Subscriber access provided by READING UNIV
C: Surfaces, Interfaces, Porous Materials, and Catalysis
Developing Distinct Chemical Environments in Ionic Liquid Films Radhika S Anaredy, and Scott K. Shaw J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06608 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
The Journal of Physical Chemistry
Title: Developing Distinct Chemical Environments in Ionic Liquid Films
2 3
Authors:
1) Radhika S. Anaredy,
[email protected] 2) Scott K. Shaw*,
[email protected] Institutions:
Department of Chemistry, University of Iowa, Iowa City, Iowa, 52242
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
ABSTRACT We report the reversible transitions from two distinct chemical environments (interfacial and bulk), to a single globally ordered dominant environment that extends to 800 nm, within two triflate based ionic liquid films. Vibrational spectra for supported ionic liquid films exhibit multiple peaks for the same vibrational mode, indicating the presence of multiple chemical environments (interfacial and bulk) in the film. After a quiescent maturation time, vibrational spectra show much simpler absorption profiles indicating coalescence of the ionic liquid molecules into a global, preferred phase that resembles interfacial environment that propagates throughout the film as a function of time. Data analysis suggests significant reorientation of the triflate anion with small changes of the cation, indicating a weakly interacting cation-anion pair. The distal extent of the self-organization is much thicker than generally reported for solid-fluid interfaces. The ordering is reversible on replenishing the film with fresh fluid. This report, describing propagation of interfacial molecular orientation to form extended ordered structures, is a motivation for future studies to apply this phenomenon towards the thoughtful design of new IL systems for use in materials and devices.
23 25 26 27 28 29 30 31 32 33 34 35 36 37
INTRODUCTION Ionic liquids (ILs) are pure salts with low melting temperatures. They exhibit several unique and useful properties, including negligible vapor pressures, moderate conductivities, stability in extreme environments, and tunable solvency.1-3 These properties have created significant interest in developing ionic liquid materials for applications in lubrication, catalysis, electrochemistry, separations and extractions.2, 4 Research efforts aiming at tuning ILs’ physical properties have seen success in carbon dioxide capture,5-6 CO2 reduction,7 and dissolution of carbohydrates.8 The bulk and interfacial structures of ILs are being studied heavily. Significant evidence now supports the spontaneous formation of nano-domains that permeate the entire volume of ionic liquids, the dimensions of which rely on the alkyl chain length, cation symmetry and temperature.9-10 However, benefits in performance are often influenced by the behavior of ILs at or near a solid or vapor interface. It is well known that molecular solvents form interfacial regions that are distinct from the bulk,11-12 and our work contributes to a better understanding of the same interfacial region in ionic liquids systems.
38 39 40 41 42 43
Studies on ILs under nanoconfinement have reported ordering of the IL molecules up to a few tens of nanometers.13-20 Few recent studies, including one from our group21 have shown so called ‘extended’ ordering of IL molecules. One such study on 1-hexyl-3-methyl-imidazolium ethylsulfate under nanoconfinement and significant shear forces show irreversible, solid-like structures which extend up to 60 nm from an adjacent solid surface as confirmed by surface force apparatus, AFM, and WAXS studies.22 Blanchard et al. reported charge-induced long-range
24
ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
ordering in 1-butyl-3-methylimidazolium tetrafluoroborate which extends up to 100 µm from the silica surface by measuring the anisotropic decay of chromophores in the IL as a function of distance from the surface.23 Computational studies show the transformation from sponge-like morphology to lamellar structure at vacuum interfaces that extend throughout a simulated slab of IL molecules, (ca. 10 nm x 10 nm) and predict that the ordering phenomenon might extend beyond these dimensions.24 A recent study of supported ionic liquid membranes showed that the IL dynamics under confinement are slower than the bulk liquid which is attributed to the change in the IL’s calculated viscosity under confinement (4 times that of the bulk viscosity) and is predicted to be a result of long-ranged influence (ca. 100 nm) of the interfaces.25 A recent report by Siria et al. shows formation of solid-like IL phases in films of thickness characteristic of the adjacent solid material. The thickest sample observed to undergo phase transition is 160 nm thick, on platinum.26 Our previous publication shows reversible, long-range ordering in bis(trifluoromethylsulfonyl)imide (TFSI) based ILs which extended up to 2 µm from a solid substrate under the influence of gravity driven flow and intermolecular interactions. This ordering shows very little dependence on the substrate material or presence of adventitious water. The data indicate significant reorientation of TFSI anions over time. The behavior of several cations examined is less clear, but they appear to play a minor role in the ordering process.21 The triflate anion, which is essentially one half of the TFSI anion, also contains C-F and S-O bonds but is smaller and has a more localized charge than TFSI. We predicted that the similar functionality and higher charge density would lead to a stronger interaction between anion and cation in triflate based IL and reasoned this might induce cations to reorient with the anions, allowing a clearer interpretation of cation behavior in the ordering process. This work describes the structures in films of two triflate based ILs, (1-butyl-3-methylimidazolium triflate (BMIM OTf) and diethylmethylammonium triflate (N221 OTf)) and reports their behaviors under similar conditions as described above. Previous studies on triflate anion have investigated the behavior of the anion based on its interaction with the cation in a salt or ionic liquid. For example, varying concentrations of lithium triflate salt in acetonitrile was studied using FTIR to report the changing vibrational bands of triflate anion over a range of concentrations. It was found that increasing concentration of the salt caused the absorption features to shift to higher frequencies. This is interpreted as triflate coordinating more strongly to Li+.27 Other work studying the orientation of BMIM OTf at air/liquid interfaces presents SFG and IR data that show significant (ca 10 cm-1) shifts of the SO3 stretching mode to higher energy. These shifts are ascribed to formation of aggregates of ions in solution. The SFG results showed a broader peak for the SO3 symmetric stretching mode in LiOTf, indicating multiple forms of cation-anion aggregates; whereas a sharp, narrow peak in case of BMIM OTf represented a homogeneous environment around the anion with strong and specific interactions between the ions. Theoretical calculations on BMIM OTf at the air/liquid interface reveal that a preference for the CF3 functional group to point from liquid into the gas phase, while SO3 remains in the liquid to interacts with the cation through hydrogen bonding.28 The first study discussed here revealed the behavior of -OTf anion as a function of its interaction with the cation in an isotropic solution, while second study specifically compared the air/liquid interface of two OTf containing salts (one being metallic salt and the other being an ionic liquid) to describe the influence of the cation-anion interaction on the interfacial structure. A number of reports on triflate based ILs describe arrangements of ions and thicknesses of ordered layers that extend < 115 nm.29-30 A study on 1-ethyl-3-methylimidazolium triflate
ACS Paragon Plus Environment
Page 2 of 16
Page 3 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
The Journal of Physical Chemistry
using angle resolved X-ray photoelectron spectroscopy showed that this IL exhibits a checkerboard pattern on the Au (111) surface up to 0.5 ML coverage. Beyond this low coverage the film forms droplets, which become isotropic. Deposition of 0.7 ML of palladium before or after IL deposition switches the 3D growth to 2D (wetting) growth but with layer by layer configuration.31 These prior results for triflate based ILs and salts show that cation-anion interactions have major impacts on the behaviors of ILs at interfaces as well as in bulk. Herein we report the behavior of two triflate based IL films at solid-liquid interfaces and outline three major findings- i) While our previous work on TFSI based ILs proved the possible long-range ordering in IL films, this report provides insight on this ordering process. The combination of dynamic wetting technique with IRRAS allows probing the transition of a twochemical environment (interfacial and bulk) film to a single dominant environment under the influence of slight shear. This transition occurs through propagation of the interfacial environment as a function of time and is evidenced from the increased intensity of IR peaks corresponding to the interfacial orientation; ii) Despite the stronger interactions of triflate anions with cations as compared to TFSI anions, the results show that anions undergo reorientation with no major influence from the cation; iii) While previous studies on interfacial ordering of triflate ILs show that the ordered layer persists in monolayer thick films and up to a reported maximum thickness of 115 nm.29-30 We report the ordering of 1-butyl-3-methylimidazolium triflate and diethylmethylammonium triflate ionic liquids that extends up to 800 nm. We believe that these findings not only provide better understanding of ionic liquid behavior but also depict that their interfacial processes are very different from molecular systems. Revealing the mechanism for this curious effect in IL systems will aid in developing new avenues in the field of materials research and design.
24 25
EXPERIMENTAL SECTION
26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
Materials: Disk shaped substrates to support IL films are cut from 14 mm diameter polycrystalline silver rods (99.99% purity, ESPI metals, Portland, OR). The disk surfaces are mechanically polished to 0.3 µm grit size alumina powder to obtain a mirror finish. Subsequent chemical polishing uses a well-established method32 including an acid etch which employs H2SO4 (ACS grade, BDH), HClO4 (70%, Sigma) and NH4OH (28-30%, BDH) all used as received, along with an aqueous solution of 4 M CrO3 (99.9%, Aldrich), and 0.6 M HCl (ACS grade, BDH). All solutions are prepared with ultrapure water (18.2 MΩ cm-1 with TOC ≤ 4 ppb) generated by a Milli-Q UV Plus System (Millipore Corp). The surface is characterized for cleanliness and roughness using ellipsometry and atomic force microscopy (AFM) respectively. The RMS roughnesses over 1 x 1 µm areas of the substrates are determined to be < 5 nm, while the n and k values for these substrates are determined from ellipsometry to be in agreement with literature values for pure, clean, silver.33 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 99% (BMIM OTf) and diethylmethylammonium trifluoromethanesulfonate, 99% (N221 OTf) were purchased from Iolitec, USA and placed under vacuum for > 24 hours at 60 °C while being stirred to remove volatile impurities and water. After removing from vacuum, ILs are stored in a glove box (Genesis, Vacuum Atmospheres Company) under nitrogen with water and O2 levels
