11936
J. Phys. Chem. B 2008, 112, 11936–11941
Anion Configuration at the Air/Liquid Interface of Ionic Liquid [bmim]OTf Studied by Sum-Frequency Generation Spectroscopy Takashi Iwahashi,† Takayuki Miyamae,‡ Kaname Kanai,§ Kazuhiko Seki,† Doseok Kim,⊥ and Yukio Ouchi*,† Department of Chemistry, Graduate School of Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan, Research Center for Materials Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan, Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul, 121-742, Korea ReceiVed: March 13, 2008; ReVised Manuscript ReceiVed: July 17, 2008
The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF3-symmetric stretching (ss) mode and SO3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF3-ss peak of the OTf anion has the opposite sign with respect to that of the SO3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF3 group points away from the bulk into the air, whereas the SO3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk. 1. Introduction Room-temperature ionic liquids (RTILs), which are salts with a liquid phase at room temperature, have attracted much interest due to their unique nature.1 Because the surface and the interface of RTILs are key issues for numerous potential applications,2 interfacial structures of RTILs are of emergent interest and have been investigated using various surface analysis techniques,3 such as direct recoil spectroscopy,3,4 neutron reflectometry,5 X-ray reflectivity,6 electron spectroscopies,7-9 and computer simulations.10-13 Among the various experimental methods, infrared (IR)-visible sum-frequency generation (SFG) spectroscopy14 has recently been employed to investigate the surface structure of RTILs.15-23 Although SFG is a powerful tool for investigating the surface and the interface of various media, structural and environmental information of RTILs obtained by SFG has been restricted to mostly that of cations due to limitations in the tuning range of the IR light source. Very recently, Baldelli et al. have performed SFG orientational analysis for both cations and anions of RTILs with 1-butyl-3methylimidazolium ([bmim]+) and hexyltributylammonium as cations and methyl sulfate, methanesulfonate, and dicyanamide as anions.18,19 However, many unresolved issues about the anions at the interface, such as the ion-ion interaction, surface structure, and the chemical environment, remain. In this paper, we report the first SFG observation of a RTILanion containing CF3 and SO3 groups and investigate the anion configurations at the air/RTIL interface by extending the IR * To whom correspondence should be addressed. E-mail: ohuchi@ mat.chem.nagoya-u.ac.jp. Phone: +81-52-789-2485. Fax: +81-52-789-2944. † Graduate School of Science, Nagoya University. ‡ AIST. § Research Center for Materials Science, Nagoya University. ⊥ Sogang University.
Figure 1. Structures of the ionic liquid components, (a) [bmim]+ cation and (b) OTf anion.
tuning range to 1000 cm-1. The sample used in our experiment was 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), which is shown in Figure 1. In addition, we investigated a lithium trifluoromethanesulfonate (LiOTf) aqueous solution (aq) as a reference. The orientational analysis clearly showed that the OTf anions indeed have polar ordering at the surface: the CF3 group points away from the bulk, whereas the SO3 group points toward the bulk. Differences in the position and the line width of the SO3-symmetric stretching (ss) peak between the bulk IR and SFG spectra strongly suggested that the structures surrounding the anion differ between the interface and the bulk. Particularly, the fact that the SF signal of the submerged SO3 group was observed in a neat liquid system also implies the presence of some structural differences between the topmost surface layer and the bulk. The results of the line width and the shift of the SO3-ss peak suggest [bmim]+ cations and OTf anions interact in a specific way at the surface as compared to the bulk. 2. Theoretical Background The basic theory of surface SFG spectroscopy has been previously described.24 However, we sketch the essentials necessary for later discussion. SFG is based on a second-order nonlinear optical process where an IR beam at frequency ωir and a visible beam at frequency ωvis overlap at the surface and
10.1021/jp8021908 CCC: $40.75 2008 American Chemical Society Published on Web 09/03/2008
Anion Configuration of Ionic Liquid [bmim]OTf
J. Phys. Chem. B, Vol. 112, No. 38, 2008 11937
generate a sum-frequency (SF) signal at a frequency ωsf ) ωir + ωvis.The SF signal intensity Isf is given by
|
Isf(ωir) ∝ χNR +
∑ ωir - ωqq + iΓq | A
2
(1)
q
where χNR is the nonresonant contribution to the susceptibility, and Aq, ωq, and Γq are the amplitude, frequency, and the damping constant of the qth vibrational mode, respectively. For functional groups with C3V symmetry, such as CF3 and SO3 groups, we choose a molecular-fixed axis system ((a, b, c), shown in Figure 2a) for calculation of its hyperpolarizability values and averaged it over the orientation of the ensemble in the laboratory coordinates system ((X, Y, Z), shown in Figure 2b) to evaluate the nonlinear susceptibility values. The ssp (denoting s-polarized sum frequency, s-polarized visible, and p-polarized IR fields, respectively) measurement of an azimuthally isotropic surface probes only the nonlinear susceptibility component χYYZ, to which only the vibrations bearing an IR transition moment normal to the interface can contribute. The ppp spectra originate dominantly from the χXXZ ) χYYZ and χZZZ components. We can calculate the orientational angle θ (defined by the angle between c and Z) of the functional groups using the relation between the molecular hyperpolarizabilities, βijk, and the susceptibility components of χ. In the case of the symmetric stretching mode, we have
χYYZ ) NSR[(1 + γ)〈cos θ 〉 - (1 - γ)〈cos3 θ〉
(2a)
χZZZ ) 2NSR[γ〈cos θ 〉 + (1 - γ)〈cos3 θ〉
(2b)
where the brackets denote an average over molecular orientational distribution, NS is the number density of molecules at the surface, R ) βccc, and γ ) βaac/βccc for the specific group investigated. 3. Experimental Section 3.1. Materials Preparation. [bmim]OTf was prepared according to the literature with slight modifications.1,17 [bmim]Br salt was prepared by the alkylation of 1-methylimidazole (Aldrich, purity 99+%, used as received) with a slight molar excess of 1-bromobutane (TCI, purity >98%), which was distilled under low pressure. The reaction mixture was stirred at room temperature for 2-3 days. [bmim]Br salt was obtained as a white powder, which was subsequently washed several times with ethyl acetate and dried under low pressure. The anion exchange reaction of [bmim]Br was carried out by adding a slight molar excess of LiOTf (Aldrich, purity 99.995%, used as received) to prepare the [bmim]OTf. [bmim]OTf salt was extracted from water using dichloromethane. After extracting, dichloromethane was evaporated under low pressure. The colorless liquids were further degassed by repeatedly heating and cooling in vacuo (less than 1.0 × 10-1 Pa) for at least several hours to remove water and other volatile solvents. Assays using Karl Fischer titration indicated the water content of RTILs was
