4990
J. Phys. Chem. B 2007, 111, 4990-4999
Structure and Orientation of the Imidazolium Cation at the Room-Temperature Ionic Liquid/SiO2 Interface Measured by Sum-Frequency Vibrational Spectroscopy† Julie B. Rollins, Brian D. Fitchett, and John C. Conboy* Department of Chemistry, UniVersity of Utah, 315 S. 1400 E. RM 2020, Salt Lake City, Utah 84112 ReceiVed: NoVember 1, 2006; In Final Form: March 9, 2007
In this study, we have examined both the effect of alkyl chain length and anion composition on the 1-alkyl3-methylimidazolium (Cnmim, n ) 4, 6, 8, 10, and 12) structure and orientation at the room-temperature ionic liquid (RTIL)/SiO2 interface by sum-frequency vibrational spectroscopy (SFVS). Four different anions were investigated in this study: tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethylsulfonyl)imide (BMSI), and bis(pentafluoroethylsulfonyl)imide (BETI). It was found that the alkyl chain in BMSI and BETI RTILs showed a decrease in gauche defects with an increase in chain length, whereas the alkyl chains of the BF4 and PF6 RTILs have virtually no gauche defects regardless of chain length. The tilt of the alkyl chain lies predominantly perpendicular to the surface for all the RTILs examined. A strong correlation between the HCCH Vs tilt angle and alkyl chain length was observed; as the alkyl chain is lengthened the HCCH Vs lies more perpendicular to the SiO2 surface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. To a lesser extent, the HCCH Vs tilt of the imidazolium ring of the cation also appears to be correlated to the surface charge density of the SiO2. As the SiO2 surface charge density becomes more negative the HCCH Vs tilt angle lies more parallel to the surface.
Introduction Room-temperature ionic liquids (RTILs) are a novel class of materials similar to high temperature molten salts. They have high thermal stabilities, melting points below 25 °C, and are nonvolatile. RTILs are typically composed of a variety of bulky cations, such as tetraalkylammonium, N,N-pyrrolidinium, tetraalkylphosphonium, and N,N′-dialkylimidazolium, and a weakly associated anion. Common anions found in air and water-stable RTILs are halides, tetrafluoroborate (BF4), hexafluorophosphate (PF6), nitrate, and bis(perfluoroalkylsulfonyl)imides. The many different combinations of cations and anions that form RTILs allow them to be “tuned” for specific physical and chemical properties. Due to their variable nature, RTILs have found uses in many areas of chemistry including organic synthesis,1-9 catalysis,3,8,10-12 electrochemistry,12-15 biphasic separations,16-20 and as lubricants.21,22 Although the bulk physical and chemical properties of the more common RTILs are well studied11,13,19,23-33 few studies have characterized their interfacial properties. Previous studies have been performed on various RTIL interfaces using a variety of techniques such as surface-enhanced IR absorption spectroscopy to study the RTIL/gold electrode interface34 and X-ray reflectivity to study the RTIL/air interface.35 Recently, reports involving the RTIL/air,36-38 RTIL/ platinum electrode,39-41 and the RTIL/SiO242,43 interfaces using sum-frequency vibrational spectroscopy (SFVS) have also appeared in the literature. An understanding of the interfacial properties of RTILs is especially important in the areas of heterogeneous separations, biphasic catalysis, and electrochemistry. For the RTIL/solid interface, the arrangement of the cations and anions in the interfacial †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * To whom correspondence should be addressed. E-mail: conboy@ chem.utah.edu.
region will affect the solvation and diffusion of molecules at the surface and, in electrochemical systems, will dictate electrontransfer rates. As a model system, we have chosen to investigate the structure of the RTIL/SiO2 interface by SFVS. The RTIL/ SiO2 interface was examined in this study since its surface chemistry is similar to other oxides commonly used in catalytic systems such as titanium oxide and aluminum oxide.44-46 Using SFVS we have investigated the influence of the RTIL composition, specifically the chemical identity of the anion and the alkyl chain length of the 1-alkyl-3-methylimidazolium cation, on the orientation and conformation of the imidazolium ion at the SiO2 surface. Romero and Baldelli have suggested that the size of the anion affects the orientation of the cation 1-butyl3-methylimidazolium in contact with quartz.43 In addition, we have previously examined the effect the cation alkyl chain length has on the interfacial structure of a series of water-equilibrated 1-alkyl-3-methylimidazolium (Cnmim, n ) 6, 8, and 10) bis(trifluoromethylsulfonyl)imide (BMSI) and bis(pentafluoroethylsulfonyl)imide (BETI) RTILs at the SiO2 surface. In the study presented here, an extensive investigation of the influence of the anion composition and cation alkyl chain length on the structure and orientation of the imidizolium cation at the charged silica surface is presented. The structures of the RTIL ions examined in this study are shown in Figure 1. Unlike our prior study and the recent report by Romero and Baldelli, the water concentration was precisely controlled in order to eliminate the effect that solvation might have on the observed structure of the interface. The results of this study suggest that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface regardless of anion composition. To a lesser extent, changes in the charge density of the silica surface, modulated by the properties of the RTIL, also affect the orientation of the cation at the RTIL/SiO2 interface.
10.1021/jp0671906 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007
SFVS Study of the RTIL/SiO
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4991 polarization combination (s-polarized sum-frequency, p-polarized visible, and s-polarized IR) information about vibrational transitions perpendicular to the surface normal is obtained. Combining the data from both polarization combinations enables a full orientation analysis of the molecule at the surface to be perfromed.48 Experimental
Figure 1. Structures of the 1-alkyl-3-methylimidazolium cations and tetrafluoroborate, hexafluorophosphate, and bis(perfluoroalkylsulfonyl)imide anions used in this study.
Sum-Frequency Vibrational Spectroscopy. SFVS was used to study the RTIL/SiO2 interface in order to better understand the effect cation alkyl chain length and anion structure has on interfacial order. SFVS is a surface specific second-order nonlinear optical technique. SFVS probes the vibrational modes of molecules which are both IR and Raman active. Details on the theory can be found elsewhere in the literature.47 Briefly, SFVS is executed by spatially and temporally overlapping a fixed visible and tunable IR laser beam at an interface. The two beams combine to produce a third photon at the sum of their frequencies. Systems which possess inversion symmetry do not produce SFVS, such as the bulk of isotropic liquids. However, SFVS does occur at an interface where the inversion symmetry is broken. The intensity of the sum-frequency signal, ISF, is given by
ISF ) |f˜SF fvis fIRχ(2)|2
(1)
where ˜fSF, fvis, and fIR are the geometric Fresnel coefficients for the sum, visible, and IR fields, respectively, and χ(2) is the second order nonlinear susceptibility tensor. χ(2) is dependent on both a resonant (R) and nonresonant (NR) contribution
χ(2) ) χ(2) NR +
∑ν χ(2)R
(2)
where
χ(2) R )
N〈AkMij〉 . ωV - ωIR - iΓV
(3)
In eq 3, N is the number density of the molecules at the surface, Ak is the IR transition probability, Mij is the Raman transition probability, ωIR is the input IR frequency, ωV is the frequency of the normal mode vibrational transition, and ΓV is the line width of the transition. The brackets, 〈 〉, in eq 3 symbolize an average over all possible orientations. One major advantage of using SFVS to probe the RTIL/SiO2 interface is the technique’s ability to obtain information regarding the orientation of particular vibrational transitions with respect to the surface normal. By using a ssp polarization combination (s-polarized sum-frequency, s-polarized visible, and p-polarized IR) information about vibrational transitions parallel to the surface normal is obtained. By switching to a sps
Materials. 1-Iodobutane (99%), 1-bromohexane (98%), 1-bromooctane (99%), 1-bromodecane (98%), 1-bromododecane (97%), and 1-methylimidazole (99%) were purchased from Aldrich and distilled prior to use. Lithium bis(trifluoromethylsulfonyl)imide (LiBMSI) (99%) was purchased from Fluka, and lithium bis(pentafluoroethylsulfonyl)imide (LiBETI) was purchased from 3M Corporation. Potassium hexafluorophosphate (KPF6) (98%) and silver tetrafluoroborate (AgBF4) (98%) were purchased from Aldrich and used as received. Synthesis. The C4mimBF4 and C4mimPF6 RTILs used in this study were purchased from Fluka and used without further purification. C12mimBMSI, C12mimBETI, and C12mimPF6 were synthesized by reacting 1-bromododecane with a slight excess of 1-methylimidazole. The reaction was carried out for approximately 4 h at 65 °C. The excess 1-methylimidazole was then distilled off. The remaining viscous product was recrystallized with 1,1,1-trichloroethane until a white powder was obtained. C12mimBr was dissolved in Nanopure water (Barnstead) with a resistivity of at least 18.2 MΩ cm. The resulting clear, colorless solution was added to an aqueous solution of either LiBMSI, LiBETI, or KPF6. The RTILs, which are denser than water, settled to the bottom and were washed with copious amounts of Nanopure water to remove any unreacted starting materials. C12mimPF6 is a solid at room temperature; therefore, no data was collected for this study. The remaining RTILs were synthesized as previously described in the literature.25 Briefly, the CnmimBMSI, CnmimBETI, and CnmimPF6, n-bromoalkanes (C6, C8, and C10) were added to a slight excess of 1-methylimidazole and reacted for 3-4 h at 65-70 °C in the presence of K2CO3. The unreacted starting materials were distilled off, and the remaining viscous liquids were dissolved in Nanopure water and neutralized with HCl. The aqueous solutions were then added to an aqueous solution of LiBMSI, LiBETI, or KPF6. Unreacted salts were removed by washing the RTIL with copious amounts of Nanopure water. C4mimBMSI and C4mimBETI were prepared following the above procedure using 1-butyliodide instead of the bromoalkane. No data was collected for C10mimPF6 since it is a solid at room temperature. CnmimBF4 RTILs were synthesized as follows. The aqueous CnmimBr solution was distilled under vacuum to remove all of the water. CnmimBr was then dissolved in acetone and added to an acetone solution containing an equimolar amount of AgBF4. The reaction was carried out overnight at room temperature, and the solid AgBr was filtered off. The acetone was then distilled off to obtain the RTIL. The remaining viscous liquid was clear and slightly yellow in color. C6mimBF4, C8mimBF4, C10mimBF4, and C12mimBF4 were synthesized following this procedure. However, C12mimBF4 is a solid, so no data is presented for this compound. In order to more accurately compare the 17 RTILs used in this study, the water concentration in the samples was kept at a constant value. All RTILs were dried under a vacuum at 62 °C for at least 14 h prior to use. 0.28 M D2O was added to the dried RTIL samples. This concentration was chosen because it is the amount of water present in the water-equilibrated C12mim BETI which is the driest water-equilibrated RTIL used in
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Rollins et al.
Figure 2. SFVS spectra of CnmimBF4 (a), CnmimPF6 (b), CnmimBMSI (c), and CnmimBETI (d), recorded with ssp polarization. The spectra are arbitrarily offset for clarity. The spectra are an average of three individual samples. The spectra are normalized to the peak at 2970 cm-1.
this study. D2O was used instead of H2O in the samples to prevent any interference from H2O in the SFVS spectra. The molarity of the RTILs after the addition of 0.28 M D2O changed by less than 1% from the neat RTILs. The RTILs were stored under Argon gas and allowed to equilibrate overnight. Samples were transferred, without exposure to ambient atmosphere, to a closed SFVS cell. A description of the cell used in these studies can be found elsewhere.42 In order to check for laser degradation and other environmental contamination, such as adsorption of atmospheric water which may have penetrated the cell, the initial spectra were compared with those obtained after several hours, and no observable changes were seen in the spectra. The pH of all of the RTIL samples studied was 5.5. The pH of the solutions was measured using pH indicator strips. The value obtained on the pH indictor strips was within error of the corrected value for pD. Therefore, the pH measured is equivalent to the pD. Surface Tension. The surface tension of the RTIL/air interface was determined using the drop volume method. A Gilmont 2.0 mL micrometer syringe was used to accurately dispense drops from a flat, stainless steal capillary with an outer tip radius of 508 µm. The average drop volume was calculated using the density of the RTIL and the weight of 10 drops. The average surface tension was calculated from three individual measurements. The Gilmont micrometer syringe was calibrated using the liquid/air surface tension of ethanol (21.97 mN m-1)49 and acetonitrile (28.66 mN m-1).49 The calibration was then tested using the toluene/air surface tension and found to be within 98% of the actual value (27.90 mN m-1).49 For all measurements, the RTILs were equilibrated with 0.28 M D2O. Contact Angle Measurements. Three drops of RTIL were dispensed from a 1 mL disposable syringe and deposited on the surface of a clean SiO2 prism. The prism was the same as that used to obtain the SFVS spectra. Multiple pictures of the drop were taken using a Canon Powershot SD550. The RTIL/ SiO2 contact angle was determined by measuring the tangent to the drop at the point of contact with the SiO2 surface using the free image processing software, Scion Image (http://
www.scioncorp.com). The average contact angle for each RTIL was calculated from six individual measurements. Sum-Frequency Vibrational Spectroscopy. The SFVS experimental setup and cell have been described elsewhere.41 The frequency range used in this study was 2750-3300 cm-1. The output IR energy at 3000 cm-1 was 3 mJ/pulse with a 4 mm2 beam size. The output energy of the 532 nm beam was 5 mJ/pulse with a 4 mm2 beam size. All experiments were done under a total internal reflection geometry with an incident angle between 77° and 83° for the 532 nm beam and 67° and 73° for the IR beam.50,51 All spectra shown are an average of three samples unless otherwise stated. Results and Discussion SFVS Peak Assignments. SFVS spectra of the RTILs studied at the SiO2 interface are shown in Figures 2 (ssp polarization) and 3 (sps polarization). The spectra shown are normalized to the peak at 2970 cm-1. Only the cation contributes to the spectra in the region between 2800 and 3250 cm-1 since the anions studied are perfluoronated and the C-F stretches occur between 100 and 1400 cm-1.52 The vibrational mode peak assignments for the 1-alkyl-3-methylimidazolium cation have been determined previously in our laboratory.42 Four peaks arise from the alkyl chain: the CH2 symmetric stretch (Vs) at 2842 cm-1, the CH3 Vs at 2875 cm-1, the CH3 Fermi resonance (VFR) at 2930 cm-1, and the CH3 antisymmetric stretch (Vas) at 2950 cm-1. The CH2 Vs, CH3 Vs, and CH3 VFR can be seen clearly, whereas the CH3 Vas overlaps with the N-CH3 Vs at 2991 cm-1 giving rise to the band at about 2970 cm-1. The broad peak between 3000 and 3100 cm-1 is from the C-H stretch on the C2 carbon of the imidazolium ring.42 Two other peaks from the imidazolium ring due to the hydrogens at the C4 and C5 positions are also visible, the HCCH Vas at 3125 cm-1 and the HCCH Vs at 3175 cm-1.42 In the ssp SFVS spectra of the BF4 and PF6 imidazolium RTILs (Figure 2, panels a and b), the intensity of the CH3 Vs at 2875 cm-1 does not vary with a change in the alkyl chain length.
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J. Phys. Chem. B, Vol. 111, No. 18, 2007 4993
Figure 3. SFVS spectra of CnmimBF4 (a), CnmimPF6 (b), CnmimBMSI (c), and CnmimBETI (d), recorded with sps polarization. The spectra are arbitrarily offset for clarity. The spectra are an average of three individual samples. The spectra are normalized to the peak at 2970 cm-1 from the ssp spectra shown in Figure 2.
In the BMSI and BETI RTILs (Figure 2, panels c and d), the CH3 Vs at 2875 cm-1 increases in intensity with an increase in alkyl chain length. An increase in the CH3 VFR (2930 cm-1) intensity for the BMSI and BETI RTILs is also observed as the alkyl chain length increases from C4 to C12. This increase in the CH3 VFR is expected since the CH3 Vs and the CH3 VFR vibrational modes are coupled.53 Examination of the sps vibrational spectra in the BMSI and BETI RTILs (Figure 3) clearly shows that the peak centered at about 2970 cm-1 also increases in intensity with an increase in alkyl chain length, further suggesting a change in the alkyl chain orientation as a function of composition of the RTIL. A quantitative analysis of the CH2 Vs (2842 cm-1), CH3 Vs (2875 cm-1), and HCCH Vs (3175 cm-1) resonances have been used to determine the structure and orientation of the imidazolium cation at the SiO2 interface and are discussed in the following sections. Conformation of the n-Alkyl Chain. The relative gauche defect content of the imidazolium alkyl chain is determined by using the ratio of the CH2 Vs to the CH3 Vs at 2842 and 2875 cm-1, respectfully, obtained from the ssp spectra.54-56 If the alkyl chain is completely trans, no CH2 Vs will be detected by SFVS due to the symmetric arrangement of the methylene units along the hydrocarbon backbone. If a gauche defect is present, the measured CH2 Vs intensity is proportional to the number of gauche defects.54,56 By normalizing the CH2 Vs to the CH3 Vs, the relative number of gauche defects can be compared between samples. Figure 4 compares the decrease in the CH2 Vs resonance as the imidazolium alkyl chain is increased from butyl to dodecyl for the BMSI and BETI RTILs. The spectral fits to the CH2 Vs and the CH3 Vs are also shown for clarity. A small CH2 Vs resonance is observed for C4mimBMSI and C4mimBETI which completely disappears as the alkyl chain increases in length. The ratios for the butyl are 0.32 ( 0.04 for BMSI and 0.30 ( 0.02 for BETI, whereas the dodecyl appears to be in an alltrans configuration since the CH2 Vs is not observed. This trend of decreasing CH2 Vs with increasing alkyl chain length was seen in our previous work.42 The CH2 Vs/CH3 Vs ratios calculated for the C6mim, C8mim, and C10mim BMSI and BETI RTILs
Figure 4. Spectra of the CH2 Vs and CH3 Vs region between 2800 cm-1 to 2900 cm-1 for CnmimBMSI (a) and CnmimBETI (b). The open circles are the raw data and the solid lines (___) represent the fit to the CH2 Vs (gray) and CH3 Vs (black) using eq 3.
TABLE 1: CH2 Ws/CH3 Ws Ratios Measured for the Alkyl Chain of the Imidazolium Cations at the SiO2 Surface as a Function of RTIL Composition C4mim C6mim C8mim C10mim C12mim a
BF4
PF6
BMSI
BETI
0.04 ( 0.01 0.00 ( 0.03 0.02 ( 0.01 0.00 ( 0.03
0.04 ( 0.01 0.00 ( 0.03 0.00 ( 0.03
0.32 ( 0.04 0.26 ( 0.01a 0.10 ( 0.02a 0.05 ( 0.01a 0.00 ( 0.03
0.30 ( 0.02 0.24 ( 0.02a 0.12 ( 0.02a 0.04 ( 0.01a 0.00 ( 0.03
Includes data from Fitchett et al.42
are all within the standard deviation of the CH2 Vs/CH3 Vs ratios calculated previously from our laboratory.42 As a result, the values obtained from our previous study for these RTILs are included in the values presented in this study. The gauche defects in the BF4 and PF6 RTILs are almost nonexistent with their CH2 Vs/CH3 Vs ratios all less than 0.04 regardless of alkyl chain
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Rollins et al. TABLE 2: Tilt Angles in Degrees for the Imidazolium Alkyl Chain (θc) and HCCH Ws (θr) as Determined from the SFVS Intensity Dataa
Figure 5. χssp/χsps ratio versus tilt angle for the alkyl CH3 Vs (a) and the imidazolium HCCH Vs (b).
length. The CH2 Vs/CH3 Vs ratios for all of the RTILs studied are summarized in Table 1. A similar decrease in the number of gauche defects with increasing chain length was also seen for dialkylammonium monolayers adsorbed on mica.57 For comparison, the RTILs examined in this study have fewer relative gauche defects than surfactants at an octadecanethiol monolayer adsorbed on a gold surface, which have a CH2 Vs/ CH3 Vs ratio ranging from 0.2 to 7.56 Even though the exact number of gauche defects cannot be determined using the CH2 Vs/CH3 Vs ratio, the small value is indicative of a high degree of conformational order in the RTIL alkyl chain; therefore, it will be assumed the alkyl chain is predominately in an all-trans conformation. This assumption will be used in the alkyl chain tilt calculations below. Orientation of the n-Alkyl Chain. Using the method of Zhang et al.,48 the orientation of the alkyl chain was determined from the ssp and sps SFVS spectra. By taking the ratio of χ(2) ssp and χ(2) sps for a particular vibrational transition, the orientation of that transition dipole moment can be determined. The tilt from the surface normal (θ) is calculated using the equation
θ ) arccot
[(
χ(2) ssp
χ(2) sps
-
) ]
1+R 1-R 1 - R 2R
1/2
(4)
where R is dependent on the Raman depolarization ratio (F) through the following equations:
R)
Q-1 Q+2
and
Q)
[53(F1 - 34)]
1/2
.
(5)
The proper form of the equation used to calculate R was determined from an independent set of polarization measurements.48 The tilt of the terminal CH3 group on the alkyl chain was determined by using a Raman depolarization ratio of 0.02342 and the intensity of the CH3 Vs in both ssp and sps polarizations. A plot of χssp/χsps versus the CH3 tilt angle using eq 4 is shown in Figure 5a. The CH3 tilt for the BF4 RTILs is nearly constant at about 57°, the tilt of the PF6 RTILs range between 46° ( 6° to 68° ( 15°, the BMSI RTILs range from 37° ( 3° to 56° ( 2°, and the BETI RTILs range from 26° ( 3° to 47° ( 2° (Table
RTIL
-CH3 Vs tilt
alkyl chain tiltb (θc)
HCCH Vs tilt (θr)
C4mimBF4 C6mimBF4 C8mimBF4 C10mimBF4 C4mimPF6 C6mimPF6 C8mimPF6 C4mimBMSI C6mimBMSIc C8mimBMSIc C10mimBMSIc C12mimBMSI C4mimBETI C6mimBETIc C8mimBETIc C10mimBETIc C12mimBETI
57 ( 4 55 ( 3 58 ( 4 57 ( 2 46 ( 6 68 ( 15 54 ( 8 56 ( 2 37 ( 3 42 ( 4 38 ( 3 37 ( 2 47 ( 2 26 ( 3 36 ( 4 41 ( 4 40 ( 8
22 ( 4 20 ( 3 23 ( 4 22 ( 2 11 ( 6 33 ( 15 19 ( 8 21 ( 2 4(3 7(4 3(3 2(2 12 ( 2 9(3 1(4 6(4 5(8
32 ( 4 30 ( 4 25 ( 3 22 ( 1 36 ( 4 29 ( 5 20 ( 4 36 ( 8 31 ( 2 24 ( 4 29 ( 4 28 ( 10 33 ( 6 19 ( 3 20 ( 3 21 ( 3 7 ( 7d
a The uncertainty is due to errors in the measurements and does not reflect the angular distribution. b Assumes an all-trans conformation. c Includes data from Fitchett et al.42 d See text.
2). The use of eqs 4 and 5 assumes a delta function distribution with regards to the orientation angle. Therefore, the standard deviations reported above reflect the error in the measurement and not the width of the angular distribution. Data from a previous study performed in our laboratory on a subset of BMSI and BETI RTILs, where the RTILs were equilibrated with D2O of varying concentrations, are within the standard deviation of those reported in this paper42 and are included in the values presented in this study. The agreement between the values calculated in this study and the values calculated in the previous study demonstrate that the water content of the BETI and BMSI RTILs does not have a large effect on the CH3 tilt angle of these RTILs.42 Assuming an all-trans conformation, the tilt of the alkyl chain was calculated58 and is recorded in Table 2. The data reveals that the alkyl chain on the imidazolium cation is oriented between 1° and 33° from the SiO2 surface normal for all four of the anions studied. The smallest alkyl chain angles were found in the hydrophobic BMSI and BETI RTILs. Orientation of the Imidazolium Ring. Figures 2 and 3 show that the HCCH Vs peak at 3175 cm-1 can be seen in both the ssp and sps spectra. Given that this peak is observed in both polarizations, the HCCH Vs transition dipole must be tilted at some angle greater than 0° and less than 90° from the SiO2 surface normal. Closer inspection of the sps HCCH Vs peak shows a decrease in intensity as the alkyl chain length increases suggesting that the transition dipole moment is becoming more normal to the surface. The transition dipole of the HCCH Vs at 3175 cm-1 lies within the plane of the imidazolium ring. The imidazolium ring can twist around the axis defined by the HCCH transition dipole. However, the twist of the imidazolium ring (φ) cannot be determined using only the HCCH Vs peak. Figure 6 shows an illustration of the HCCH Vs tilt (θr) and twist angle (φ), and the orientation of the alkyl chain (θc) of the cation at the SiO2 surface. In principle, the twist of the imidazolium ring can be determined by using the orientation of both the HCCH Vs dipole and the N-CH3 Vs dipole.42 However, our previous attempts to obtain a self-consistent geometry for the HCCH Vs dipole and the N-CH3 Vs dipole were not successful. It was concluded that the N-CH3 Vs group can adopt a wide range of possible orientations; therefore, the exact twist angle of the imidazolium ring cannot be solved using eqs 4 and 5.42 Some restraints on the twist of the ring can be determined from our
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J. Phys. Chem. B, Vol. 111, No. 18, 2007 4995
Figure 7. The HCCH Vs tilt angle as a function of alkyl chain length for the BF4 (b), PF6 (O), BMSI (9), and BETI (0) RTILs. Figure 6. Schematic of the HCCH Vs tilt angle (θr), twist angle (φ) and alkyl chain orientation (θc) of the imidazolium cation at the silica surface.
previous SFVS interference measurements42 and the chemical structure of the imidazolium ring. From the SFVS interference measurements, it was determined that the terminal CH3 Vs on the alkyl chain points away from the silica surface.42 Conversely, the N-CH3 Vs is directed toward the silica surface.42 A twist angle of 0° is arbitrarily defined as the HCCH Vs and N-CH3 Vs transition dipoles lying in the x-z plane. Knowing that the relative positions of the CH3 Vs and N-CH3 Vs dipoles are oriented antiparallel to each other, and that the N-CH3 points toward the silica surface, the twist of the imidazolium ring is limited to between 0° and less than 90° from the x-z plane. It should be noted that the HCCH Vas was not used to calculate the twist angle of the ring because its Raman depolarization ratio is 0.74 ( 0.13 which is completely depolarized;42 therefore, no real solution exists for eq 4 using the HCCH Vas. The tilt of the HCCH Vs transition dipole has been calculated using eqs 4 and 5 and the HCCH Vs intensity at 3175 cm-1 obtained from the ssp and sps spectra. The Raman depolarization ratio for the HCCH Vs has previously been determined in our laboratory to be 0.09142 which corresponds to a Q value of 2.4. C4mimBF4 has a calculated HCCH Vs tilt of 32° ( 4° that decreases to 22° ( 1° for C10mimBF4. The PF6 RTILs have a similar trend with the tilt angle decreasing from 36° ( 4° to 20° ( 4° for C4mimPF6 and C8mimPF6, respectively. The calculated tilt angles for both BMSI RTILs and BETI RTILs also show a decrease as the alkyl chain length is increased. The HCCH Vs is not observed in the C12mimBETI sps spectra. This suggests the imidazolium ring lies completely perpendicular to the SiO2 surface with an HCCH Vs tilt angle of 0° from the surface normal. Figure 5b plots the χssp/χsps ratio as a function of the HCCH Vs tilt angle using eq 4 and an R value of 0.319. As the χssp/χsps ratio approaches infinity, the HCCH Vs tilt angle approaches 0°. In order to determine a possible range of HCCH Vs tilt angles for C12mimBETI, the background noise in the sps spectra was analyzed in a region void of any vibrational resonances. The standard deviation of the noise was determined from the C12mimBETI sps spectra from 2800 to 2830 cm-1. This standard deviation was used to find the detection limit of the experiment, with the detection limit defined as three times the standard deviation of the background noise.59 The intensity of the smallest possible detectable signal was used to solve eq 4. Given this calculated limit of detection, the possible range of tilt angles for the HCCH Vs of C12mimBETI is between 0° and 14° from the surface normal. As a result of this analysis,
an approximate value of 7° ( 7° was used for the HCCH Vs tilt angle of the C12mimBETI to include all possible tilt angles. The HCCH Vs tilt angles for all of the RTILs examined are summarized in Table 2 and shown as a function of alkyl chain length in Figure 7. Figure 7 shows that the HCCH Vs tilt angle decreases as the alkyl chain length increases. In contrast to the effect of the alkyl chain length on ring orientation, there appears to be little change in the tilt angle as a function of the anion. This is contrary to what was reported by Romero and Baldelli for a series of C4mim RTILs coupled with BF4, PF6, BMSI, and BETI anions at a RTIL/quartz interface.43 For the BF4 RTILs the HCCH Vs tilt angle decreases from 32° ( 4° to 22° ( 1° as the alkyl chain length increases from C4 to C10. The HCCH Vs tilt angle decreases from 36° ( 4° to 20° ( 4° as the alkyl chain length increases from C4 to C8 in the PF6 RTILs. The HCCH Vs tilt angle decreases from 36° ( 8° to 28° ( 10° as the alkyl chain length increases from C4 to C12 in the BMSI RTILs, and for the BETI RTILs the HCCH Vs tilt angle decreases from 33° ( 6° to 7° ( 7° as the alkyl chain length increases from C4 to C12. In addition to the HCCH Vs tilt angle being influenced by the alkyl chain length, other physical properties may also affect the orientation of the imidazolium ring at the SiO2 surface. Effect of SiO2 Surface Charge Density on the Orientation of the Imidazolium Ring. A clear correlation between the imidazolium HCCH Vs tilt and alkyl chain length is seen in Figure 7. This observation supports the simplistic hypothesis that a longer alkyl chain orients the ring at the SiO2 surface. However, it must also be remembered that the physical properties of the RTILs, such as density and ionic strength, are also influenced by the length of the alkyl chain. One possible explanation for the correlation between the alkyl chain length and the HCCH Vs tilt angle is the difference in density between the RTIL samples studied. The density may affect the RTIL cation packing at the SiO2 surface. As the alkyl chain length is increased, the density decreases as shown in Figure 8. As the density decreases, the imidazolium ring lies more perpendicular to the silica surface. If the packing density was solely responsible for the changes in the HCCH Vs tilt angle, the imidazolium ring should lie more parallel to the surface with a decrease in the density. There is also a change in the density of the RTILs as the anion is varied (Figure 8). The HCCH Vs tilt would also be affected by the anion if the tilt only depended on the packing density of the RTIL. The data in Figure 7 shows there is little dependence on the HCCH Vs tilt angle with anion composition, which is counter to the observation previously reported by Romero and Baldelli for water-free RTILs.43 In addition to the packing density of the RTIL, there must be some other influence dictating the orientation of the imidazolium ring.
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Figure 8. RTIL density as a function of alkyl chain length for BF4 (b), PF6 (O), BMSI (9), and BETI (0) RTILs.
The density of the RTILs also directly affects the concentration of the ions in the bulk. Varying the ionic strength will modulate the surface charge density of the underlying silica surface in contact with the RTIL.60 In addition, changes in the alkyl chain length of the cation will also modulate the surface activity of the RTIL.61 The orientation of the imidazolium ring has been shown previously to be dependent on the interfacial potential at the RTIL/platinum electrode interface.39,41 These previous studies showed an increase in the tilt angle of the C4mim cation, when paired with the dicyanamide, BF4, and PF6 anions, as more negative potentials were applied to the electrode surface. It is conceivable that the changes in the surface charge density of silica will have a similar effect on the imidazolium ring orientation. An estimate of the surface charge density of the silica surface in contact with a RTIL can be calculated using the method of Janssens-Maenhout and Schulenberg.62 The surface charge density (q) is determined from the contact angle (Θ) of the RTIL/SiO2 interface using the following equation:
q)
[
) )]
2γ0 ze 4nkT φ -1 cos Θ + 1 + cosh φ0 κγ0 2kT 0
( (
(6)
where γ0 is the surface tension, n is the bulk ion concentration in mol m-3, k is Boltzmann’s constant, T is the temperature in Kelvin, z is the charge of the ions, e is 1.602 × 10-19 C, κ is the inverse Debye length, and the surface potential, φ0, is defined as
φ0 )
2kT κq . arcsinh ze 4zen
( )
(7)
The inverse Debye length, κ, can be calculated from
κ)
x
2z2e2n 0kT
(8)
where is the permittivity of the liquid and 0 is the permittivity of free space. Equations 6 and 7 are solved self-consistently to obtain both q and φ0 for a measured value of Θ and γ0. Solving for q and φ0 is challenging because not all values of for the RTIL samples examined here are known. In addition, the extremely high ion concentrations of the RTILs lead to small Debye lengths, κ-1. As a result, ion size becomes important when calculating the electrochemical double-layer thickness in these systems. Few studies can be found in the literature in which the dielectric constants of RTILs have been measured.64,65 It is difficult to directly measure their permittivity since the RTILs are conductive media. Wakai, et al.65 used dielectric spectros-
Figure 9. Percent error in the permittivity () versus the percent error in the surface charge density (q) determined from eqs 10 and 11.
copy to determine the static dielectric constants of C2mimBMSI (15.2 ( 0.3), C2mimBF4 (12.8 ( 0.6), C4mimBF4 (11.7 ( 0.6), C4mimPF6 (11.4 ( 0.6), and C6mimPF6 (8.9 ( 0.9). From this data, alkyl chain length versus permittivity plots were constructed for both the BF4 and PF6 RTILs. The dielectric constants for the unknown BF4 and PF6 RTILs were estimated assuming a linear correlation between the alkyl chain length and the permittivity. In the absence of any data on the influence of alkyl chain length for the BMSI and BETI RTILs, the change in dielectric as a function of alkyl chain length for these materials was assumed to be an average of that measured for the BF4 and PF6 RTILs. The average slope from the BF4 and PF6 plots and the measured dielectric constant of C2mimBMSI were used to determine for the BMSI and BETI RTILs as a function of alkyl chain length. Wakai et al. stated that the permittivity of a RTIL with a water content of less than 0.5 volume% was within the error of their measurements, which ranged from 2% for C2mimBMSI to 10% for C6mimPF6.65 Since the RTILs examined in this study contain less than 0.5 volume% D2O, no adjustments were made for the D2O content. It is important to see if the assumptions used above in calculating the dielectric constants adversely affect the calculations of q and φ0. In order to quantify the dependence of on the calculated values of q and φ0, the method of random error propagation was used. The propagation of random errors was used instead of the propagation of systematic errors because the sign and magnitude of the error in is not known.66 The propagation of random errors for an arbitrary function, F, is given by the following equation:
S2(F) )
2
2
2
(∂F∂x ) S (x) + (∂F∂y ) S (y) + (∂F∂z ) S (z) + ... 2
2
2
(9)
where S is the variance in the variable and x, y, and z represent individual experimental variables.66 Using eqs 6 and 7, the error in the surface charge density and surface potential due to errors in can be found using the following:
S2(q) )
[
2γ0 x2n2z2e2 eφ + 2 2 1.5 cosh -1 φ0 2kT zen 2 γ0 0 0kT
( ( ) )
( )
S2(φ0) )
[
q2z4kT
(
8z2e2n30 1 +
2 2
]
)
zq 80kTn
2
]
2
S2() (10)
S2()
(11)
where S2() is the variance in the permittivity. Equations 10 and 11 were solved simultaneously to determine how errors in affect the calculated values of q. Figure 9 shows the relationship between the percent error in versus the percent error in q. To
SFVS Study of the RTIL/SiO
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4997
determine the largest variance in from the assumptions made above, an average and standard deviation were determined using all of the calculated and known dielectric constants for the RTILs, regardless of alkyl chain length or anion composition. The average calculated was 9.43 ((2.54). Using this average and standard deviation, an error in q of only 0.25% was calculated. To validate if the assumptions incorporated into are indeed negligible, the SiO2 surface charge density (q) for C4mimBF4 was calculated using the average of 9.43 ((2.54). This value was compared to the q calculated when using the known value of , 12.05 ((5%).63 C4mimBF4 was chosen because its known is the farthest from the average calculated using the assumptions stated above. The q value calculated using the known value of (52.1 ( 0.3 mC m-2) lies within the 99% confidence interval of the q value calculated from the average (52.0 ( 0.3 mC m-2). This error analysis proves that although large uncertainties may exist in the actual values of they have little effect on q and φ0. Since the errors in have been shown to be negligible compared to other experimental errors, such as, measurement of the contact angle and surface tension, the average was used when solving q and φ0 in order to consider the largest possible distribution of errors. The previously measured values of were used for C4mimBF4, C4mimPF6, and C6mimPF6.65 In addition to the problem of determining in these systems, the high salt concentration also needs to be addressed. To more accurately describe the double-layer in contact with a charged surface at high salt concentrations Smagala and Fawcett incorporated two adjustable, dimensionless parameters, R and β, into the Gouy-Chapmann theory.60 These dimensionless parameters account for the effects of ion size which limits how close the ions can approach the charged solid surface. In dilute solutions, the effect of ion size can be neglected because the Debye lengths in these systems are long enough to extend well into solution. However, at high salt concentrations the Debye lengths are much smaller. To accurately describe the doublelayer in these systems ion size cannot be ignored. The adjustable parameter β is defined by the equation
(
)
(1 + 2κd)1/2 1 β) 2 2
1/2
(1 + 2κd)1/2 1 + 4 4
(12)
where d is the diameter of the ion. The dimensionless parameter R is assumed to be 1.49, the average value found by best fit parameters to Monte Carlo simulations.60 The value of d used for the Cnmim cations was determined by calculating the volume, assuming a cylindrical shape, with a radius of 2.19 Å and a length of 10.05, 12.35, 14.84, 17.05, and 19.45 Å for the C4mim, C6mim, C8mim, C10mim, and C12mim cations respectively.25 These values were then used to calculate the diameter of a sphere with an equivalent volume. The value of d used for the BMSI and BETI anions was calculated similar to the procedure for determining the diameter of the cations using a radius of 2.34 and 2.39 Å, and a length of 7.78 and 10.1 Å for BMSI and BETI respectively.25 The BF4 and PF6 anions were assumed to be spheres with a radius of 2.25 Å for the BF4 and 2.53 Å for the PF6.67 Since the double-layer is composed of both cations and anions an average of their diameters was used to solve for β. The dimensionless parameters R and β adjust κ for the effects of the ion size at the double-layer using the following equation:
κ′ ) κ(Rβ)1/2.
(13)
Figure 10. Illustration of the potential profile near the SiO2 interface showing the potential drop from the surface (φ0) to the bulk solution (φbulk) using the theory by Smagala and Fawcett.60
Figure 11. HCCH Vs tilt as a function of SiO2 surface charge density for CnmimBF4 (a), CnmimPF6 (b), CnmimBMSI (c), and CnmimBETI (d).
To incorporate the effects of ion size when solving eqs 6 and 7 for the SiO2 surface charge density and surface potential, κ′ was used instead of κ. An illustration of the Smagala and Fawcett model showing the potential profile from the SiO2 surface into the bulk RTIL is shown in Figure 10 for C8mimBMSI. This potential profile was constructed using the following equation:
[ ] [ ]
Rψ0F 4RT exp[-κ′xd] ) Rψ(x)F tanh 4RT tanh
(14)
where F is Faraday’s constant, R is the molar gas constant, x is a dimensionless distance, which is converted into units of length by multiplying it by the diameter of the ion, ψ0 is the dimensionless surface potential at x ) 0 or the outer Hemholtz plane, and ψ(x) is the dimensionless potential at position x. The dimensionless potential is related to actual potential, φ(x), by59
φ(x) )
ψ(x)RT . F
(15)
The potential profiles calculated using eq 14 are similar for all of the RTILs studied with the Debye length ranging from 0.27
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TABLE 3: RTIL Concentrations (n), Contact Angles (Θ), and Surface Tensions (γ0) Used in Eqs 6 and 7 to Calculate the SiO2 Surface Charge Densities (q)
RTIL
RTIL conc (M) (( 1%)
contact angle (degrees)
surface tension (mN/m)
SiO2 surface charge density (mC/m2)
C4mimBF4 C4mimPF6 C4mimBMSI C4mimBETI C6mimBF4 C6mimPF6 C6mimBMSI C6mimBETI C8mimBF4 C8mimPF6 C8mimBMSI C8mimBETI C10mimBF4 C10mimBMSI C10mimBETI C12mimBMSI C12mimBETI
5.28 4.80 3.42 2.86 4.54 4.05 3.05 2.58 3.86 3.51 2.70 2.37 3.40 2.49 2.18 2.26 2.02
21.6 ( 2.1 33.0 ( 2.6 27.6 ( 2.2 30.7 ( 1.4 29.3 ( 4.1 32.8 ( 2.6 28.5 ( 1.7 31.8 ( 2.6 34.9 ( 1.9 36.3 ( 3.4 32.4 ( 1.5 37.0 ( 1.1 37.5 ( 3.3 34.9 ( 1.7 43.1 ( 2.9 40.5 ( 2.5 47.5 ( 2.1
37.7 ( 0.1 48.8 ( 0.6 35.3 ( 0.3 31.7 ( 0.4 37.0 ( 0.4 45.2 ( 0.3 36.2 ( 0.1a 32.0 ( 0.2a 36.9 ( 0.3 40.4 ( 0.2 36.3 ( 0.3a 32.6 ( 0.3a 35.3 ( 0.4 37.0 ( 0.3a 34.0 ( 0.3a 36.9 ( 0.6a 34.6 ( 0.2a
52.1 ( 0.3 64.1 ( 0.9 47.5 ( 0.4 42.1 ( 0.3 49.4 ( 1.0 59.1 ( 0.8 48.4 ( 0.3 42.2 ( 0.5 47.8 ( 0.5 51.9 ( 1.0 47.6 ( 0.4 41.8 ( 0.3 45.0 ( 0.9 47.8 ( 0.4 41.8 ( 0.8 46.1 ( 0.7 41.2 ( 0.7
a
From Fitchet et al.61
to 0.46 Å which clearly never extends beyond the diameter of the RTIL ions. The final variable needed to solve for the SiO2 surface charge density in these systems is the RTIL surface tension. The surface tension of the RTILs was calculated from the drop volume method using the following equation:
γ0 )
∆FgV r 2πrδ 1/3 V
( )
(16)
where ∆F is the difference in the densities between air and the RTIL, g is the gravitational constant, V is the volume of the detached drop, r is the outer tip radius, and δ(r/V1/3) is a constant determined by Earnshaw et al.68 The constant is needed to correct γ0 for liquid that remains on the tip of the needle, nonvertical boundary tension forces, and pressure differences across the curved surface.68 The values of the surface tension, γ0, contact angles, Θ, and concentration, n, are summarized in Table 3. The surface charge density was calculated by simultaneously solving eqs 6 and 7 using the values listed in Table 3, a corrected inverse Debye length (κ’) based on the theory by Smagala and Fawcett, and an average (except when is known) as discussed above. The surface charge density values are recorded in Table 3. In order to determine if the change in the surface charge of SiO2 has any affect on the orientation of the positively charged imidazolium ring, the correlation between the HCCH Vs tilt angle and the SiO2 surface charge density was examined (Figure 11). Figure 11 shows that the HCCH Vs tilt angle increases (the imidazolium ring is lying more parallel to the silica surface) with an increase in the negative charge on the SiO2 surface. This correlation is anion dependent. The positive charged imidazolium ring would be expected to lie more parallel to the surface as the surface charge density becomes more negative in order to more effectively screen the increasing negative charge on the silica surface. These results are consistent with those previously observed for imidazolium cations at the platinum electrode surface.39,41 The largest changes in the calculated surface charge densities are seen for the BF4 and PF6 RTILs where there is a change of 7.1 ( 0.9 and 12.2 ( 1.3 mC m-2, respectively (Figure 11).
Conversely, the smallest changes in the calculated surface charge densities appear in the BMSI and BETI RTILs where there is a change of only 1.4 ( 0.8 and 0.9 ( 0.8 mC m-2, respectively (Figure 11). Examination of eqs 6 and 7 reveals that the calculated surface charge density is dependent on the RTIL concentration, surface tension, and contact angle. A positive correlation on the calculated surface charge density is seen for both the RTIL concentration and the surface tension. However, increases in the contact angle decrease the calculated surface charge density. Table 3 shows that the RTIL concentration and surface tension decrease, while the contact angle increases with increasing alkyl chain length for the BF4 and PF6 RTILs. These changes will all decrease the calculated surface charge density. However, for the BMSI and BETI RTILs the surface tension and contact angle increase while the concentration decreases as the alkyl chain is lengthened. The negative correlation of the surface tension with alkyl chain length is competing with the changes due to the RTIL concentration and contact angle. These counteracting forces may account for the small calculated changes in the SiO2 surface charge densities for the BMSI and BETI RTILs. It should be noted that while there is a correlation between the surface charge density and the HCCH Vs tilt angle the strongest dependence on the HCCH Vs tilt angle is with the alkyl chain length as shown in Figure 7. Conclusion The surface specific technique of SFVS was used to measure the structure and orientation of the 1-alkyl-3-methylimidazolium cation at the SiO2 surface as a function of anion and cation composition. The CH2 Vs/CH3 Vs ratio was used to determine the number of relative gauche defects in the alkyl chain of the imidazolium cation. The relative number of gauche defects were found to be minimal and decreased with an increase in the alkyl chain length especially for the BMSI and BETI RTILs. Assuming an all-trans conformation the tilt of the alkyl chain was found to be between 1° and 33° from the surface normal for all of the RTILs studied. The HCCH Vs tilt of the ring was also examined and found to be dependent on the change in alkyl chain length and surface charge density of the SiO2. The strongest correlation was seen between the alkyl chain length and the HCCH Vs tilt angle. As the chain length decreases the imidazolium ring orients more parallel to the silica surface. This trend was found to be independent of the anion composition. A similar tend was also observed as a function of the SiO2 surface charge density, with the imidazolium ring lying more parallel to the surface with increasing negative surface charge. This study gives new insight into the forces that affect the imidazolium cation structure and orientation at the RTIL/SiO2 interface. Acknowledgment. This work was supported by the National Science Foundation (CHE 0515940). References and Notes (1) Anjaiah, S.; Chandrasekhar, S.; Gree, R. AdV. Synth. Catal. 2004, 346, 1329-1334. (2) Clavel, G.; Larionova, J.; Guari, Y.; Guerin, C. Chem.-Eur. J. 2006, 12, 3798-3804. (3) McNulty, J.; Capretta, A.; Cheekoori, S.; Clyburne, J. A. C.; Robertson, A. J. Chimica Oggi 2004, 22, 13-16. (4) Ngo, H. L.; Hu, A.; Lin, W. Tetrahedron Lett. 2005, 46, 595597. (5) Pedireddi, V. R.; Shimpi, M. R.; Yakhmi, J. V. Macromol. Symp. 2006, 241, 83-87. (6) Vallette, H.; Ferron, L.; Coquerel, G.; Gaumont, A.-C.; Plaquevent, J.-C. Tetrahedron Lett. 2004, 45, 1617-1619. (7) Zhang, H.; Hong, K.; Mays, J. W. Macromolecules 2002, 35, 57385741.
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