Langmuir 1999, 15, 8429-8434
8429
First Observation of Molecular Composition and Orientation at the Surface of a Room-Temperature Ionic Liquid Thomas J. Gannon, George Law, and Philip R. Watson* Department of Chemistry, Oregon State University, Gilbert Hall 153, Corvallis, Oregon 97331
Adrian J. Carmichael and Kenneth R. Seddon The QUILL Centre, The Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, U.K. Received July 30, 1999 We report the first measurements of the composition and molecular orientation at the surface of a room-temperature ionic liquids1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Recoil spectrometry using rare gas ions on continuously refreshed liquid surfaces in vacuo shows that neither ion is significantly enriched in the surface. The average orientation of the cation is with the plane of the ring vertical. The cation ring is rotated about an axis through its center such that the nitrogen atoms and side chains are deeper in the surface with the surface normal passing between the two nitrogen atoms (with an estimated error of (30°).
1. Introduction Salts that are fluid at room temperature were first reported over 40 years ago1 and have been extensively studied as battery electrolytes. Recently, they are exciting interest as environmentally friendly solvents for a range of chemical processes.2,3 Ambient temperature ionic liquids typically consist of a heterocyclic cation based on substituted imidazole or pyridine and an inorganic anion such as [AlCl4]-,[BF4]-, or [PF6]-. The chloroaluminates are usually water and/or air sensitive, while those salts with anions containing fluorine are generally air- and waterstable compounds.1 Ionic liquids of this type have many benefits; they can dissolve an enormous range of inorganic, organic, and polymeric materials at very high concentrations, are noncorrosive, have low viscosities, and have no significant vapor pressures. A rapidly developing application area for ionic liquids is that of two-phase homogeneous catalytic reactions4,5 where one phase is chosen both to dissolve the catalyst and to be immiscible with the second phase that contains the reactants and products. Such catalysis is believed to occur at the surface of the ionic liquid, at the interface with the overlying organic phase. The chemistry must depend on the access of the catalyst to the surface and the transfer of material across the interface. There are a number of similarities with conventional phase transfer catalysis (PTC) using an aqueous catalytic solution and an organic second phase.6,7 In the most common type of PTC using quaternary ammonium salt catalysts, the reaction rate depends on the surface concentration of the * Address correspondence to this author (E-mail watsonp@ chem.orst.edu). (1) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68 (4), 351. (2) Freemantle, M. Chem. Eng. News 1998, 76 (13), 32. (3) Freemantle, M. Chem. Eng. News 1999, 77 (1), 23. (4) Chauvin, Y.; Olivier-Bourbigou, H. CHEMTECH 1995, 25, (9), 26. (5) Olivier, H.; Chauvin, Y. Chem. Ind. 1996, 68 (Catalysis of Organic Reactions), 249. (6) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis; Verlag Chemie: Wienheim, 1980; Vol. 11.
catalyst, which in turn depends on the quaternary cation structure and the anion activity at the interface.7 Direct investigations of surface phenomena in PTC are few, but forms of photoelectron spectroscopy have been employed8 to investigate the surfaces of solutions of PTC catalysts (aqueous solutions are not possible for these vacuum-based techniques and the authors typically used methanamide as a model solvent). They have provided some important insights into interfacial processes in PTC: large, poorly solvated anions are closely associated in ion pairs, while more highly solvated anions are much less associated and tend to be removed from the interface. Our understanding of the mechanisms of catalysis in ionic liquids is still in its infancy. These potentially important clean-technology systems suffer from an almost complete lack of surface chemical information about the solvents themselves and the solvation of catalysts at the surface. In contrast to the situation in conventional aqueous/organic PTC systems (where vacuum-based spectroscopies must use surrogate low vapor-pressure polar solvents to represent water), the intrinsic involatility of ionic liquids makes them ideal candidates for study by vacuum spectroscopies. We have recently applied timeof-flight (TOF) ion scattering and recoil spectrometry, originally developed for solid surfaces,9 to probe the properties of low vapor pressure liquid surfaces.10 The method is surface sensitive, can detect all elements including hydrogen, and allows us to determine the atomic composition in the surface; hence it generates information on molecular orientation.11 We have previously used recoil spectrometry to investigate a variety of low vapor pressure (7) Starks, C. M. Modern Perspectives on the Mechanisms of Phasetransfer Catalysis. In Phase-transfer Catalysis: Mechanisms and Synthesis; Halpern, M. E., Ed.; American Chemical Society: Washington, DC, 1997; Vol. 659, p 10. (8) Moberg, R.; Bokman, F.; Bohman, O.; Siegbahn, H. 0. G. J. Am. Chem. Soc. 1991, 113, 3663. (9) Rabalais, J. W. J. Vac. Sci. Technol. 1991, A9, 1293. (10) Tassotto, M.; Gannon, T. J.; Watson, P. R. J. Chem. Phys. 1997, 107, 8899. (11) Gannon, T. J.; Tassotto, M.; Watson, P. R. Chem. Phys. Lett. 1999, 300, 163.
10.1021/la990589j CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
8430
Langmuir, Vol. 15, No. 24, 1999
Figure 1. Definition of the angle of incidence (R), scattering (θ), recoil (φ), and azimuthal angles (δ) in recoil spectrometry experiments.
Gannon et al.
Figure 3. Recoil spectrum of [bmim][PF6] taken with 2.5 keV Ar+ ions at a recoil angle of 45° and an incident angle from the surface of 8°.
energies between 1.5 and 2.5 keV and angles of incidence from 8° to 36° for a fixed scattering angle of 45°. 3. Results Figure 2. Molecular structure of the ionic liquid [bmim][PF6].
liquids, a siloxane,10 glycerol,10 hydrocarbons,12 and phthalate esters;11 we report here the use of this technique in the first experimental investigation of the surface properties of an ionic liquid. 2. Experimental Section The experiment involves projecting a pulsed beam of inert gas ions onto a freshly prepared liquid surface at a specified incidence angle R, measured away from the plane of the surface (Figure 1). Typically, the ion current in a small Faraday cup mounted at the sample position is about 100-300 nA before pulsing. The pulsed current (at ∼10 kHz repetition rate) is about a factor of 104 smaller. Ions are scattered into an angle θ, and surface atoms are recoiled into an angle φ; these angles are fixed and equal in our case. The physics of scattering and recoil phenomena under ion bombardment is thoroughly documented9,12 and is well described as classical binary collisions subject to conservation of energy and linear momentum. We record the elapsed flight time for scattered ions and atoms that recoil out of the liquid surface as they traverse a 1.125 m flight path. From these times we can determine the kinetic energy of the recoil atom and establish the identity of the surface atom involved in the collision from well-known formulas.9 Details of the apparatus are available elsewhere.10 The ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] [PF6], Figure 2) was prepared as described elsewhere.14,15 The liquid was transferred to the sample cell and thoroughly degassed in the spectrometer. We obtained data using Ar+ and Ne+ ions at incident (12) Gannon, T. J.; Watson, P. R. To be published. (13) Mashkova, E. S.; Molchanov, V. A. Medium Energy Ion Reflection from Solids; North-Holland: Amsterdam, 1985. (14) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (15) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem 1998, 8, 2627.
Figure 3 shows a spectrum taken with 2.5 keV Ar+ ions at an incident angle R ) 8°. By use of this ion and an experimental scattering angle of 45°, no single scattering of the incident ion is seen and the spectra consist solely of atomic recoils with some underlying background due to multiply scattered incident ions. The small peak at the start of the spectrum is due to photons produced during collision events and is used to establish time zero. The spectrum shows a H recoil peak near 7 µs and a second complex feature near 10 µs. Under these experimental conditions the C and N recoils fall too close together to resolve cleanly (predicted TOFs are C ) 9.09 and N ) 9.44 µs) and the F recoils form a shoulder (predicted TOF of 10.31 µs). There is no appreciable phosphorus recoil signal (predicted TOF of 12.40 µs). This is to be expected as the fluoride ligands in [PF6]- will effectively shadow the central P atom from the ion beam and also block the flight of any recoiling phosphorus atoms. The broad feature visible at ∼14 µs is probably associated with multiply scattered Ar ions that have a wide range of flight times. Using Ne+ ions produced spectra with an improved separation of the recoil features and less background. Figure 4 shows 2.0 keV Ne+ spectra taken at R ) 8 and 36°. The C and F recoils are much better separated than those in the Ar+ spectrum (predicted TOFs of 8.85 and 10.78 µs, respectively). Under these experimental conditions, we can also expect to see a signal from Ne+ scattered from F. It transpires that the TOF for this type of event is only slightly later than that of F recoils themselves and the cross sections for the two processes are quite similar. We expect N recoils to occur at 9.40 µs. This is still too close to resolve completely from the C signal, but examination of the spectra in Figure 4 reveals that at an incident angle of 8° the C peak stands out cleanly, whereas the spectral area where we expect N recoils to occur fills in substantially at the higher incidence angle. Once again there is no sign of a P recoil signal (predicted to occur at 14.08 µs). The relative intensities of the recoil peaks in the Ne+ spectrum are quite different from those in the Ar+ spectrum. This is due to the markedly different recoil cross sections when using the lighter projectile and does not imply any contradiction.
Surface of Ionic Liquid
Langmuir, Vol. 15, No. 24, 1999 8431 Table 1. Experimental Intensity Ratios and Derived Atomic Ratios from Recoil Peak Maxima from [bmim][PF6] Using 2.0 keV Ne+ Ions at a Scattering Angle of 45° and Incidence Angles (from the Surface) of 8° and 36° a intensity ratio R (deg) 8 36
H/C
H/Fb
atomic ratio C/Fb
H/C
H/F
C/F
1.3-1.4 2.3-2.8 1.7-2.1 1.5-1.9 3.1-4.0 1.7-2.0 1.1-1.2 2.1-2.6 1.7-2.0 1.3-1.7 2.5-3.3 1.8- 2.0
a Atomic ratios are calculated using factors accounting for crosssections and detector efficiency as described in the text. The listed atomic ratio ranges reflect the combined uncertainties in background removal when measuring intensities and detector efficiency. b Assumes 50% of F intensity can be assigned to F recoils.
Figure 4. Recoil spectrum of [bmim][PF6] taken with 2.0 keV Ne+ ions at a recoil angle of 45° and incidence angles from the surface of 8° and 36°.
In order to extract quantitative information from the spectra, we need to extract the true signal intensities and correct for the above-mentioned cross-section effects and for differing detector sensitivities. Spectral deconvolution is complicated by a poor knowledge of the functional form of the obvious peak tails. Work in progress in this laboratory indicates that the tails contain some depth informationsit appears that additional collisions shift some of the recoils to longer flight times. These later signals may include recoils that result from collisions involving incident ions that have lost energy in traversing some depth of liquid, recoils that originate at some depth into the liquid and have lost energy traveling to the surface, or a combination of both mechanisms. While the theory of the tails is yet incomplete, it is clear that the signal at the expected single-collision recoil TOF for a particular atomic mass with no other energy losses arises from atoms originating in the very topmost layers of the liquid, within approximately the top 2 Å. Hence, the intensity of the H recoil signal at the peak in Figure 4 (i.e., at the expected TOF of 5.80 µs) is proportional to the H atomic concentration in the first molecular layer of the liquid. Furthermore, if we construct a reasonable ad-hoc background function for the H tail, we can extract an estimate of the C recoil intensity from surface C atoms and similarly for the F recoils. The left side of Table 1 shows intensity ratios for H, C, and F atoms at the surface of this ionic liquid. The H and C recoil intensities are measured straightforwardly from the peak maximum to the background function. As discussed earlier, the fluorine-containing area of the spectrum contains contributions from both F recoils and Ne+ ions singly scattered from F atoms. The cross sections for these two processes are very similar. Hence, after subtracting the background from the F signal, we took the F recoil intensity as 50% of the total signal intensity at the peak maximum. Obtaining atomic concentration ratios from intensity ratios necessitates a correction for cross-section and detector efficiency. Cross sections for recoiling were calculated using the Ziegler, Biersack, and Littmark
potential using standard codes,17 Channel electron multipliers of the type used here respond to any particles (charged or otherwise) that have sufficient momentum to create secondary electrons in the oxide coating of the multiplier. The mass response of such multipliers is a matter of some debate. The minimum particle velocity (vo) needed to create secondary electrons in the multiplier is generally regarded as independent of the nature of the particle, while the overall efficiency () depends on the particle atomic number (Z) and not the mass. Work in this laboratory indicates that a reasonable expression for the detector efficiency is
) R(v - vo) Zn where vo ) 5.5 × 104 m/s,16 n ∼ 0.8, and R depends on the particular multiplier. Fortunately, as we are here dealing exclusively with ratios, the actual value of R is not needed. The right side of Table 1 shows the values of the ratios of atomic concentrations derived from intensity ratios modified by cross-section and detector efficiency effects. The combined uncertainties of background removal and detector efficiency are reflected in the range of values quoted for the atomic ratios. 4. Discussion 4.1. Cation/Anion Surface Composition. Turning to the Ne+ data of Figure 4, we can immediately conclude that the observation of recoil signals from both the cation (C, H) and anion (F) allows us to reject extreme composition variations such as the surface being completely cation or anion dominated. To put this on a more quantitative footing, we note that Table 1 shows that the surface atomic C/F ratio is about 2. If the surface were highly anion enriched, then the fluorine-containing anion layer would prevent access of the incident ions to the underlying carbon-containing cations resulting in C/F , 1. On the other hand, if the surface were highly cation-enriched a similar argument would lead us to expect C/F . 1. As a result we are forced to conclude that both the cation and anion are present in the surface. Unfortunately, [bmim][PF6] is not amenable to crystallization and has not been characterized by X-ray diffraction. However, the crystallography of a series of higher hexafluorophosphate homolog (Cn-mim][PF6] where n ) 12, 14, 16, 18 has been published.15 In these compounds the anion is located over the cation ring with a typical closest cation-anion contact that is close to the van der Waals distance. Hydrogen bonding is thought to occur when the anion in a [Cn-mim]+ ionic liquid system contains (16) Dietz, L. A.; Sheffield, J. C. J. Appl. Phys. 1975, 46 (10), 4361. (17) Rabalais, J. W. CRC Crit. Rev. Solid State Mater. Sci. 1988, 14, 319.
8432
Langmuir, Vol. 15, No. 24, 1999
Gannon et al.
halide atoms with a charge density >118 (for instance, in [MC14]2- systems), but not for systems where the charge density is
