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J. Phys. Chem. B 2009, 113, 923–933

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Alkyl Chain Interaction at the Surface of Room Temperature Ionic Liquids: Systematic Variation of Alkyl Chain Length (R ) C1-C4, C8) in both Cation and Anion of [RMIM][R-OSO3] by Sum Frequency Generation and Surface Tension Cherry S. Santos and Steven Baldelli* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003 ReceiVed: September 6, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

The gas-liquid interface of halide-free 1,3-dialkylimidazolium alkyl sulfates [RMIM][R-OSO3] with R chain length from C1-C4 and C8 has been studied systematically using the surface-specific sum frequency generation (SFG) vibrational spectroscopy and surface tension measurements. From the SFG spectra, vibrational modes from the methyl group of both cation and anion are observed for all ionic liquid samples considered in the present study. These results suggest the presence of both ions at the gas-liquid interface, which is further supported by surface tension measurements. Surface tension data show a decreasing trend as the alkyl chain in the imidazolium cation is varied from methyl to butyl chain, with a specific anion. A similar trend is observed when the alkyl chain of the anion is modified and the cation is fixed. Introduction A comprehensive understanding of the surface properties of room-temperature ionic liquids is essential for advancement in this field, as the reactions for a wide range of applications only occur at the interface. For instance, as media in the gasseparation processes, this entails an understanding of the chemical nature of the liquid surface, since this influences the interaction between the gas molecules and the surface molecules of the liquid sample. Similarly, an efficient process in the electron transfer of an electrochemical reaction is dependent on the nature of the interface, and this requires the knowledge of the properties and structural description of these systems at a molecular level to be able to control reactions at interfaces. Other applications of ionic liquids include liquid-liquid extraction, biphasic catalysis, organic synthesis, and others.1-12 Room-temperature ionic liquids have been utilized for a variety of applications that involved interfacial reactions; but until now, studies on these systems are still limited. Hence, a systematic investigation is fundamental to broaden the range of application in this field. Here, the effect of systematic modification in the alkyl chain length of both cation and anion is explored by employing the halide-free 1,3-diakylimidazolium alkyl sulfate. The study involves the variation of the alkyl chain length at position 1 of the imidazolium cation ring (see Figure 1) with a fixed anion and the variation of the anion chain length with a fixed cation. The ionic liquids that have been utilized in this study have the general formula of [RMIM][R-OSO3] with R chain length from C1-C4 and C8. The following ionic liquids are considered in the present study: 1,3-dimethylimidazolium [MMIM]+ cation with methyl sulfate [MeOSO3]-, ethyl sulfate [EtOSO3]-, propyl sulfate [PrOSO3]-, and butyl sulfate [BuOSO3]- anions; 1-ethyl-3-methylimidazolium [EMIM]+ cation with [MeOSO3]+, [EtOSO3]+, [PrOSO3]+, and [BuOSO3]+ anions; 1-propyl-3-methylimidazolium [PMIM]+ cation with [MeOSO3]-, [EtOSO3]-, [PrOSO3]-, and [BuOSO3]- anions; 1-butyl-3-methylimidazolium [BMIM]+ cation with [MeOSO3]-, [EtOSO3]-, [PrOSO3]-, [BuOSO3]-, and octyl sulfate * To whom correspondence should be addressed.

Figure 1. Numbering scheme for imidazolium cation.

[OcOSO3]- anions; and 1-octyl-3-methylimidazolium [OMIM]+ cation with [BuOSO3]- and [OcOSO3]- anions. An increased interest in this field is evident in the number of published papers from 2000 to 2006.13 Surface and bulk properties of these salts have been investigated in recent years; however, a systematic study on the surface structure and thermodynamics is still limited.14 Surface techniques that have been employed for surface orientation and composition analysis include the direct recoil spectrometry,15-17 neutron and X-ray reflectometry,18,19 sum frequency generation vibrational spectroscopy,14,20-26 and ultrahigh-vacuum-based techniques, such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), low-energy ion scattering (LEIS), metastable impact electron spectroscopy (MIES), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and highresolution electron energy loss spectroscopy (HREELS).27-32 Surface tension data on the imidazolium-based ionic liquids are also available in the literature; however, some values are not in agreement with one another.33-39 The discrepancy in the values may likely be due to the hygroscopic nature of these liquids, which makes it difficult to achieve a moisture-free sample. In addition, purity is another factor that needs to be considered in preparing these compounds. They are often contaminated by colored impurities and halide ions. Some purification methods are mentioned in the literature, which include treatment with active charcoal, silica gel, and alumina for colored impurities40-42 and electrolysis for removal of halide ions.43

10.1021/jp807924g CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

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Figure 2. Synthesis of imidazolium alkyl sulfate ionic liquids.

A comprehensive review of the different surface techniques used in the investigation of the ion orientation at the gas-liquid and solid-liquid interfaces has been presented in the literature.44 Both experimental and computational studies reveal that the gas-liquid interface of ionic liquids consists of both cation and anion. However, discrepancies in the surface orientation arise from experimental investigations, as indicated by the DRS and SFG results.15,20 From the DRS study, it is suggested that the anion is located near the most acidic carbon atom, C(2). Conversely, SFG results suggested that the ions are located adjacent to one another with the methyl group directed to the gas phase as indicated by the crystallographic evidence and phase simulations.25 The simulation studies, however, demonstrate that orientations observed experimentally are acceptable and the preferential ordering occurs at the first layer.45-49 In addition, results from all surface techniques agree with regard to the orientation of the butyl chain at the surface in which it is projected toward the gas phase. However, the imidazolium ring orientation has not been mentioned in most cases, except for the DRS and SFG techniques.15,17,20,22,25,26 The relative location of the anion and cation at the liquid surface can vary depending on the chemical species that comprise the ionic liquid. For instance, the SFG spectra of [EMIM][MeOSO3] and [EMIM][PF6] show different spectral features.26 This illustrates the effect of anion on the orientation of the cation at the gas-liquid interface. The fact that the cation orientation is distinct for each liquid suggests a relatively different arrangement of ions at the given surface. An investigation on [BMIM][I] and [BMIM][BF4] employing attenuated total reflection infrared absorption (ATR-IR) spectroscopy present two distinct spectra.50 The observed differences were believed to be due to the relative positioning of the anion with respect to the imidazolium cation. Systematic modification of the molecular structures is essential in understanding the relationship between the molecular structure and physical properties. Investigations on the effect of cation and anion variation in ionic liquids on their bulk physical properties have been extensively studied.51-55 Conversely, systematic studies on the surface orientation of these ions have been limitedly explored. Hence, the present study aims to contribute into the surface analysis of the ionic liquids field, which has become of interest in recent years. Competition between the Coulombic interaction and the chain-chain interaction is investigated to recognize the driving force responsible for the surface partitioning and structure of the ions. For this purpose, a systematic modification on the alkyl chain substituent of the cation and anion is considered.

Background SFG spectroscopy is a second-order nonlinear technique in which two pulsed laser beams, a tunable IR (2000-4000 cm-1) and a fixed visible (532 nm), are overlapped at the surface of the material, and the generated third beam is detected at the sum of the frequencies of the two input light fields. This technique is only sensitive to molecules present at the surface where the centrosymmetry is broken; hence, it is surfacespecific. This feature enables SFG to differentiate molecules at the surface from that of the bulk. SFG theory has been described thoroughly in several papers.56-59 The intensity of the SFG signal, I(ωSF), is proportional to the square of the induced polarization, where E refers to the electric fields of the incoming visible and IR beams as indicated in eq 1. χ(2) is the second-order nonlinear susceptibility that relates the interface response to the two input light fields. According to eq 2, the nonlinear susceptibility contains all the information on the molecule through the molecular hyperpolarizability, β(2), containing contributions from the Raman polarizability and IR dipole transition and averaged over all orientations of molecules at the surface, as indicated by the brackets, 〈 〉. χNR is the nonresonant contribution from the background of the surface. N indicates the number of modes contributing to the SFG signal, ωIR and ωn refer to the frequencies of the incoming IR and the normal mode, respectively, and Γn is the damping constant.

I(ωSF) ∝ |P(2) ) χ(2):EvisEIR | 2 χ(2) ) χNR +

∑ ωIR -N〈βωn +〉 iΓn

(1)

(2)

(2)

By using different polarization combinations of incoming and outgoing laser beams, the molecular orientation of surface molecules with respect to the surface normal can be deduced by using the polarization intensity ratio method.22,60-64 Surface tension is a measure of cohesion of liquid molecules present at the surface and is dependent upon the structure and orientation of the liquid.65 It is generally a quantification of force per unit length or free energy per unit area. The Du Nou¨y ring method was employed in the present study. The technique is used to determine the force associated with the withdrawal of the platinum-iridium (Pt-Ir) ring from the gas-liquid interface at a slow upward motion. The ring on the surface of the solution experiences a downward force due to the resistance of the solution and is measured by a balance. Before the ring pulls

Cation and Anion of [RMIM][R-OSO3]

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Figure 3. SFG spectra for [RMIM][R-OSO3] (R ) C1-C4) ionic liquids for ssp polarization.

free of the surface, a maximum force is reached, which is related to the surface tension as given by the equation γ ) f/4πR. The γ is the surface tension, f is the maximum force exerted on the ring (in dyn or mN), and R is the diameter of the ring (in cm or m).66 Experimental Section Synthesis and Sample Preparation. 1,3-Dialkylimidazolium Alkyl Sulfates [RMIM][R-OSO3]. The synthesis of alkyl sulfate ionic liquids follows the method published previously, and a schematic of the procedure is shown in Figure 2.67,68 Step 1 involved the alkylation of the R-imidazole (R ) methyl to butyl) with a dropwise addition of dimethyl sulfate (or diethyl sulfate) in toluene, maintaining the temperature below 40 °C.67 The mixture was stirred at room temperature for ∼1 h after the addition of the alkylating agent. All samples were prepared using dimethyl sulfate except for [EMIM][EtOSO3], which was prepared from methylimidazole with diethyl sulfate as an alkylating agent. Step 2 follows the transesterification method reported by Wasserscheid et al.,68 but sulfuric acid was used as the catalyst instead of the reported methanesulfonic acid. In this step, a corresponding alcohol (ethanol to butanol) and the acid catalyst were added to the compound obtained from step 1. The reaction mixture was heated to ∼70 °C (no more than 90 °C) with constant stirring and under dry conditions. A vacuum distillation was applied for efficient removal of the methanol formed during the reaction.

[BMIM][OcOSO3], [OMIM][BuOSO3], and [OMIM][OcOSO3]. Synthesis of these salts follows the procedure outlined by Bo¨smann et al.69 The corresponding imidazolium chloride salt and the sodium alkyl sulfate were dissolved in hot water, ∼60 °C. The water was removed under vacuum, and a white solid precipitate was obtained. The product was extracted using dichloromethane and washed with water several times until the washing was chloride-free. The solvent was removed under vacuum. Sodium butyl sulfate and sodium octyl sulfate were also synthesized according to the procedure by Sandler and Karo.70 Butyl and octyl alcohols were used as starting materials. The corresponding alcohol was dissolved in dioxane, and the mixture was cooled to ∼10-15 °C. A 20% fuming sulfuric was added to the mixture, stirred, and neutralized with cold 10% NaOH. After neutralization, it was concentrated under vacuum to obtain the product. [MMIM][PF6], [EMIM][PF6], [PMIM][PF6], and [BMIM][PF6]. These compounds were prepared by following the procedure published previously by Rogers et al.67 except for [BMIM][PF6], which was synthesized according to the literature procedure of Gra¨tzel et al.71 All synthesized compounds were characterized using 1H NMR spectroscopy, and charcoal purification was performed for all yellowish samples until colorless. Prior to conducting each SFG experiment, the sample was transferred to the glass SFG cell with Kalrez O-rings and Teflon

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Figure 4. SFG spectra for [RMIM][R-OSO3] (R ) C1-C4) ionic liquids for ppp polarization.

stopcocks and was dried under high vacuum at ∼70 °C in a glass vacuum line equipped with liquid nitrogen traps until it reached a pressure of 5 × 10-5 Torr. The cell was then backfilled with argon gas prior to the SFG experiment. SFG Setup. The optical setup consists of a 1064 nm fundamental mode from picosecond Nd:YAG laser (Ekspla), which is used to pump an optical parametric generation/ amplification, OPG/OPA, system (Laser Vision). The OPG/OPA is based on a set of KTP/KTA crystals, which are used to create the tunable IR beam (2000-4000 cm-1) and the fixed visible light (532 nm). The visible and IR beams probe the surface at an angle of 50° and 60° from the surface normal, respectively. The computer controls data collections and the SFG setup using the LabVIEW program. The spectra for all samples were acquired at room temperature except for [MMIM][PF6] (∼90 °C), [EMIM][PF6] (∼75 °C), [MMIM][MeOSO3] (∼45 °C), and [MMIM][EtOSO3] (∼40 °C). Each spectrum was averaged over five scans of 20 shots/point at 1 cm-1/s. SFG data were corrected for IR fluctuations, and the intensity was normalized relative to CH3 symmetric stretch peak in ssp (s-polarized SFG beam, s-polarized IR light, and p-polarized visible beam) spectrum. The fitting of the final SFG spectra was performed using Origin 7.0 Professional nonlinear curve fitting. Equation 2 was used in the fitting function with instrumental setting method for the error bars. Surface Tension. All measurements were performed using a Kru¨ss K12 Du Nou¨y ring tensiometer with a platinum-iridium

(Pt-Ir) ring. Samples were dried according to the drying procedure described in the SFG experiment. A volume of ∼10 mL sample was transferred into the sample dish. The glass housing, which has a glass plate cover with a hole, was built to create an enclosed system, and in order to achieve a moisturefree environment, the surface was bathed with a slow flow of nitrogen gas. Each sample was analyzed in three independent sets of measurements in different days with 30 readings each to ensure that no relevant variables were affecting the measurements, and the results of the independent measurements were averaged. All glassware was soaked in 1:1 ratio of HNO3/H2SO4 mixture overnight and subsequently rinsed with Milli-Q water (18.2 MΩ · cm resistivity). Prior to each measurement, the sample dish and the Pt-Ir ring were flame-annealed. The system was calibrated using pure liquids with known surface tension values:72 ethanol (21.97 mN/m), acetone (23.46 mN/m), toluene (27.93 mN/m), acetonitrile (28.66 mN/m), dimethyl sulfoxide (42.92 mN/m), glycerol (64.00 mN/m), and water (72.9 mN/ m). All readings were acquired at room temperature, except for [MMIM][MeOSO3] (∼45 °C) and [MMIM][EtOSO3] (∼40 °C). Results and Discussion Peak Assignments. The SFG spectra taken at two different polarization combinations of ssp and ppp are presented in Figures 3 and 4, respectively, for the [RMIM][R-OSO3] (R ) C1-C4) ionic liquids. Table 1 displays all the peak assignments for the vibrational modes observed in both polarization

Cation and Anion of [RMIM][R-OSO3]

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TABLE 1: C-H Stretching Mode Assignments for [RMIM][R-OSO3] (R ) C1-C4) Ionic Liquids peak assignment (frequency, cm-1) ionic liquid

OCH3 (sym), ssp

OCH3 (FR), ssp

OCH3 (asym), ssp

OCH3 (asym), ppp

[MMIM][MeOSO3] [EMIM][MeOSO3] [PMIM][MeOSO3] [BMIM][MeOSO3] [MMIM][EtOSO3] [EMIM][EtOSO3] [PMIM][EtOSO3] [BMIM][EtOSO3] [MMIM][PrOSO3] [EMIM][PrOSO3] [PMIM][PrOSO3] [BMIM][Pr OSO3] [MMIM][BuOSO3] [EMIM][BuOSO3] [PMIM][BuOSO3] [BMIM][BuOSO3]

2830 2825 2835 2825

2905 2915 2920 2915

2955 2950 2955 2945

2975 2985

76-78

75-77

76-78

76-78

OCH2 (sym), ssp

NCH322,64,79 (sym), ssp

NCH322,64,79 (asym), ppp

2955

3000

2901 2900 2910

2970 2974 2980 2965 2975

CCH322,64,80 (sym), ssp

CCH322,64,80 (FR), ssp

CCH322,64,80 (asym), ppp

2880 2885 2875 2940 2940 2945 2945 2880 2880 2880 2880 2875 2880 2880 2875

2950 2955 2945 2870 2875 2880 2880 2940 2945 2940 2945 2945 2945 2950 2940

2960 2970 2960 2980 2980 2980 2965 2965 2970 2970 2960 2960 2960 2970 2960

TABLE 2: C-H Stretching Mode Assignments for [RMIM][PF6] Ionic Liquids peak assignment (frequency, cm-1) ionic liquid

[MMIM][PF6] [EMIM][PF6] [PMIM][PF6] [BMIM][PF6]

CCH3 (sym),22,64,80 ssp

CCH3 (FR),22,64,80 ssp

NCH2 (sym),81 ssp

NCH3 (sym),22,64,79 ssp

2945 2880 2885

2880 2940 2950

2925

2975 2970

combinations.22,64,73-81 A few characteristics in the spectra are noticeable as the alkyl chain length at position 1 of the imidazolium cation and the anion is varied from methyl to butyl. [C1-C4MIM][MeOSO3]. The SFG spectra and vibrational modes for [MMIM]-, [EMIM]-, and [BMIM][MeOSO3] have been reported previously.26 Here, [PMIM][MeOSO3] is added in the series. The spectra for ionic liquids with ethyl, propyl, and butyl substituents exhibit methyl symmetric stretch, C-CH3 (sym), O-CH3 (sym); methyl Fermi resonance, C-CH3 (FR), O-CH3 (FR); and methyl antisymmetric stretch, C-CH3 (asym), O-CH3 (asym); from both cation and anion, as shown in Table 1. The peak at ∼2950 cm-1 in ssp spectra contains both the O-CH3 (sym) and C-CH3 (FR) modes. [MMIM][MeOSO3], on the other hand, displays different spectral features from other ionic liquids considered here.26 Here, the substituent at position 1 is only one methyl group, N-CH3, giving a symmetric structure for the cation. O-CH3 and N-CH3 symmetric and antisymmetric stretches are visible in ssp and ppp spectra, respectively. The O-CH3 (sym) from the [MeOSO3]- anion is observed in all four samples at ∼2830 cm-1. However, from ppp spectra, the O-CH3 (asym) mode is only seen in [MMIM][MeOSO3] and [EMIM][MeOSO3] samples, which is at ∼2975 and ∼2985 cm-1, respectively. [C1-C4MIM][EtOSO3]. This set of samples displays four peaks in the ssp spectra except for [BMIM][EtOSO3], which only shows two distinct modes. In addition to the appearance of vibrational modes C-CH3 (sym) and C-CH3 (FR) from the terminal methyl in the ssp spectra; resonances at ∼2900 and ∼2970 cm-1, which are ascribed to the methylene symmetric stretch, O-CH2 (sym), and methyl symmetric stretch, N-CH3 (sym), are noted. However, both peaks decrease dramatically as the chain length of the cation increases and are no longer visible in the ssp spectrum of [BMIM][EtOSO3]. In this instance, the spectrum is dominated by the vibrational modes from the terminal methyl of both the cation and anion.

H-C(4)-C(5)-H (sym),22,64,79 ssp 3175 3175

Conversely, as opposed to the normal or conventional peak assignments for C-CH3 (sym) and C-CH3 (FR) at ∼2880 and ∼2945 cm-1 of long alkyl chain, respectively,80,82,83 peak assignments of ethyl chain are exchanged. The ∼2945 cm-1 peak is ascribed to the C-CH3 (sym), and the ∼2880 cm-1 mode is attributed to C-CH3 (FR). This switch in mode assignments is considered since the spectra show that the higher frequency mode is more intense.80 In addition, the component of the symmetric C-H stretching mode which is more intense can be naturally assumed to contain more of the fundamental.80,83 Hence, in this case, the ∼2945 cm-1 is designated to the symmetric methyl stretch and the ∼2880 cm-1 is assigned to the Fermi resonance. [C1-C4MIM][PrOSO3]. In this sequence, only the [MMIM][PrOSO3] sample exhibits different spectral features from the other three salts. In addition to resonances from terminal methyl group, a prominent peak is observed at ∼2965 cm-1, which can be attributed to the N-CH3 (sym). As the cation chain length proceeds from methyl to ethyl, the peak only appears as a shoulder at ∼2975 cm-1. However, for the longer chain salts, [PMIM][EtOSO3] and [BMIM][PrOSO3], this mode is no longer visible. [C1-C4MIM][BuOSO3]. An apparent characteristic in this series of alkyl sulfate-based ionic liquids is the resemblance of the ssp spectra to [BMIM][BF4] or [BMIM][PF6] ssp spectrum, where the only features come from the terminal methyl group of the chain length, the C-CH3 (sym) and C-CH3 (FR).22 The [BMIM][PF6] ssp spectrum is presented in Figure 5. The imidazolium ring modes, H-C(4)-C(5)-H at ∼3175 cm-1 (sym) and ∼3150 cm-1 (asym) and H-C(2) at ∼3020 cm-1, were not observed for all the alkyl sulfate ionic liquids. [C1-C4MIM][PF6]. The ssp spectra for [MMIM]-, [EMIM], and [BMIM][PF6] have already been published previously;25,26 but for comparison purposes, they are shown here. To complete the sequence, [PMIM][PF6] is added in the present study, which is presented in Figure 5. Only the N-CH3 (sym) mode at ∼2975

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Figure 5. SFG spectra for [RMIM][PF6] (R ) C1-C4) ionic liquids for ssp polarization combinations.

Figure 6. Simulated SFG spectrum of [EMIM][PF6]. Values in the simulation are taken from data in Figure 5.

cm-1 is observed in [MMIM][PF6], while [EMIM][PF6] and [PMIM][PF6] spectra demonstrate interesting characteristics. For [EMIM][PF6], all symmetric modes (C-CH3, N-CH3, and N-CH2) are observed, including the ring mode (H-C(4)-C(5)-H).26 The vibrational peak assignments are shown in Table 2. Similar to the ethyl chain described above, there is a switch in mode assignments for [EMIM][PF6], which differ from the assignments that were previously published.26 In addition, the presence of the ring modes in the spectrum indicates a change in the orientation of the imidazolium ring on the surface. However, an extra methylene group in the cation chain resulted in the disappearance of the N-CH3 and N-CH2 symmetric modes as seen in the [PMIM][PF6] spectrum. For [BMIM[PF6], only the resonances from the terminal methyl group are noticeable as reported previously.22,25,26,84 The IR and Raman vibrational peak assignments found in the literature for n-alkanes and 1-methylimidazole were used to differentiate the stretching vibrations of the methyl groups and the imidazolium ring.73,74,79,80 In addition, isotopic labeling with deuterium on some ionic liquids has been performed previously to facilitate the peak assignment in the C-H frequency range.22 From the investigation, IR and Raman spectra were obtained for [(d9)-BMIM][Br], [(d9)-BMIM][PF6], and [(d3)-BMIM][PF6] salts. The SFG resonances were identified using the established peak assignment from IR and Raman spectra. Moreover, the SFG spectra acquired for [(d9)BMIM][Br] and [(d9)-BMIM][PF6] confirmed that the peaks observed for [BMIM][Br] and [BMIM][PF6] were solely due to methyl from the butyl chain.22 Another SFG study has been performed on 1-methyl-1-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide with the deuterated butyl chain.24 This has provided unambiguous assignments of the vibrational modes in the C-H frequency range and a more accurate description of the structure of ions at the gas-liquid interface. Furthermore, an experimental vibrational analysis was also performed on 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][(CF3SO3)2N] and [EMIM][EtOSO3].85 The IR and Raman spectra were presented for both liquids, and it was found

that the C-H stretching region for IR was remarkably weak in comparison with the Raman. In addition, the IR spectrum in the C-H stretching region for [EMIM][EtOSO3] is marginally stronger compared to [EMIM][(CF3SO3)2N]. Talaty et al.81 have also presented Raman spectra for [EMIM]-, [PMIM]-, and [BMIM][PF6] along with ab initio calculations. The [EMIM]+ bands of [EMIM][PF6] in Raman spectrum are in good agreement with that of the [EMIM][(CF3SO3)2N]. In the present study, a selective or partial deuteration of the cation seems necessary for a more accurate determination of various peaks, especially for the ethyl series: [MMIM]-, [EMIM]-, and [PMIM][EtOSO3]. However, for the purpose of confirmation of both cation and anion occupancy at the gas-liquid interface, it is believed that the changes in the spectra as a function of ion identity is evidence of the presence of both ions at the given surface. For instance, the observed O-CH3 mode in the SFG spectra of [C1-C4MIM][MeOSO3] series, as shown in Figure 3, confirms the presence of anion. In addition, comparison of SFG spectra of the alkyl chain series ([C1-C4MIM][MeOSO3], [C1-C4MIM][EtOSO3], and [MMIM][PrOSO3]) and the hexafluorophophate-based ionic liquids reveals different spectral features. However, the relatively longer alkyl chain salts ([C2-C4MIM][PrOSO3], [C1-C4MIM][BuOSO3]) possess similar spectral features as the [BMIM][PF6]. In order to further demonstrate that both ions are at the gas-ionic liquid interface, the spectroscopy of the cation and anion is explored. However, since both ions contain C-H vibrations, it is difficult to have a 100% identification of each ion’s contribution to the total SFG spectrum. As mentioned above, the most reliable method to distinguish the two ions without affecting the surface energy is to isotopically label (with deuterium) one of the ions. This is currently underway in our laboratory. Another method is to spectroscopically deconvolute the spectra into their individual components and then to simulate the resulting spectra of the ionic liquid of interest. This is a difficult procedure in nonlinear optics since the spectra are not a simple summation but a sum squared of the individual contributions; thus, there is interference between the different peaks. However, given enough data sets, this procedure is possible as shown below. Our goal is to show that the spectrum of [EMIM][EtOSO3] has contributions from both ions, that is, the N-CH2CH3 portion of the cation and the O-CH2CH3 part of the anion. Thus, we want to obtain independent SFG spectra of [EMIM]+ and [EtOSO3]-. First, we let the [EtOSO3]- spectrum be χ[EtOSO3] and the cation spectrum be χ[EMIM]. The experimental spectrum of χ[EMIM] is simulated, from the experimental parameters of [EMIM][PF6], in Figure 6. Next, we need the SFG spectrum of [EtOSO3], which is represented by χ[EtOSO3] ) χ[MMIM][EtOSO3] χ[MMIM][PF6]. Thus, from the two measured spectra, the anion component is extracted essentially, since [PF6]- does not contribute to the spectrum and the [MMIM]+ part is subtracted, which leaves the anion portion of the spectrum. The

Cation and Anion of [RMIM][R-OSO3]

Figure 7. Simulated SFG spectra of [MMIM][PF6] and [MMIM][EtOSO3] Values in the simulation are taken from data in Figure 5.

Figure 8. SFG spectrum of the synthesized [EtOSO3]- anion.

Figure 9. SFG spectra of [EMIM][EtOSO3] (experimental and simulated).

[MMIM][PF6] and [MMIM][EtOSO3] spectra, simulated from the measured values, are shown in Figure 7. The SFG spectrum of synthesized [EtOSO3]- is shown in Figure 8. The spectrum resembles that of other ethoxy groups, such as ethanol76,86,87 and the ethyl group26 of the [EMIM]+; however, the notable feature, which is unique to the [EtOSO3]-, is the peak at 2900 cm-1, which is possibly a vibration from the O-CH2 group. Finally, if we combine the χ[EMIM] used to produce the [EMIM]+ spectrum in Figure 6 and χ[EtOSO3] used to create the [EtOSO3]spectrum in Figure 8, the resulting spectrum of the synthesized [EMIM][EtOSO3] is presented in Figure 9, along with the experimental SFG spectrum of [EMIM][EtOSO3] taken with ssp polarization. The results show that the surface spectrum contains clear contributions from cation and anion. Despite the congestion

J. Phys. Chem. B, Vol. 113, No. 4, 2009 929 in the C-H stretching region, each ion has a unique contribution and clearly shows the presence of both ions. This is especially clear considering the peak at 2900 cm-1 from the anion. This analysis is valid as long as the orientation of the alkyl chain remains the same for the ions in different systems and their concentrations are similar. Both these assumptions have been shown to be true for ionic liquid systems studied here. It should be noted that the peak in the high frequency (aromatic C-H stretching range) is influenced by the identity of the anion and thus is present in [PF6]- but absent in the alkyl sulfate anions.26 The surface tension data reinforce observation on the presence of both ions at the interface since the surface tension values decrease as a function of methylene unit added, whether to the cation or anion (Figure 11), as described in detail below. Ions at the Gas-liquid Interface. Previous experimental and computational studies have already demonstrated the presence of both cation and anion on the surface of gas-ionic liquid interface.15,17,30,46,48 However, in this study, the investigation focuses on the systematic variation of the alkyl chain on both cation and anion. This intends to investigate if a competition between Coulombic interaction, which can be deduced from the ring orientation, and chain-chain interaction exists and to determine the driving force responsible for the orientation and partitioning of the ions at the gas-liquid interface. Initially, the present study was formulated with alkyl chain length from methyl to octyl as the substituent in the anion as well as in position 1 of the cation. This method allows a systematic variation on the surface energy of cation and anion independently. However, after C4 substitution, the ring is not observed to change as evident in the absence of ring modes in all the spectra (see Figure 3). Furthermore, Figure 10 displays the ssp spectra of [BMIM][OcOSO3], [OMIM][BuOSO3], and [OMIM][OcOSO3], which show similar spectral features with the butyl-substituted ionic liquids, except for the prominent methylene feature at 2850 cm-1, which is ascribed to the symmetric CH2 stretch.76 The appearance of methylene feature probably arises from the gauche defects at the terminal methyl in the alkyl chain, which results in a conformational defect in the surface orientation of the ions.14,24,76,87,88 The intensity of the CH2 peak increases as the chain substituent proceeds from butyl to octyl, whether it is attached to the cation or the anion. Since the ionic liquids considered here exhibit similar vibrational features with chain substituent greater than propyl, the number of carbon atoms in the chain length was limited to four. The presence of the methyl group vibrations from the shortest chain cation and anion, [MMIM][MeOSO3], in the spectra suggests that both ions are present at the gas-liquid interface for all the halide-free samples considered in the present study. Moreover, modes from the terminal methyl group of the alkyl substituent (ethyl to butyl) on the cation and anion indicates that the chains are projecting toward the gas phase, while the absence of any modes from the imidazolium ring suggests that the ring is lying parallel to the surface plane. The suggested orientation of the imidazolium ring at the gas-liquid interface has been supported by the SFG study of deuterated [BMIM][PF6] samples.22 The surface orientations of [MMIM], [EMIM]-, and [BMIM][MeOSO3] were described in the preceding paper.26 From the results, [MMIM]+ cation is believed to have one N-CH3 group pointing toward the liquid phase while the other group is projecting toward the gas phase. Furthermore, replacement of the alkyl sulfate anion with the non-alkyl one, such as [PF6]-, resulted in a different orientation for [EMIM]+ and [PMIM]+ cations, which is noticeable in the appearance of additional peaks as described above (see Figure

930 J. Phys. Chem. B, Vol. 113, No. 4, 2009

Santos and Baldelli

Figure 10. SFG spectra of [BMIM][OcOSO3], OMIM][BuOSO3], and [OMIM][OcOSO3] for ssp and ppp polarization combinations.

Figure 11. Surface tension data (A) for [RMIM][R-OSO3] (with fixed cation) and (B) for [RMIM][R-OSO3] (with fixed anion).

5). The orientation of [MMIM]+ cation in [MMIM][PF6] is similar to the one described above for [MMIM][MeOSO3].26 On the other hand, the orientation of [EMIM]+ cation, with [PF6]- anion, is believed to have a slightly tilted imidazolium ring orientation with N-CH3 substituent pointing into the liquid phase, and the ethyl chain propagates toward the gas phase. In the present study, the spectrum of [PMIM][MeOSO3] appears to have similar spectral features as the [BMIM][MeOSO3]. However, substitution of [PF6]- anion into [PMIM][MeOSO3] gives a spectrum that is analogous to [BMIM][PF6], except for the H-C(4)-C(5)-H mode at ∼3175 cm-1. In this case, the ring is not lying parallel to the surface but has a slight tilt showing the H-C(4)-C(5)-H ring mode, as opposed to the flat orientation of the ring for [BMIM][PF6] suggested in earlier SFG studies.20,22,84 Furthermore, previous investigations on 1-butyl-3-methylimidazolium-based ionic liquids with other nonalkyl anions, e.g., [BF4]-, [(CN)2N]–, [Br]-, [I]-, [(CF3SO3)2N]-, [CH3SO3]-, and [SCN]-, illustrated that the anion does not show any effect on the orientation of butylimidazolium cation.20,22,23Even in the current study, the ssp spectra for [EMIM]-, [PMIM]-, and [BMIM][PrOSO3] and [MMIM]-, [EMIM]-, [PMIM]-, and [BMIM][BuOSO3] resemble the spectrum of [BMIM][PF6]. The results suggest that for alkyl sulfate ionic liquids with chain length greater than C1 the hydrophobic effect dominates the surface, creating a charge neutral environment. In fact, the SFG spectrum of [BMIM][OcOSO3] shows resonances similar to that of the shorter alkyl chain, except for the prominent methylene feature at ∼2850 cm-1. Therefore, the results demonstrate that the organic functional group dominates the surface even in a pure

ionic system. This illustrates a surface excess that is driven by the presence of the alkyl chain. This behavior of the ions at the gas-liquid interface can be compared with that of the pure alcohols. Investigation on the surface orientation of pure alcohols at the gas-liquid interface provides spectra that are dominated by the CH3 groups.76,87,89 Shen et al.87 reported the ssp spectra for alcohols (methanol to octanol), and the results showed that all alcohols are polaroriented with the alkyl chain pointing toward the gas phase and away from the liquid phase. Methanol exhibits methyl group resonances in the spectrum in which the symmetric stretch is observed at 2832 cm-1. The ethanol shows three features in which the peak at 2875 cm-1 is attributed to CH2 symmetric stretch, the mode at 2933 cm-1 is due to CH3 symmetric stretch, and the shoulder at 2975 cm-1 can be assigned to the asymmetric CH3 and/or CH2 stretches. Spectra of propanol, butanol, and pentanol are all similar and dominated by the terminal methyl group in which the symmetric stretch is observed at 2875 cm-1 with its Fermi resonance at 2940 cm-1. Similar spectral features are observed for [EMIM]-, [PMIM]-, [BMIM][PrOSO3], [MMIM]-, [EMIM]-, [PMIM]-, and [BMIM][BuOSO3] ionic liquids. For hexanol, heptanol, and octanol, the spectra demonstrate the presence of gauche defects in the alkyl chain, which is comparable to the spectra of [BMIM][OcOSO3], [OMIM][BuOSO3], and [OMIM][OcOSO3] observed here. Another investigation on this alcohol series by Wang et al.76 reveals consistent results with the ssp spectra described above. However, peak assignments on ethanol differ for the two studies.76,86,87

Cation and Anion of [RMIM][R-OSO3]

J. Phys. Chem. B, Vol. 113, No. 4, 2009 931

TABLE 3: Room Temperature (20-25 °C) Surface Tension Data with Literature Values at 25 °C surface tension (mN/m) ionic liquids

exptl

[MMIM][MeOSO3] [EMIM][MeOSO3] [PMIM][MeOSO3] [BMIM][MeOSO3] [MMIM][EtOSO3] [EMIM][EtOSO3]

65.1 62.9 52.3 44.1 58.3 50.5

[PMIM][EtOSO3] [BMIM][EtOSO3] [BMIM][OcOSO3]

48.6 41.7 25.2

surface tension (mN/m)

lit. 33

59.8

43.339 46.96,34 46.5 (20 °C),35 45.43,36 47.31 (20 °C),92 42.5 (22-25 °C),37 48.7938

ionic liquids

exptl

[MMIM][PrOSO3] [EMIM][PrOSO3] [PMIM][PrOSO3] [BMIM][PrOSO3] [MMIM][BuOSO3] [EMIM][BuOSO3]

51.9 44.0 42.1 36.7 41.8 40.8

[PMIM][BuOSO3] [BMIM][BuOSO3]

38.2 31.1

lit.

26.7136

A similar investigation on a systematic modification of the cation chain length on imidazolium-based ionic liquid was reported by Ouchi et al.14 In the study, the chain length is varied from C4-C11 with a tetrafluoroborate as the anion. SFG results at the gas-liquid interface show that the alkyl chain protrudes into the gas phase. In addition, it is observed that as the number of carbon atoms in the chain is increased, the gauche defect is decreased. Surface Orientation. A surface energy change is associated with the formation of an interface in which it is represented by an excess surface energy of the functional group, which is the alkyl chain, present at a given interface. A molecule at the surface orients itself in such a way that the surface energy is always at a minimum.65,90 Therefore, a model in which the charged groups are always located toward the bulk liquid and the alkyl group is always exposed to the gas phase provides a lower surface energy. Ionic liquids, although charged, have a hydrophobic alkyl tail and positively charged head groups (imidazolium ring) and the negatively charged sulfate anion which adhere to this model, as evident in the SFG spectra that are dominated by the vibrational modes from the CH3 groups. The presence of an organic moiety at the gas-liquid interface is further confirmed by the surface tension measurements illustrated in Figure 11. The surface tension of pure liquids is largely dependent on the nature of the functional group that forms the surface layer of a given interface.90,91 In this instance, the alkyl group forms the actual surface of the liquid by the orientation of the molecule. The surface tension values for [RMIM][R-OSO3] (R ) C1-C4, C8) series is presented in Table 3. Only [MMIM][MeOSO3], [EMIM][EtOSO3], [BMIM][MeOSO3], and [BMIM][OcOSO3] have available surface tension values in the literature to compare with.33-39 The surface tension values measured in the present study do not coincide with that of the data from the literature for [MMIM][MeOSO3] (hanging drop method)33 and [EMIM][EtOSO3]. In fact, surface tension data for [EMIM][EtOSO3] reported from the literature vary from one another in which measurements were performed with hanging drop,34 pendant drop,35,36,92 capillary rise,37 and forced bubble38 methods. On the other hand, measured surface tension values for [BMIM][MeOSO3] and [BMIM][OcOSO3] are close to the reported values from hanging39 and pendant drop36 methods, respectively. The surface tension values shown in Figure 11 are lower than that of water, 72.9 mN/m,72 but higher than those of alcohols (methanol to butanol), 21-25 mN/m, and n-alkane solvents, typically 16-27 mN/m.35,36,65,93 The alcohols have surface energies that are comparable with those of the hydrocarbons due to the similar surface layer in which both layers consist of methyl groups.90

The surface tension values decrease monotonously as the chain length is varied as apparent in the plots given in Figure 11. The effect of increasing the number of carbon atoms is shown, and clearly, the surface tension decreases with increasing number of carbon atoms. The observed trends remain regardless of whether the increase in methylene groups is in the anion (Figure 11A) or in the cation (Figure 11B). This trend is in agreement with the Langmuir principle90,91 as well as with the experiment by Watson and Law on [CnMIM][PF6] and [CnMIM][BF4] (n ) 4, 8, 12),16 and a molecular dynamic simulations on ionic liquids with alkyl substituents of different lengths.94 As observed in Figures 3, 5, and 10, the alkyl group has a significant role in the surface structures of these salts.94 As the chain becomes longer, the polar group shifts away from the surface that causes the polarity of the surface to become weaker, which results in a decrease in surface tension. Correspondingly, a salt with a relatively longer alkyl chain, such as [BMIM][OcOSO3], creates a less polar surface, and its stability is possibly a result of the interactions among the alkyl chains. This certain amount of hydrophobicity is less than that of the n-alkane due to the presence of charged groups in the vicinity.46 In the SFG spectra of [C2-C4MIM][MeOSO3] series, it is noted that the intensities of both methyl groups from the anion and cation increase as the chain length is extended from ethyl to butyl. An analogous trend is observed in the SFG spectra of [MMIM][C2-C4OSO3] ionic liquid series. Therefore, the observed manner of increased peak intensity as the chain is extended is applicable for both anion and cation. Another possible explanation for the observed monotonous decrease in surface tension values as a function of alkyl chain length may be due to the volume occupied by the ions. The liquid is composed of charged head groups and uncharged alkyl chains. The volume occupied by the alkyl chains is available for these ions and provides a closer proximity for the ions, which results to an energy gain. As the chain gets longer, a larger volume is available for these charged groups; hence, more energy is gained by the system. Previous SFG studies demonstrate that the alkyl chain has an average orientation of projecting into the gas phase.20-22,24-26 Moreover, this chain conformation has been established using other experimental and computational studies.30,32,45,46,48,49,94 In addition, a phase simulation performed on the CH3 groups of the butyl chain and methyl sulfate anion clearly shows that both groups are pointing in the same upward direction, suggesting they are located adjacent to each other.25 Moreover, since SFG only probes the vibrations with net polar orientation, the absence of C(2)-H and H-C(4)-C(5)-H modes from the imidazolium ring for all [RMIM][R-OSO3] series indicates that the ring is parallel to the surface plane.

932 J. Phys. Chem. B, Vol. 113, No. 4, 2009 For ssp spectra of [MMIM][EtOSO3], [EMIM][EtOSO3], and [PMIM][EtOSO3], only vibration from N-CH3 among the ring modes is visible. The peak strength is noticed to be decreasing from C1-C4, and it is no longer observable for [BMIM][EtOSO3]. In this case, the ring is likely to be tilted from the surface plane. On the other hand, ssp spectra of [RMIM][PF6] series exhibit additional features other than the vibrational modes from the C-CH3 group, except [BMIM][PF6]. For [MMIM][PF6], the presence of C-H mode from N-CH3 group indicates that the cation is polar oriented with one CH3 group projecting into the liquid phase. It is vertically aligned at the surface with its C2 axis approximately parallel to the surface plane. An analogous orientation is observed for [MMIM][MeOSO3].26 Consequently, addition of one methylene unit modifies the net orientation of the cation. Here, the orientation for [MMIM][MeOSO3] and [MMIM][PF6] deviates from the suggested SFG model of [EMIM][MeOSO3] described above. Modes from N-CH3 and H-C(4)-C(5)-H are also noticeable in addition to the C-CH3 group vibrations. Therefore, the [EMIM]+ ring could not possibly orient parallel to the surface plane but instead has a slight tilt. The SFG spectrum of [PMIM][PF6] exhibits similar features to that of [BMIM][PF6], except the presence of ∼3175 cm-1 ring mode. Hence, the imidazolium ring of [PMIM][PF6] is also slightly tilted at the surface. The combination of SFG spectroscopy, surface tension measurement, and the systematic variation in the cation and anion of the ionic liquid structure shows how the balance between surface charge and dispersion forces are manifested at the surface. As the chain becomes longer than ethyl group, the alkyl chains essentially dominate the surface, and this influences the surface chemistry. Conclusion A systematic investigation of the surface composition and orientation of [RMIM][R-OSO3] (R ) C1-C4) ionic liquid series at the gas-liquid interface is realized using sum frequency generation vibrational spectroscopy and surface tension measurements. SFG spectra show that the surface is dominated by the alkyl functional group as evident in the CH3 stretching modes. This is further supported by the surface tension measurements in which a decreasing trend is observed regardless if an increase in the alkyl chain substituent is in the cation or in the anion. Acknowledgment. We are thankful to the R. A. Welch Foundation (E-1531) for support of this work and to Imee Su Martinez for restoring the Du Noüy ring tensiometer used in the surface tension measurements. References and Notes (1) Sheldon, R. Chem. Commun. 2001, 2399–2407. (2) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Ind. Eng. Chem. Res. 2004, 43, 3049–3054. (3) Blanchard, L. A.; Brennecke, J. F. Ind. Eng. Chem. Res. 2001, 40, 287–292. (4) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315–7320. (5) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156–164. (6) Seddon, K. R. J. Chem. Tech. Biotechnol. 1997, 68, 351–356. (7) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391– 1398. (8) Suarez, P. A. Z.; Einloft, S.; Dullius, J. E. L.; de Souza, R. F.; Dupont, J. J. Chim. Phys. 1998, 95, 1626–1639. (9) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667–3692.

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