liquid, the longer jump-to-contact distance

Figure 3. Visual appearance of colloidal silica suspensions in BmimBF4 with and without added water.

water-added system. We also performed Turbiscan (Formulaction MA2000) measurements in order to monitor the dispersion stability as a function of storage time (see Supporting Information Figure S2). The resultant data indicate that the destabilization occurs mainly through slow sedimentation (rather than flocculation) in the water-free system whereas rapid flocculation is clearly observed in the 7 and 15 wt % water-added systems. These experimental results strengthen our hypothesis that water molecules can be adsorbed onto silica from the binary solutions of BmimBF4 with water. A similar water-induced destabilization mechanism has been suggested in the silica particles/EAN22 and gold nanoparticles/imidazoliumtype RT-IL systems.30 Force curve measurements were also performed on mica in the presence of added water (1, 7, and 15 wt %). The results are shown in Figure 4. Interestingly, oscillating solvation forces are still observed at these water concentrations, although the number of detectable layers is gradually decreased with increasing water concentration. These results are significantly different from the results shown in Figure 2. The comparison

Figure 4. Inward force curves measured in BmimBF4 with added water. The solid substance employed here is mica and the water concentrations are (a) 1, (b) 7, and (c) 15 wt %. 6088

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Figure 5. Inward force curves measured in EmimTFSI with and without added water. The solid substance employed here is silica, and the water concentrations measured by the Karl Fischer method are (a) 3 × 10−4, (b) 0.64, (c) 1.3, and (d) 2.0 wt % (saturation level).

Figure 6. Inward force curves measured in EmimTFSI with and without added water. The solid substance employed here is mica, and the water concentrations measured by the Karl Fischer method are (a) 3 × 10−4, (b) 1.2, (c) 1.4, and (d) 2.0 wt % (saturation level).

observed in the EmimTFSI system suggests the increased thickness of the water phase when compared with that formed

in the BmimBF4 system. This may be attributed to the hydrophobic nature of EmimTFSI. 6089

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Figure 7. Outward force curves measured in EmimTFSI with added water. The solid substances employed here are (a) silica and (b) mica, and the water concentration was estimated as 2.0 wt % (saturation level).

an interesting report focusing on the formation of interfacial water phase on silica from cyclohexane−water binary mixtures.31 In this study, the interfacial energy was also estimated on the basis of adhesion force data. With the combination of sum frequency generation vibrational spectroscopy data, the authors in this reference concluded that the interfacial water molecules formed an ice-like ordered structure on silica when water was added at its saturation concentration in cyclohexane, whereas the structure became less ordered when the water concentration exceeded the saturation concentration (i.e., phase separation). The interfacial energy (i.e., the adhesion force) was observed to be increased when the water molecules are highly ordered on silica and significantly greater than the bulk water/cyclohexane interfacial tension or energy.31 In our case, the observed difference in the outward force curve data results from the solid material, and hence the hydrogen bonding of water molecules with surface silanol groups plays a key role in forming the interfacial water phase with the greater adhesion force and interfacial energy. It seems likely that this is caused by the ordered structure of water molecules adsorbed on silica, when taking into account the finding reported for the cyclohexane−water system.31

We also performed dispersion stability experiments using the colloidal silica particles in a manner similar to the BmimBF4 system. The stability was greater for the water-free sample than for the added water system, as shown in Supporting Information Figure S3. We found significant differences in the force curve results between the two solid systems. The inward force curves measured on mica are shown in Figure 6. In the absence of added water (Figure 6a), oscillating solvation forces are detected. The gradual change in the force curve observed below the apparent separation of ca. 0.5 nm suggests the normal resilience of the innermost solvation layer, caused by the strong attractive interaction of Emim+ ions with the mica surface. Surprisingly, the addition of water results in the attractive jump-to-contact, suggesting the formation of a water phase on mica from the mixed EmimTFSI−water solutions. This was not expected because of the absence of hydrogenbonding sites on mica. Here it should be recalled that BmimBF4 is a hydrophilic RT-IL whereas EmimTFSI is a hydrophobic one. Hence, we suggest a possibility that water molecules dissolved in EmimTFSI are “expelled” from the liquid phase to the hydrophilic mica surface and form the interfacial water phase. We also suggest an additional possibility that if the strongly adsorbed innermost Emim+ layer is able to remain completely or partially on the mica surface even in the presence of water, the Emim+ ions assist the formation of the interfacial water phase. Finally, we analyzed outward force curves measured on the two solid surfaces. This is based on a hypothesis that, if the adsorption mechanisms of water on the two solid surfaces are different, the nature of the interfacial water phases must be different between the two solid systems. Figure 7 shows typical outward force curve data obtained for the EmimTFSI−silica and EmimTFSI−mica systems in the presence of 2.0 wt % added water. As is seen in this figure (and repeated experimental results not shown here; at least 10 repeated experiments were performed under each condition), a much greater pull-off or adhesion force was observed on silica (5.9 ± 0.27 nN) than on mica (1.6 ± 0.08 nN). It is known that the adhesion force (Fadhesion) is relevant to the interfacial energy (γ), according to the equation28,31 Fadhesion = 4πγR

4. CONCLUSION We have presented effects of the addition of water on the solvation layers of RT-ILs formed on the two hydrophilic solid surfaces (i.e., silica and mica). The addition of water into the hydrophilic RT-IL (BmimBF4) resulted in the disruption of the solvation layers and then the formation of the interfacial water phase on silica. In contrast, the formation of the interfacial water phase was not evidenced on mica because of the absence of hydrogen-bonding sites on the solid surface. Interestingly, in the hydrophobic RT-IL (EmimTFSI), the addition of water induced the formation of the interfacial water phase on the two solid surfaces. In the EmimTFSI system, however, significantly greater adhesion forces were observed on silica than on mica, being reflective of the different formation mechanisms of the interfacial water phase on the two solid surfaces. The conclusion of this study is therefore that the hydrogen bonding is a key factor in determining whether water molecules can be adsorbed on the solid surfaces, but it is also necessary to take into account the hydrophilic/hydrophobic nature of the RTILs. We anticipate that the finding reported in this paper is useful when understanding solid/RT-IL interfacial phenomena that are seen in various applications such as lubrications, electrochemical devices, and inorganic material fabrications.

(1)

where R is assumed to be the radius of cantilever tip. This equation suggests that the greater adhesion force on silica than on mica results from the greater interfacial energy between the interfacial water phase formed on silica (rather than on mica) and the bulk liquid, under an assumption that the R values are almost the same with each other.32 Here we note that there is 6090

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(18) Okazawa, A.; Kinoshita, Y.; Misono, T.; Endo, T.; Sakai, K.; Abe, M.; Sakai, H. Abstracts of the 65th Division Meeting on Colloid and Surface Chemistry, The Chemical Society of Japan, 2014. (19) Iwahashi, T.; Sakai, Y.; Kim, D.; Ishiyama, T.; Morita, A.; Ouchi, Y. Nonlinear Vibrational Spectroscopic Studies on Water/Ionic Liquid ([Cnmim]TFSA: n = 4, 8) Interfaces. Faraday Discuss. 2012, 154, 289−301. (20) Horn, R. G.; Evans, D. F.; Ninham, B. W. Double-Layer and Solvation Forces Measured in a Molten Salt and Its Mixtures with Water. J. Phys. Chem. 1988, 92, 3531−3537. (21) Fitchett, B. D.; Conboy, J. C. Structure of the RoomTemperature Ionic Liquid/SiO2 Interface Studied by Sum-Frequency Vibrational Spectroscopy. J. Phys. Chem. B 2004, 108, 20255−20262. (22) Smith, J. A.; Werzer, O.; Webber, G. B.; Warr, G. G.; Atkin, R. Surprising Particle Stability and Rapid Sedimentation Rates in an Ionic Liquid. J. Phys. Chem. Lett. 2010, 1, 64−68. (23) Espinosa-Marzal, R. M.; Arcifa, A.; Rossi, A.; Spencer, N. D. Microslips to “Avalanches” in Confined, Molecular Layers of Ionic Liquids. J. Phys. Chem. Lett. 2014, 5, 179−184. (24) Espinosa-Marzal, R. M.; Arcifa, A.; Rossi, A.; Spencer, N. D. Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and Lubricity in the Presence of Water. J. Phys. Chem. C 2014, 118, 6491− 6503. (25) Heintz, A.; Lehmann, J. K.; Wertz, C.; Jacquemin, J. Thermodynamic Properties of Mixtures Containing Ionic Liquids. 4. LLE of Binary Mixtures of [C2MIM][NTf2] with Propan-1-ol, Butan1-ol, and Pentan-1-ol and [C4MIM][NTf2] with Cyclohexanol and 1,2-Hexanediol Including Studies of the Influence of Small Amounts of Water. J. Chem. Eng. Data 2005, 50, 956−960. (26) Freire, M. G.; Neves, C. M. S. S.; Marrucho, I. M.; Coutinho, J. A. P.; Fernandes, A. M. Hydrolysis of Tetrafluoroborate and Hexafluorophosphate Counter Ions in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2010, 114, 3744−3749. (27) Li, H.; Endres, F.; Atkin, R. Effect of Alkyl Chain Length and Anion Species on the Interfacial Nanostructure of Ionic Liquids at the Au(111)-Ionic Liquid Interface as a Function of Potential. Phys. Chem. Chem. Phys. 2013, 15, 14624−14633. (28) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press: New York, 2011 (Japanese version translated by Ohshima, H. Asakura Publishing: Tokyo, 2013). (29) Mizukami, M.; Moteki, M.; Kurihara, K. Hydrogen-Bonded Macrocluster Formation of Ethanol on Silica Surfaces in Cyclohexane. J. Am. Chem. Soc. 2002, 124, 12889−12897. (30) Vanecht, E.; Binnemans, K.; Patskovsky, S.; Meunier, M.; Seo, J. W.; Stappers, L.; Fransaer, J. Stability of Sputter-Deposited Gold Nanoparticles in Imidazolium Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 5662−5671. (31) Mizukami, M.; Kobayashi, A.; Kurihara, K. Structuring of Interfacial Water on Silica Surface in Cyclohexane Studied by Surface Forces Measurement and Sum Frequency Generation Vibrational Spectroscopy. Langmuir 2012, 28, 14284−14290. (32) The nominal R value of our cantilever tip is 15 nm; however, it is required to measure exact R value of each cantilever tip in order to estimate the exact γ value from eq 1. On the basis of this difficulty, we focus on our adhesion and interfacial energy data only from the qualitative viewpoint here.

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: 19F NMR spectra of BmimBF4 at various water concentrations; Figure S2: Turbiscan results obtained for the colloidal silica suspensions in BmimBF4 with and without added water; Figure S3: visual appearance of colloidal silica suspensions in EmimTFSI with and without added water. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01184.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.S.). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b01184 Langmuir 2015, 31, 6085−6091