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Surface Ordering of Amphiphilic Ionic Liquids James Bowers* and Marcos C. Vergara-Gutierrez Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom
John R. P. Webster ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom Received August 14, 2003. In Final Form: November 10, 2003 Neutron reflectometry measurements suggest that the interfacial structure normal to the free surface of two short-chain amphiphilic 1-alkyl-3-methylimidazolium-based ionic liquids is inhomogeneous, with the inhomogeneity extending ∼40 Å. For the two liquids studied (1-butyl-3-methylimidazolium tetrafluoroborate and 1-octyl-3-methylimidazolium hexafluorophosphate), the data are consistent with a stratified surface model with distinct regions of alkyl chains and ionic headgroups.
1. Introduction In modern chemistry, an ionic liquid (IL) can be considered as a salt composed of an organic cation and, commonly, an inorganic anion, X, that is molten at, or close to, room temperature. Their many favorable properties have led to their development as solvents for a wide range of applications.1 Recently, ILs based on the 1-alkyl3-methylimidazolium cation, Cnmim, have received much attention because these ILs can be synthesized containing water-stable anions. Further to this, the Cnmim cations possess an inherent amphiphilicity and may be considered as short-chain cationic surfactants or hydrotropes. This amphiphilicity together with charge ordering and hydrogen bonding capabilities suggest that extended ordering may be found in such liquids. Indeed, thermotropic liquidcrystalline mesophases in longer-chain ILs2-4 and lyotropic mesophases in concentrated aqueous solutions of ILs with moderate alkyl chain lengths5 have been reported. The similarity in ordering in the liquid and crystalline states is currently receiving attention.6 Further to X-ray and neutron diffraction studies,7-10 the structure in the liquid state has been investigated using computer simulations, in which emphasis is placed on determining the anion distribution.11-14 Recently, the gas-liquid interface of C1* Author to whom correspondence should be addressed. (1) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley: New York, 2003. (2) Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1996, 1625. (3) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (4) Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McGrath, S. E. J.; Nieuwenhuyzen, M. Chem. Mater. 2002, 14, 629. (5) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258. (6) Hardacre, C. Order in the Liquid State and Structure. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley: New York, 2003; Chapter 4, p 127. (7) See ref 6 for details. (8) Takahashi, S.; Suzuya, K.; Kohara, S.; Koura, N.; Curtiss, L. A.; Saboungi, M. L. Z. Phys. Chem. 1999, 209, 209. (9) Hardacre, C.; Holbrey, J. D.; McMath, S. E. J.; Bowron, D. T.; Soper, A. K. J. Chem. Phys. 2003, 118, 273. (10) Hagiwara, R.; Matsumoto, K.; Tsuda, T.; Ito, Y.; Kohara, S.; Suzuya, K.; Matsumoto, H.; Miyasaki, Y. J. Non-Cryst. Solids 2002, 312, 414. Matsumoto, K.; Hagiwara, R.; Ito, Y.; Kohara, S.; Suzuya, K. Nucl. Instrum. Methods 2003, B199, 29. (11) Hanke, C. G.; Price, S. L.; Lynden-Bell, R. M. Mol. Phys. 2001, 99, 801.
mim-Clhas been considered by Lynden-Bell using atomistic simulation.15 The results imply that the headgroups at the surface are significantly ordered. The interface between ILs and other bulk phases is of interest for applications involving heterogeneous catalysis and for situations in which the wettability of substrates by an IL (or IL-rich) phase is significant. A knowledge of the interfacial structure, of both pure ILs and of mixed systems containing ILs, is required to obtain a better understanding of these processes. However, there have been few dedicated structural studies of the surfaces of ILs. Watson and co-workers have measured the surface tension16,17 of Cnmim-based ILs and have applied direct recoil spectroscopy (DRS) to elucidate the molecular orientation at the free surfaces of the liquids.18,19 Carmichael et al. have examined the structure of spin-coated films of Cnmim-X salts in their liquid, liquid-crystalline, and solid states using X-ray reflectometry (XR) and have found evidence for multilayer ordering.20 Interestingly, the DRS and XR studies suggest that the orientation of the salt molecules is such that the charged units are situated adjacent to the vapor phase. Here, we report the findings of a preliminary neutron reflection (NR) study examining the structure of the free surface of two ILs: 1-butyl-3-methylimidazolium tetrafluoroborate, C4mim-BF4, and 1-octyl-3-methylimidazolium hexafluorophosphate, C8mim-PF6. Our principal objective was to seek evidence for segregation at the surface, specifically the formation of a hydrocarbon-rich region. The ability to employ deuterium labeling of the alkyl chain, as we have done here, facilitates our investigations. (12) Morrow, T. I.; Maginn, E. J. J. Phys. Chem. 2002, B106, 12807. (13) de Andrade, J.; Bo¨es, E. S.; Stassen, H. J. Phys. Chem. 2002, B106, 3546. de Andrade, J.; Bo¨es, E. S.; Stassen, H. J. Phys. Chem. 2002, B106, 13344. (14) Shah, J. K.; Brennecke, J. F.; Maginn, E. J. Green Chem. 2002, 4, 112. (15) Lynden-Bell, R. M. Mol. Phys. 2003, 101, 2625. (16) Law, G.; Watson, P. R. Langmuir 2001, 17, 6138. (17) Law, G.; Watson, P. R. Chem. Phys. Lett. 2001, 345, 1. (18) Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429. (19) Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Phys. Chem. Chem. Phys. 2001, 3, 2879. (20) Carmichael, A.; Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Seddon, K. R. Mol. Phys. 2001, 99, 795.
10.1021/la035495v CCC: $27.50 © 2004 American Chemical Society Published on Web 12/11/2003
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Table 1. Scattering Length Densities of Components and Molecular Segments Relevant to This Study scattering length density, 10-6 Å-2 moiety
C4mim-BF4
C8mim-PF6
notesa
whole molecule alkyl-d2n+1 chain cationic headgroup anion headgroup (including anion)
4.47 6.65 1.73 2.08 3.79
4.92 6.28 1.73 2.91 4.62
based on bulk liquid density based on octane density based on density of 1-methylimidazole based on density of 1-methylimidazole based on density of 1-methylimidazole
a See ref 1 for the densities of the ILs; all other densities are taken from the CRC Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995-1996.
2. Experimental Section The ILs were synthesized according to standard methods by a quaternization reaction of 1-methylimidazole using a bromoalkane.1 For these experiments, the alkyl chain is perdeuterated, and the deuterated bromoalkanes were purchased from Cambridge Isotope Laboratories (C4D9Br, 98 D atom %; C8D17Br, 98 D atom %). The bromide salts were repeatedly washed with ethyl acetate, and the postmetathesis products were washed with dichloromethane and dried under a high vacuum. The purity of the products was checked using 1H NMR spectroscopy. After drying, precautions were taken to minimize the uptake of atmospheric water by these hygroscopic ILs. For the reflectivity measurements, the samples were located in a low-volume poly(tetrafluoroethylene) trough sited inside a vapor-tight stainless steel cell with quartz windows.21 NR is an established surface structure determination technique, and the reflectivity is related to the composition profile normal to the interface.22 The measurements were conducted on the SURF reflectometer at the ISIS Facility, Rutherford Appleton Laboratory, U.K. SURF is a time-of-flight (TOF) instrument and, operating at 50 Hz, utilizes neutrons with wavelengths 0.5 < λ < 7.0 Å. The reflectivity measurements were performed with a constant illuminated area and resolution (δQ/Q ≈ 7%) over the entire Q range, where Q is the momentum transfer normal to the interface, defined as Q ) 4π sin θ/λ, in which λ is the neutron wavelength and θ is the grazing angle of incidence. The TOF spectrum was converted to the wavelength spectrum, and after normalization by monitor counts and correcting for detector and monitor efficiency, the reflectivity data were placed on an absolute scale by normalization with a scale factor determined from the measurement of the reflectivity from D2O for the same instrument geometry. Further details of the operation of TOF methods can be found in refs 23 and 24. The reflectivity data have been modeled using optical layer models, rather than by inversion, to yield a scattering length density profile consistent with the reflectivity data. A discussion of subjective and objective data analysis methods can be found in the review article by Lovell and Richardson.25 The constant resolution and a flat background term have been included in the modeling of the reflectivity data. The composition is directly related to the scattering length density, Nb, via Nb ) ∑iφi(Nb)i, where φi and (Nb)i are the volume fraction and scattering length density, respectively, of component i in the bulk. The scattering length densities relevant to this study are given in Table 1. Reflectivity calculation and modeling was performed using the MULF and free-form SLAB_FIT26 fitting programs, which evaluate the reflectivity using the optical matrix (21) Zarbakhsh, A.; Bowers, J.; Webster, J. R. P. ISIS Annual Report; RB10753; Rutherford Appleton Laboratory: Didcot, U.K., 2000. (22) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (23) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (24) Penfold, J. Physica B 1991, 173, 1. (25) Lovell, M. R.; Richardson, R. M. Curr. Opin. Colloid Interface Sci. 1999, 4, 197. (26) Penfold, J. MULF; Rutherford Appleton Laboratory: Didcot, U.K., 1988 (reflectivity analysis program). Sivia, D. S. SLAB_FIT; Rutherford Appleton Laboratory: Didcot, U.K., 1991 (reflectivity analysis program).
method. In SLAB_FIT, at the start of each analysis the overall thickness of the interfacial region, τ, is fixed, as are the maximum and minimum scattering length densities.
3. Results and Discussion The measured reflectivity data from the surface of C8mim-PF6 has been modeled in terms of optical layer models. Evidence for the presence of an interfacial inhomogeneity normal to the surface is obtained from modeling using zero- and one-layer models. To model the data using a bare-substrate (zero-layer) model requires the scattering length density of the subphase to be unreasonably large (Nb ) 5.83 × 10-6 Å-2 compared with the calculated value Nbsub ) 4.92 × 10-6 Å-2), thus indicating the unsuitability of the model. One-layer models, using the calculated Nbsub, that reasonably reproduce the experimental data indicate that the surface region is of the order 40-55 Å in extent with a reasonably well-defined scattering length density of ∼5.27 × 10-6 Å-2, which is in excess of Nbsub. This enhancement could be associated with deuterium enrichment in the surface region or a more ordered surface layer. However, surface tension measurements suggest that the surfaces of ILs are more disordered than the bulk16 and the spatial extent of this region is clearly greater than the length of the molecule (∼17 Å), indicating that this model only coarsely represents the structure at the interface. Further guided reflectivity simulations have been applied to assess the data in terms of a chemically sensible structural model. It transpires that a three-layer model provides a simple model that is consistent with the data and possesses realistic molecular dimensions and chemical composition. The measured and modeled reflectivity data and the modeled scattering length density profiles are shown in Figure 1 for an indicative three-layer model. Also shown are the profiles for zero- and one-layer models using Nbsub. The three-layer model reproduces the experimental data more faithfully than the one-layer model and is supported, but not purely justified, by the presence of an enhancement in the signal in the region 0.15 < Q < 0.5 Å-1. We believe that this enhancement is a genuine interference fringe, but unfortunately the fringe is not sufficiently wellresolved to allow us to determine the thickness of the interfacial inhomogeneity unambiguously. If this feature is considered as background, then the background is ∼8 × 10-6, which is significantly greater than the background of ∼2 × 10-6 commonly found for liquid samples on SURF. It is notable that the background for the C4mim-BF4 sample is 2.2 × 10-6. Both the C4mim-BF4 and C8mimPF6 samples contain a similar concentration of protons, and the nuclei in the two counterions do not possess significant incoherent scattering cross sections and should, thus, contribute similarly to the background signal. A physical interpretation of the three-layer model is that multilayer ordering is occurring: an alkyl-rich region is located on top of a region representing the cationic
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Figure 1. Measured (b) and modeled reflectivity data and modeled scattering length density profiles (inset) for the free surface of C8mim-PF6: zero-layer (dashed line), one-layer (dotted line), and three-layer (solid line) models are also shown. See text for details.
Figure 2. Measured (b) and modeled reflectivity data and modeled scattering length density profiles (inset) for the free surface of C4mim-BF4: zero-layer (dashed line), one-layer (dotted line), and three-layer (solid line) models are also shown. See text for details.
headgroups and anions, which is, in turn, on top of a further alkyl-rich layer. The dimensions and scattering length densities of all the layers in this model correspond to sensible values (see Table 1). We must stress here that the scattering length density profile presented for the three-layer model is by no means uniquely determined from the data. Further simulations reveal that the scattering length density profile containing more oscillations, representing a more extended structure than the three-layer structure modeled here, also generates a reflectivity profile that reproduces the experimental values. Qualitatively similar oscillatory profiles have been reported to occur at the surfaces of aqueous surfactant solutions27 and microemulsions.28 In addition to this ambiguity regarding the depth of the ordering, four-layer models (as refinements of the three-layer model) can also be generated that allow for charged groups at the vapor side of the interface. Surprisingly, despite its shorter alkyl-chain length, a qualitatively similar result emerges for C4mim-BF4. In this case, for the bare substrate model, the liquid requires an unreasonable scattering length density of 5.19 × 10-6 Å-2 compared with Nbsub ) 4.47 × 10-6 Å-2, and the onelayer models with calculated Nbsub indicate a structural inhomogeneity with Nb ) 4.68 × 10-6 Å-2 extending ∼4055 Å normal to the surface. As for the C8mim-PF6 case, such models are unsuitable. Further simulations were performed, and a three-layer model is the simplest model that is consistent with the data and makes chemical sense. The measured and simulated reflectivity data and model scattering length density profiles are shown in Figure 2 for zero-, one-, and three-layer models. As for the C8mimPF6 case, a four-layer refinement of the three-layer model accommodating an extra layer representing the headgroups adjacent to the vapor is also consistent with the data. It is notable that distinctions regarding the length of the alkyl chains and identity of the anions of the two ILs studied here are contained in these models. Figure 3 compares representative scattering length density profiles corresponding to the three-layer models for the two ILs. The results suggest that the surfaces of these two ILs
Figure 3. Sample scattering length density profiles through the free surfaces of C4mim-BF4 (dotted line) and C8mim-PF6 (solid line). The regions occupied by the charged groups have been overlapped to demonstrate that the regions of enhanced scattering length density correspond to the molecular dimensions of the alkyl groups.
(27) Steitz, R.; Braun, C.; Lang, P.; Reiss, G.; Findenegg, G. H. Physica B 1997, 234, 377. (28) Lee, D. D.; Chen, S. H.; Majkrzak, C. F.; Satija, S. K. Phys. Rev. E 1995, 52, R29.
are ordered with the headgroups and tailgroups segregating to form a lamellar structure and that the structure extends to at least two tiers of alkyl groups. In addition to the uncertainties present in resolving the precise layer thicknesses, uncertainties in the structure determination concern the depth to which the ordering extends and whether the charged groups reside on the vapor side of the surface. Further experiments are required, using more extensive isotopic substitution, to resolve these issues. With such extensive structures at surfaces, it is useful to turn to free-form data analysis to enable further models to be considered. Some example scattering length density profiles resulting from such analysis are shown in Figure 4 for both ILs; a number of overall widths, τ, for the interfacial region were used. The profiles shown in Figure 4b,c, for C4mim-BF4, can be discounted on physical grounds, leaving models in agreement with those discussed previously. However, models incorporating extended oscillations representing regions of segregation have not been found despite allowing an accommodating interfacial width.
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Figure 4. Alternative scattering length density profiles for the surface of (a-c) C4mim-BF4 and (d-f) C8mim-PF6. See text for details.
The orientations of the cations and anions of a range of ILs have been studied by Watson and co-workers18,19 using DRS. NR is able to probe a greater depth in to the bulk of the liquid than DRS, which probes the uppermost few angstroms portion of the surface region. We have obtained evidence suggesting that surface ordering occurs normal to the free surfaces of the ILs studied and have, thus, provided a complementary view of the surface structure. For the ILs considered here, the results of Watson et al.’s DRS measurements indicate that the cation is oriented with the imidazolium ring perpendicular rather than parallel to the surface and with a tendency for the N-methyl group to be directed toward the gaseous phase.
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Unfortunately, we are unable to unambiguously verify these observations. However, the simulation results of Lynden-Bell15 are consistent with the majority of Watson et al.’s findings and also suggest that the headgroups of Cnmim-based ILs have a tendency to order in discrete layers near the surface. Such alignment may be present in ILs with longer alkyl chains and may give rise to complex structures at interfaces. With the exception of density oscillations at the free surface of liquid metals29 and in a molecular liquid adjacent to a solid substrate,30 such ordering in a single non-liquidcrystalline fluid composed of small molecules has hitherto not been reported. Accordingly, the origin of the ordering is of interest because it has implications for the ordering found in mixed systems such as those mentioned earlier.27,28 A further important issue is whether traces of water in these hygroscopic liquids have a profound influence on the surface/ordering behavior or whether the native amphiphilicity of the molecules is sufficient to drive the organization. Acknowledgment. The authors would like to thank the EPSRC for funding this project (GR/R56129) and the CCLRC of the U.K. for provision of beamtime on SURF. We wish to thank Dr. C. Hardacre and colleagues at the QUILL Centre, Queen’s University, Belfast and Dr. C. P. Butts and Mr. J. C. D. Lord (University of Exeter) for assistance, advice, and discussions. LA035495V (29) For example, see Shpyrko, O.; Huber, P.; Grigoriev, A.; Pershan, P.; Ocko, B.; Tostmann, H.; Deutsch, M. Phys. Rev. B 2003, 67, 115405 and references therein. (30) Yu, C.-J.; Richter, A. G.; Datta, A.; Durbin, M. K.; Dutta, P. Phys. Rev. Lett. 1999, 82, 2326.
