Ionic Liquids - American Chemical Society

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Chapter 23

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Ionic Liquids: Contained and Characterised Imee Su Martinez and Steven Baldelli University of Houston, Houston, TX 77204-5003

Three ionic liquids - [C4mim][BF4], [C4mim][N(CN)2] and [C4mim][MeOSO3] - were studied using three complementary surface techniques; SFG-polarisation mapping, surface tension measurement and surface potential measurement. Custom vacuum cells were designed for each technique to be able to perform measurements in a highly controlled environment, minimising the presence of water and other contaminants which may compromise measured values. SFG results depicted both anions and cations to be present on the surface with the butyl chain of the cation positioned toward the gas phase and the imidazolium ring parallel to the surface plane. Surface potential measurements showed an excess positive charge on the surface for all three ionic liquids. The surface tension of [C4mim][N(CN)2] was determined.

Introduction and Rationale Interest in ionic liquids has grown a great deal since the synthesis of the first ionic liquid called “red oil” in the mid-19th century (1). At present, studies and papers regarding ionic liquids are being published at a rate corresponding to thousands per year. The special properties that these liquids possess, and their countless possible applications, are what attract the scientific community to study them. Ionic liquids, in particular room-temperature ionic liquids, are pure liquid salts at temperatures below 100 °C (2). The composition of these liquids is typically a combination of an organic cation and an inorganic anion (1). Depending on the constituting ions, some ionic liquids have a wide liquidus range, wide electrochemical window, high conductivity, low vapour pressure, © 2009 American Chemical Society In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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336 and thermal stability (3-5). This group of liquids have tuneable properties, such that different combinations of the cation or anion can be made allowing for modifications in their properties such as hydrophobicity, viscosity, density and solvation. The properties mentioned are what led to the current most common applications of ionic liquids, specifically liquid-liquid extraction, biphasic catalysis, corrosion, lubrication and solar cells or electrochemical applications (6-8). The possible application of ionic liquids in the gas absorption of certain anthropogenic gas pollutants such as CO2, SO2, NH3, and CFCs, can be used to address global issues concerning climate changes and ozone depletion (9,10). Studies done on this particular application, however, were leaning towards the solvation properties of ionic liquids with respect to these gases (11-13). Gas uptake, which is also a surface interaction, creates a need to study in detail gasliquid interfaces of ionic liquids. Determining the molecular orientation, excess charge, ion size and geometry, or probing the interface at the molecular level in order to understand how these gases are absorbed into the liquid, becomes very significant. Parallel to this, the necessity of characterising these interfaces in a controlled and very clean environment becomes crucial in order to produce reliable results. The presence of organic contaminants, chloride and water alters values of measured physical properties in ionic liquids. Seddon et al. investigated the effect of these contaminants on the viscosity, density and 1H NMR shifts in some ionic liquids (14). Chloride, even in low concentrations of 0.01 mol kg-1 in [C4mim][BF4] caused a dramatic decrease in viscosity, a nonlinear decrease in density and a downfield shift in the 1H NMR signals of the imidazolium ring of the cation. Co-solvents such as ethanenitrile, trimethylethanenitrile, 2-propenenitrile, 1-methylimidazole, toluene, 1,4-dimethylbenzene and 1,2-dimethoxyethane, when added incrementally to [C4mim][BF4] and [C4mim][PF6], caused the measured viscosity to decrease exponentially. A more pronounced decrease in viscosity was caused by water alongside ethanenitrile, and 2-propenenitrile. Density decreased rapidly as well, at excess mole fractions of water (> 0.5). In an atmosphere filled with moisture, studying the effect of water on the physical properties of a substance is very important. Bowers and co-workers plotted surface tension and conductivity isotherms of imidazolium ionic liquids including [C4mim][BF4] in water (15). The general trend was the surface tension and conductivity decreased with increasing ionic liquid concentration until a critical concentration is reached wherein the values plateau. An experiment on the solubility of water vapour in ionic liquids including [C4mim][BF4] was performed by Anthony et al. using a gravimetric microbalance (16). This provides an idea of how volatile components can actually dissolve into the ionic liquid and affect its properties. The determined enthalpies and entropies of water absorption into the ionic liquid were found to be similar to those of polar solvents, indicating a strong infinity of these ionic liquids with water. The effect of water on the vapour-liquid interfaces of ionic liquids has been studied using Sum Frequency Generation (SFG). Previous work in our group on [C4mim][BF4] showed that water is only probed at ionic liquid concentrations of

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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337 ≤0.02 mole fractions, whereas at higher concentrations the surface showed SFG spectra similar to that of the pure ionic liquid (17). Sung and co-workers studied the effects of water on the vapour-liquid interface of [C4mim][BF4] using surface tension and SFG (18,19). A rapid decrease in surface tension was observed from 0 to 0.016 mole fraction of ionic liquid, which corresponds to the lowest value of the measured surface tension. A slight increase was observed at around 0.05 mole fraction, which evened out with the increase in ionic liquid concentration. The mole fraction which corresponds to a minimum in the measured surface tension exhibited an unusually intense signal in SFG at ssp and ppp polarisations compared to the pure ionic liquid. The group explained this phenomenon in terms of the surface being covered purely by cations at low concentrations up to 0.02 mole fraction where the anions start to appear on the surface. A mole fraction of 0.05 signalled that the surface is being equally populated by both cations and anions. What makes ionic liquids interesting systems for research, in addition to the mentioned applications, is the fact that these liquids are composed of pure ions making them a relevant system for studying the surface structure of charged species minus solvent effects. The role of charge size, intermolecular interactions, and polarisability on surface conformation can be determined to test existing theories such as the Gouy-Chapman model of the double layer and to explore more likely models. The goal of this study is to probe the gas-liquid interface of room temperature ionic liquids using sum frequency generation-polarisation mapping method, surface tension measurements using the ADSA (Axisymmetric Drop Shape Analysis) method, and surface potential measurements using the compensation/vibrating plate method. Special cells for these three different techniques were designed to be able to perform measurements in vacuum at 10-5-10-6 Torr in order to perform measurements in a clean and controlled environment. These three techniques combined will complement each other and will provide a better understanding on how the ions of these liquids are structured at the surface. SFG-polarisation mapping will give a better molecular level description of the interface. Surface potential measurements will be able to determine the excess charge on the surface and therefore determine which species prevail or are dominant over the other. Surface tension will determine excess surface energy and will correspond to the functional group as well as to the intermolecular forces prevalent on the surface. The results from these three techniques tied together will therefore determine the arrangement of these ions with respect to each other and how each ion whether cation or anion is oriented at the gas-liquid interface. Three different ionic liquids were used for this study, with the same cation 1-butyl-3-methylimidazolium, [C4mim]+, and three different anions tetrafluoroborate [BF4]-, dicyanamide [N(CN)2]- and methyl sulfate [MeOSO3]-, which have different sizes and geometries.

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Figure 1. Ionic liquids under study: (a) 1-butyl-3-methylimidazolium cation (b) dicyanamide anion (c) tetrafluoroborate anion (d) methyl sulfate anion

Background Sum Frequency Generation (SFG)-Polarisation Mapping Method SFG is a nonlinear vibrational spectroscopic technique that involves two input laser beams; visible and tuneable IR that overlap in a medium to generate an output beam that has a frequency equal to the sum of the frequencies of the two incoming beams (20). It is a highly surface specific technique, since it is forbidden in a medium with inversion symmetry (20,21). The intensity, I(ωSF), of the generated sum frequency beam is proportional to the square of the induced polarisation P(2) on the surface due to the coming together of the electric fields of the two incident beams (EIR, Evis) (22). The term that relates the induced polarisation response to the electric fields is the second order nonlinear susceptibility tensor χeff(2). This has two components, the χnr coming from the non-resonant background of the surface and the resonant term, which contains the vibrational spectroscopic information (20). β(2) is the hyperpolarisability, which contains this information averaged over all molecular orientations composed of the Raman polarisability and the IR dipole transition. ωIR, ωq, and Γq are the frequency of the IR beam, frequency of the normal mode and the damping constant of the qth vibrational mode, respectively (23).

The polar orientation of a molecule on the surface is what dictates the magnitude of the χeff(2). By varying the polarisations of the input and output beams, the Cartesian components of the susceptibility tensor can be determined, which allows for the determination of the molecular orientation relating to the surface normal (20,21). The structural orientation of the ionic liquids mentioned above has already been determined using SFG (24-27). Results of previous authors in the group showed that both cation and anion are present on the surface. The ring of the

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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339 imidazolium cation lies flat at the gas-liquid interface, while the butyl chain is extended towards the gas phase at an angle from the surface normal. These previous SFG studies used the usual polarisation combinations in particular ssp, ppp, sps and pss to observe the surface. Results from studies, both simulation and experimental, presented by other authors vary in relation to the results published by our group. Balasubramanian and Bhargava used atomistic molecular dynamic simulations to investigate [C4mim][PF6] (28). Both anions and cations enrich the surface with the anions contributing to the enhanced calculated electron density. The butyl chains are parallel to the surface normal protruding out of the liquid. The ring positioned closer to the vapour phase is parallel to the surface except at the densest part, where it is perpendicular. X-ray reflectivity and surface tensiometry were performed by Sloutskin et al. to probe the surface of alkylimidazolium ionic liquids with anions [PF6]- and [BF4]- (29). Electron density results indicated that both anions and cations are present on the surface. Assuming that electroneutrality governs the surface, they have calculated a nett negative charge at the interface. The observed surface layer thickness was close to the length of a butyl chain, implying that the cations are standing up, although a stack of lying down molecules which are three layers in thickness cannot be discounted. Capillary wave spectra and surface tension were measured by Halka and coworkers for [C4mim][PF6] (30). The calculated negative surface entropy from the surface tension values agreed with simulations which predicted anisotropic alignment of imidazolium cations on the surface. The resulting wave spectra gave surface dipole moment density described by the authors as a condensed liquid monolayer with the cations strongly aligned capable of sliding past each other. Direct recoil spectrometry used to probe the surface structure of imidazolium ionic liquids showed atomic ratios implying the presence of both cations and anions without segregation occurring amongst them (31). According to this group, the cation is oriented perpendicular to the surface plane where the nitrogen atoms of the ring are positioned at the top. Increasing the alkyl chain length of the cations of [C4mim][PF6] and [C4mim][BF4] from four to twelve increased the rotation angular spread of the cations from 30° to 45°. Orientations however of certain ionic liquids such as [C8mim]X (X = [BF4], Cl or Br) cannot be clearly determined. Jeon et al., using X-ray reflectivity and SFG, postulated that the alkyl chains of the imidazolium cations are oriented toward the gas interface while the anions and the cation cores are in contact with the liquid (32). The SFG intensity for [C4mim]I was double the intensities of [C4mim][BF4] and [C4mim][PF6], which made them conclude that the number densities for the cations of the last two ionic liquids are smaller than that for [C4mim]I. These results are congruent to the X-ray results, which showed that the layer thickness for [C4mim][BF4] and [C4mim][PF6] are shorter than the extended chain length of the butyl chain, suggesting the chain is tilted at an angle. In contrast, that of the [C4mim]I has a larger thickness compared to the length of the butyl chain suggesting otherwise. The electron density of [C4mim]I is also higher, implying

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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340 that the anions are not coexisting with the imidazolium cations on one layer but are situated directly below the cations. Recently, Iwahashi and co-workers performed SFG on 1-butyl-3methylimidazolium trifluoromethanesulfonate, [C4mim][OTf], and observed that polar ordering exists at the surface (33). The polar SO3 groups are pointed toward the bulk and the non-polar CH3 functional groups are directed towards the vapour phase. The SFG signal coming from the SO3 suggests that the SO3 on the surface is different from the bulk SO3, even though both are in contact with the bulk. Also, blue shifting of the SO3 peak implies that there is a strong interaction between the imidazolium cation and the [OTf]- anions, indicating that ions form an aggregated configuration on the surface. The narrow line width of the SO3 peak implies a specific configuration of this so-called aggregation. Polarisation mapping methods provide a better approach to analyse interfaces using SFG (34). This can improve fitting results by means of probing the interface using polarisation combinations other than the normal ssp, ppp, sps, and pss, obtaining more reliable spectral information if not a larger set of data with which to perform analysis. χeff(2) is a summation of its 27 Cartesian components, which reduces to four independent non-vanishing components considering an azimuthally isotropic interface (20-23,35). The contribution of these components to the intensity of the sum frequency beam or the χeff(2) varies according to the polarisation of the incoming and outgoing beams (20-22,34). Setting the visible beam s-polarised and the IR beam p-polarised leads to χyyz contributing to the induced polarisation on the surface, producing an s-polarised emitted beam. When both incoming beams are p-polarised, the other four Cartesian components χxxz, χxzx, χzxx andχzzz contribute with their relative importance determined by the electric field on the surface to produce a p-polarised beam. Performing polarisation mapping, which is setting the polarisation of the IR p-polarised, the visible 45° from the s-polarisation direction and varying the angle (σs) of the polariser in front of the detector, the resulting contribution to the χeff(2) is a combination or interference between the ssp and ppp χeff(2) at half the intensity. The resulting series of spectra is therefore an interference of the ssp and ppp spectra. The intensity of the sum frequency beam at various polarisations is represented below:

A 2D contour plot of wavenumber versus incrementing polarisation angles σs can therefore be constructed to improve spectral analysis. This map will allow extraction of phase information since different vibrational modes will reach maximum peak intensity at different signal beam polarisation angles (34). It also means it can separate overlapping peaks according to phase difference, damping factor difference, and difference in intensity based on the fact that an SFG spectrum with very different spectral features can be collected by varying σs.

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341 Increased fitting resolution because of SFG-polarisation mapping will enhance the usefulness of SFG in orientational analysis. Having seven different polarisation combinations compared to only four will probe the surface of ionic liquids at a more rigorous manner. Simultaneous fitting of spectra from these polarisation combinations will lessen bias, which is typical in fitting SFG spectra. Results from this mapping technique in terms of orientation of ions on the surface will be compared to previous studies performed on imidazolium ionic liquids. Orientational analysis using the method of Wang and Hirose has been carried out on imidazolium ionic liquids previously in our group. In fact, the orientation of cation and anion of [C4mim][N(CN)2] has been studied in detail (26). The butyl chain of the cation based on the C-H stretching region of the terminal CH3 functional group was determined to be 52°-80°, with a distribution of 0°-30° from the surface normal. The dicyanamide anion for the same ionic liquid was observed to have tilt angles of 52°-80° in a dipping configuration perpendicular to the length of the molecule and twisting angles along the length of the ion of 0°-30°. The tilt angles of the terminal carbon of the butyl chain from the cation of [C4mim][PF6] and [C4mim]Br were also determined (24). The tilt angles around the axis of the terminal methyl functional group were (47 ± 2)° and (54 ± 1)° for the [C4mim]Br and [C4mim][PF6], respectively. Surface Potential Measurement Using Vibrating Plate Methods At the interface, the distribution of ions, electrons, and electric field due to permanent or induced dipoles leads to a potential difference. This difference in potential in turn causes redistribution of charges in the interface forming the electric double layer (36). There are several models used to interpret the measured Volta potential, but the Gouy-Chapman Model of the electrical double layer is a simple approach to begin the discussion. This model looks at ions as point charges distributed in a solution according to the Boltzmann equation, where in at an infinite distance from the surface the electrical potential φ(x), must be equal to the inner potential, which changes as the surface is approached (37,38). Boltzmann Equation

where N is the number of ions at distance x and ∞ from the surface, zi the valency of the ion and e the electronic charge. Poisson’s Equation relates electrical potential φ(x) to the nett charge density per unit volume ρ(x) in a planar double layer such that:

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Since the double layer, surface plus solution is electrically neutral, the charge per unit area of surface σ is balanced by the charge in solution such that:

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Combining these equations lead to an equation that relates surface charge density to the electrical potential at the surface.

Now, the reciprocal double layer thickness can be described as

such that when zeφ(x)