An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper is investigated by sum frequency generation (SFG) imaging microscopy and electron backscattering diffraction (EBSD). The strong SFG signal contrast across the domain boundary indicates the existence of grain structures on copper surface, which is further verified by EBSD measurements. The nonresonant contribution of SFG response shows anisotropy with respect to the in-plane rotation of the sample relative to the surface normal and varies in each crystal domain area. The resonant contribution of the monolayer, such as the amplitude ratio of CH3-sym/CH3-asym, is azimuthally nearly isotropic. Since the zzz tensor component of nonlinear susceptibility dominates, the resonant part of SFG spectra on the metal surface does not show any anisotropy. Further, a strong correlation between the local metal structures with the top monolayer packing behaviors is identified based on the statistical distribution analysis. Using the methyl group as an illustrative case, the variations in tilt angle of methyl group for different crystal grains, visualized in the SFG image, suggest that the underneath local grain structure contributes significantly to the overall monolayer packing behaviors measured on the macroscale.

•Methanol adsorption on copper surface was studied in high pressure.•Methoxy monolayer is oriented closer to the surface normal with introduction of oxygen.•Oxygen induces the methoxy monolayer densely packed on copper surface with less defects.The adsorption of atmospheric pressure methanol on the polycrystalline copper surface has been studied by a combination of sum frequency generation imaging microscopy (SFGIM) and temperature programmed desorption (TPD). Methoxy species can be generated by exposing the polycrystalline copper surface to methanol vapor at room temperature. SFGIM results demonstrate that oxygen promotes the surface adsorption of methanol and the increase in the amount of methoxy produced on copper surface. SFGIM orientation analysis suggests the methoxy monolayer is oriented closer to the surface normal with introduction of oxygen. Employing the image statistical analysis approach, the heterogeneities and conformation distribution of methoxy monolayers on copper surface with and without oxygen adsorption are compared. These results illustrate SFGIM indeed could provide more insight for understanding the heterogeneous metal/metal oxide surface in the molecular level.

Adsorption of dimethyldodecylamine oxide (DDAO) and its mixtures with Triton X-100 (TX-100) at the hydrophilic silica/water interface has been studied using total internal reflection (TIR) Raman spectroscopy and target factor analysis (TFA). The use of a linear vibrational spectroscopic technique helps obtain information on molecular behavior, adsorbed amount, and conformational order of surfactant molecules at the interface. The results obtained from polarized Raman measurements of pure DDAO show insignificant changes in the orientation and conformational order of surface molecules as a function of DDAO bulk concentrations. The adsorption isotherm of pure DDAO shows a change in the structure of the adsorbed layer at concentrations close to the critical micelle concentration (cmc). TFA reveals that, for a low concentration of DDAO (0.30 mM in this study), adsorption of both DDAO and TX-100 in the mixed surfactants was enhanced at low TX-100 concentrations. The synergistic effect is dominant at low concentrations of TX-100, with enhanced adsorption of both surfactants. Although competitive adsorption is effective at high concentrations of TX-100, the presence of a small amount of DDAO at the interface still enhances TX-100 adsorption. When DDAO concentrations are increased to 1.00 mM, TX-100 replaces DDAO molecules on the surface when TX-100 concentration is increased.

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper was investigated by sum frequency generation (SFG) imaging microscopy. The crystal grain and grain boundaries of the copper surface were mapped in the SFG image based on the strong brightness contrast of the SFG signal across the boundary. Local SFG spectra reveal significant difference with each other as well as the average SFG spectra, indicating the heterogeneity of the copper surface resulting from copper grains with distinct crystallographic facets and orientations. It is demonstrated that the SFG signal of crystalline domain areas contains azimuthal anisotropy with respect to the plane of incidence. In addition, the statistical orientation analyses of amplitude ratio of CH3-sym/CH3-asym and corresponding contour maps imply that the orientation of ODT molecules is affected by the underlying copper.

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed molecules on the gold surface, held in place by the van der Waals interactions between the alkyl chains.

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA+) chloride has been studied at the hydrophilic silica–water interface by Raman spectroscopy in total internal reflection geometry (TIR Raman). This Raman spectroscopic technique takes advantage of an evanescent electric field that is generated at the silica–water interface in TIR mode with specific probing depth. The present study demonstrates the capabilities of the TIR Raman sampling configuration to provide structural information and simultaneously serve as an experimental platform for studying thermodynamic and kinetic properties of BDMHA+Cl– at the silica–water interface at neutral pH and compare its adsorption behavior with the modified adsorption properties in the presence of four different concentrations of a divalent metal salt. Spectral analysis of the Raman scattering intensities as a function of time and concentration provided the input data for evaluating adsorption properties of the surfactant in the absence and presence of the metal salt additive. Addition of the magnesium metal salt lowered the cmc, altered the surface excess of the surfactant, and increased the Langmuir adsorption constants, as well as the magnitude of the free energy of adsorption, and adsorption kinetics, proportional to the concentrations of the metal salt. Adsorption isotherms based on a modified Langmuir adsorption model were established for five systems: the pure surfactant in aqueous solution, and the surfactant in the presence of 5, 10, 50, and 100 mM of magnesium chloride. The metal salt did not enhance surfactant adsorption at very low surfactant concentrations below 5 μM, where adsorption occurs by electrostatic attraction; the divalent metal salt, however, favorably influenced the adsorption behavior in the aggregate formation region by reducing the electrostatic repulsion between the polar surfactant head groups, and enhancing the hydrophobic effect between the hydrophobic surfactant alkyl chains and the polar water molecules.

A surface-sensitive nonlinear vibrational spectroscopic technique, sum frequency generation (SFG), has been used to study cyano-containing ionic liquids in contact with two different solid salt surfaces. Specifically, the interfacial chemistry of BaF2(111) single-crystal and solid NaCl{100} surfaces in contact with ionic liquids such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM], and [EMIM][TCB] has been investigated. Spectral features in both C–H and C–N stretching regions were assigned, with a detailed discussion of the nature of surface interactions and ordering of the ionic liquid ions at the interface of the different crystals. Results showed that [BMIM]+ cations adhered closely via Coulombic interactions to the negatively charged NaCl{100} surface, while [SCN]−, [TCM]−, and [DCA]− anions revealed a strong electrostatic affinity to the positively charged BaF2(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer. The linear [SCN]− anion has a particularly strong affinity to the BaF2(111) surface, resulting in a first layer of anions directly in contact with BaF2(111) containing an effective negative surface excess charge. This promoted ordering of the cations in the second layer to counter the charge excess. At the BaF2(111)–[EMIM][TCB] interface, however, a strongly bound layer of anions populating the first layer resulted in a much larger counterion charge delivered near the crystal salt surface than required to effectively neutralize the initial surface charge from the crystal. As a result, strong resonances from the cation were observed at the BaF2(111) surface, suggesting a more complicated structure of the double layer at the interface than a simple Helmholtz-type model.

Sum frequency generation (SFG) spectroscopic techniques are used to investigate the molecular orientation of adsorbed acetonitrile on rutile TiO2 (110) at the solid–vapor interface. Generally, most molecular orientation analyses using SFG have been performed on dielectric substrates, to avoid the spectral interference between resonant and the near-resonant background signal. Although rutile crystal can be treated as a dielectric substrate, its electronic state contributes to the intensity and interferes with the resonant signal when the SFG frequency is close to its band gap energy. In addition, the rutile crystal is a uniaxial birefringent material, and the (110) surface is anisotropic, which further complicates the spectral analysis. In this study, various SFG measurement techniques were applied, and quantitative analytical methods were established to interpret the surface orientation of an adsorbed molecule. SFG vibrational spectra of acetonitrile on rutile TiO2 (110) surface have been measured using distinct polarization combinations, polarization mapping, and null angle method. By varying the polarization combinations of SFG, the magnitude and shape of the spectra undergo substantial change, which originate from the interference between the near-resonant signal from the rutile substrate and the resonance signal from the acetonitrile. Theory, simulation, and analytical methods for obtaining quantitative orientation information of a molecule on an anisotropic semiconductor substrate in the presence of a near-resonant signal are presented.

Sum frequency generation (SFG) vibrational spectroscopy analyses were performed on self-assembled monolayers (SAMs) formed via the adsorption of 2-(4-(octadecyloxy)phenylethanethiol (R1ArMT), 2-(3,5-bis(octadecyloxy)phenylethanethiol (R2ArMT), and 2-(3,4,5-tris(octadecyloxy)phenyl-ethanethiol (R3ArMT) on gold. SFG spectra showed that the monolayers formed were closely packed and well ordered. It was determined, using orientation analysis, that as additional octadecyloxy chains were attached to the aromatic ring the apparent methyl group tilt increased and conformational order decreased. Aromatic C–H modes exhibited in the SFG spectra suggest the aromatic rings have a C2 axis along the surface normal. Model phenylethanethiols with different symmetries along the benzene ring were synthesized and used to identify vibrational modes and the surface orientation of the alkoxyphenylethanethiols. SAMs derived from R1ArMT exhibit aromatic C–H stretching modes due to the C2v symmetry at the ring. In contrast, SAMs derived from R2ArMT and R3ArMT exhibit no aromatic C–H stretching modes due to their low Raman cross section.

Sum frequency generation imaging microscopy (SFGIM) was used to study a binary system of self-assembled monolayers (SAMs) on evaporated gold. A two-step process that consists of microcontact printing (μCP) and the solution backfilling method is used to generate a sample surface with two distinct SAMs domains. The high contrast chemical identification and the homogeneity map of the SAMs for two different head groups are revealed by processing the three-dimensional images (xy-surface and infrared wavenumbers) obtained from SFGIM. For conformation order analysis, the amplitude ratios of the methylene and methyl symmetric resonance obtained by a nonlinear curve fit of the spatially and spectrally resolved SFG images are used to reconstruct spatial image maps. The conformation order map shows the distribution of gauche defects in SAMs with improved contrast across monodentate and bidentate alkanethiol monolayer regions. The results show that variation of the head group attached on the surface dominantly influences the conformation disorder and surface coverage of the molecules, while the terminal methyl group in both monodentate and bidentate SAMs shows a similar orientation angle.

In order to evaluate the interfacial monolayer order on copper surfaces in two-dimensions, the tilt angle distribution and orientation-conformation correlation maps were investigated using sum frequency generation imaging microscopy (SFGIM). In this paper, these methods of analyses were further utilized in evaluating the effect of the formation of a thin oxide film on the monolayer order of octadecanethiol (ODT) deposited on Cu. By chemically stripping the native oxide of the copper surface using amidosulfonic acid (ASA), the spontaneous atmospheric oxidation of Cu from an initial reduced state was observed as a change in the nonresonant phase response of the sum frequency generation (SFG) spectral line shape obtained from the SFGIM setup configured with a 1064 nm probe beam. Statistical calculations and contour maps for the CH-stretch modes of the terminal methyl and methylene group showed the overall decrease in the mean tilt angle and increase in the amount of alkyl gauche defects as a result of the oxidation of the metal surface underneath ODT. With the growth of the oxidized film, the changes in the methyl group tilt orientation are spatially correlated to the changes in the nonresonant phase. Regions with the highest concentration of reduced surface Cu registered lower gauche defect signals and low relative nonresonant phases. Over the exposure time, these regions of well-ordered monolayers were observed to gradually undergo Cu oxidation, initiating from the domain boundaries moving inward.

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA+) chloride was studied at an octadecyltrichlorosilane (OTS)-monolayer-modified silica–water interface by Raman spectroscopy in total internal reflection (TIR) geometry. The present study demonstrates the capabilities of this spectroscopic technique to evaluate thermodynamic and kinetic BDMHA+Cl– adsorption properties at the hydrophobic silica surface. The surface coverage of BDMHA+ decreased by 50% at the hydrophobic OTS-silica surface relative to the surface coverage on bare silica; the dominating driving mechanisms for surfactant adsorption were identified as hydrophobic effects and head group charge screening by the electrolyte counterions. Addition of magnesium metal salt (MgCl2) to the aqueous solution (∼ neutral pH) lowered the surface coverage and moderately increased the Langmuir adsorption constants relative to those of the pure surfactant. These trends were previously observed at the hydrophilic, negatively charged silica surface but with a smaller change in the Gibbs free energy of adsorption at the hydrophobic silica surface. The hydrophobic OTS-silica surface properties resulted in shorter times for the surfactant to reach steady-state adsorption conditions compared to the slow adsorption kinetics previously seen with the surfactant at the hydrophilic surface. Adsorption isotherms, based on Raman signal intensities from spectral analysis, were developed according to the Langmuir adsorption model for the pure surfactant at the OTS-silica–water interface; the modified Langmuir model was applied to the surfactant adsorption in the presence of 5, 10, 50, and 100 mM magnesium chloride. Spectral analysis of the Raman scattering intensities and geometric considerations suggests a hemimicelle-type surface aggregate as the most likely surfactant structure at the OTS-silica surface. The different kinetics observed at the hydrophilic versus the hydrophobic silica surface further indicate that the surface charge and potential influence the surfactant diffusion and kinetic rates of adsorption at the silica–water interface.

Advancement in the field of ionic liquid technology requires a comprehensive understanding of their surface properties, as a wide range of chemical reactions occur mainly at interfaces. As essential media currently used in several technological applications, their accurate molecular level description at the gas–liquid interface is of utmost importance. Due to the high degree of chemical information provided in the vibrational spectrum, vibrational spectroscopy gives the most detailed model for molecular structure. The inherently surface-sensitive technique, sum frequency generation (SFG) spectroscopy, in combination with bulk-sensitive vibrational spectroscopic techniques such as FTIR and Raman, has been used in this report to characterize the surface of cyano-containing ionic liquids, such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM] and [EMIM][TCB] at the gas–liquid interface. By structural variation of the anion while keeping the cation constant, emphasis on the molecular arrangement of the anion at the gas–liquid interface is reported, and its subsequent role (if any) in determining the surface molecular orientation of the cation. Vibrational modes seen in the C–H stretching region revealed the presence of the cation at the gas–liquid interface. The cation orientation is independent of the type of cyano-containing anion, however, a similar arrangement at the surface as reported in previous studies was found, with the imidazolium ring lying flat at the surface, and the alkyl chains pointing towards the gas phase. SFG results show that all three anions of varying symmetry, namely, [DCA]− (C2v), [TCM]−(D3h) and [TCB]− (Td) in ionic liquids [BMIM]DCA], [BMIM][TCM] and [EMIM][TCB] are significantly tilted from the surface plane, while the linear [SCN]− in [BMIM][SCN] exhibited poor ordering, as seen in the absence of its C–N stretching mode in the SFG vibrational spectra.

A systematic approach of experimentally measuring the average orientation and distribution of octadecanethiol (ODT) monolayer on gold was performed by using sum frequency generation imaging microscopy (SFGIM). By constructing maps from region of interests (ROIs) of different length scales, measuring from 5 to 1000 μm, two-dimensional domains could be distinguished to contain less-oriented monolayers. By generating the tilt angle distributions of the scaled ROIs, SFGIM determines the experimental orientation, θ, and distribution width, σ0, as dictated by the SFGIM resolution limit. Using the same statistical methods, the heterogeneities of ODT monolayers on Au and Cu were compared. The quantitative analysis of these orientation parameters is essential for future work on characterizing monolayer integrity on more complex surfaces such as reactive metals and alloys. Composite maps that merge orientation and conformational information were demonstrated as potential tools for spectromicroscopic measurement of the reactivity of metal substrates to surface processes such as corrosion and wettability.

Sum frequency generation (SFG) vibrational spectroscopy of the ionic liquid, 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA], in contact with two different solid salt surfaces, BaF2(111) single crystal and solid NaCl{100}, are discussed in this Letter. This investigation describes the nature of an ionic liquid–(solid) salt interface using SFG, contributing a new understanding to the molecular-level interactions involved in salts, which are conceptually similar compounds (of purely ionic character) but of different physical properties (liquid versus solid at room temperature). Results show the presence of [BMIM]+ at the NaCl{100} surface and [DCA]— at the BaF2(111) surface. [BMIM]+ cations adhere closely via Coulombic interactions to the negatively charged NaCl{100} surface, while [DCA]− anions subsequently have a strong electrostatic affinity to the positively charged BaF2(111) surface. Ions of the ionic liquid adsorb to the solid salt surface to form a Helmholtz-like electric double layer.Keywords: ionic liquids; solid−liquid interface; sum frequency generation;

Sum frequency generation (SFG) microscopy was used to investigate the structure and orientation of multidentate alkanethiolate monolayers on gold surfaces. Adsorbate molecules in which one, two, or three sulfur atoms coordinated to the surface of gold afforded self-assembled monolayers (SAMs) with controlled spacing between the alkyl chains. SFG imaging was used to determine the orientation of the molecules on the surface and to visualize spatial variations across the surface. Using SFG imaging, the average orientation of the terminal methyl group is measured as well as the standard deviation. Further, the presence of film defects is measured by the CH2/CH3 intensity ratio and the distribution quantified through the standard deviation. The results demonstrate that the tridentate adsorbate forms a monolayer in which the alkyl chains are highly disordered on the surface when compared to SAMs derived from normal alkanethiols (i.e., monodentate adsorbates). The results are consistent with a model in which the tridentate alkanethiols generate SAMs with lower alkyl chain density. SFG microscopy provides a detailed view of the origin of the monolayer disorder by statistical spectroscopic analysis of the surface.

This paper presents a model on the electrochemical behaviors of n-alkanethiol self-assembled monolayers (SAMs) on gold electrodes with different chain length (C10, C12, C16, and C18) using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and sum frequency generation spectroscopy (SFG). The SAMs studied reached their maximum density with a deposition time of ≈72 h according to the kinetics results shown by EIS Bode phase plots. For shorter chain SAMs, C10 and C12 monolayers, the results from CV, EIS, and SFG were consistent with each other. Significant amounts of thiols were removed by reductive desorption of the films for C10 and C12 SAMs, as indicated by decreases in the electrochemical waves and SFG peaks. Surprising differences were observed for longer-chain SAMs made from C16 and C18 monolayers. The CV results suggested that a large amount of the C16 and C18 monolayers chemically desorbed from the gold surface, while the EIS and SFG results illustrated that C16 and C18 layers still remained in the vicinity of the double-layer region near the gold surface and retained their two-dimensional, densely packed ordered structure, respectively. Only small decreases in phase angle were observed from EIS, and all the CH vibrational modes were nearly unchanged in SFG spectra for C16 and C18 SAMs. Furthermore, SFG orientation analysis of the C16 and C18 SAMs revealed a molecular structure for C16 and C18 SAMs after 30 CV cycles similar to that of their original films. Cyclic voltammograms also showed no observable oxidative readsorption waves for C10 and C12 SAMs, while for C16 and C18 monolayers detectable reoxidation peaks appeared. The results from EIS, SFG, and CV strongly suggest that the commonly accepted notion, the formation of aggregates and micelles for alkanethiol SAMs after reductive desorption, does not apply to the long-chain SAMs that were studied. After applying the potential to the long-chain SAMs, both the chemically desorbed and intact SAM molecules maintained molecular structures similar to that of the freshly prepared monolayers. The persistent crystallinity of longer-chain SAMs against negative potentials could be explained by dense packing and much lower solubility of the molecules in aqueous solutions. Implication for corrosion inhibited by the monolayer is discussed.

The oxidation of octadecanethiol (ODT, CH3(CH2)17SH)-covered copper in dry air has been studied by in situ vibrational sum frequency spectroscopy (VSFS), infrared reflection absorption spectroscopy (IRAS), and cathodic reduction (CR). During the first 10 h of exposure, the VSF spectral line shape in the CH stretching region changed significantly, with resonances observed as dips being transformed into peaks. This was attributed to a phase change in the nonresonant sum frequency signal due to the formation of a thin layer of copper(I) oxide beneath the ODT. Complementary cathodic reduction and infrared reflection/absorption spectroscopy studies yielded a thickness of the oxide layer of <2 nm after 19 h exposure. An orientation analysis on the adsorbed molecules by VSFS indicated a decreased tilt angle of the terminating methyl groups with respect to the surface normal during the formation of the oxide layer.

The organization of ions at the interface of ionic liquids and the vacuum is an ideal system to test new ideas and concepts on the interfacial chemistry of electrolyte systems in the limit of no solvent medium. Whilst electrolyte systems have numerous theoretical and experimental methods used to investigate their properties, the ionic liquids are relatively new and our understanding of the interfacial properties is just beginning to be explored. In this critical review, the gas–liquid interface is reviewed, as this interface does not depend on the preparation of another medium and thus produces a natural interface. The interface has been investigated by sum frequency generation vibrational spectroscopy and ultra-high vacuum techniques. The results provide a detailed molecular-level view of the surface composition and structure. These have been complemented by theoretical studies. The combinations of treatments on this interface are starting to provide a somewhat convergent description of how the ions are organized at this neat interface (108 references).

In situ observations of surface hydroxyl groups and the subsequent ligand exchange during initial atmospheric corrosion conditions of Zn are reported. The data have been obtained with sum frequency generation spectroscopy (SFG) during exposure of polycrystalline zinc to humidified or dry nitrogen gas, N2, to which 100 parts per billion of formic acid, HCOOH, was added. The SFG data provide evidence of surface hydroxyl groups and their gradual replacement by formate species through a ligand exchange, representing the initial step of formate-induced dissolution of zinc. In addition, the aqueous adlayer on the Zn surface is probed, and the occurrence of the free OH vibration confirms a bulk-like water film.Keywords (keywords): corrosion; sum frequency generation; surface; water; zinc;

Vibrational sum frequency spectroscopy (VSFS) and ab initio density functional theory (DFT) calculations of formic acid on ZnO/Zn have been performed in order to understand the first step of atmospheric corrosion on zinc initiated by formic acid. In addition, infrared reflection absorption spectroscopy (IRAS) has been employed to complement the surface sensitive VSFS results to identify the corrosion products. Oxidized polycrystalline zinc samples were exposed to 120 ppb formic acid in either humid or dry air where, the formic acid adsorption on ZnO/Zn is observed to have a low dependence on the humidity, as deduced by VSFS. Formate is formed on the surface in both dry and humid air and stabilized in configuration after about 90 min exposure in 120 ppb formic acid as seen in the VSFS results. This is evidenced by the occurrence of the CH and symmetric COO− vibrations of the formate ion. The DFT calculations support the VSFS results, showing a coordination of the formate to zinc ions without participation from water molecules.

Advancement in the field of ionic liquid technology requires a comprehensive understanding of their surface properties, as a wide range of chemical reactions occur mainly at interfaces. As essential media currently used in several technological applications, their accurate molecular level description at the gas–liquid interface is of utmost importance. Due to the high degree of chemical information provided in the vibrational spectrum, vibrational spectroscopy gives the most detailed model for molecular structure. The inherently surface-sensitive technique, sum frequency generation (SFG) spectroscopy, in combination with bulk-sensitive vibrational spectroscopic techniques such as FTIR and Raman, has been used in this report to characterize the surface of cyano-containing ionic liquids, such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM] and [EMIM][TCB] at the gas–liquid interface. By structural variation of the anion while keeping the cation constant, emphasis on the molecular arrangement of the anion at the gas–liquid interface is reported, and its subsequent role (if any) in determining the surface molecular orientation of the cation. Vibrational modes seen in the C–H stretching region revealed the presence of the cation at the gas–liquid interface. The cation orientation is independent of the type of cyano-containing anion, however, a similar arrangement at the surface as reported in previous studies was found, with the imidazolium ring lying flat at the surface, and the alkyl chains pointing towards the gas phase. SFG results show that all three anions of varying symmetry, namely, [DCA]− (C2v), [TCM]−(D3h) and [TCB]− (Td) in ionic liquids [BMIM]DCA], [BMIM][TCM] and [EMIM][TCB] are significantly tilted from the surface plane, while the linear [SCN]− in [BMIM][SCN] exhibited poor ordering, as seen in the absence of its C–N stretching mode in the SFG vibrational spectra.

The organization of ions at the interface of ionic liquids and the vacuum is an ideal system to test new ideas and concepts on the interfacial chemistry of electrolyte systems in the limit of no solvent medium. Whilst electrolyte systems have numerous theoretical and experimental methods used to investigate their properties, the ionic liquids are relatively new and our understanding of the interfacial properties is just beginning to be explored. In this critical review, the gas–liquid interface is reviewed, as this interface does not depend on the preparation of another medium and thus produces a natural interface. The interface has been investigated by sum frequency generation vibrational spectroscopy and ultra-high vacuum techniques. The results provide a detailed molecular-level view of the surface composition and structure. These have been complemented by theoretical studies. The combinations of treatments on this interface are starting to provide a somewhat convergent description of how the ions are organized at this neat interface (108 references).