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The adsorption of dimethyldodecylamine oxide (DDAO) and Triton X-100 (TX) as single components and mixed systems at the silica-water interface has been studied using total internal reflection (TIR) Raman spectroscopy combined with multivariate curve resolution (MCR). In this study, the mixtures of DDAO and TX indicate minimal synergism in the bulk solution; however, the cooperative adsorption behavior on the silica surface is shown with various mixtures of DDAO (up to 1.3 mM) and TX (up to 1.1 mM). Adding the DDAO (up to 0.3 mM) to TX solution, the surface excess of TX shows 30% enhancement, from 1.2 to 1.8 μmol m

Sum frequency generation microscopy is a label-free optical imaging technique with intrinsic molecular vibrational contrast for surface studies. Recent developments of compressive sensing broad-band hyperspectral SFG microscopy have demonstrated the potential application for imaging monolayer at metal surfaces with micrometer spatial resolution. Here is presented the capability of chemical imaging of spatially patterned monolayers of 1-octadecanethiol (ODT) and 16-methoxy-1-hexadecanethiol (MeOHT) molecules assembled on a copper surface. The spatial distributions of the monolayer with vibrational-spectral contrast are well-demonstrated at different frequency regions through reconstruction of the hypercube using a 3-dimensional total variation regularization algorithm (3DTV). Spatial-chemical distributions of each component are also reconstructed directly from the compressive measurements by endmember unmixing (CEU) scheme. Compared to 3DTV algorithm, the reconstruction from CEU shows spatial distribution of each component on the surfaces, and demonstrates the ability to characterize the domains of mixed-molecules on surfaces.

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.

The gas-liquid interface of halide-free 1,3-dialkylimidazolium alkyl sulfates [RMIM][R-OSO(3)] with R chain length from C(1)-C(4) and C(8) has been studied systematically using the surface-specific sum frequency generation (SFG) vibrational spectroscopy and surface tension measurements. From the SFG spectra, vibrational modes from the methyl group of both cation and anion are observed for all ionic liquid samples considered in the present study. These results suggest the presence of both ions at the gas-liquid interface, which is further supported by surface tension measurements. Surface tension data show a decreasing trend as the alkyl chain in the imidazolium cation is varied from methyl to butyl chain, with a specific anion. A similar trend is observed when the alkyl chain of the anion is modified and the cation is fixed.

A systematic sum frequency generation vibrational spectroscopy study is conducted on the gas-liquid interface of room-temperature ionic liquids. The compounds contain ammonium and pyrrolidinium based cations, to which alkyl substituents of different length and/or functional groups are attached, and they are all combined with the bis-trifluoromethanesulfonimide anion ([imide]-). The alkyl chain length shows a strong effect on the ordering of the chains at the topmost layer, reaching the maximum order for (C4H9)N+(CH3)2(C3H7) and (C6H13)N+C4H8(CH3). There is also evidence of gauche defects for longer alkyl chains, and changes in the spectral features for the shortest ones. The substitution of a carbon atom in the chain by oxygen, and its per-deuteration, provides the means to acquire a more complete description of the surface structure, and an unambiguous assignment of the vibrations detected in the SFG spectra. Finally, a brief comparison between alkylpyrrolidinium, and previously studied alkylimidazolium, cations is also established.

Phosphatidyl-nucleosides are a class of functional amphiphiles, where a nucleic acid monomer is conjugated to a lipid skeleton. These derivatives self-organize in aqueous solution as assemblies of various size, shape, and interfacial curvature. This paper presents a comparison of the aggregation behavior of different 1-R,2-R-sn-glycero-3-phosphatidyl-nucleosides, where R = 8 (DiC8PN) or R = 12 (DLPN) and N is either adenosine (a purine) or uridine (a pyrimidine), a complementary pair in RNA. Surface tension, small angle neutron scattering, cryo-TEM, and circular dichroism are used to highlight and distinguish the impact of the hydrophobic assembler and of the base substitution on the solution phase behavior. Our main conclusion is that the nucleic functionalization provides an additional parameter to control self-assembly through specific interactions among the polar heads. Further nonideal effects are induced by mixing nucleolipids with complementary base substitution. We show that these contributions alter the aggregation thresholds and modulate properties of the aggregates on the mesoscale.

The orientation of the cation and the anion of room-temperature ionic liquids using sum frequency generation vibrational spectroscopy is reported. The ionic liquids are based on butyl-methyl imidazolium [BMIM]+ and hexyl-tributyl ammonium [N6444]+ together with dicyanamide [DCA]- as the anion. The tilt angle of the C3 axis of the methyl group from the alkyl chain in the cations was found to vary from 52 degrees to 80 degrees as a function of the distribution width sigma (which ranges from 0 degrees to 30 degrees with respect to the surface normal) for [BMIM][DCA] and similarly for [N6444][DCA]. The orientation of the C2 axis in the dicyanamide anion as a function of the twist angle phi, varied between 46 degrees and 90 degrees for [BMIM][DCA] and from 53 degrees to 90 degrees for [N6444][DCA]. These results suggest the presence of both ionic species at the gas-liquid interface and help describe the behavior of a simple inorganic anion at the surface.

The orientation of the cation and the anion of 1-butyl-3-methylimidazolium dicyanamide at the platinum-liquid interface, using sum frequency generation (SFG) spectroscopy is reported. Sum frequency spectra at two different polarizations and different potentials are recorded, and analysis of polarization-dependent spectra is performed to estimate the orientation of the dicyanamide anion and the alkyl and ring moieties in the cation as a function of the potential applied to the platinum electrode. In addition, cyclic voltammetry and electrochemical impedance spectroscopy are conducted. A model of the double-layer structure at the electrified interface is presented from the analysis of capacitance and SFG data.

In situ sum frequency generation vibrational spectroscopy, at varied potentials and polarization combinations, was performed on polycrystalline copper, polycrystalline platinum, and polycrystalline gold samples in 0.5 M HClO4 with 50 mM 5-methylbenzotriazole (5-methylBTAH) added. These studies were performed to determine the orientation of 5-methylBTAH on the surface at different potentials. For copper surfaces, orientation of the molecule on the surface is not affected by potential within the potential window studied (-500 to -100 mV vs saturated calomel electrode (SCE)). Sum frequency generation spectra of 5-methylBTAH on platinum show a change in orientation over the potential range studied (-250 to 750 mV vs SCE). The orientation of the methyl group tilts more toward the plane of the interface as the potential is scanned in the positive direction. This orientation change is correlated to hydrogen coadsorption on the platinum surface at low potentials. 5-Methylbenzotriazole lies in the surface plane or does not orient on gold at lower potentials but the orientation is tilted toward normal at more positive potentials over the potential range studied (-500 to 900 mV vs SCE). To compliment these results, cyclic voltammetry and electrochemical impedance spectroscopy measurements were performed. Cyclic voltammograms of copper show that addition of 5-methylBTAH protects the surface from copper dissolution, increasing the electrochemical window by 450 mV. Cyclic voltammetry of 5-methylBTAH on platinum showed a partial blockage of adsorbed hydrogen and also prevented the adsorption of oxygenated species at 450-600 mV. Cyclic voltammetry on gold shows that 5-methylBTAH blocks oxide formation for 400 mV thus increasing the electrochemical window. Electrochemical impedance spectroscopy has been performed to determine the potential of zero charge of 5-methylBTAH on copper.

The purpose of this investigation is to study the ionic liquid/quartz interface with sum frequency generation vibrational spectroscopy (SFG). SFG spectroscopy was chosen for this study because of its unique ability to yield vibrational spectra of molecules at an interface. Different polarization combinations are used, which probe different susceptibilities, giving SFG the ability to determine molecular orientation at the interface. The ionic liquids used were 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF(4)], and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF(6)]. To determine the influence of the molecular structure and charge on orientation at the interface, neutral, 1-methylimidazole, and 1-butylimidazole were also studied. Raman spectra and depolarization ratios were obtained for neat samples of 1-methylimidazole, 1-butylimidazole, and 1-butyl-3-methylimidazolium tetrafluoroborate recorded from 2700 to 3300 cm(-1). SFG spectra of the 1-methylimidazole/quartz interface showed both methyl and aromatic C-H vibrations. Orientation calculations determined that the ring of the molecule is tilted 45-68 degrees from normal, with the methyl group oriented 32-35 degrees from normal. The SFG spectra of 1-butylimidazole contain several resonances from the alkyl chain with only one weak resonance from the aromatic ring. Orientation calculations suggest that the ring is lying in the plane of the surface with the methyl group pointing 43-47 degrees from normal. The orientation of the [BMIM][PF(6)] ionic liquid was sensitive to trace amounts of water and had to be evacuated to3 x 10(-5) Torr for the water to be removed. SFG spectra of both ionic liquids were similar, displaying resonances from the alkyl chain as well as the aromatic ring. Orientation analysis suggests the aromatic ring was tilted 45-90 degrees from normal for [BMIM][BF(4)], while the ring for [BMIM][PF(6)] was tilted 38-58 degrees from normal. This suggests the orientation of the molecule is influenced by the size of the anion.

Sum frequency generation spectroscopy, SFG, was used for the surface characterization at the gas−liquid interface of the 1-butyl-3-methylimidazolium cation combined with the following anions: Br-, I-, PF6-, BF4-, (CF3SO2)2N- (imide), SCN-, CH3SO3- (MeSO3), CH3SO4- (MS), and (CN)2N- (DCN). The SFG spectra obtained for the different ionic liquids were similar independent of the anion selected; therefore, a comprehensive analysis for the surface characterization of the ionic liquids' cation was focused only on the PF6- and Br- anion combinations. For an accurate identification of the vibrational modes observed, FT-IR and Raman spectroscopy in combination with isotopic labeling with deuterium and polarized Raman spectroscopy was used. The cation orientation was determined by analysis of polarization-dependent SFG spectra. For a compound dried in a vacuum to ≤2 × 10-5 Torr, the cation appears to be oriented with the ring laying flat along the surface plane and the butyl chain projecting into the gas phase ind...

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.

Sum frequency generation vibrational spectroscopy (SFG) and cyclic voltammetry (CV) measurements have been conducted on the ionic liquid/platinum electrode system. The room-temperature ionic liquid investigated is 1-butyl-3-methylimidazolium [BMIM]+ with [PF6]- or [BF4]- anions. SFG spectra are taken in situ under potential control from 2750 to 3300 cm-1 (C−H stretching region). Polarization-dependent SFG spectra are used to extract orientation information for the cation at the electrode surface. The SFG spectra indicate that the cation changes orientation as the electrode potential is changed within the double-layer region. The plane of the imidazolium ring tips from 35° at positive surface charge to ∼60° from the surface normal at negative surface charge. Further, the orientation change is different for [BMIM][PF6] than for [BMIM][BF4]. A model for ions at the surface is presented based on these spectroscopic and electrochemical measurements.

Sum frequency generation vibrational spectroscopy was used to determine the orientation of the cations in a room-temperature ionic liquid at the air−liquid interface. The ionic liquid that was studied was 1-butyl-3-methylimidazolium bis-trifluoromethylsulfonimide, [BMIM]+[imide]-. The orientation of the cation, [BMIM]+, was studied as a function of water pressure in the vapor over the range of 10-5 to 200 Torr. At water pressures below 10-4 Torr, the imidazolium ring was oriented parallel to the surface plane. On increasing the water pressure to greater than 10-4 Torr, the imidazolium ring tipped up from the surface with an angle along the surface normal of 40−55°. No water signal was observed in the spectra obtained at the water pressures studied here.

Sum frequency generation (SFG) vibrational spectroscopy in combination with cyclic voltammetry (CV) was used to investigate the oxidation of carbon monoxide (CO) in acetonitrile (CH3CN)/water (H2O) solutions at the platinum (Pt) electrode surface. CO oxidation on Pt was studied as a function of water concentration and electrode potential. In electrolyte solutions containing small concentrations of water, a preoxidation wave was observed at +900 mV vs SCE whereby a small amount of the adsorbed CO was oxidized. This preoxidation wave was followed by the main oxidation of CO on Pt at approximately +1700 mV vs SCE. Spectroscopy and voltammetry data in these essentially nonaqueous electrolytes indicate an oxidation intermediate of CO to CO2 that was nearly parallel to the Pt electrode surface.

Sum frequency generation, SFG, vibrational spectroscopy is used to investigate the orientation of acetonitrile (CH3CN) at the Pt(111) electrode. Acetonitrile is studied as a function of water concentration and electrode potential to determine how these factors affect the adsorbate structure at the interface. The SFG results indicate acetonitrile orients in response to the electrode potential with the C−C bond, C3 symmetry axis, perpendicular to the surface. The orientation is predominately with the CH3 group toward the metal between 200 and 600 mV and with the CN group toward the metal above 800 mV, (vs NHE, normal hydrogen electrode). The orientation is reversible between 0 and 1400 mV. Addition of water, >0.05x, disrupts the orientation of acetonitrile at the interface. The alignment of acetonitrile along the surface normal is in response to the surface charge at the electrode.

The vapor−liquid interface of aqueous inorganic acid and salt solutions are examined using sum frequency generation (SFG). The results show that the SFG intensity of hydrogen-bonded water on 0.01x acid (HCl, HNO3, and H2SO4) solutions, where x = mole fraction, is greatly enhanced compared to the 0.01x solution of the corresponding salts (NaCl, NaNO3, and KHSO4). This suggests that either surface water molecules on the acid solutions orient with the dipole in a more vertically aligned fashion than those on the salt solutions or that more layers of water are ordered. These results are interpreted with an electric double layer model, in which the double layer is composed of subsurface anions and cations. The weak association of protons, as opposed to Na+ or K+, with anions results in a greater electric field at the surface on acid solutions. The perturbation of surface water on ionic solutions is discussed in terms of electrostatic and displacement mechanisms.

The vibrational spectroscopy sum frequency generation (SFG) is used to investigate the adsorption of carbon monoxide on the single crystal (111) and polycrystalline platinum surfaces. By varying the frequency and polarization of the light beams, different surface species of CO species are probed. SFG signal intensities for different polarization indicate that adsorbed CO polarizability is significantly perturbed from the gas-phase molecule. The SFG signal of CO disappears well below the main oxidation potential of CO to CO2. The disappearance of the CO signal is interpreted as a transformation in the CO layer to a state which is invisible to SFG. The invisible state is suggested to be CO with the bond axis nearly parallel to the platinum surface.

The surface-specific vibrational spectroscopy, sum frequency generation, has been used to study the structure of water at air/liquid interfaces of inorganic, ionic solutions. The experiments suggest that ions in solution reorient surface water into a more structured, hydrogen-bonded network compared with the neat water/air interface. In more concentrated solutions, there is a high degree of ion association. A model based on Hard Soft Acid Base theory indicates that associated ions are able to penetrate closer to the air interface than unassociated ions. Associated ions incorporate surface water into the ion-pair hydration shell.

Structural data about water at the air/water interface of sulfuric acid solutions have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a “free OH”, decreases in concentration as the sulfuric acid concentration is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solutions from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solutions change from ionic in nature at low concentrations ( 0.4x).

The OH and CH regions of glycerol/water mixtures in the concentration range 0.0−1.0 mole fraction have been investigated with vibrational sum frequency generation. Glycerol is found to partition to the surface of these solutions in all concentrations. Neat glycerol surfaces contain no free OH groups projecting into the vapor. The surface orientation of glycerol is constant through most of the concentration range.

The arrangement of ions at the platinum electrode in the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate has been determined using sum frequency generation vibrational spectroscopy (SFG), electrochemical impedance spectroscopy (EIS), and the vibrational Stark effect. The results indicate that CO adsorbed on the Pt electrode has a Stark shift of 30-35 cm(-1)/V in the ionic liquid. The potential of zero charge (PZC) of the ionic liquid-Pt system is approximately -500 mV (vs Ag wire), with a capacitance of 0.12 F/m2. Further, polarization-dependent SFG experiments suggest the ions reorganize at the surface depending on the electrode charge. In combination, all these results indicate that the ions of a neat ionic liquid are organized in a Helmholtz layer at the electrified metal electrode interface.