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On the Arrangement of Ions in Imidazolium-Based Room Temperature Ionic Liquids at the Gas-Liquid Interface, Using Sum Frequency Generation, Surface Potential, and Surface Tension Measurements Imee Su Martinez and Steven Baldelli* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003 ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: June 2, 2010
The characterization of three ionic liquids [BMIM][BF4], [BMIM][DCA], and [BMIM][MS] having a common cation and with anions of varying sizes and shapes was performed with three complementary surface techniques: sum frequency generation-polarization mapping, surface tension measurement, and surface potential measurement. Custom vacuum cells were designed for each technique to be able to perform measurements in a highly controlled environment minimizing the presence of water and other contaminants, which may compromise measured values. SFG results show evidence of having anions and cations present on the surface with the butyl chain of the cation positioned toward the gas phase and the imidazolium ring mostly parallel to the surface plane. Results from the surface potential measurements reveal the relative positions of the ions where the anions are located at a slightly lower plane compared to the cations. Observed values from the surface tension measurements denote surface intermolecular interactions indicative of both van der Waals and Coulombic interactions suggesting the presence of alkyl chains as well as ions on the surface. A model on the gas-liquid interface of ionic liquids is described based on the concurring results from these three surface characterization techniques, as well as current literature. Introduction Fundamental understanding of the molecular arrangement of ionic liquids at the gas-liquid interface is a study of vast interest because of the known applications these liquids can offer. The most common and current applications of these liquids are liquid-liquid extraction, biphasic catalysis, corrosion, lubrication, and solar cells or electrochemical applications.1-3 Obtaining a well-defined picture of the surface can possibly unravel myriad applications that are yet to be discovered. Most important, however, is the curiosity that these new class of liquids being liquid salts evoke among the scientific community. These liquids are pure ions making them relevant systems for studying the surface structure of charged species minus solvent effects. Questions on the role of charge size, intermolecular interactions, and polarizability on surface organization emerge. Whether these liquids conform to existing theories of electrolyte structure on the surface such as the Gouy-Chapman model is yet to be answered. Experimental results on the supposed structure are somewhat contradicting posing more questions whether there is indeed layering of ions or coexistence of cations and anions on the surface. The countless number of cation-anion combinations makes one wonder if there is single model to describe the surface structure of all ionic liquids, if possible. One application of ionic liquids in the gas absorption of certain anthropogenic gas pollutants such as CO2, SO2, NH3, and CFC’s can be used to address global issues concerning climate changes and ozone depletion.4,5 Studies done on this particular application, however, were leaning toward the solvation thermodynamics of ionic liquids with respect to these gases.6-8 Gas uptake, which is also a surface interaction, creates a need to study in detail gas-liquid interfaces of ionic liquids. Determining the molecular orientation, excess charge, ion size, * To whom correspondence should be addressed.
and geometry or probing the interface at the molecular level in order to understand how these gases are absorbed into the liquid becomes very significant. Parallel to this, the necessity of characterizing these interfaces at a controlled surrounding particularly with regards to the presence of volatile components becomes crucial in order to produce reliable results. The structural orientation of imidazolium-based room temperature ionic liquids has already been determined with SFG.9-13 Results of previous studies showed that both cations and anions are present on the surface. The ring of the imidazolium cation lies flat at the gas-liquid interface, while the alkyl chain is extended toward the gas phase at an angle from the surface normal. Iwahashi and co-workers performed SFG on 1-butyl3-methylimidazolium trifluoromethane sulfonate, [BMIM][OTf], to observe the orientation of the anion on the surface in detail.14 The polar SO3 groups are pointed toward the bulk and the nonpolar CH3 functional groups are directed toward the vapor phase. This provides a picture of the charged functional groups sitting on the liquid part of the gas-liquid boundary. Blue shifting of the SO3 peak also implied that there is a strong interaction between the imidazolium cations and the OTf anions. This indicates the presence of both species as well as the formation of aggregated configuration of ions on the surface. The narrow line width of the SO3 peak suggests a specific configuration of this so-called aggregation. Santos et al. observed a similar orientation of the methylsulfate anion with SFG where the methyl functional group is oriented toward the gas phase while the sulfate is toward the direction of the liquid phase.15 Phase simulation studies showed that the methyl group from the anion is oriented in the same manner as the butyl chain of the imidazolium ring. Jeon et al., using X-ray reflectivity and SFG, observed analogous results for their studies on [BMIM][BF4] and [BMIM][PF6]. They have observed the alkyl chains of the
10.1021/jp1039095 2010 American Chemical Society Published on Web 06/14/2010
Characterization of Ionic Liquids imidazolium cations to be oriented toward the gas phase while the anion and the cation cores are in contact with the liquid. Their result for [BMIM][I], however, is different, where the SFG intensity for the [BMIM][I] was double the intensities of [BMIM][BF4] and [BMIM][PF6]. This suggests that the number densities for the cations of the latter two ionic liquids are smaller than that of the [BMIM][I]. X-ray results were congruent, which showed that the layer thicknesses for [BMIM][BF4] and [BMIM][PF6] are shorter than the extended chain length of the butyl chain implying that the chain is tilted at an angle from the surface normal. In contrast, the layer thickness of [BMIM][I] is longer compared to the length of the butyl chain suggesting a different configuration. The electron density of [BMIM][I] was observed to be higher denoting that the anions are not coexisting with the imidazolium cations on one layer but are situated directly below the cations. These observations are very important as this can mean that depending on the constituent ions of the ionic liquid, the surface structure can vary. Other techniques such as X-ray reflectivity, capillary wave spectra, direct recoil spectroscopy, and Rutherford backscattering spectroscopy gave similar results, in terms of having both ions present on the surface as well the anisotropic alignment of the imidazolium cations.16-20 However, other details on the results from these techniques do not necessarily agree with SFG. In particular is the direct recoil spectroscopy study done by Law et al., wherein the imidazolium cation was observed to be oriented perpendicular to the surface plane with the nitrogen atoms of the ring postured at the top.18 Another study is the X-ray reflectivity and surface tensiometry studies performed by Sloutskin et al. to probe the surface of alkylimidazolium ionic liquids with anions [PF6]- and [BF4]-. They have stated that more anions are present on the surface with two possible arrangement of the alkyl chain, which is perpendicular and parallel to the surface plane.17 Similarly, recent experiments with oxygen atom scattering suggest, contrary to most spectroscopic results, that the alkyl chains are parallel to the surface plane.21 A theoretical study performed by Balasubramanian and Bhargava used atomistic molecular dynamic simulations to investigate [BMIM][PF6].22,23 Concurrent to the SFG results, both ions enrich the surface with the anions contributing to the enhanced calculated electron density. The butyl chains were found to be parallel to the surface normal protruding out of the liquid. The ring positioned closer to the vapor phase is parallel to the surface except at the densest subsurface part where it is perpendicular. It is a known fact that organic contaminants, chloride, and water alter values of measured physical properties in ionic liquids. Seddon et al. investigated the effect of these contaminants on the viscosity, density, and H NMR shifts in some ionic liquids.24 Chloride, even in low concentrations of 0.01 mol/kg in [BMIM][BF4], caused a dramatic drop in viscosity, a nonlinear decrease in density, and a downfield shift in the H NMR signals of the imidazolium ring of the cation. Cosolvents such as ethanenitrile, trimethylethanenitrile, 2-propenenitrile, 1-methylimidazole, toluene, 1,4-dimethylbenzene, and 1,2dimethoxyethane when added incrementally to [BMIM][BF4] and [BMIM][PF6] caused the measured viscosity to decrease exponentially. A more pronounced decrease in viscosity was caused by water alongside ethanenitrile, and 2-propenenitrile. Density decreased rapidly as well, at excess mole fractions of water (>0.5). In an atmosphere filled with moisture, studying the effect of water on the physical properties of a substance is very important. Bowers and co-workers plotted surface tension and conductivity
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11565 isotherms of imidazolium ionic liquids including [BMIM][BF4] in water.25 The general trend was the surface tension and conductivity decreased with increasing ionic liquid concentration until a critical concentration is reached wherein the values plateau. The effect of water on the vapor-liquid interfaces of ionic liquids has been studied with Sum Frequency Generation (SFG). Previous studies on [BMIM][BF4] showed that water is only probed at ionic liquid concentrations of e0.02 mol fractions, whereas at higher concentrations the surface showed SFG spectra similar to that of the pure ionic liquid.26 In fact at mole fractions of ionic liquids g0.05, vibrational peaks from the water have totally disappeared. A more significant effect was observed in ionic liquids [BMIM][PF6] and [BMIM][imide] where at water pressures greater than 5 × 10-4 Torr, the ring of the cation seems to tilt away from the plane showing ring vibrational modes H-C(4)C(5)-H symmetric stretch at ∼3175 cm-1 and antisymmetric at ∼3130 cm-1.27,28 Sung and co-workers studied the effects of water on the vapor-liquid interface of [BMIM][BF4] using surface tension and SFG.29 A rapid decrease in surface tension was observed from 0 mol fractions of ionic liquids to 0.016 mol fraction, which corresponds to the lowest value of the measured surface tension. A slight increase was observed at around 0.05 mol fraction, which evened out with the increase in ionic liquid concentration. The mole fraction, which corresponds to a minimum in the measured surface tensions, exhibited an unusually intense signal in SFG at ssp and ppp polarizations compared to the pure ionic liquid. This phenomenon was explained in terms of the surface being covered purely by cations at low concentrations of up to 0.02 mol fraction where the anions start to appear on the surface. A mole fraction of 0.05 showed that the surface is being equally populated by both cations and anions. However, it is also observed that the water signal on the surface disappears, and might be related to the surface tension minimum.26 Rutherford backscattering and X-ray photoelectron spectroscopy studies performed by Hashimoto et al. showed contamination issues compromising the integrity of their results. The surface of [BMIM][DCA] was found to be covered with ester or carboxylic moieties leading to the deviation from the stoichiometric ratio of the atoms on the surface.30 Results on the same study done on [BMIM][PF6] with a neat surface showed stoichiometric surface composition expected of surfaces having both ions on the surface. It is therefore imperative that measurements are performed in a controlled environment. In addition, surface techniques are highly sensitive to impurities making the issue of contamination very important. The goal of this study is to probe the gas-liquid interface of room temperature ionic liquids with the sum frequency generation-polarization mapping method, surface tension measurements with the Axisymmetric Drop Shape Analysis (ADSA), and surface potential measurements with the compensation/vibrating plate method. Special cells for these three different techniques were designed to be able to perform measurements under vacuum at 10-5-10-6 Torr or in a clean and controlled environment. These three techniques combined will relate together to provide a better understanding on how the ions of these liquids are structured at the surface. SFG-polarization mapping will give a better molecular level description of the interface in terms of speciation and orientation of the ions. Surface potential measurements will be able to determine the excess charge on the surface, and therefore determine which species prevail and how
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Figure 1. Ionic liquids under study: (a) 1-n-butyl-3-methylimidazolium cation, (b) dicyanamide anion, (c) tetrafluoroborate anion, and (d) methyl sulfate anion.
the ions are arranged at the surface. Surface tension will determine excess surface energy, and will correspond to the functional group as well as to the intermolecular forces prevalent on the surface. Results are combined to develop a coherent molecular-level description of the gas-liquid interface. Three different ionic liquids will be used for this study (Figure 1). These ionic liquids have the same cation, 1-butyl-3methylimidazolium [BMIM]+, and three different anions; tetrafluoroborate [BF4]-, dicyanamide [DCA]-, and methyl sulfate [MS]-;which have different sizes and geometries. Background Sum Frequency Generation (SFG)-Polarization Mapping Method. SFG is a nonlinear vibrational spectroscopic technique that involves two input laser beams;visible and tunable IR;that overlap in a medium to generate an output beam that has a frequency equal to the sum of the frequencies of the two incoming beams.31 It is a highly surface specific technique, since it is forbidden in a medium with inversion symmetry.31,32 The intensity I(ωSF) of the generated sum frequency beam is proportional to the square of the induced polarization P(2) on the surface due to the coming together of the electric fields of the two incident beams (Evis, EIR).33 The term that relates the induced polarization response to the electric fields is the second (2) . This has two compoorder nonlinear susceptibility tensor χeff (2) nents, the χnr from the nonresonant background of the surface and the resonant term (χr(2)), which contains the vibrational spectroscopic information.31 The Raman polarizability and the IR dipole transition contribute to this vibrational information expressed as the hyperpolarizability, β(2), when averaged over all molecular orientations on the surface. ωIR, ωq, and Γq are the frequency of the IR beam, frequency of the normal mode, and the damping constant of the qth vibrational mode, respectively.34 (2) I(ωSF) ∝ |P(2) | ) |χeff :EvisEIR | 2
(2) χeff ) χ(2) nr +
∑ (ωIR -N〈βωq +〉 iΓq)
(1)
(2)
(2)
q
The orientation of a molecule on the surface contributes to (2) . By varying the polarizations of the the magnitude of the χeff input and output beams, the Cartesian components of the susceptibility tensor can be determined, which allows for the determination of the molecular orientation relating to the surface normal.31,32 The polarization mapping method provides a better approach to analyze interfaces with SFG.35 This method presents more reliable spectral information as well as improves fitting resolution by means of probing the interface using seven polarization combinations other than the normal ssp, ppp, sps, and pss. Simultaneous fitting of spectra from these seven polarization
combinations will provide a unique solution and reduce bias, typical in fitting SFG spectra. A 2D contour plot of wavenumber versus incrementing polarization angles σs can therefore be constructed to improve spectral analysis. This map will allow extraction of phase information since different vibrational modes will reach maximum peak intensity at different signal beam polarization angles.35 Ambiguity in the resulting fitting parameters, for example, the relative phase (() of the amplitude, can be resolved. This also means that it can separate overlapping peaks according to phase difference, damping factor difference, and difference in intensity based on the fact that an SFG spectrum with very different spectral features can be collected by varying σs.34,35 Results from this mapping technique in terms of orientation of ions on the surface will be compared to those of previous studies performed on imidazolium ionic liquids. Surface Potential Measurement with the Vibrating Plate Method. Voltage detection or charge measurement on the surface with a vibrating sensor is based on the principle of capacitive coupling between the sample surface and the probe.36-40 A voltage difference (U) between the sample (U2) and the probe (U1) is related to the charge (Q) on the test surface according to the equation:
Q)U·
oA D
(3)
where ∈ is the permittivity of the material in between the sample and the probe, o is the electric permittivity of the vacuum, A is the area of the sample surface, and D is the distance between the probe and the sample. A detailed description of this technique is provided in the Supporting Information. The Kelvin probe measures the outer potential difference also known as the Volta potential difference.41 This term is best defined as the electrostatic work done to bring a test charge from infinity or from vacuum to a point just outside the phase concerned.42 The outer potential is proportional to the excess charge on the surface and becomes zero if the charge is zero. In relation to this, surface potential is the potential difference between vacuum and a certain phase such that for a metalvacuum interface the surface potential comes from the discontinuity that a test charge experiences as it travels from the vacuum to just outside or inside the metal surface. In the same way, a solution can have a surface potential, which is the work required to bring a charge from vacuum to the solution surface. The test charge will experience a layer of dipoles orthogonally oriented with respect to the surface as it approaches the solution. At the interface, the distribution of ions, electrons, and electric field due to permanent or induced dipoles leads to a potential difference. This difference in potential in turn causes redistribution of charges in the interface forming the electric double layer.43 There are several models used to interpret the measured Volta potential, but the Gouy-Chapman model of the electrical double
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layer is a basic approach to begin the discussion, as presented in the Supporting Information. Surface Tension Measurement with the Pendant Drop Method. Surface tension (γ) is defined as force per unit length (mN/m), which arises from the imbalance of forces on molecules at the interface (i.e., gas-liquid).44 Attraction of the molecules in the liquid by various intermolecular forces leads to this phenomenon. There are several ways to measure surface tension, the most common of these are the Dunuoy ring method, the Wilhelmy plate method, the maximum bubble pressure method, the capillary rise method, and the drop method.43 Among these techniques, the drop method is selected since it requires a minimum amount of sample, gives accurate and precise results, and is useful in monitoring surface aging.45 Also, the method can be readily contained in an enclosed chamber unlike its bulkier counterparts previously mentioned. This allows measurements to be performed under vacuum or in a controlled environment. The pendant drop method will be used in this particular study since our concern is the gas-liquid interface. This method involves a drop of liquid suspended by a syringe needle. Several algorithms have been derived to accurately calculate the surface tension of liquids from the shape of the drop. These algorithms are derived from the Young-Laplace equation (eq 4), which describes the mechanical equilibrium in a suspended drop46
(
γ
)
1 1 + ) ∆p R1 R2
(4)
where γ is the surface tension, R1 and R2 are the two radii of curvature, and ∆p is the change in pressure due to the change in surface area of the drop. Computer iterations are used to find the best surface tension value to fit the drop profile as shown in the Supporting Information. Experimental Section Synthesis and Sample Preparation. All the materials used for synthesizing the ionic liquids were ACS reagents purchased from Aldrich, except for the sodium dicyanamide, which was from Alfa-Aesar. The water used was deionized with a Millipore A10 system with a resisitivity of 18 MΩ · cm and TOC index of [BMIM][DCA] > [BMIM][MS]. Ionic liquids being pure ions are a highly concentrated charged media. The ions on the surface are specifically oriented as distinctively shown by SFG where the alkyl chains of the imidazolium cations are tilted at a certain angle from the surface normal and ring parallel to the surface plane. The ions in the bulk are considered as isotropically arranged at the macroscopic level having equal amounts of cations and anions creating an atmosphere of electroneutrality. The measured surface potential can therefore be attributed to two contributing factors first coming from the ions in the Stern layer and second from the dipole contribution, which can come from the alkyl chain or from the head groups. The diffuse layer term in eq 4 is omitted based on the electroneutrality as well as due to the concentration of the system being 100% ions, where a diffuse layer no longer exists. Concurrently for ionic liquids, the contribution to the potential difference can be expressed as
∆V )
Na σδs eff (Γcµ⊥c cos θc) + os ot
(7)
Equation 6 illustrates that the measured surface potentials are dependent on the concentration of the ions on the surface, their relative arrangement toward each other as well as the corresponding dipole contribution from the cation. The different measured surface potential values observed for the ionic liquids can therefore give us a very insightful picture of the ions on the surface.
TABLE 3: Values Used for the Estimation of Surface Potential on the Surface IL
εt
µc (D)
θc (deg)
θa (deg)
δs,max (Å)
εs
Ac (Å2)
Aa (Å2)
[BMIM][BF4] [BMIM][DCA] [BMIM][MS]
2 2 2
0.38 0.38 0.38
47 47 45
70 6512
3.27 3.57 3.65
11.770 11.374 14.875
22.2571 22.25 22.25
19.6372,73 24.6372,73 26.0671,76,77
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TABLE 4: Calculated Surface Potentials at Specific Cation-Anion Configurations
TABLE 5: Surface Tension of Ionic Liquids γ (V) ( ∆
[BMIM]+ ionic liquid [BF4]-
[DCA]-
[MS]-
configuration
δs
∆V
δs
∆V
δs
∆V
A B
0 3.27
0.12 2.49
0 3.57
0.10 2.79
0 3.65
0.11 2.23
ration assumes surface coverage similar to configuration A, where the ions are both still considered to be on the surface. Figure 9 shows these different configurations, which define the possible arrangement of the ions on the surface. The results of the calculation for configurations A and B for the three ionic liquids are shown in Table 4. The experimental values in Table 3 are between these values but closer to the values of the side by side configuration. In fact the measured potentials are slightly higher than the values considering configuration A, which tells us that the arrangement of ions is similar to configuration C. The anions, therefore, are more submerged toward the liquid phase depending on the Stern length on the surface. The measured values also show that the probability of the ions forming layers on the surface as in configuration B is not probable. SFG and other techniques support this claim as well since both ions are actually observed on the surface. The question then would be how does the Stern length contribute to the measured surface potential? In simpler terms, how does the position of the ions relative to each other affect the measured values? To estimate the length of the Stern layer from the measured values, incrementing values of the Stern length as well as charge density where varied to calculate the surface potential with eq 6. Since both ions are on the surface, and the ions are essentially in the side by side configuration A, values for the Stern length and the charge density used for the calculation where varied near zero. Figure 10 shows the calculated surface potential values with respect to a range of Stern length and surface charge density. The values that were close to the experimental values for each ionic liquid were labeled accordingly. Results show that [BMIM][MS] has the lowest Stern potential contribution with a Stern length of ca. 0.15-0.30 Å. This means that the methyl sulfate anion is closest to the gas phase and very close to the level of the imidazolium
Figure 10. Calculated surface potentials with respect to the arrangement of ions.
IL name
trial 1
trial 2
trial 3
average
[BMIM] [BF4] [BMIM] [DCA] [BMIM] [MS]
42.76 ( 0.53
40.12 ( 0.47
41.47 ( 1.53
41.29 ( 1.53
43.36 ( 1.33
44.97 ( 2.49
42.23 ( 1.50
43.55 ( 1.50
43.25 ( 2.03
44.07 ( 1.37
43.17 ( 2.03
43.47 ( 1.53
ring. This is followed by the [BMIM][DCA] with a stern length of ∼0.4-0.8 Å and then [BMIM][BF4], which has a Stern length of ∼0.5-0.8 Å very close to that of [BMIM][DCA]. The measured positive potential, therefore, can be attributed to the dipole contribution from the alkyl chain as well as to the Stern potential dictated by the relative position of the ions with respect to each other. The reasons, however, as to why some anions have more affinity toward the gas phase than others can only be speculated. Among the three anions, methyl sulfate has the highest tendency to position closer to the gas phase followed by dicyanamide then tetrafluoroborate. It is possible that in this case, methyl sulfate being the largest and most polarizable prefers this position.81 The presence of a less polar methyl functional group and a more polar sulfate group on methyl sulfate can also be the reason it prefers to position closer to the interface to allow for such a discrepancy in polarity and the methyl group should lower the surface energy. In fact SFG has previously shown the preferred orientation of the methyl sulfate anion.15 The affinity of the ions toward each other can also possibly affect their arrangement on the surface. One interaction that is plausible would be hydrogen bonding between the cations and the anions.82-87 Experimental results on the bulk have shown that hydrogen bonding exists between the hydrogens on the rings and the anions. ESI-MS studies by Siciliano et al. on various ionic liquids including [BMIM][BF4] and [BMIM][DCA] showed the H-bonds to be in-plane with the ring and higher affinity of the dicyanamide anion than the tetrafluoroborate anion to the [BMIM] cation.88 This can be the reason why dicyanamide has a higher tendency to be on the same plane as the ring. Also, Holbrey et al., using XRD, observed significant hydrogen bonding between hydrogens in the imidazolium rings of 1,3dimethylimidazolium methyl sulfate and the three terminal oxygens of its methyl sulfate counterion.50 There is no experimental proof, however, of the strengths of these hydrogen bonds and their exact placement in the gas-liquid interface so the effect cannot be quantified. For the cation, its closer proximity to the gas phase can be due to the positive dipole contribution from its alkyl chain. It is also possible that it is mainly because the less polar chain will tend to protrude toward the gas phase while the charged ring faces the bulk, as is usually seen in SFG.9,89 The chains interacting through dispersion forces can also increase the tendency of the cation to be closer to the gas phase.13,66 Surface Tension. The surface tension values are shown in Table 5. The measured values are relatively lower than values published by other groups, which might be a result of the method of preparation and sample measurement under controlled dry conditions. As is the case for most imidazolium ionic liquids, the measured value is higher than the surface tension values of organic compounds such as alkanes where dispersion forces are prevalent but lower than the surface tension of aqueous electrolytic solutions where hydrogen bonding and ion-dipole forces are the dominant intermolecular interactions.90 These surface tension values are even much lower compared to ionic
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Figure 11. Arrangement of ions on the surface and their contribution to the surface potential.
systems and molten salts.91-94 van der Waals forces must be present on the surface knowing that the alkyl chains are protruding toward the vapor phase, which is observed through SFG.9,15 The presence of both ions on the surface implies the presence of ionic interaction or ion-dipole interactions, although not as dominant judging from the measured surface tension value. This can mean that the alkyl chain chains are protruding into the gas phase while the charged head groups are situated closer to the liquid phase. The trend in the measured surface tension values is [BMIM][DCA] ≈ [BMIM][MS] > [BMIM][BF4]. The standard deviations incurred, however, show that the values of these ionic liquids are similar. Hydrogen bonding between the ions can also add to the degree of force on the surface leading to the possibility of interplay between hydrogen bonding, Coulombic interactions, and dispersion forces on the surface. In fact an IR study of imidazolium ionic liquids performed by Fumino et al. showed that localized directional hydrogen bonding and alkyl interactions tend to perturb Coulombic interactions.95 The disruption or lowering of Coulombic interactions in ionic liquids is also observed in their melting points, which are much lower compared to that of ionic crystals where Coulombic interactions are the sole dominating interactions.96 The surface excesses of these three ionic liquids can be viewed as similar considering that their surface tension values are quite comparable. This can imply that the most stable configuration on the surface with the lowest surface energy is achieved when the alkyl chains are next to the gas phase followed by the charged counterparts.
An Overall Representation at the Gas-Liquid Interface. Ionic configurations discussed here are based on results from the three techniques as well as results from previous studies both experimental and simulation. The first two configurations where the surface is comprised purely of cations or anions are not supported by SFG and other techniques.9,11,12,16,19,97,98 The SFG results of this study as well as previous studies showed vibrational modes coming from both the cation and anion on the surface. SFG in itself cannot determine the detailed distribution of the ions on the surface with respect to surface charge. Surface potential measurements can account for this limitation by determining positions of the ions at the gas-liquid interface and how these contribute to the overall potential. Results from the surface potential measurements in this study showed positive values on all the ionic liquids characterized coming from both the dipole and charge contribution. The anions are slightly more submerged into the bulk phase compared to the cations significantly contributing a positive charge density to the Stern potential. As in previous studies, SFG results show that the alkyl chains are oriented toward the gas phase at a certain orientation from the surface normal, while the imidazolium rings are mostly parallel to the surface. Surface tension results complement both the SFG and surface potential results where the values are between that of alkanes and molten salts indicating a mixture of dipole interactions from the alkyl chains and head groups as well as ionic interactions.
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Comparable measured surface tension values indicate similar surface excesses for all three ionic liquids. Figure 11 shows a probable picture of the arrangement of ions on the surface. Conclusion This study shows novel methods of characterizing ionic liquids in a highly clean, controlled, and contaminant-free environment. Since surface techniques are highly sensitive to impurities including the techniques used here, these methods will provide substantially reliable measurement results. The results from the SFG polarization mapping method concur with the results from previous studies, which used only the four usual polarization parameters ssp, ppp, sps, and pss. This verified the previous results and showed that for simple molecules with minimal vibrational modes like the ionic liquids studied, the previous method is acceptable. The mapping technique in this study, however, was able to probe vibrational modes that appear in intermediate polarizations such as the ring modes. A better estimate of the tilt angle of the rings from the surface normal, therefore, was determined. The imidazolium rings were observed to be tilted ∼70° for [BF4]-, ∼80° for [MS]-, and >70° for the [DCA]- anions. On the basis of previous studies and results from this study at this point, the gas-liquid interface of the three imidazolium ionic liquids comprises both cations and anions on the surface. Positive surface potential readings mean the prevalence of dipole and dispersion interactions on the surface as well as contribution from the surface charge coming from the ions due to their characteristic positions on the surface. Theoretical calculations will be very helpful in the understanding of the surface potential values. Concurrently, the butyl chain of the cation is extended to the vapor phase at a certain angle while the charge constituent of the ions lies on the liquid phase positioning the imidazolium ring parallel to the surface as established by SFG results as well as surface tension measurements. From the three techniques, it can be concluded that factors affecting the arrangement of ions on the surface are probably ionic interactions of the head groups, dipole contribution from the alkyl chain and of the anions, affinity of the ions toward each other, and perhaps sizes and shapes of the counteranions. It is important to mention that experiments with surface potential and surface tension measurements on varying alkyl chains of cations and anions of these alkylimidazolium-type ionic liquids are currently being performed in order to provide a clearer picture on ion distribution on the surface. Alkylimidazolium alkyl sufate ionic liquid, which was previously characterized in the group by using SFG and surface tension measurements, is considered a good system for this study.66 Increasing chain lengths whether on the cation or anion exhibited a decrease in the surface tension values and the SFG results showed the surface to be dominated by -CH vibrational stretches. In addition, the experiments performed in this study with the three techniques all combined can only probe the first molecular layer of the sample. Although some studies showed layering of ionic liquids particularly in solid-liquid interfaces, this study can only comment on the layer directly in contact with the vacuum or gas phase. Acknowledgment. We are grateful to the R. A. Welch Foundation (E-1531) for the financial support for this study.
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