Liquid-Mercury-Supported Langmuir Films of Ionic Liquids: Isotherms

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Liquid-mercury-supported Langmuir Films of Ionic Liquids: Isotherms, Structure and Time Evolution Eitan Elfassy, Yitzhak Mastai, Diego Pontoni, and Moshe Deutsch Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00196 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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Liquid-mercury-supported Langmuir Films of Ionic Liquids: Isotherms, Structure and Time Evolution Eitan Elfassy1, Yitzhak Mastai2, Diego Pontoni3 and Moshe Deutsch4 1,2

Chemistry and 4Physics Departments, and Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan, 5290002, Israel 3

ESRF - The European Synchrotron, 71 Avenue des Martyrs, Grenoble, France

1

[email protected]

2

[email protected]

ACS Paragon Plus Environment

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Abstract Ionic liquids have been intensively developed for the last few decades and are now used in a wide range of applications, from electrochemistry to catalysis and nanotechnology. Many of these applications involve ionic liquid interfaces with other liquids and solids, the sub-nanometric experimental study of which is highly demanding, and has been little studied to date. We present here a study of mercury-supported Langmuir films of imidazolium-based ionic liquids by surface tensiometry and X-ray reflectivity. The charge-delocalized ionic liquids studied here exhibit no 2D lateral order but show diffuse surface-normal electron density profiles exhibiting gradual mercury penetration into the ionic liquid film, and surface-normal structure evolution over a period of hours. The effect of increasing the non-polar alkyl chain length was also investigated. The results obtained provide insights into the interactions between these ionic liquids and liquid mercury and about the time evolution of the structure and composition of their interface.


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1. Introduction Ionic liquids are typically composed of stable organic cations and organic or inorganic anions, often presenting asymmetric shapes and delocalized charges1. Ionic liquids have been developed for few decades and are now used in a wide range of applications, from electrochemistry2 to extraction3, catalysis4 and nanotechnology5. The extremely low vapor pressure of ionic liquids together with their high chemical and thermal stability can help to solve many safety and environmental issues, including ignition risks and industrial contamination of the environment. Also, the reduced reactivity of the ions often endows the ionic liquids with a broad electrochemical window, rendering them optimal for use in electrochemical applications. The potential of ionic liquids as a medium for electrochemical reactions6-8, electrodeposition9,10 and electrocatalysis11,12 was confirmed by many studies during the last twenty years. Moreover, ionic liquids are intensely investigated as useful electrolytes in energy storage devices like supercapacitors13,14 and lithium ion batteries15-18. All these electrochemical applications strongly depend on the structure and properties of the interface between the ionic liquid and the electrode, usually a metal. However, the standard PoissonBoltzmann-equation-based double layer models19, like the Gouy-Chapman-Stern one20-22, used for diluted electrolytes are not relevant for the ionic liquids. These liquids are solventless and hence infinite-concentration electrolytes, and thus subject to strong Coulomb correlations between the ions, showing non-standard effects23. For these reasons, the ionic liquid/metal interface is of much current research interest, and a deep understanding of the sub-nanoscale structure of these interfaces could improve many of the applications of the ionic liquids. One of the challenges is to


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determine the distribution of the ions near the interface and to get an accurate molecular picture. Some of the experimental methods used for this purpose are X-ray24 and neutron25 scattering, X-ray photoelectron spectroscopy26 (XPS), scanning tunneling microscopy27 (STM), surface force apparatus (SFA)28 and atomic force microscopy (AFM)29. The structure of the interface between the ionic liquid N,N-diethyl-N-methylN-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([MOENE2M]+ [NTf2]) and an Au(111) electrode surface was studied by X-ray reflectometry at two different applied voltages24. A layering structure, similar to those found at an ionic liquid/insulator30 and ionic liquid/air interfaces31, was observed also at the electrode surface, with alternating cation and anion layers exhibiting decreasing order with distance from the interface. The reported voltage dependence was consistent with previously reported molecular dynamics (MD) simulations32. In another study, the interface between the

ionic

liquid

1-butyl-1-methylpyrrolidinium

bis(trifluoromethylsulfonyl)imide

([C4mpyr]+ [NTf2]-) and an Au(111) electrode was studied by neutron reflectometry at three different voltages25. A monolayer of cations was found at the electrode surface and some surface excess of cations was observed even when the electrode was positively charged. This last point was linked to the occurrence of a specific adsorption of the pyrrolidinium cations to the gold electrode. These two studies illustrate that even if some general behavior can be recognized, changing the chemical nature of the ions (here only the cation) has critical consequences on the structure of the interface. Similar conclusions were previously found in a study combining AFM, STM and cyclic voltametry experiments for the interfacial structure of the ionic liquids [C4mpyr]+ [NTf2]- and


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[C4mim]+ [NTf2]-§ at the Au(111) interface29. In both cases three to four solvation layers were detected but the interaction strength between the Au(111) surface and the inner layer of the pyrrolidinium-based ionic liquid was four times higher than for the imidazolium-based ionic liquid, again suggesting stronger interactions between the pyrrolidinium cation and the Au(111) surface. In a recent study, multilayers of the ionic liquid 1-octyl-3methylimidazolium tetrafluoroborate ([C8C1Im]+ [BF4]-) were deposited on a Cu(111) surface by evaporation in ultra-high vacuum (UHV)33. Nanodroplets of this ionic liquid were observed at room temperature on the Cu(111) surface by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS), indicating a stronger attractive interaction between the liquid’s ions and molecules than between them and the copper substrate. Although several high-resolution studies were reported in this area over the last few years34-36, the huge chemical variability of ionic liquids has inhibited so far the emergence of a broad consensus on the general principles governing the interfacial structure between ionic liquids and metals. This is particularly true for ionic liquid interfaces with liquid metals, where, unlike at solid metals surfaces, the metal atoms are mobile, lack surface-parallel order, and some admixing of the species from the two sides of the interface may be possible. In fact, to the best of our knowledge, only a single atomic-resolution x-ray study of an ionic liquid/liquid metal interface has been published to date52. The chemical nature of the ions is also a key for the understanding of the interfaces between ionic liquids and metallic nanoparticles. For example, ionic liquids with imidazolium cations can stabilize gold37 and transition-metal38 nanoparticles and so

§

1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide


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can serve as a medium for the synthesis of these nanoparticles. Furthermore, the strong interactions between the imidazolium cation and the metal were confirmed by density functional theory (DFT) calculations and molecular dynamics (MD) simulations39. Once again, the vast chemical variability of ionic liquids precludes generalizations at this stage and a systematic evaluation encompassing many different ionic liquids and many different metals is still lacking. An additional relevant important question is: to what extent the local structure in the bulk is conserved at the interface40,41? For example, for ionic liquids with non-polar alkyl chains greater than or equal to ethyl42, the alkyl chains tend to segregate giving rise to nanoscopic heterogeneities in the bulk43-45, with hydrophilic and hydrophobic regions. Changing the length of the alkyl chains46-48, the amount of charge delocalization49, using perfluorinated ions50 or adding some functional groups to the ions15,18,51, have all been used to investigate the effects of the chemical variability on the ionic liquids' structure. For a comprehensive review, outside the scope of this research paper, see Refs. 40-41. We studied Langmuir films of ionic liquids supported on bulk liquid mercury. The thermodynamics and structure of the Langmuir films on the surface of liquid mercury were investigated, respectively, by surface tensiometry and by surface-specific X-ray methods. Compared to water, mercury provides a very smooth surface which enables carrying out X-ray reflectivity measurements with sub-Ångström resolution, as demonstrated by the only X-ray study of a mercury-supported ionic liquid Langmuir film published to date52, that of 1-methylpyrrolidinium tris(perfluoroalkyl)trifluorophosphate ([C4pyrr]+ [FAP]-, see figure 1). Here we studied a different family of ionic liquids which contains

an

aromatic

imidazolium

ring,

the

1-alkyl-3-methylimidazolium


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bis(trifluoromethylsulfonyl)imide ([Cnmim]+ [NTf2]-, see figure 1). We also address the effect of increasing the alkyl chain length (n=4, 12, 18) and of changing the anion from [NTf2]- to [FAP]-. The time evolution of the interface structure is also studied.

2. Materials and methods The same experimental methods for the study of Langmuir films on mercury were discussed in details in previous papers from our group52,53, so we give here only a summary. 2.1 Materials The high purity grade ionic liquids [C4mim]+ [NTf2]-, [C12mim]+ [NTf2]- and [C18mim]+ [NTf2]- were purchased from IoLiTec GmbH, Heilbronn, Germany. The high purity grade ionic liquid [C18mim]+ [FAP]- and the mercury (triple distilled, 99.999% pure), were purchased from Merck KGaA, Darmstadt, Germany. Before the experiments, the ionic liquids were maintained in a vacuum oven at 70 ⁰C for at least 24 hours to remove humidity traces, usually at very low levels for this kind of hydrophobic ionic liquids. Stock solutions were freshly prepared, with molarities from 0.1 to 0.4 mM, using high purity grade acetonitrile from Sigma-Aldrich (CHROMASOLV®, HPLC, ≥99.9%).


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2.2 The trough The Langmuir trough resided in a hermetically sealed aluminium box designed for simultaneous surface tension and X-ray measurements. For the X-ray measurements a slow flow of pure He was maintained trough the box to minimize background scattering and oxidation of the mercury surface. A nitrogen flow was sufficient when only surface tension measurements were performed. The temperature of the mercury surface was kept at 25 ⁰C by a water circulation system. For further details of the trough and enclosure see reference 52.

2.3 Surface tension measurements The Wilhelmy plate method53 was employed for the surface tension measurements. In short, an Hg-amalgamated platinum plate touching the mercury surface was attached to a leaf spring and the position of the plate, proportional to the surface tension, was measured using a linear variable differential transformer (LVDT). In this way we could measure the surface pressure π, which is the difference between the surface tension of bare and film-covered mercury: π=γ0 - γ. The surface pressure as a function of surface area per molecule, known as π-A isotherms, were measured point after point by deposition at regular time intervals (1.5 min) of accurately measured volumes (3 to 5 μL) of the stock solutions mentioned above. Control experiments were performed with pure acetonitrile to ensure that all the acetonitrile evaporates and does not leave impurities on the mercury surface, which was indeed found to be the case.


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2.4 X-ray measurements The structure of the Langmuir film at the molecular scale was studied by surface specific X-ray methods. The X-ray measurements were performed at the liquid surface and interface scattering (EH1) station of the ID10 beamline, European Synchrotron Radiation Facility (ESRF), Grenoble, France. The wavelength of the radiation was λ = 0.5414 Å (22.90 keV). The trough was supported on an active vibration isolation unit to eliminate vibrations from the environment. The surface-normal electron density profile was investigated by X-ray reflectivity (XR) measurements. In this method, the reflected fraction, R(qz), of the incident intensity was measured as a function of q z, the surfacenormal component of the wavevector transfer q. As a function of the incident angle α, qz is given by qz = 4πsin(α)/λ. Two types of reflectivity scans were carried out: long scans out to 2.4 Å-1 , and short scans out to 1.4 Å-1 . The former go past the layering peak of the free surface of mercury54,55 at 2.2 Å-1 and were used for measurements immediately following film deposition. However, the fast-evolving shape of the reflectivity curve and increasing surface roughness at longer time scales, strongly decreased the reflected signal at high qz, rendering measured points at qz> 1.4 Å-1 unusable, and thus allowing only short scans to be carried out. The in-plane order was also investigated by grazing incidence diffraction (GID) measurements with a 2D detector, but no GID peaks were found. The possibility of beam-induced damage was explored, and such effects were minimized by adjusting the incident beam's intensity and translating the sample periodically transverse to the beam to expose fresh surface locations.


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3. Results and discussion. 3.1 Surface tension measurements. Fig. 2a presents a measured π-A isotherm for [C4mim]+ [NTf2]-, with isotherms for additional [Cnmim]+ [NTf2]- ionic liquids shown in Fig. S1. It should be noted that the π-A isotherm for [C4mim]+ [NTf2]- isotherm differs from the previously measured isotherms for the [C4C1pyr]+ [FAP]- ionic liquid on mercury52. Indeed, an increase in both surface pressure and molecular area was observed in the former over the later. We hypothesize that this may be connected to the increase in the charge delocalization of the ions of [C4mim]+ [NTf2]- as compared to [C4C1pyr]+ [FAP]-. For the [C4mim]+ [NTf2]-, both the cation and the anion have resonance structures which are missing in the [C4C1pyr]+ [FAP]-, as shown by the structure of the ions in Fig. 1. The high π50 mN/m at low-A limit of [C4mim]+ [NTf2]- indicates a stronger interaction between the ionic liquid molecules and the mercury surface, so that the ions with strong charge delocalization reduce more efficiently the surface energy of mercury. The end position of the steep π rise and start of the π plateau, at around 120 Å2 for [C4mim]+ [NTf2]-, cannot be solely explained by the surface area of the ions [C4mim]+ and [NTf2]-, since the calculated area of the ions, lying flat on the surface, yields only 66 Å2 (see supplementary material, figure S2). Stronger interactions between the mercury atoms and the ionic liquid molecules can lead to the intercalation of some mercury atoms into the Langmuir film, thus increasing the observed surface area. This suggestion is further supported by the fact that the fast-rising part of the isotherm cannot be fitted well by a Volmer



1-methylpyrrolidinium tris(perfluoroalkyl)trifluorophosphate


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isotherm of a 2D gas of non-interacting molecules, thus implying the presence of additional molecule-molecule and molecule-surface interactions. These points are further discussed below, along with the X-ray reflectivity results. Finally, after film collapse at very high surface coverage (25 Å2/molecule), the spreading of the acetonitrile on the mercury surface was much more difficult for the ionic liquids measured, which implies a strong tendency to form 3D structures. The isotherms for the ionic liquids with longer alkyl chains present the same mentioned characteristics, with higher surface area and increasing number of steps, as expected for ionic liquids with increasing alkyl chains53. 3.2 X-ray reflectivity measurements' phenomenology. X-ray reflectivities were measured for the ionic liquids [C4mim]+ [NTf2]-, [C12mim]+ [NTf2]-, [C18mim]+ [NTf2]- and [C18mim]+ [FAP]-. The measurements were done in the plateau regions of the π-A isotherms to ensure a densely packed monolayer above the mercury, at 90 Å2/molecule for the shorter ionic liquids [C4mim]+ [NTf2]- and [C12mim]+ [NTf2]-, at 145 Å2/molecule for the two ionic liquids with the longer alkyl chain length. A set of reflectivity curves measured for [C12mim]+ [NTf2]- as a function of time are shown in Fig. 3. It clearly exhibits a time evolution from a smooth low-surfaceroughness curve upon deposition to a modulated curve as time goes on. A dip is observed to develop with time, and move towards lower qz values. Generally speaking, such a dip indicates the presence, on the surface of the supporting liquid mercury, of a layer having an optical density different from that of the underlying liquid. Such dip results from destructive interference between rays reflected from the top and bottom of the adlayer. As a rough estimate, the layer’s thickness is given by W=2π/qdip where qdip is the position


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of the dip. The time-dependence of the layer’s thickness W for the reflectivities of Fig. 3 is shown in Fig. 4A. These values could be reasonably well fitted by a simple phenomenological function of the form W(t)= W∞-K*exp(-(t-to)/τ), where W∞,K, t0 and τ were the fitted parameters (dashed line of Fig. 4A). Similar fits were performed for the other ionic liquids studied, [C4mim]+ [NTf2]-, [C18mim]+ [NTf2]- and [C18mim]+ [FAP]-. The time constants τ and infinite-time layer thicknesses W∞ are plotted for all ionic liquids studied in Figs. 4B and 4C, respectively. The observed time constants of hours suggest slow reorganization of the molecules on the mercury surface. Such time constants of hours were observed for the reorientation of alkylthiols on gold during self-assembly processes56,57. In our case, the evolution of the shorter [C4mim]+ [NTf2]- ionic liquids seems to be more prolonged than for the longer ones, as shown in Fig. 4B. This trend is probably a consequence of the increased affinity of the shorter [C4mim]+ [NTf2]- ionic liquid for the mercury as we will see later, the mercury atoms acting as defects disturbing the reorganization of the molecules. This stronger affinity of the shorter [C4mim]+ [NTf2]- ionic liquid for mercury can also explain the increase in the maximal layer’s thickness W∞ shown in Fig. 4C, and the longer stabilization times required to reach that larger W∞. This trend is further confirmed below by the full-curve model fits of the measured reflectivity curves. It should be mentioned that the early stage curves for the layer growth do not show a clear dip and thus are not included in the phenomenological analysis presented above. 3.3 Full-curve model fits of the X-ray reflectivity measurements To obtain a more detailed description of the interface's structure than that emerging from the phenomenological analysis above, we employed full curve fits of


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R(qz) by a modified version of the distorted crystal model (DCM), successfully employed to described the decaying atomic layering of mercury at the mercury/vapor54,55 and mercury/liquid58,59 interfaces. In this model, the mercury is represented by an infinite sum of regularly spaced Gaussians with increasing widths, leading to the constant density of mercury deep in the bulk. For the modeling of the ionic liquid adlayer, three more Gaussians were added so that the laterally-averaged surface-normal electron density is given by: 𝜌𝑏𝑢𝑙𝑘

=∑

∞

d

𝑛=0 σn √2π

exp [

−(z−nd)2 2σ2n

]+∑

3

ρk

𝑘=1 ρ𝑏𝑢𝑙𝑘

∗σ

d

k √2π

exp [

−(z−pk )2 2σ2k

] (1)

The short range of the measurable reflectivity (see below) could not support inclusion of finer details of the mercury layering like the somewhat different first layer spacing previously reported55. The mercury’s bulk electron density calculated from the roomtemperature mass density of mercury is 𝜌𝑏𝑢𝑙𝑘 = 3.25 electrons/Å3. The first term in the equation represents the mercury layers where d is the distance between the atomic layers, each layer having a gradually increasing Gaussian width: 𝜎𝑛2 = 𝜎02 + 𝑛𝜎̅ 2 . Here σ0 is the first layer width accounting for the interfacial roughness due to thermal capillary surface waves and any possible intrinsic non-thermal roughness. The parameter 𝜎̅ controls the decay length of the layering. The second term is the sum of the three additional Gaussians, of positions, widths and densities pk, k and k, respectively, representing the ionic liquid film. In the following we name the additional Gaussians according to their relative distance from the mercury surface as G1, G2 and G3, G1 being the closest to the mercury surface. The reflectivity curve corresponding to this density profile is then calculated through a Fourier transform60, R(qz)=RF(qz)|bulk-1(d/dz)exp(-iqzz)|2,


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where RF(qz) is the Fresnel reflectivity curve of an ideally flat and abrupt interface60. The 2

Fresnel reflectivity is given by RF(qz) = |

𝑞𝑧 − √𝑞𝑧2 − 𝑞𝑐2 𝑞𝑧 + √𝑞𝑧2 − 𝑞𝑐2

| where qc = (4π/λ) sin(αc) is the

critical scattering vector and αc is the critical incidence angle for total external reflection, which depends on the electron density of the sample at the interface region. Then the Fourier transform yields:

𝑅(𝑞𝑧 ) 𝑅𝐹 (𝑞𝑧 )

2 −𝑞2 𝑧 𝜎0 𝑖𝑑𝑞𝑧 𝑒 2 ̅2 𝑞2 𝜎 𝑖𝑑𝑞 − 𝑧 1−𝑒 𝑧 2

=|

3

+ρ

𝑖𝑞𝑧

𝑏𝑢𝑙𝑘

∑

𝜌𝑘 𝑑𝑒

2 𝑞2 𝑧 𝜎𝑘 𝑖𝑞𝑧 𝑝𝑘 − 2

2

|

(2)

𝑘=1

The parameter values defining the model of Eq. 1 above are then obtained by least-squares fitting Eq. 2 to the measured R(qz) curves. The model was first used to fit a long R(qz), measured on a bare mercury surface over the range 0 Å-1 qz  2.4 Å-1. A good fit was obtained yielding d = 2.72 Å, 𝜎̅ = 0.46 Å and σ0=1.2 Å, in agreement with, but with a slightly higher σ0 than the ~1 Å of, previous measurements54,55, carried out under H2 atmosphere or vacuum. Upon deposition of the ionic liquid film, the fitted roughness σ0 was found to increase with time to values ~1.6 Å. This, and the shape evolution of R(qz), manifested by the appearance with time of the dip (Fig. 4), reduce the reflected intensity at qz > 0.6-0.8 Å-1 by a factor of 100 below RF, as highlighted in the inset to Fig. 5, rendering the reflected signal practically unmeasurable above background for qz > 1.4-1.5 Å-1. This restricted the measurement for times t ≥ 4-5 h (when the dips first started appearing) to short reflectivities over the range 0 Å-1 qz  1.4 Å-1, as mentioned above. Moreover, the increased σ0 washes out the layering-generated oscilations in below the mercury


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surface, which are distinguishable for the lower roughnesses of refs. 54-55. The consequent smooth contribution to from the mercury is demonstrated in Fig. 5 by the blue dashed line. The fits of the measured R(qz) for the adlayer-covered mercury surface were performed for qz values from 0.25 to 2.4 Å-1 for the long scans and from 0.25 to 1.4 Å-1 for the short scans. For all the reflectivity data, good fits could be obtained with d = 2.72 Å and 𝜎̅ = 0.46 Å, as discussed above. In order to restrict the total number of free parameters, the reflectivity data was first fitted with all parameters (σ0, p1, 1, 1, p2, 2, 2, p3, 3, 3) of the model free to vary. The d and 𝜎̅ parameters were kept fixed at the values reported above since their values depend strongly on the shape of the measured layering peak at ~2.2 Å-1, which could not be measured with adequate error bars for all but the very early curves after deposition, as discussed above. Then in a second round σ0 was also fixed at the average value obtained in the first round of fits. From these fits the positions of the two mercuryadjacent Gaussians G1 and G2, observed to vary only little with time for a given ionic liquid, were averaged for each ionic liquid, and served as starting values for a third round of fits, in which only small variations in these values (up to 0.15Å) were allowed. The amplitudes and widths of G1 and G2, as well as all parameters of the vapor-adjacent G3 Gaussian were allowed to vary freely in this third and final round of fits. The average position of the G1 and G2 Gaussians turned out to be respectively 2.6 Å and 5.6 Å above the mercury surface, for all ionic liquids studied. The fact that these positions are very close to the atomic layers' spacing of mercury (respectively 2.7 Å


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and 2 × 2.7 = 5.4 Å) together with the high densities observed at these positions, suggest that these two additional Gaussians represent two strongly altered layers of mercury, most likely by ionic liquid penetration. Alternatively, these layers can be regarded as ionic liquid layers with a high penetration of mercury atoms for the first reflectivity scans, when the mercury atoms just begin to diffuse into the layer. The fitted densities of the Gaussian G3 turned out to be significantly lower for all ionic liquids than those of the two Gaussians closer to the mercury surface. An illustration of the fit results is given in figure 5 for the 12h42min reflectivity curve (the lowest curve in figure 3) of [C12mim]+ [NTf2]-. This reflectivity curve is very well fitted by the model discussed above. The inset of figure 5 shows a measured, Fresnel normalized, reflectivity (symbols) and the corresponding fit (dashed line). The corresponding laterally averaged surface-normal electron density profile is shown in the main figure 5 as a black solid line, with the mercury contribution shown as a blue dashed line, and those of the three Gaussians G1, G2 and G3 representing the adlayer shown in cyan, green and magenta dashed lines, respectively. These contributions are discussed below. 3.4 Effect of the alkyl chain length on the reflectivities and electron density profiles. The Fresnel-normalized reflectivity data measured at different times after deposition and their final fits by the model discussed above, are shown in figure 6A and 6B for all the ionic liquids addressed here. As the first two reflectivities for [C4mim]+ [NTf2]- in figure 6A show, no dip develops during the first two hours and only a slight decrease in the intensity was


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observed. However, reflectivity measurements that were taken after about four hours showed the development of two dips at qz ≃ 0.45 Å-1 and qz ≃ 1.15 Å-1. Those dips evolved with time, became sharper and moved toward lower qz values. After about 9h the reflectivity dips reached qz ≃ 0.35 Å-1 and qz ≃ 1 Å-1 as shown in the last scan in figure 6A. In the [C12mim]+ [NTf2]- reflectivities only a single dip is observed at qz ≃ 0.9 Å1

. It started to develop after about six hours, as shown by the second scan. The dip

became sharper and moved toward lower qz values reaching qz ≃ 0.75 Å-1 after 7h, as shown by the third scan. It sharpened and reached qz ≃ 0.55 Å-1 in the last scan, measured 12h42min after film deposition. For [C18mim]+ [NTf2]-, also only a single dip is observed, at qz ≃ 0.8 Å-1, first emerging after about six hours, as shown by the third scan. The dip slightly moved to lower qz values, sharpened, and reached qz ≃ 0.65 Å-1 after the last measured curve, 6.6h after deposition. Finally, to check the effect of an anion change, we measured reflectivities for the [C18mim]+ [FAP]- ionic liquid. Here also a single dip appeared after about four hours at qz ≃ 0.7 Å-1, as shown by the third scan. The dip moved to lower qz values, only slightly sharpened and after about 9h reached qz ≃ 0.55 Å-1, as shown by the last scan. The X-ray results for [Cnmim]+ [NTf2]- presented here are very different from the only previously reported study of a mercury supported ionic liquid film, of the chargelocalized short-chain [C4C1pyr]+ [FAP]- ionic liquid52. For this compound a 2D-smectic order was observed in the mercury-supported Langmuir film. In our case the ionic liquids


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are charge-delocalized, and did not show any 2D order. We speculate that the different nature of the charge distribution within the two cations yields a different degree of the cation interaction with the underlying mercury, which results, in turn, in a different film structure for the two ionic liquids. As seen on figure 6A, the reflectivities of the four different ionic liquids exhibit different evolutions, emphasizing the importance of the specifics of the molecules on this evolution. The formation of two dips in the X-ray reflectivities was seen only for the [C4mim]+ [NTf2]- ionic liquid whereas for all the other ionic liquids only one dip was seen, even though the reflectivity curves extend to twice (short scans) or four times (long scans) the qz position of the dip. Moreover, there seems to be a correlation between the length of the alkyl chain and the qz position reached by the dip. The shorter the length of the alkyl chain, the smaller were the qz values which the dip reached after 10-12 hours, and the sharper were the dips. As seen on figure 6A, for [C4mim]+ [NTf2]- the sharp dip reached ~0.35 Å-1, for [C12mim]+ [NTf2]- ~ 0.55 Å-1 and for [C18mim]+ [NTf2]- the less sharp dip reached ~0.65 Å-1. The behavior found for [C18mim]+ [FAP]- was slightly different at first glance, for which a less sharp dip reached ~0.55 Å-1, with higher values of R/RF than for [C18mim]+ [NTf2]-. Figure 6B shows the electron density profiles derived from the reflectivities using the final fits by the distorted-crystal model discussed above. The contribution of the mercury in equation (1), identical for all reflectivity fits, was subtracted off and only the contribution of the ionic liquid film, consisting of the three gaussians of the second term of equation (1), is shown, with the mercury surface being at z = 0 Å. The same color code was used for both the reflectivities and the density profiles. The density profile for the


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first reflectivity curve after deposition, whose time was taken as the reference t = 0h, is shown in a blue solid line for all the ionic liquids measured. This first curve was similar for all the ionic liquids, with the two near-interface Gaussians G1 and G2 having a mercury-relative electron density of about 0.3 and the third Gaussian G3 having a mercury-relative density of about 0.1. The electron densities of the ionic liquids calculated from their room-temperature mass density (from literature48 and from our measurements) varies between 0.44 electron/Å3 for [C4mim]+ [NTf2]- and 0.30 electron/Å3 for [C18mim]+ [NTf2]-. Relative to mercury, the electron density of the ionic liquids varies between 0.14 and 0.09. When considering the density profiles just after deposition (blue lines in the right panels of Figure 6), the topmost ~5 Å-thick layer, which has a relative electron density of about 0.1 can be interpreted as a diffuse monolayer of surface-parallel ionic liquid molecules. However, the lower, mercuryadjacent, 7-8 Å-thick layer (corresponding to the two near-mercury gaussians) which has a relative electron density of about 0.2-0.3 cannot be composed only of ionic liquid molecules and must incorporate some mercury atoms. The general time evolution trend (though not the time constant τ discussed above) of the density profile is similar for all the ionic liquids, as shown by the arrows in figure 6. Overall, the maximal relative electron density increases with time and reaches values of about 0.8-0.9, i.e. close to, but below, the bulk density of mercury. This indicates the formation of a region mostly composed of mercury atoms. During the time evolution, it was qualitatively observed that the density of the first Gaussian G1 grew faster, whereas those of the two outer Gaussians G2 and G3 grew over a longer time scale. The widths of G1 and G2 didn’t vary significantly whereas the width of the outermost Gaussian increased with time. Also, the


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position of this Gaussian, which was not fixed in the fit, moved slightly further away from the mercury surface with time. The effect of the cation's alkyl chain length on the density profiles at long times is shown on figure 7. Note first that the near-mercury density is slightly lower for the longest-chain ionic liquids than the roughly-equal ones for the shorter chains. Moreover, the density further from the mercury surface, near the vapor side of the film, say at -15 to -20 Å, increases significantly as the chain length decreases. These features indicate a generally lower penetration trend of mercury atoms into the ionic liquid film as the chain length increases. This follows the trend observed in Fig. 4C for W∞. Note also that the apparent width-at-half-height of the density profiles in the figure decreases with increasing chain length. This counterintuitive result is a direct consequence of the lower mercury penetration into the layer, in particular to its near-vapor part, with increasing chain length. This, in turn, reduces in particular the amplitude of the vapor-adjacent Gaussian G3, rendering the highly asymmetric density profiles narrower with increasing chain length. Further insight into the experimental results above can be gained from a consideration of the interaction of ionic liquids with metals. Pensado and Pádua studied the structure of the [C4mim]+ [NTf2]- ionic liquid at the interface of a ruthenium metallic nanoparticle by density functional (DFT) and molecular dynamics (MD) methods39. This study was motivated by the observed stabilization of transition-metal38 and gold37 nanoparticles in imidazolium-based ionic liquids. The atomic model of Pensado and Pádua takes into account the high polarizability of the metallic atoms and can be adapted to other metallic surfaces39. Although the interface between ionic liquid and liquid


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mercury is much more complex, the interactions at the atomic level should be similar, and the Pensado-Padua model is therefore relevant to the present study. They found that the physical adsorption of the ionic liquid ions on the metal surface gave rise to a one ion thick solvation layer, with the charged moieties of both the imidazolium cation and the [NTf2]- anion residing at the metal surface. The CF3 groups of the [NTf2]- anions and the non polar side chains of the [C4mim]+ cations were preferentially directed away from the metal surface. Inside the charged moieties, the oxygen atoms of the [NTf2]- anions and the C2 carbons of the imidazolium rings (between the two nitrogen atoms) were preferentially in contact with the metal surface. Porting these results to our mercury-ionic liquid interface we observe that similar interactions between the mercury atoms and the charge-delocalized ions can become large enough to give rise to substantial adsorption of mercury atoms at specific locations on the ionic liquid molecule. The most probable locations for such substantial adsorption of mercury atoms on the ionic liquid molecule are given at the top of figure 8. These locations are hypothesized based on the studies discussed above, and on steric hindrance considerations. Unlike the atoms of a solid metal surface, the atoms of liquid mercury are free to move so the C4 carbons of the imidazolium rings can also become an adsorption site. Based on these interactions, molecular models for the density profiles just after deposition and after stabilization are given on the bottom of figure 8. Just after deposition, the upper 5 Å of the density profiles, of a relative density around 0.1, consists of lying down alkyl chains of the [Cnmim]+ cations and the CF3 groups of the anions. With alkyl chain length increasing from 4 to 18 carbons, this layer becomes less diffuse


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and closer to the expected relative electron density of pure ionic liquid molecules (0.09 for [C18mim]+ [NTf2]- but 0.14 for [C4mim]+ [NTf2]-) as shown by the blue solid lines of the density profiles for each ionic liquid (right panels in Figure 6). The inner 7-8 Å of the density profiles just after deposition, with a relative density of about 0.2-0.3, consists of the charged moieties of the ions with intercalated mercury atoms. The intercalation of mercury atoms not only explains the higher electron density but also the shift to higher surface area in the isotherms discussed above, since few mercury atoms around each imidazolium ring can significantly increase the surface area of the ionic liquid. At long times after deposition, the layer has a total width of about 20 Å and the electron density is very close to the mercury density for the mercury-adjacent 8-10 Å, then decreasing with distance from the mercury interface. Reorganization of the ions into domains of few ion pairs in which the charged moieties mount one over the other with intercalated mercury atoms, and the non polar moieties pointing on average more toward the inside than the outside, is shown on the bottom of figure 8. Such self-assembly is consistent with previously reported MD simulations where the imidazolium ionic liquids with alkyl side chains longer than or equal to C4 showed local nanoscale segregation into polar and non polar domains42. This explains the interfacial growth, where the 7-8 Å near the mercury are mostly composed of mercury atoms, while further above the mercury interface a progressive decrease in the intercalation of the mercury atoms is observed. Also, with increasing alkyl chain length the polar domains are proportionally less important, so less mercury atoms are found at the interface. As seen in figure 7 the interface for [C4mim]+ [NTf2]- is thicker and higher relative electron densities are reached, about 0.9 instead of 0.8 for [C18mim]+ [NTf2]-.


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4. Conclusion The present X-ray and surface tension study demonstrates that the interface between imidazolium ionic liquids and liquid mercury is highly disordered, with a decrease in the concentration of mercury atoms with distance towards the vapor. The concentration evolves over a period of hours. Unlike the pyrrolidinium ionic liquid, where charge is more localized, the high charge delocalization of both the imidazolium cation and the [NTf2]- anion increases the affinity of the ionic liquid for the mercury atoms at the sites of charge delocalization. In fact, for ionic liquids with longer non-polar alkyl chains this affinity is reduced and mercury concentration gradients with lower electron densities are observed. Also, the kinetics observed may point towards some lateral segregation of the ions at the interface, probably into non-polar and polar domains including mercury atoms. Finally, these results highlight the importance of the chemical nature of the ions on the structure of the interface between ionic liquids and metals. Detailed theoretical modeling and simulations of this complex system should provide more insights into the nature and structure of this important liquid metal-ionic liquid interface.

Supporting information (accessible electronically) Including pressure-area isotherms for all the ionic liquids and calculated dimensions of the ions.


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(52) Tamam, L.; Ocko, B. M.; Reichert, H.; Deutsch, M.: Checkerboard Self-Patterning of an Ionic Liquid Film on Mercury. Physical Review Letters 2011, 106, 1-4. (53) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Deutsch, M.: Langmuir films of normal-alkanes on the surface of liquid mercury. Journal of Chemical Physics 2003, 119, 10339-10349. (54) Magnussen, O. M.; Ocko, B. M.; Regan, M. J.; Penanen, K.; Pershan, P. S.; Deutsch, M.: X-ray reflectivity measurements of surface layering in liquid mercury. Physical Review Letters 1995, 74, 4444-4447. (55) DiMasi, E.; Tostmann, H.; Ocko, B. M.; Pershan, P. S.; Deutsch, M.: X-ray reflectivity study of temperature-dependent surface layering in liquid Hg. Physical Review B 1998, 58, 13419-13422. (56) Calvente, J. J.; Molero, M.; Andreu, R.; Lopez-Perez, G.: Aliphatic Alcohols Facilitate Interfacial Reorientation of Thiols: Correlation with Alcohol Adsorptivity. Langmuir 2010, 26, 5254-5261. (57) Peterlinz, K. A.; Georgiadis, R.: In situ kinetics of self-assembly by surface plasmon resonance spectroscopy. Langmuir 1996, 12, 4731-4740. (58) Duval, J. F. L.; Bera, S.; Michot, L. J.; Daillant, J.; Belloni, L.; Konovalov, O.; Pontoni, D.: X-Ray Reflectivity at Polarized Liquid-Hg-Aqueous-Electrolyte Interface: Challenging Macroscopic Approaches for Ion-Specificity Issues. Physical Review Letters 2012, 108, 1-5. (59) Elsen, A.; Festersen, S.; Runge, B.; Koops, C. T.; Ocko, B. M.; Deutsch, M.; Seeck, O. H.; Murphy, B. M.; Magnussen, O. M.: In situ X-ray studies of adlayer-induced crystal nucleation at the liquid-liquid interface. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 6663-6668. (60) Pershan, P. S.; Schlossman, M. Liquid Surfaces and Interfaces Synchrotron X-ray Methods; Cambridge University Press: Cambridge, 2012.

Figure 1:

Structures, nomenclature and abbreviations of the cations and anions studied.


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Figure 2: 50

[C mim]+ [NTf ]4

2

+

-

[C mpyrr] [FAP] 4

40

 (mN/m)

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Langmuir

30 a 20

b

10 0

50

100

150

A (Å2)

200

250

a: Measured surface pressure as a function of surface area (π-A isotherm) for the mercury-supported [C4mim]+ [NTf2]- ionic liquid (blue symbols). b: Previously published52 measured π-A isotherm for [C4C1pyr]+ [FAP]- ionic liquid (red symbols).


Langmuir

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Figure 3:

Measured x-ray reflectivity curves for the indicated mercury-supported ionic liquid at the specified times after film deposition. Curves are shifted by a decade each for clarity. Error bars are shown for the two first and the two last long scans. The lowest curve is the one shown in more detail Figure 5.


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Figure 4:

A: Time-dependence of the layer’s thickness W=2π/qdip for the ionic liquid [C12mim]+ [NTf2]- on mercury (data points), as derived from the positions qdip of the dips in the reflectivity curves in Fig. 3. The dashed line is a fit to an exponential curve as explained in the text. B,C: Time constants τ and terminal thickness W∞ of the adlayer extracted from the exponential fits for [Cnmim]+ [NTf2]- (circles) and [C18mim]+ [FAP]- (cross).


Langmuir

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Figure 5:

Details of the fit of the x-ray reflectivity curve of the mercury-supported [C12mim]+ [NTf2]- film by the modified distorded crystal model (DCM) discussed in the text. Main: the best-fit surface-normal electron density profile (normalized to the electron density of mercury Hg=3.25 electrons/Å3) derived from the fit of the 12h42min measurement (lowest reflectivity curve in figure 3). The black solid line is the density profile. This profile can be decomposed into two parts: the three Gaussians of the ionic liquid adlayer G1, G2 and G3 (cyan, green and magenta dashed lines) and the underlying mercury (blue dashed line). Inset: measured, Fresnel normalized, reflectivity at 12h42min after deposition (symbols) and its best fit (dashed line).


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Figure 6:

A: Fresnel-normalized reflectivity curves (symbols) and fits (dashed lines) at different times for the ionic liquids [C4mim]+ [NTf2]-, [C12mim]+ [NTf2]-, [C18mim]+ [NTf2]-, and [C18mim]+ [FAP]-. B: Ionic liquid films' surface-normal electron density profile corresponding to the fits in A, at different times and for the same ionic liquids. These profiles were obtained by subtracting the mercury contribution from the fitted total electron density profile. The electron density is normalized to that of mercury and the


Langmuir

mercury surface resides at z = 0 Å. The same color code was used for the reflectivity and electron density curves.

Figure 7: [C4mim]+ [NTf2]-

0.8

[(z)-DCM(z)]/Hg

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0.6

[C12mim]+ [NTf2][C18mim]+ [NTf2][C18mim]+ [FAP]-

0.4 0.2 0 -20

-10 z (Å)

0

10

Comparison between the electron density profiles of the adlayers for the longest measured times for the different ionic liquids [C4mim]+ [NTf2]-, [C12mim]+ [NTf2]-, [C18mim]+ [NTf2]-, and [C18mim]+ [FAP]-.


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Figure 8:

Top: The interactions between the charge-delocalized ions and the polarized mercury atoms, with the probable locations of the mercury atoms for which the forces are the strongest. Bottom: Molecular model based on these interactions for the evolution of the density profiles just after deposition and at the longest measured times. The time evolution is generated by a self-assembly mechanism in which the polar moieties of the ions segregate into domains with intercalated mercury atoms.


Liquid-Mercury-Supported Langmuir Films of Ionic Liquids: Isotherms

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