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Surface chemistry is notoriously difficult to study, in part, due to the decreased number of molecules that contribute to the properties compared to the bulk phase but often has significant effects on the chemical activity of the material. This is especially true in topics such as corrosion, catalysis, wetting, and many others in nature and industry. Sum frequency generation (SFG) spectroscopy was developed for interface studies due to its high molecular selectivity and surface sensitivity, which is quite useful to study the effects of structural inhomogeneity in microscopy. Compressive sensing (CS) combined with SFG spectroscopy minimizes the imaging time while still producing quality images. Selected systems are presented here to demonstrate the capability of CS-SFG microscopy. CS-SFG microscopy successfully distinguished the static monolayer molecular mixtures, the orientations and adsorption of adsorbed molecules by the dip-coating technique, and the localized CO behaviors on polycrystalline Pt electrodes. Further discussion includes dynamic imaging as a future direction in CS-SFG microscopy. As materials and surfaces become more complex, imaging with chemical contrast becomes indispensable to understanding their performance and CS-SFG microscopy seems highly beneficial in this respect. [ABSTRACT FROM AUTHOR]

Molecular interactions at interfaces have a significant effect on the wetting properties of surfaces on a macroscale. Sum frequency generation (SFG) spectroscopy, one of a few techniques capable of probing such interactions, generates a surface vibrational spectrum sensitive to molecular structures and has been used to determine the orientation of molecules at interfaces. The purpose of this review is to assess SFG spectroscopy's ability to determine the molecular orientations of interfaces composed of fluorinated organic molecules. We will explore three different types of fluorinated organic material-based interfaces, naming liquid–air, solid–air, and solid–liquid interfaces, to see how SFG spectroscopy can be used to gain valuable and unique information regarding the molecular orientation of each interface. We hope this review will help to broaden the understanding of how to employ SFG spectroscopy to obtain more complex structural information for various fluorinated organic material-based interfaces in the future. [ABSTRACT FROM AUTHOR]

Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C—H stretching region (2750–3100 cm−1) for [P8 8 8 n][Cl], where n = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for n = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of n = 12 or n = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with n = 8, 10, or 12) confirmed gauche defects, with enhancement for the long alkyl chain and an abrupt increase of gauche occurrence from n = 8 to n = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented. [ABSTRACT FROM AUTHOR]

The orientation of the cation and the anion of 1-butyl-3-methylimidazolium dicyanamide at the platinum-liquid interface are reported, using sum frequency generation (SFG) spectroscopy. Results suggest that the imidazolium ring in the cation tends to lie flat at the surface of the electrode at negative potentials but is repelled at positive ones, while the anion is strongly adsorbed at the surface of the platinum at more positive potentials with its [C.sub.2] axis aligned with the surface normal.

Sum frequency generation spectroscopy is used to study the influence of water on the surface of the water-miscible ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]. The results suggest that water has no drastic effect on the surface orientation of [BMIM][BF4] cation for concentrations from 0.0002[X.sub.IL] to 1[X.sub.IL].

In situ sum frequency generation vibrational spectroscopy was performed at varied potentials and polarization combinations on polycrystalline copper, platinum and gold samples in 0.5M HCl[O.sub.4] with 50mM 5-methylbenzotriazole 5-methylBTAH added. For copper surfaces orientation of the molecule on the surface was not affected by potential within the potential window studied and sum frequency generation spectra of 5-methyBTAH on platinum show a change in orientation over the potential range studied.

Sum frequency generation vibrational spectroscopy was used to characterize the interfacial properties of two ionic liquids as well as 1-methylimidazole and 1-butylimidazole on a quartz surface and Raman spectroscopy was used to identify vibrations and conclude whether they were symmetric or asymmetric. It was concluded that larger the anion the more the imidazolium ring was tilted towards normal.

Sum frequency generation spectroscopy (SFG) is used for the surface characterization at the gas-liquid interface of the 1-butyl-3-methylimidazolium cation combined with Br(super -), I(super -), PF(sub 6)(super -), BF(sub 4)(super -), (CF(sub 3)SO(sub 2))(sub 2)N(super -), SCN(super -), CH(sub 3)SO(sub 3)(super -), SH(sub 3)SO(sub 4)(super -), and [CN(sub 2)]N(super -) (DCN) anions. The analysis of the polarization-dependent SFG spectra has provided a reliable characterization of the cation at the liquid-gas interface.

The sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has a significant mixing with the molecules deposited from the backfilling solution.

Sum frequency generation vibrational spectroscopy (SFG) and cyclic voltammetry (CV) measurements are conducted on the ionic liquid/platinum electrode system. The SFG spectra indicate that the cation changes orientation as the electrode potential is changed within the double-layer region.

A new sum frequency generation imaging microscope using a novel sampling theory, compressive sensing (CS), has been developed for surface studies. CS differentiates itself from the conventional sampling methods by collecting fewer measurements than the traditional methods to reconstruct a high quality image. Pseudorandom patterns were applied to a light modulator and reflected the sum frequency (SF) signal generated from the sample into a photomultiplier tube detector. The image of the sample was reconstructed using sparsity preserving algorithms from the SF signal. The influences of the number of CS testing patterns applied and the number of SF pulses acquired for each pattern on the quality of the images was investigated and a comparison of the image quality with the traditional raster scan was made at varying resolutions for a gold patterned Si surface. Our results demonstrate the CS technique achieved 16 times the pixel density beyond the resolution where the raster scan strategy lost its ability to image the sample due to the dilution of the SF signal below the detection limit of the detector. [ABSTRACT FROM AUTHOR]

The study employs the technique of sum frequency generation (SFG) imaging microscopy to analyze the reactions taking place between cyanide and gold at the solid-liquid interface. The continuous exposure of the gold film to cyanide is shown to lead to the formation of higher-coordinated gold-cyanide complexes.

The consequences of a pure electrolyte at the interface in an electrochemical system are presented. It was noticed that all techniques were measured as a function of applied potential to give complete description of the interface for numerous imidazolium-based ionic liquids.

Sum frequency generation vibrational spectroscopy is used for studying the gas-liquid interface of room-temperature ammonium and pyrrolidinium ionic liquids. The chain length dependence studies have shown that the contribution of the N[C[H.sub.3] vibration is masked by the chain for lengths of four carbons and more.

Sum frequency generation vibrational spectroscopy is used for studying the orientation of the cation and the anion of room-temperature ionic liquids. The results have shown that presence of both ionic species at the gas-liquid interface and have described the behavior of a simple inorganic anion at the surface.

The surface orientation of ionic liquids, 1,3-dimethylimidazolium methyl sulfate, [MMIM][MS], 1-ethyl-3-methylimidazolium methyl sulfate [EMIM][MS], and 1-butyl-3-methylimidazolium methylsulfate, [BMIM][MS] at the gas-liquid interface was determined. The observations exhibited that both ions are present at the first layer of the gas-liquid interface and the alkyl chains are extended towards the gas state and far away from the liquid state.

A home-built frequency generation spectroscopy (SFG) imaging microscope is used to obtain the vibrational spectrum and images of the monolayer at the same time. Results demonstrate the importance of considering that defects and inhomogeneities exist on the surface, which influence the interpretation of SFG data and images.

The sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to show that octadecanethiol (ODT) forms ordered monolayers at either electrochemically reduced or oxidized Zn surfaces. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt and acid media, but the protection efficiency is reduced with immersion due to the presence of defects within the monolayers.