Letter pubs.acs.org/JPCL
Surface Ordering in Binary Mixtures of Protic Ionic Liquids Christiaan Ridings,† Gregory G. Warr,‡ and Gunther G. Andersson*,† †
Centre for NanoScale Science and Technology, Flinders University, Adelaide, SA 5001, Australia School of Chemistry and Australian Institute for Nanoscale Science and Technology, The University of Sydney, NSW 2006, Australia
‡
S Supporting Information *
ABSTRACT: The surface composition of binary mixtures of the protic ionic liquids ethylammonium nitrate and propylammonium nitrate has been investigated using surface tension measurements and the perfectly surface sensitive method metastable induced electron spectroscopy. Given that the latter technique is sensitive only to the outermost layer, it allows for the determination of the surface fraction occupied by a given species. The piecewise linear relationship between surface fraction and surface tension found in this study can be described by a phase separation within the surface layer.
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interfaces has been detected by surface force measurements and AFM imaging.8,9 This is not limited to an adsorbed or oriented monolayer of ions but propagates as an anisotropic near-surface structure that gradually decays into the isotropic bulk nanostructure. Such surface and near-surface layers strongly affect the lubrication of adjacent sliding layers, with some ILs producing near-zero friction coefficients.10−12 Despite their obvious importance for low-pressure applications, there have been fewer studies of the IL−air or IL− vacuum interface. X-ray reflectometry13 and surface spectroscopy studies14,15 have shown that IL−air interfaces are more complex than a simple monolayer of amphiphilic cation; the interface itself is quite diffuse. Some alkyl chains are oriented out of the liquid, but the outermost liquid layer comprises both cations and anions, suggesting that there may be ion pairs or clusters present.16,17 Between the interface and the bulk liquid, there is also an oscillatory, near-surface structure reminiscent of the layering at solid−IL interfaces. There is a striking difference with the effect that cation structure has on both the bulk and interfacial nanostructure. Xray reflectometry of EtAN also shows a diffuse IL−air interface but no near-surface layering, consistent with the absence of bulk liquid nanostructure.18 However, vibrational sumfrequency spectroscopy, neutral impact collision ion scattering,18 and metastable induced electron spectroscopy (MIES)14 all yielded a consistent picture of the outermost part of the pure EtAN surface being enriched in the methylenes of the ethanolammonium cation. Despite their complete miscibility, however, surface tensiometry shows that EAN forms a saturated adsorbed layer at the surface of EtAN at all compositions above 5% w/w.19
he structure and properties of ionic liquids (ILs) have attracted significant attention over the past two decades as they have found application in diverse areas. Formed solely of ions, and with melting points below 100 °C (and frequently below room temperature), ILs have negligible vapor pressures, making them potentially attractive as “green” replacements for conventional organic solvents and as lubricants and electrolytes, especially at high temperatures and low pressures.1,2 Protic ionic liquids (PILs) are particularly attractive because of their low cost and relative ease of synthesis from readily available acid and base starting materials.3 A wide range of alkyl- or alkanol- ammoniums readily form PILs with simple anions, exemplified by ethylammonium nitrate (EAN, C2H5NH4+NO3−).4 In the bulk, many ILs exhibit amphiphilic nanostructure, in which strong electrostatic attractions between polar cation and anion moieties solvophobically exclude pendant alkyl groups into nonpolar domains.5 PILs with even very short ethylammonium cations like EAN are nanostructured, with propylammonium nitrate (PAN, C3H7NH4+NO3−) exhibiting an even more pronounced nanostructure compared to the ethylammonium analogue. The nonpolar domain size and extent of segregation depend on the molecular structure of both the cation and anion.6 Whereas alkylammonium salts are nanostructured in the bulk phase, hydroxyl-terminated cations such as ethanolammonium nitrate (EtAN) are not, as the hydroxyl is incorporated into the cation−anion polar domains by H-bonding.6 Most PILs are miscible in all proportions, enabling liquid structure to be tuned through composition.7 The extent of nanostructure can be controlled by changing the proportion of alkyl- versus alkanol- ammonium cations, and the domain sizes can be controlled through mixing alkyl chain lengths. The consequences of this nanostructure are also evident at macroscopic IL interfaces. Layering at various solid−IL © XXXX American Chemical Society
Received: June 27, 2017 Accepted: August 20, 2017 Published: August 20, 2017 4264
DOI: 10.1021/acs.jpclett.7b01654 J. Phys. Chem. Lett. 2017, 8, 4264−4267
Letter
The Journal of Physical Chemistry Letters The complex nature of the liquid surface and bulk liquid nanostructure in alkylammonium PILs prompts us to ask whether surface segregation might occur even between PILs with cations such as PAN and EAN. This may be suggested by the fact that the aforementioned bulk nanostructure noticed in EAN is even more pronounced in PAN.8,20,21 Although lateral structure and demixing is well-known in spread or insoluble (Langmuir) films, the corresponding phenomenon in mixed adsorbed (Gibbs) layers is not. Early models, such as that of Eberhart,22 proposed that the surface tension of binary mixtures depends linearly on the surface composition (mole fraction), but these rely on the Gibbs isotherm for surface excess or a model for surface composition. To determine directly the composition of the outermost layer of mixtures of ILs, MIES is employed here. MIES is a technique perfectly sensitive to the outermost layer of condensed matter.23 Unlike other electron spectroscopy methods, such as X-ray photoelectron spectroscopy, its surface sensitivity is not derived from the inelastic mean free path of emitted electrons but the probing depth of the technique. In this case the large cross section for deexcitation of the metastable helium atoms (projectiles) leads to a very high probability of the electron excitation energy being transferred from the projectile to surface molecules. The low excitation energy (19.8 eV) leads to molecular orbitals of the surface being probed; hence, the measured energy spectrum of emitted electrons is characteristic of the composition of the outermost layer of the sample. In this way, a surface consisting of two individual components will have a measured spectrum, S, that can be deconvoluted as a linear combination of the spectra of both pure components, A and B, such that
liquid−vacuum interface and whether this surface organization mirrors the bulk miscibility of the two liquids. MIE spectra were recorded for eight binary mixtures of EAN and PAN which, along with the pure EAN and PAN spectra, were used as the input set for the SVD procedure. This procedure determined that two reference spectra were needed to fit the input set, i.e. the entire input set could be fit as a linear combination of two individual reference spectra. In this case it makes sense that these two reference spectra are those of the pure EAN and PAN, with the weighting factors αA and αB (normalized to unity) being a direct indication of the surface fraction of that component. The average EAN and PAN spectra were used as the two reference spectra for fitting the binary mixtures; further details of this are provided in the Supporting Information. The surface tension of EAN was measured as 49.9 mN/m, while PAN was 39.1 mN/m, which are in good agreement with values reported in literature,2 although it must be noted that there is a small range between reported results, likely due to the highly hygroscopic nature of the ILs and the effect of water as an active surface impurity.26,27 Figure 1 gives the surface fraction of PAN and surface tension for each sample as a function of the bulk mole fraction of PAN.
S = αA SA + αBSB
where the coefficients αA and αB are the fraction of the surface occupied by each component and thus add up to unity.24 In order to practically evaluate these components, the singular value decomposition (SVD) algorithm is applied. In brief, SVD is used to determine the number of individual reference spectra that are required to fit a set of measured spectra. Once the number of reference spectra is determined, it is then possible using a separate procedure to reconstruct each reference spectrum as fits to the measured spectra. Thus, all measured spectra in that series can be thought of as a linear combination of the reference spectra. Further discussion of the details of this procedure is provided in previous publications.23,25 By combining surface composition determined directly from MIES with surface tensiometry, Kirmse and Morgner24 have shown that the surface tensions of binary mixtures frequently vary linearly with the surface area fraction (hereafter referred to as surface fraction). That this, rather than surface mole fraction, is the determining factor is not surprising. A few large molecules at the surface should have more effect than the same number of small ones. Strikingly, some binary mixtures were also observed to deviate from linearity, exhibiting piecewise linear variation of surface tension with area composition. This was explained as being analogous to bulk phase miscibility gaps, but where two distinct surface phases coexist, each with their own chemical potential. In this work we examine binary mixtures of EAN and PAN using the combination of MIES and surface tensiometry in order to understand the adsorption behavior of ions at the
Figure 1. Surface fraction and surface tension as a function of bulk molar fraction of PAN. The lines are to guide the eye. The error bars of the surface tension reflect the reproducibility of the surface tension measurements. The error bars of the surface fraction reflect the confidence interval for the linear combination fitting procedure. Residual water can influence both surface tension and the fitting procedure.
Figure 2 shows the surface tension as a function of the surface fraction of PAN for all samples. It can clearly be seen that the data points cannot be fitted with a linear relationship within the uncertainties given by the error bars, as has been demonstrated for fully miscible binary mixtures of, e.g., formamide, polyethylene glycol, and 3-hydroxypropionitrile.24 Linear relationships between surface tension and surface fraction suggest that each molecule at the surface contributes its own free energy to the surface energy and that this happens regardless of its neighbors in the surface layer. The surface tension of EAN/PAN mixtures shows a positive deviation, which is best described as two linear segments intersecting at a surface fraction of PAN of 0.73. This implies a free energy cost of mixing components in the surface, which is resolved by segregation. Previous results on molecular mixtures have shown that similar behavior is observed when the surface tensions of the two components differ by 10 mN/m or more, such as mixtures 4265
DOI: 10.1021/acs.jpclett.7b01654 J. Phys. Chem. Lett. 2017, 8, 4264−4267
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The Journal of Physical Chemistry Letters
segregation can lead to larger scale nanostructure or phase separation.32 EAN and PAN are protic ionic liquids and are fully miscible. Full miscibility at the surface would mean that there is a linear relationship between the surface fraction and the surface tension. The mixtures of EAN and PAN do not show such a linear relationship, and the conclusion can be drawn that a miscibility gap exists at the surface of these mixtures.
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EXPERIMENTAL SECTION The ILs were prepared by the dropwise addition of nitric acid (Sigma-Aldrich) to the amine base (ethylamine, propylamine, Fluka) in equimolar amounts. The solution was stirred over the course of the addition and kept below 15 °C. Water was removed from the product by rotary evaporation at 50 °C, then by purging with nitrogen while being heated to 110 °C overnight. Using the pure EAN and PAN, eight binary mixtures were made at PAN molar fractions of 0.17%, 0.25%, 0.40%, 0.50%, 0.60%, 0.75%, 0.83%, and 0.90%. Surface tension was recorded using a profile analysis tensiometer (PAT1) from Sinterface Technologies. The equipment was flushed with ethanol and Milli-Q water prior to use, with the absence of surface active impurities checked by the surface tension of the Milli-Q water. Approximately 5 mL of sample was then flushed through prior to each measurement. MIES experiments are performed in an ultrahigh vacuum chamber with a base pressure of a few 10−10 mbar, although this pressure is not required for these measurements nor is it readily attainable because of the evaporation of trace amounts of water and gases in the ILs. The chamber pressure during measurements is typically in the low 10−6 mbar range. The technical details of the instrument are described elsewhere.14 The instrument is also equipped with a 12 kV, 200 W nonmonochromatic X-ray source (SPECS, Berlin) with both Mg and Al anodes. The XPS was used to detect surface active impurities. A high level of sample purity was confirmed by both the absence of unexpected elements and stoichiometric ratios of expected elements (C, N, O). A specialized target is used to generate a liquid surface for measurement by MIES/UPS, which is described in detail elsewhere.33,34 In brief, an upright, flat stainless steel disc is immersed roughly halfway in a reservoir containing a few milliliters of the liquid being investigated. As the disc rotates, a fresh liquid film is generated on the surface of the disc which is then exposed for analysis near the apex of rotation. The reservoir is enclosed in a cell to minimize evaporation and contamination of the vacuum chambers. A cap can also be placed on the target covering the disc to further minimize evaporation of the liquid; however, the cap was not used in this study. Instead the ILs were exposed overnight to a vacuum of approximately 10−7 mbar in a preparation chamber with the disc rotating in order to maximize evaporation of absorbed water. A previous study has shown that this order of time is needed to minimize the effect of water as a surface impurity in ILs.35
Figure 2. Surface tension as a function of the surface fraction of PAN for all samples. A piecewise linear fit is given to best describe the system.
of benzyl alcohol and formamide.28 There the surface tension displayed three piecewise linear segments.24 Surface fraction and surface tension as a function of the bulk molar fraction show continuous relationship and no breaks because the surface fraction and surface tension depend on the composition of the outermost layer rather than the bulk composition. Calculations of the Gibbs energy of mixing at the surface deviated strongly from that of ideal solutions, from which a miscibility gap at the surface was inferred.28 The piecewise linear surface tension behavior of EAN/PAN mixtures shown in Figure 2 thus also implies a miscibility gap at the surface. (The uncertainties in surface tension and surface composition are such that the true phase separation may not be present, but the surface may only consist of PAN- and EANenriched regions that can be described within a regular-solution style model,29−31 for which phase separation is a limiting case.) This means that the surface of EAN/PAN mixtures can be divided into two regions: below a PAN surface fraction of 0.73 the surface contains a mixture of EAN and PAN, but at higher PAN content, separate domains of (almost) pure PAN are present and coexist with the 0.73:0.27 mixture. It is possible to derive the Gibbs energy of mixing from the data in Figure 2 based on the procedure outlined in ref 28, and the result is shown in the Supporting Information. The Gibbs energy of mixing has its minimum around 0.4 PAN bulk molar fraction and is −25 J/mol. It needs to be emphasized that the error is rather large, as discussed in the Supporting Information. Previous investigations into the surface structure of pure PAN and EAN have shown that both the cation and anion are present in the outermost layer; however, there is more cation at the surface of PAN than EAN.14 Surface tensiometry is also consistent with EAN alkyl chains not being closely packed at the surface.19 As discussed above, neutron scattering studies have demonstrated that PAN has a much more pronounced amphiphilic bulk nanostructure than EAN. Taken together, this suggests that the simplest model of the EAN/PAN surface consists of ion pairs. The EAN-rich surface domain can accommodate substantial amounts of PAN, but the structure of the second, PAN-rich domain, which is dominated by the cation orientation of PAN, does not readily incorporate EAN. This would also support the more rapid decline in surface tension for the second domain. Such structure-driven immiscibility due to chain-length mismatch in the surface is not unprecedented. It is in many ways analogous to the bulk phase miscibility of these ionic liquids with aliphatic alcohols, in which polar−apolar
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01654. MIE spectra showing fitting procedures (PDF) 4266
DOI: 10.1021/acs.jpclett.7b01654 J. Phys. Chem. Lett. 2017, 8, 4264−4267
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AUTHOR INFORMATION
Corresponding Author
*E-mail: Gunther.andersson@flinders.edu.au. ORCID
Gregory G. Warr: 0000-0002-6893-1253 Gunther G. Andersson: 0000-0001-5742-3037 Notes
The authors declare no competing financial interest.
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REFERENCES
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