Long-Range Ordering of Ionic Liquid Fluid Films - Langmuir (ACS

May 3, 2016 - Interestingly, the ionic liquids do not form solids upon ordering but do exhibit strong preferential alignments of molecules that persis...
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Long-Range Ordering of Ionic Liquid Fluid Films Radhika S Anaredy, and Scott K. Shaw Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00304 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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TITLE:

Long-Range Ordering of Ionic Liquid Fluid Films

AUTHORS:

1) Radhika S Anaredy, University of Iowa, [email protected] 2) Scott K Shaw, University of Iowa, [email protected]

ABSTRACT: We report the transformation of ionic liquid films from isotropic bulk to a fluid-ordered state over micron length-scales. Data from infrared and non-linear spectroscopy measurements show clear transitions that, for varying ionic liquids, occur over time frames of ten minutes to two hours. These maturation times depend linearly on the chosen ionic liquids’ bulk viscosities. Interestingly, the ionic liquids do not form solids upon ordering, but do exhibit strong preferential alignments of molecules that persist throughout the fluid films’ thicknesses. Our measurements characterize this ordering process; and show that it is largely insensitive to substrate surface chemistry or small amounts of absorbed water. Additional experiments show the transition is observed across several of the most common ionic liquid cations; and that the process is completely reversible. The driving force for this organization is attributed to electrostatic and steric forces combined with a slow shearing of the viscous ionic liquid. These interactions work together to slowly bring the molecules within the film to a preferred, global orientation. The physical length and time scale of this transformation is unexpected and intriguing, and invites additional studies to develop understanding and control of ionic liquid materials’ behavior, particularly near surfaces, to benefit their uses in lubrication, capacitive energy storage, and heterogeneous catalysis. INTRODUCTION: Room temperature ionic liquids (ILs) are defined as salts with melting points below 100 °C. They display low vapor pressures, thermal and electrochemical stability, and low flammability.1 These properties have led to ILs being heralded as, task-specific solvents2 useful in extreme environments3, 4, 5, 6 and as tunable reaction matrices7, 8 in which otherwise intractable chemical problems might be addressed. The study of bulk IL structure via experiment and calculation has produced a rich literature. X-ray and neutron diffraction studies have been particularly productive and results indicate nanoscale structural heterogeneities throughout the bulk phase. These lamellar or micelle-like nanoscale domains are embedded in the polar network formed due the electrostatic or hydrophobic interactions between the IL cations and anions. The size of these domains depends on various factors such as alkyl chain length, cation symmetry and temperature.9, 10 In case of triphilic ILs, an additional fluorous domain contributes to the molecular interactions.11, 12, 13 Margulis et al showed formation of filamentous structure made from the charged groups that hydrogen bond amongst themselves. The alkyl and fluorinated tails associated with the charged head groups decorate the surface of the filament with the two types of tails segregating into alternate strips thereby leading to formation of polar charged, nonpolar, and fluorous domains within the IL.11 The balance of the electrostatic interactions between the charged groups and the van der Waals forces between the alkyl chains contribute to the properties and structure of the bulk IL.9 In many applications, ILs are employed at or near solid surfaces. It is known that molecules near surfaces behave very differently from their surrounding bulk phases, often exhibiting unique properties within an interfacial region.14, 15 These surface-induced effects have been used in many positive ways such as capacitive touch-screens, anti-fouling surfaces, and in heterogeneous catalysis.16 Molecular, or van der Waals fluids respond to the presence of a solid surface with a nearly instantaneous re-organization to minimize the surface free energy.17, 18, 19 By comparison, ILs are reported to have relatively slow re-organization dynamics (on timescales of tens of seconds or even minutes) as demonstrated by significant hysteresis in cyclic voltammetry measurements, capacitive measurements, spectroscopic measurements, and simulation20, 21, 22, 23, 24, 25 of the same. These behaviors can be influenced by multiple factors, including intermolecular forces and Newtonian forces such as shearing.17, 18, 19 Effects Manuscript Text ACS Paragon Plus Environment

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of shear on liquid films have been studied directly by the surface force apparatus, in which a fluid film is pressed between two smooth, parallel surfaces which are slid past each other. Results from molecular fluid studies show forces between the solid surfaces alternate between attraction and repulsion with a periodicity equivalent to molecular radii.26 These results are interpreted to arise from ordering of the thin fluid film which ultimately manifest (at small plate separations and increased pressures) as reversible phase transitions from liquid-like to solid-like behavior.26, 27 In these studies, asymmetric chain-like molecules have been shown to become oriented under shear.28 The viscosity of water and tetradecane films (as thin as 50 Å) between two mica surfaces was determined using the same apparatus and found to be 10% within the bulk viscosity of the fluids. This shows that molecular ordering at the interface does not necessarily have significant effect on viscosity.29 Ionic liquid films over thicknesses of 0.23-5 nm have been shown to form layers that easily slide past each other, exhibiting excellent lubricating properties attributed to irregular ion shapes and the inherent ionic charges.30 Additional studies using emersion31 have shown Newtonian fluid films experience shearing forces that are directly proportional to fluid viscosity and that higher shearing forces are inversely proportional to film thickness. There have also been a large number of studies showing ordering of IL molecules in naoconfinement. Irreversible long range ordering of 1-Hexyl-3-methylimidazolium ethylsulfate extending up to ~50 nm was shown to occur under nanoconfinement and shear.32 In very recent work, over-crowded layers of ions were observed at an electrode surface, accompanied by a significant increase in differential capacitance.33 At the air-solution interface, Langmuir films of 1methyl-3-octadecylimidazolium bis(trifluoromethylsufonyl)imide have been shown to reversibly form crystalline structures upon compression/relaxation. This observation is supported by molecular dynamic simulations that show formation of checkered board pattern by the charged groups at the water surface with the alkyl chain pointing towards air.34 Dutta et al studied the interfacial layer of spherical molecule fluid and polymeric fluid on silicon surface under shear. Spherical molecule fluid shows decrease in the fraction of interfacial layered region under shear as opposed to the polymeric fluid which shows formation of layers due to shear induced disentanglement. Studies of ILs in confinement have also been carried out, and show that there is often strong layering near surfaces,35 and that even strong (simulated) shearing does not necessarily disrupt the ordering of the nanoscale IL domains seen in the bulk.36 Many studies examine relatively thin interfacial layers of fluids, confined in volumes that are near to solid surfaces. A few reports have suggested longer-range ordering may be possible at >10 nm distances from a solid surface.37 In fact, using various instrumental methods, the presence of IL phase ordering up to 1-2 nm,38, 39 1-10 nm,40, 41, 42, 43, 44 or as far as 50-60 nm32, 45 from a solid surface has been reported. In addition, molecular dynamics simulations have predicted the presence of ‘sponge-like’ ordering structures,47 that permeate the bulk phase of ILs, and unexpected aggregation behavior of bulk-phase imidazolium based ILs has been recently reported by Sarkar et al.48 The chemical models that guide the community’s understanding, and definition, of the interfacial region of ILs are evolving even as the reported thicknesses and magnitude of the interfacial region is diverging. Our most recent results add a new and intriguing layer of intricacy to this field. Here we report reversibly formed, highly ordered layers of ILs which extend over 1000 nm from any surface (solid or vapor). Using infrared reflection absorption spectroscopy, ellipsometry, and second harmonic generation, we track the properties and behavior of these IL films as a function of thickness and time. Our data confirm a long-ranging molecular organization throughout the films’ entire thickness. We attribute the formation of these extended structures to a combined shearing effect coupled with intermolecular interactions. These forces act on the IL film as it is supported on a vertically aligned substrate, causing a preferred orientation to be adopted over the course of several minutes to hours. The study of this slow transition phenomenon could be helpful in several areas. Many open questions (nicely listed in a recent paper by Rotenberg et. al)23 remain regarding the structural transitions that would advance ILs’ utility in applications as lubricants, capacitive energy storage, and general solvents. These include effects of 1) impurities (i.e. water and other solvents), 2) geometric registry with substrate, and 3) Manuscript Text ACS Paragon Plus Environment

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temperature. This work begins to address the first two questions, and we hope our results will motivate further investigations of ILs’ extended interfacial regions and long-range ordering. We predict that ILs of varying functionality will allow tuning of these extended structures, which may lead to directed selfassembly into micro- and mesoscale materials. Such self-assembly is a major target of fundamental chemical research and materials development. EXPERIMENTAL SECTION Materials: Polycrystalline silver substrates were cut from a 14 mm diameter rod (99.999%, ESPI Metals) and polished to a mirror finish using 600 and 1000 grit sand papers (3M), followed by 9.5, 3.0, 1.0, and 0.3 µm aluminum oxide powder and appropriate felt or synthetic polishing pads (Buehler). Chromic acid etching of silver was done using 4.8 M CrO2 (99.9%, Aldrich), and 0.6 M HCl (ACS grade, BDH), prepared with ultrapure water (18.2 MΩ cm-1 with TOC ≤ 4 ppb) generated by a Milli-Q UV Plus System (Millipore Corp) and H2SO4 (ACS grade, BDH), HClO4 (70%, Sigma) and NH4OH (28-30%, BDH) as received. 5mM solution of 11-Mercapto-1-undecanol (Aldrich) was prepared in ethanol (200 Proof, Pharmco-Aaper), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 99% (EMIM TFSI), 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 99% (BMIM TFSI), 1-hexyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, 99% (HMIM TFSI), 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide, 99% (OMIM TFSI), 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 99% (P14 TFSI) 1-butylpyridinium bis(trifluoromethylsulfonyl)imide, 99% (BPy TFSI) (Iolitec, USA), Butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, 99% (N1114 TFSI) 1-Butyl-3-methylimidazolium thiocyanate, 95% (BMIM SCN) (Sigma-Aldrich) were purchased and placed under vacuum (> 24 hours at 50 C°) to remove water and other volatile impurities before measurements were made. Once removed from vacuum, the ILs were stored under dry conditions in a glove box (Genesis, Vacuum Atmospheres Company) with water and O2 levels of 6) of dispensing, resting, and retracting fluid droplets. ImageJ software was used to determine the contact angle for each surface/fluid combination. Dynamic Wetting: The substrate is mounted to a brass shaft and connected to a gearhead (Micromo) and variable voltage 12-volt DC motor (Micromo) which allows control of the rotational velocity of the substrate. For this work, rotational velocity of 60 µm/s is used consistently. The velocity is measured at the perimeter of the 14 mm diameter substrate, equaling approximately 0.1 r.p.m. This motor/substrate assembly is inserted into a custom, air-tight, PTFE cell designed to maintain a controlled, gas-phase (dry N2) environment for the duration of experiments. The cell’s nitrogen atmosphere is maintained at a slight positive pressure (ca. 3-4 psi) to ensure no water vapor or atmospheric gases enter the cell. The cell’s two, 25 mm diameter, CaF2 windows (Casix, U.K.) that flank the substrate allow for direct spectroscopic Manuscript Text ACS Paragon Plus Environment

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probing of the substrate and wetting films. These windows are fitted with gas tight seals to maintain environmental control. To introduce fluid to the cell, a glass capillary (1 mm inner diameter) filled with IL is first brought near to the face of the substrate. A small drop of liquid ( 17 hours in the case of BMIM TFSI to a final thickness of 430 nm (see Figure S7 and similar traces observed in ellipsometry). We separate this thinning time from the reorganization process by virtue of the order of magnitude longer period required for the film to reach a steady state thickness. We also note that the absorption intensity of the thinned films at ca. 17 hours is reduced due to the thinner films, but that the absorption profile matches that of ‘mature’ films taken at earlier times (see Figure S7). Clearly, the IL film remains fluid throughout the time required for the re-orientation transition, and throughout our experimental timeframe. We have proposed that the ordering of the IL film could result at least in part because of a shearing force. Such shearing action has been reported to create liquid crystalline phases in ILs.60 Microphase segregation has also been shown to act as the driving force for liquid crystalline phase formation in ILs with longer alkyl chains.61, 62 In ILs with shorter alkyl chain, the electrostatic interactions between the charged groups overcome the van der Waals interaction between the alkyl chains. Hence, the alkyl chains aggregate to form domains. With longer alkyl chains, stronger van der Waals forces allow the chains to align to form IL crystals.62 In this work, we note that the ordering takes place regardless of the cation structure or alkyl chain lengths, albeit at longer or shorter times respectively. After observing these ordering effects, we tested several factors that might control the phenomena. These results are described here. Water is commonly considered an impurity in IL, and its presence and concentration is a significant concern for our work. Previous studies have shown how water can affect the interfacial structure of ILs. Adsorption of water on the mica surface is shown to cause layering of IL at the surface which extends to >17 nm. Water is thought to facilitate the removal of K+ ions from the mica surface rendering it negatively charged. Cations occupy the charged sites initiating an ordered structure of cations and anions.63 The influence of water was also studied on imidazolium based ILs using sum frequency generation spectroscopy. The cation ring is shown to orient parallel to the surface under relatively dry conditions, to rotate towards surface normal as water content increases.64 In light of these Manuscript Text ACS Paragon Plus Environment

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and similar effects reported for water, we also carried out measurements on IL films that were exposed to a humid atmosphere, 35-40% relative humidity, at room temperature (25 °C). This experiment serves two purposes: it alters the chemical properties of the liquid-vapor interface, and it tests for possible effects of water absorption into the film. The data from these measurements show that the films undergo an identical reorientation processes regardless of humidity or water content. We conclude that neither the vapor phase water nor water impurities significantly affect this process (see Figure S8). Karl Fischer titration data to quantify water content in all of our IL samples is provided in the supporting information Table S9. To see whether the reorientation of IL films is a more general phenomena, we performed additional measurements on a series of TFSI based ILs with cations listed in Table 1. Each of these ILs shows similar behavior in that the four previously identified vibrational modes of TFSI anion undergo a distinct transition (Figure 3), albeit with varying amounts of time required for the films to complete their reorientation. Within the error of our measurements, the maturation times across our selected series of ILs trend linearly with the IL viscosity as is shown in Figure 4. Previous studies have shown that IL organization at a surface is influenced by the surface material.44, 63, 65 Amorphous carbon facilitates parallel orientation of the imidazolium ring with respect to the surface due to π-π stacking44 whereas mica surface being negatively charged shows the adsorption of cation molecules on the surface charge sites.66 X-ray studies with applied voltages on H-terminated silicon substrates showed crowding of anions at higher voltages at the surface33 as opposed to alternate layers of cations and anions on graphene surfaces.24 Hence, we also considered the possibility that the substrate may play a role in templating the ordering of the IL films. To test this, we prepared additional IL films on gold substrates and on –OH terminated self-assembled monolayer (SAM) modified silver, as shown in Figure S2b and S2c. These surfaces produced nearly identical film maturation processes as seen on bare silver, but maturation times do differ significantly between substrates. Given the similar results of IL ordering, but the different surface chemistry of bare Ag, bare Au, and the –OH terminated SAM layer, we rule out specific interactions of the substrate as a controlling factor in the reorientation process. Finally, we extended our studies to include ILs with anions other than TFSI. As expected, measurements carried out on BMIM SCN give significantly different IR profiles, and importantly, they do not show any evidence of reorganization as a function of time (Figure S10). We propose a few possible reason for this change in behavior. The TFSI anion is much larger than SCN and the charge on TFSI is more delocalized. Hence, interactions between TFSI and the cation may be weaker which allows a more facile reorientation under the influence of shearing and other intermolecular forces of the maturing IL film. The smaller SCN anion with more localized charge interacts relatively strongly with the cation. The shearing force may not be strong enough to overcome this interaction. Also, differential scanning calorimetry studies show that BMIM SCN does not crystallize upon cooling, but rather forms a glass.67, 68 This is opposed to BMIM TFSI which crystallizes completely at -44 °C. This indicates that BMIM TFSI may be more likely to form a highly ordered phase than BMIM SCN.35 Based on this, we propose that future studies including IL’s with clear crystallization temperatures may be promising candidates to show similar, highly organized phases, similar to what we have reported here. CONCLUSION A series of IL films supported on three different solid surfaces were examined using spectroscopic methods and found to form long-ranging, liquid-ordered structures over periods of time ca. 30 minutes to 120 minutes. This is evidenced by characteristic changes in IRRAS absorption profiles and SHG measurements. The ordering is found to occur throughout the film to thicknesses of ca. 2 µm, and appears to be independent of substrate material, cation structure, overlying vapor phase, and the presence of water impurities. The time required for the films to mature varies substantially as a linear function of the fluids’ bulk viscosities. We propose that the relatively high viscosity of IL slows the rate of reorientation, making the adoption of the locally preferred orientation significantly slower than what is seen for surface induced Manuscript Text ACS Paragon Plus Environment

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(re)organizations of molecular liquids.17, 18, 19 While it is widely accepted that some degree of molecular ordering and phase transition occurs in fluids near surfaces and in fluids under shear, observation of an organized fluid layer extending some microns from a solid surface is, to our knowledge, unprecedented until now. We ascribe the long-range ordering achieved here to a combination of a weak shear force induced by the film as it flows under the influence of gravity, and the electrostatic interactions of the ions as they maximize the separation of like charges. Our results add significant new information to the understanding of IL interfaces, molecular orientation in materials, and IL materials’ behaviors. These data encourage additional studies to find if long-range ordering may be observed in a broader range of ILs as a possible avenue for forming regular 3-D molecular networks. Supporting information The Supporting Information is available free of charge on the ACS Publications website Figure S1: schematic diagram of the wetting process, Figure S2: IRRAS data showing maturation of BMIM TFSI film on three different surfaces viz. bare silver, gold and 11-MUD modified silver, Figure S3: ellipsometry data for three different time points during maturation of the film, Table S4: all the IR vibrational frequencies with corresponding modes from 100 cm-1 to 1600 cm-1, Figure S5 and S6: peak fitting data for BMIM TFSI on bare silver, Figure S7: IRRAS spectra of BMIM TFSI on bare silver 60 minutes and 17 hours after stopping rotation respectively, Figure S8: IRRAS spectra of BMIM TFSI film on bare silver under ambient conditions, Table S9 : water content of all the seven ILs studied in this work, Figure S10: IRRAS data acquired for BMIM SCN. ACKNOWLEDGEMENTS: The authors thank Prof. Cheatum and his student, Mr. Jon Humston for use of equipment and technical expertise to acquire the SHG data. The authors also thank Dr. James Hilfiker and Dr. Nina Hong at J.A. Wollam for their helpful discussions and assistance. Funding for this research was provided by the Iowa Energy Center (OG-15-002), the ACS-PRF (55279-DNI5), and the University of Iowa. References: 1. Johnson, K. E. What's an ionic liquid? Electrochem. Soc. Interface 2007, 16 (1), 38-41. 2. Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. CO2 Capture by a Task-Specific Ionic Liquid. Journal of the American Chemical Society 2002, 124 (6), 926-927. 3. Suzuki, A.; Shinka, Y.; Masuko, M. Tribological Characteristics of Imidazolium-based Room Temperature Ionic Liquids Under High Vacuum. Tribol Lett 2007, 27 (3), 307-313. 4. Jin, C.-M.; Ye, C.; Phillips, B. S.; Zabinski, J. S.; Liu, X.; Liu, W.; Shreeve, J. n. M. Polyethylene glycol functionalized dicationic ionic liquids with alkyl or polyfluoroalkyl substituents as high temperature lubricants. Journal of Materials Chemistry 2006, 16 (16), 1529-1535. 5. Phillips, B. S.; John, G.; Zabinski, J. S. Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mössbauer spectroscopy. Tribol Lett 2007, 26 (2), 85-91. 6. Jiménez, A. E.; Bermúdez, M. D. Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 2: Friction, Wear and Surface Interactions at High Temperature. Tribol Lett 2010, 37 (2), 431-443. 7. Clark, K. D.; Nacham, O.; Yu, H.; Li, T.; Yamsek, M. M.; Ronning, D. R.; Anderson, J. L. Extraction of DNA by Magnetic Ionic Liquids: Tunable Solvents for Rapid and Selective DNA Analysis. Analytical Chemistry 2015, 87 (3), 1552-1559. 8. Kwak, K.; Kumar, S. S.; Pyo, K.; Lee, D. Ionic Liquid of a Gold Nanocluster: A Versatile Matrix for Electrochemical Biosensors. ACS Nano 2014, 8 (1), 671-679.

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9. Zheng, W.; Mohammed, A.; Hines, L. G.; Xiao, D.; Martinez, O. J.; Bartsch, R. A.; Simon, S. L.; Russina, O.; Triolo, A.; Quitevis, E. L. Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids. The Journal of Physical Chemistry B 2011, 115 (20), 6572-6584. 10. Triolo, A.; Russina, O.; Bleif, H.-J.; Di Cola, E. Nanoscale Segregation in Room Temperature Ionic Liquids. The Journal of Physical Chemistry B 2007, 111 (18), 4641-4644. 11. Hettige, J. J.; Araque, J. C.; Margulis, C. J. Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen-Bonding Ionic Liquids. The Journal of Physical Chemistry B 2014, 118 (44), 1270612716. 12. Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R. Mesoscopic Structural Heterogeneities in RoomTemperature Ionic Liquids. The Journal of Physical Chemistry Letters 2012, 3 (1), 27-33. 13. Shen, Y.; Kennedy, D. F.; Greaves, T. L.; Weerawardena, A.; Mulder, R. J.; Kirby, N.; Song, G.; Drummond, C. J. Protic ionic liquids with fluorous anions: physicochemical properties and self-assembly nanostructure. Physical Chemistry Chemical Physics 2012, 14 (22), 7981-7992. 14. Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; 3rd ed.; Wiley-VCH: Weinheim, Germany, 2013. p 461. 15. Yu, C. J.; Richter, A. G.; Datta, A.; Durbin, M. K.; Dutta, P. Observation of Molecular Layering in Thin Liquid Films Using X-Ray Reflectivity. Physical Review Letters 1999, 82 (11), 2326-2329. 16. Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley1994. p 667 pp. 17. Israelachvili, J. N. Intermolecular and Surface Forces; McGraw-Hill Publishing Co. Japan, Ltd.1991. p 291 pp. 18. Lipkowski, J.; Ross, P. N.; Editors. Structure of Electrified Interfaces; VCH1993. p 406 pp. 19. Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces, Third Edition; Wiley-VCH2013. p 495 pp. 20. Druschler, M.; Borisenko, N.; Wallauer, J.; Winter, C.; Huber, B.; Endres, F.; Roling, B. New insights into the interface between a single-crystalline metal electrode and an extremely pure ionic liquid: slow interfacial processes and the influence of temperature on interfacial dynamics. Phys Chem Chem Phys 2012, 14 (15), 5090-5099. 21. Druschler, M.; Huber, B.; Roling, B. On Capacitive Processes at the Interface between 1-Ethyl-3methylimidazolium tris(pentafluoroethyl)-trifluorophosphate and Au(111). J Phys Chem C 2011, 115 (14), 6802-6808. 22. Lockett, V.; Sedev, R.; Ralston, J.; Horne, M.; Rodopoulos, T. Differential capacitance of the electrical double layer in imidazolium-based ionic liquids: Influence of potential, cation size, and temperature. J Phys Chem C 2008, 112 (19), 7486-7495. 23. Rotenberg, B.; Salanne, M. Structural Transitions at Ionic Liquid Interfaces. The Journal of Physical Chemistry Letters 2015, 6 (24), 4978-4985. 24. Uysal, A.; Zhou, H.; Feng, G.; Lee, S. S.; Li, S.; Fenter, P.; Cummings, P. T.; Fulvio, P. F.; Dai, S.; McDonough, J. K.; Gogotsi, Y. Structural Origins of Potential Dependent Hysteresis at the Electrified Graphene/Ionic Liquid Interface. J Phys Chem C 2014, 118 (1), 569-574. 25. Limmer, D. T. Interfacial Ordering and Accompanying Divergent Capacitance at Ionic LiquidMetal Interfaces. Physical Review Letters 2015, 115 (25), 256102. 26. Van Alsten, J.; Granick, S. Molecular Tribometry of Ultrathin Liquid Films. Physical Review Letters 1988, 61 (22), 2570-2573. 27. Akbulut, M.; Chen, N.; Maeda, N.; Israelachvili, J.; Grunewald, T.; Helm, C. A. Crystallization in Thin Liquid Films Induced by Shear. The Journal of Physical Chemistry B 2005, 109 (25), 12509-12514. 28. Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N.; Homola, A. M. Liquid to solidlike transitions of molecularly thin films under shear. J. Chem. Phys. 1990, 93 (3), 1895-906. 29. Israelachvili, J. N. Measurement of the viscosity of liquids in very thin films. Journal of Colloid and Interface Science 1986, 110 (1), 263-271. Manuscript Text ACS Paragon Plus Environment

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30. Perkin, S.; Albrecht, T.; Klein, J. Layering and shear properties of an ionic liquid, 1-ethyl-3methylimidazolium ethylsulfate, confined to nano-films between mica surfaces. Phys Chem Chem Phys 2010, 12 (6), 1243-1247. 31. Tiani, D. J.; Pemberton, J. E. Emersion of 11-mereapto-1-undecanol-modified Ag substrates from aqueous and nonaqueous solvents: The effect of emersion velocity on emersed solvent layer thickness. Langmuir 2003, 19 (16), 6422-6429. 32. Jurado, L. A.; Kim, H.; Arcifa, A.; Rossi, A.; Leal, C.; Spencer, N. D.; Espinosa-Marzal, R. M. Irreversible structural change of a dry ionic liquid under nanoconfinement. Physical Chemistry Chemical Physics 2015, 17 (20), 13613-13624. 33. Chu, M.; Miller, M.; Dutta, P. Crowding and Anomalous Capacitance at an Electrode–Ionic Liquid Interface Observed Using Operando X-ray Scattering. ACS Central Science 2016, 2 (3), 175-180. 34. Filipe, E. J. M.; Morgado, P.; Teixeira, M.; Shimizu, K.; Bonatout, N.; Goldmann, M.; Canongia Lopes, J. N. Crystalline-like structures and multilayering in Langmuir films of ionic liquids at the air-water interface. Chemical Communications 2016. 35. Ueno, K.; Kasuya, M.; Watanabe, M.; Mizukami, M.; Kurihara, K. Resonance shear measurement of nanoconfined ionic liquids. Physical Chemistry Chemical Physics 2010, 12 (16), 4066-4071. 36. Butler, S. N.; Müller-Plathe, F. Nanostructures of ionic liquids do not break up under shear: A molecular dynamics study. Journal of Molecular Liquids 2014, 192, 114-117. 37. Shelton, D. P. Long-range orientation correlation in dipolar liquids probed by hyper-Rayleigh scattering. The Journal of Chemical Physics 2015, 143 (13), 134503. 38. Baldelli, S. Interfacial Structure of Room-Temperature Ionic Liquids at the Solid-Liquid Interface as Probed by Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2013, 4 (2), 244-252. 39. Baldelli, S. Surface Structure at the Ionic Liquid-Electrified Metal Interface. Acc. Chem. Res. 2008, 41 (3), 421-431. 40. Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. (Washington, DC, U. S.) 2015, 115 (13), 6357-6426. 41. Atkin, R.; Borisenko, N.; Druschler, M.; Endres, F.; Hayes, R.; Huber, B.; Roling, B. Structure and dynamics of the interfacial layer between ionic liquids and electrode materials. J. Mol. Liq. 2014, 192, 44-54. 42. Atkin, R.; Borisenko, N.; Drueschler, M.; El, A. S. Z.; Endres, F.; Hayes, R.; Huber, B.; Roling, B. An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface. Potential dependent solvation layers and the herringbone reconstruction. Phys. Chem. Chem. Phys. 2011, 13 (Copyright (C) 2013 American Chemical Society (ACS). All Rights Reserved.), 6849-6857. 43. Ray, A. Solvophobic Interactions and Micelle Formation in Structure Forming Nonaqueous Solvents. Nature 1971, 231 (5301), 313-&. 44. Gong, X.; Kozbial, A.; Rose, F.; Li, L. Effect of π-π stacking on layering of ionic liquids confined to amorphous carbon surface. ACS Appl. Mater. Interfaces 2015, 7 (13), 7078-7081. 45. Bovio, S.; Podestà, A.; Lenardi, C.; Milani, P. Evidence of Extended Solidlike Layering in [Bmim][NTf2] Ionic Liquid Thin Films at Room-Temperature. The Journal of Physical Chemistry B 2009, 113 (19), 6600-6603. 46. Gebbie, M. A.; Valtiner, M.; Banquy, X.; Fox, E. T.; Henderson, W. A.; Israelachvili, J. N. Ionic liquids behave as dilute electrolyte solutions. Proceedings of the National Academy of Sciences of the United States of America 2013, 110 (24), 9674-9679. 47. Carstens, T.; Gustus, R.; Höfft, O.; Borisenko, N.; Endres, F.; Li, H.; Wood, R. J.; Page, A. J.; Atkin, R. Combined STM, AFM, and DFT Study of the Highly Ordered Pyrolytic Graphite/1-Octyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Interface. The Journal of Physical Chemistry C 2014, 118 (20), 10833-10843. Manuscript Text ACS Paragon Plus Environment

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48. Majhi, D.; Pabbathi, A.; Sarkar, M. Probing the Aggregation Behavior of Neat Imidazolium-Based Alkyl Sulfate (Alkyl = Ethyl, Butyl, Hexyl, and Octyl) Ionic Liquids through Time Resolved Florescence Anisotropy and NMR and Fluorescence Correlation Spectroscopy Study. The Journal of Physical Chemistry B 2015. 49. CRC Handbook of Chemistry and Physics; 89 ed.; CRC press: Florida, 2009. 50. Smoliński, S.; Zelenay, P.; Sobkowski, J. Effect of surface order on adsorption of sulfate ions on silver electrodes. Journal of Electroanalytical Chemistry 1998, 442 (1–2), 41-47. 51. Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of Normal-Alkanethiols on the Coinage Metal-Surfaces, Cu, Ag, Au. Journal of the American Chemical Society 1991, 113 (19), 71527167. 52. Fujiwara, H.; Wiley InterScience (Online service). Spectroscopic ellipsometry principles and applications [Online]; John Wiley & Sons,: Chichester, England ; Hoboken, NJ, 2007; pp. 1 online resource (xviii, 369 p.). http://infolink.lib.uiowa.edu/sfx_local?sid=ALEPH:856&svc.fulltext=yes&rft.object_id=10000000003573 51. 53. Nania, S. L.; Shaw, S. K. Analysis of fluid film behaviour using dynamic wetting at a smooth and roughened surface. Anal Methods-Uk 2015, 7 (17), 7242-7248. 54. Talaty, E. R.; Raja, S.; Storhaug, V. J.; Dölle, A.; Carper, W. R. Raman and Infrared Spectra and ab Initio Calculations of C2-4MIM Imidazolium Hexafluorophosphate Ionic Liquids. The Journal of Physical Chemistry B 2004, 108 (35), 13177-13184. 55. Kiefer, J.; Fries, J.; Leipertz, A. Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide and 1-Ethyl-3-methylimidazolium Ethylsulfate. Applied Spectroscopy 2007, 61 (12), 1306-1311. 56. Rey, I.; Johansson, P.; Lindgren, J.; Lassègues, J. C.; Grondin, J.; Servant, L. Spectroscopic and Theoretical Study of (CF3SO2)2N- (TFSI-) and (CF3SO2)2NH (HTFSI). The Journal of Physical Chemistry A 1998, 102 (19), 3249-3258. 57. Harrick, N. J. Principles of Internal Reflection Spectroscopy. Applied Spectroscopy 1971, 25 (1), 142-&. 58. Greenler, R. G. Infrared study of adsorbed molecules on metal surfaces by reflection techniques. J. Chem. Phys. 1966, 44 (1), 310-15. 59. Boyd, R. W. Chapter 1 - The Nonlinear Optical Susceptibility. In Nonlinear Optics (Third Edition), Boyd, R. W., Ed.; Academic Press: Burlington, 2008, pp 1-67. 60. Godinho, M. H.; Cruz, C.; Teixeira, P. I. C.; Ferreira, A. J.; Costa, C.; Kulkarni, P. S.; Afonso, C. A. M. Shear-induced lamellar phase of an ionic liquid crystal at room temperature. Liquid Crystals 2008, 35 (2), 103-107. 61. D. Holbrey, J.; R. Seddon, K. The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. Journal of the Chemical Society, Dalton Transactions 1999, (13), 2133-2140. 62. Saielli, G.; Bagno, A.; Wang, Y. Insights on the Isotropic-to-Smectic A Transition in Ionic Liquid Crystals from Coarse-Grained Molecular Dynamics Simulations: The Role of Microphase Segregation. The Journal of Physical Chemistry B 2015, 119 (9), 3829-3836. 63. Gong, X.; Kozbial, A.; Li, L. What causes extended layering of ionic liquids on the mica surface? Chemical Science 2015, 6 (6), 3478-3482. 64. Baldelli, S. Influence of Water on the Orientation of Cations at the Surface of a RoomTemperature Ionic Liquid:  A Sum Frequency Generation Vibrational Spectroscopic Study. The Journal of Physical Chemistry B 2003, 107 (25), 6148-6152.

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65. Rietzler, F.; Nagengast, J.; Steinrück, H. P.; Maier, F. Interface of Ionic Liquids and Carbon: Ultrathin [C1C1Im][Tf2N] Films on Graphite and Graphene. The Journal of Physical Chemistry C 2015, 119 (50), 28068-28076. 66. Segura, J. J.; Elbourne, A.; Wanless, E. J.; Warr, G. G.; Voitchovsky, K.; Atkin, R. Adsorbed and near surface structure of ionic liquids at a solid interface. Physical Chemistry Chemical Physics 2013, 15 (9), 3320-3328. 67. Domańska, U.; Laskowska, M. Temperature and Composition Dependence of the Density and Viscosity of Binary Mixtures of {1-Butyl-3-methylimidazolium Thiocyanate + 1-Alcohols}. Journal of Chemical & Engineering Data 2009, 54 (7), 2113-2119. 68. Domańska, U.; Laskowska, M.; Pobudkowska, A. Phase Equilibria Study of the Binary Systems (1Butyl-3-methylimidazolium Thiocyanate Ionic Liquid + Organic Solvent or Water). The Journal of Physical Chemistry B 2009, 113 (18), 6397-6404.

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Table of Contents Graphic

Surface

Surface

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Table 1. Average static contact angles of ionic liquids examined on surfaces as listed. Droplet volume 20 μl, n > 6.

Ionic Liquids

Bare Ag

11-MUD modified Ag

Bare Au

EMIM TFSI, (n=2)

15.7+0.3

19+1

15.0+0.3

BMIM TFSI, (n=4)

14.7+0.7

28+2

14.6+0.2

HMIM TFSI, (n=6)

14.8+0.3

18+1

15.8+0.8

OMIM TFSI, (n=8)

20.3+0.2

21+1

15.6+0.5

N1114 TFSI

20+1

22+2

15+1

BPy TFSI

19+2

11+2

15.8+0.8

P14 TFSI

13.7+0.9

15+3

16.8+0.8

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2.0 1.5 Absorbance

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Bulk FTIR

1.0

Film @ 60 μm/s

0.5

Film stopped 60 mins

0.0 1000

1200 1400 1600 Wavenumber cm-1

1800

Fig 1. IRRAS spectra for a 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide film. The black (top) trace shows the FTIR spectra of the IL between two KBr plates. Red (centre) trace corresponds to the absorption profile as the film is freshly created on a rotating substrate (velocity as shown) and the (bottom) green trace shows the films absorption profile after the film has matured for 60 minutes. Substrate material is Ag metal. Spectra are offset vertically for clarity.

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SHG signal intensity (A.U.)

Langmuir

0

10 20 30 40 50 60 70 80 90 100 110 Time (minutes)

Fig. 2 SHG intensity measured at 400 nm from a silver substrate wetted with BMIM TFSI. The increase of the SHG signal intensity is indicative of an increasingly ordered IL film. The two traces represent independent measurements and are offset vertically for clarity.

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3.5

a

Freshly prepared films EMIM TFSI

Absorbance

3.0

BMIM TFSI

2.5

HMIM TFSI

2.0

OMIM TFSI

1.5

P14 TFSI

1.0

BPy TFSI

0.5 BtmN TFSI

0.0 3.5

Matured films

b

EMIM TFSI

3.0 Absorbance

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2.5

BMIM TFSI

2.0

HMIM TFSI OMIM TFSI

1.5

P14 TFSI

1.0

BPy TFSI

0.5

BtmN TFSI

0.0 1000

1200

1400

Wavenumber

1600 (cm-1 )

Fig. 3 IRRAS spectra of a series of ionic liquid films supported on bare Ag substrates while rotating at 60 μm/s (top) and after a reorientation time (bottom). IRRAS measurements are acquired at 4 cm-1 resolution over ten minutes.

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140 120 Time (minutes)

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R2 =

0.9591

100 80 60

HMIM TFSI P14 TFSI

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OMIM TFSI N1114 TFSI

BPy TFSI BMIM TFSI EMIM TFSI

40 20 20

40

60 80 100 Viscosity (centipoise)

120

Fig. 4 A linear relationship between the time required for film reorientation and bulk IL viscosity is observed. The required reorientation time is defined by acquiring spectra till no further changes are observed in the peak profile. Each solid square data point represents the average of n > 3 independent measurements for the associated ionic liquid. The solid black line is a linear regression of the data. We note that the acquisition time of one IRRAS measurement is ten minutes, which limits the time resolution of this measurement.

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