Sum Frequency Generation Spectroscopy Research Articles

Probing specific ion effects at air-aqueous dibutyl phosphate interfaces using vibrational sum frequency generation spectroscopy

Molecular properties at air–liquid and liquid–liquid interface hold the key to many processes involving molecular transport across phase boundaries from aerosol formation to carbon cycling and material separation using solvent extraction techniques. Using dibutyl phosphate (DBP) as a representative for partially aqueous soluble surfactants, the specific ion effect (SIE) of the Hofmeister series cations Cs+, Na+, Li+, and Mg2+ on the partition and interaction between surfactant molecules and water molecules in the air–aqueous interface are investigated using vibrational sum frequency generation spectroscopy and surface tension measurements. In the presence of 1 mM and 1M bulk aqueous phase ionic strength salt concentrations, fundamental qualitative relationships are observed for the salting out of DBP relative to bulk aqueous phase nitrate salt concentrations and the specific cations species. At 1 mM ionic strength, the interfacial charge and hence the interfacial potential modulates the electrostatic interactions; in particular, the counter cations partially screen the negatively charged interface induced by the DBP in a direct Hofmeister order. At 1M ionic strength, the electric field at the interface or interfacial potential is effectively neutralized, and the counter cations promote the partitioning of DBP to the interface depending on their specific interaction with the DBP head group and metal ion hydration properties. The present results lay a foundation to study SIEs of heavier metals on hydrophobic-aqueous DBP interfaces.

Theoretical study of the structure and vibrational sum frequency generation spectroscopy of liquid-vapor interface of aqueous acetic acid.

We have investigated the liquid-vapor interface of aqueous acetic acid solution through calculations of the structure and vibrational sum frequency generation (VSFG) spectra of the interfaces for varying concentrations of the acetic acid (AA). Our findings reveal the surface propensity of the AA molecules. As the concentration of AA increases, more AA molecules are found to be present in the interfacial region. The AA molecules at the interface are found to be oriented in a manner where the hydrophobic part (methyl chain) is oriented toward the vapor phase and the hydrophilic part is pointed toward the liquid phase. The total VSFG spectrum reveals that the intensity of the peak of dangling OH groups of water around 3750 cm-1 decreases when the concentration of AA increases, while the intensity of the peak around 3550 cm-1 due to OH groups having hydrogen bonds between water and AA molecules increases. Furthermore, the latter peak is redshifted as the AA concentration increases, which is due to the partial cancellation between the positive response of water OH modes and the negative response of the OH modes of AA molecules. It is also noted that the AA molecules make the interfacial water more structured by making hydrogen bonds with its otherwise dangling OH modes at the surface.

Frequency-Dependent Vibrational Relaxation Time of OH Stretch at the Air/Isotopically Diluted Water Interface.

Elucidation of the vibrational relaxation process of interfacial water is indispensable for understanding energy dissipation at the aqueous interface. In this study, the vibrational relaxation dynamics of the hydrogen-bonded OH (HB OH) stretch vibration was investigated at the air/isotopically diluted water (HOD-D2O) interface by time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. We observed the temporal change of the excited-state band (v = 1 → 2 transition), which enables a reliable determination of the T1 time of interfacial water. The T1 times obtained for the HB OH stretch with various pump frequencies are 0.14 ± 0.15 ps (3200 cm-1), 0.27 ± 0.05 ps (3300 cm-1), 0.34 ± 0.03 ps (3400 cm-1), and 0.63 ± 0.04 ps (3500 cm-1), indicating that T1 is comparable with the value at the air/H2O interface at the low-frequency side but is markedly longer at the high-frequency side. The observed frequency-dependent T1 time can be rationalized in terms of the frequency mismatch between the HB OH stretch and the bending overtone in HOD-D2O, supporting the conclusion that vibrational relaxation through the Fermi resonance with the bending overtone is the predominant mechanism of the vibrational relaxation of the HB OH stretch at the air/water interface.

Ordered Interfacial Water Generated at Poly(ionic liquid) Membrane Surface Imparts Ultrafast Water Transport and Superoleophobicity.

Achieving ultrahigh permeance and superoleophobicity is crucial for membrane application. Here, we demonstrated that a poly(ionic liquid)/PES hydrogel membrane can achieve dual goals. The high polarity of the ionic liquids induces the water molecules on the membrane surface to be arranged more ordered, as verified by molecular dynamics (MD) simulation and advanced femtosecond sum frequency generation (SFG) vibrational spectroscopy. Meanwhile, a large amount of water exists in membrane pores, demonstrated by water absorption, low-field nuclear magnetic resonance, and SFG spectroscopy. The interfacial water layer endows the membrane with superior anti-oil-fouling properties, and the large amount of water in membrane pores imparts membrane with ultrahigh permeability. The positive charge on the channel surface and moderate channel size confer a high rejection of metal ions. The optimal membrane exhibited a permeance of 35.1 L m-2 h-1 bar-1, 5-10 times that of conventional hydrogel membranes with similar rejection. Moreover, the membrane exhibited excellent antibacterial properties. It can be expected that highly polar poly(ionic liquid) membranes will find promising applications in the water treatment field.

Hydrogen bonding blues: Vibrational spectroscopy of the TIP3P water model.

The computational spectroscopy of water has proven to be a powerful tool for probing the structure and dynamics of chemical systems and for providing atomistic insight into experimental vibrational spectroscopic results. However, such calculations have been limited for biochemical systems due to the lack of empirical vibrational frequency maps for the TIP3P water model, which is used in many popular biomolecular force fields. Here, we develop an empirical map for the TIP3P model and evaluate its efficacy for reproducing the experimental vibrational spectroscopy of water. We observe that the calculated infrared and Raman spectra are blueshifted and narrowed compared to the experimental spectra. Further analysis finds that the blueshift originates from a shifted distribution of frequencies, rather than other dynamical effects, suggesting that the TIP3P model forms a significantly different electrostatic environment than other three-point water models. This is explored further by examining the two-dimensional infrared spectra, which demonstrates that the blueshift is significant for the first two vibrational transitions. Similarly, spectral diffusion timescales, evaluated through both the center line slope and the frequency-frequency time correlation function demonstrate that TIP3P exhibits significantly faster spectral dynamics than other three-point models. Finally, sum-frequency generation spectroscopy calculations suggest that despite these challenges, the TIP3P empirical map can provide phenomenological, qualitative, insight into the behavior of water at the air-water and lipid-water interfaces. As these interfaces are models for hydrophobic and hydrophilic environments observed in biochemical systems, the presently developed empirical map will be useful for future studies of biochemical systems.

Adhesion of the mucilage envelope of Ocimum basilicum seeds probed by sum frequency generation spectroscopy.

The attachment of seeds to natural surfaces is important for the reproductive success of plants. This study investigates the adhesion mechanisms of Ocimum basilicum seed mucilage to CaF2 and polystyrene surfaces, using sum frequency generation (SFG) vibrational spectroscopy and pull-off force measurements. The results show that the adhesion is driven by the formation of crystalline cellulose at the interface. Initially, cellulose within the mucilage envelope is disordered due to strong cellulose-water interactions. As water evaporates, cellulose interactions with the substrate increase, leading to a more ordered molecular structure, with the degree of order varying between substrates. The CaF2 surface promotes a more crystalline cellulose assembly, whereas polystyrene results in a less ordered structure. Despite the reduced order, adhesion strength is higher on the polystyrene surface, suggesting that molecular disorder enhances the ability of the mucilage to absorb mechanical stress, thereby improving adhesion. These findings highlight the significant role of substrate chemistry in seed adhesion.

Hydrodynamic slip in nanoconfined flows: a review of experimental, computational, and theoretical progress.

Nanofluidics has made significant impacts and advancements in various fields, including ultrafiltration, water desalination, biomedical applications, and energy conversion. These advancements are driven by the distinct behavior of fluids at the nanoscale, where the solid-fluid interaction characteristic lengthscale is in the same order of magnitude as the flow conduits. A key challenge in nanofluidics is understanding hydrodynamic slip, a phenomenon in which fluids flow past solid boundaries with a non-zero surface velocity, deviating from the classical no-slip boundary condition. This review consolidates experimental, computational, and theoretical efforts to elucidate the mechanisms behind hydrodynamic slip in nanoconfined flows. Key experimental methods, such as the surface force apparatus, atomic force microscopy, and micro-particle image velocimetry are evaluated alongside emerging techniques like suspended microchannel resonators, dynamic quartz crystal microbalance, and hybrid graphene/silica nanochannels, which have advanced hydrodynamic slip characterization at the nanoscale. In addition to direct slip measurement techniques, methods like sum frequency generation spectroscopy, X-ray reflectometry, and ellipsometry are discussed for their roles in probing solid-liquid interfacial interactions, shedding light on the origins of hydrodynamic slip. The review also highlights the contributions of molecular dynamics simulations, including both non-equilibrium (NEMD) and equilibrium (EMD) approaches, in modeling interfacial phenomena and slip behavior. Additionally, it explores the influence of factors such as surface wettability, shear rate, and confinement on slip, emphasizing the interaction between liquid structuring and solid-liquid interactions. Advancements made so far have uncovered more complexities in nanoconfined flows which have not been considered in the past, inviting more investigation to fully understand and control fluid behavior at the molecular level.

Local field effects of quadrupole contributions on sum frequency generation spectroscopy.

In the theory of condensed-phase spectroscopy, local field effect is of general importance to account for intermolecular electrostatic interactions. The present paper extends the microscopic treatment of local field effects on the sum frequency generation (SFG) spectroscopy to incorporate quadrupole interactions, since their roles have been increasingly recognized in the SFG spectroscopy. The extended theory involves some corrections to the conventional formulas of the nonlinear susceptibilities in both the interface and bulk regions, including the χIQB term. Fresnel transformations for the interface and bulk susceptibilities are rigorously applied, which implies inseparability of the interface and bulk signals in PSS and PPP cases. We examined the influence of the corrections with quantitative calculations of the susceptibilities, including dipolar and quadrupolar interactions.

A Tale of Two Tails: Tail Ordering of Stoichiometric 1:1 DTAB:SDS Pairs Adsorbed at the Oil-Water Interface.

Cationic:anionic surfactant mixtures adsorbed at an oil-water interface stabilize foams in the presence of oil, making them essential to the oil, gas, and firefighting industries. The oil tolerance of foams stabilized by surfactant mixtures, relative to pure (unmixed) cationic and anionic surfactants, results from the mixtures' enhanced flexibility in tailoring the physicochemical properties of the interface. To judiciously employ these mixtures, it is necessary to characterize the structure-function property relationship of their surfactant monolayers that lend to oil-tolerant/intolerant foams. In this work, we employ interfacial tensiometry and vibrational sum frequency spectroscopy to determine the composition (surfactant population and cationic:anionic ratio) and the structure (surfactant alkyl tail conformation) of monolayers prepared at the oil-water interface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium dodecyl sulfate) mixtures. We show that the interfacial surfactant density of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB and SDS at similar concentrations up to and beyond their respective critical micelle concentration. The enhanced interfacial adsorption of these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that these paired surfactants preferentially adsorb at the interface, causing the interfacial DTAB:SDS ratio to be nearly 1:1. Additionally, we find that the SDS tail is more conformationally ordered than the DTAB tail, even though they are expected to be conformationally identical along the entire tail, since they are likely conjoined through van der Waals interactions. This leads to the conclusion that the surfactant pairs are in a staggered arrangement at the interface. These findings help to uncover molecular factors that contribute to the enhanced oil tolerance, and in some cases oil intolerance, of foams stabilized by cationic:anionic surfactant mixtures.

DFT-based calculation of vibrational sum frequency generation spectral features of crystalline β-sheets in silk: Polarization and azimuth angle dependences.

Sum frequency generation (SFG) necessitates both noncentrosymmetry and coherence over multiple length scales. These requirements make vibrational SFG spectroscopy capable of probing structural information of noncentrosymmetric organic crystals interspersed in polymeric matrices and their three-dimensional spatial distributions within the matrices without spectral interferences from the amorphous components. However, this analysis is not as straightforward as simple vibrational spectroscopy or scattering experiments; it requires knowing the molecular hyperpolarizability of SFG-active vibrational modes and their interplay within the coherence length. This study demonstrates how density function theory (DFT) calculations can be used to construct the molecular hyperpolarizability of a model system and combine it with the SFG theory to predict the polarization and azimuth angle dependences of SFG intensities. A model system with short peptide chains mimicking β-sheet domains in Bombyx mori silk was chosen. SFG signals of the amide-I, II, III, and A bands and one of the CH deformation modes were simulated and compared with the experimental results and the predictions from the group theory. The SFG features of amide-I and A bands of antiparallel β-sheet could be explained with DFT-based theoretical calculations. Although vibrational coupling with neighboring groups breaks the symmetry of the D2 point group, the group theory approach and DFT calculations gave similar results for the amide-I mode. The DFT calculation results for amide-II did not match with experimental data, which suggested vibrational coupling within a larger crystalline domain may dominate the SFG spectral features of these modes. This methodology can be applied to the structural analysis of other biopolymers.

Photoswitchable Arylazopyrazole Surfactants at the Water-Air Interface: A Microscopic Perspective.

Surfactants play an important role in modifying the properties of water-air interfaces. Here, we combine information from molecular dynamics simulations, surface tensiometry, and vibrational sum-frequency generation spectroscopy to study the interfacial behavior of photoswitchable arylazopyrazole (AAP) surfactants. This combination of the experimental techniques allows a direct relation between surface tension and surface concentration rather than just the bulk concentration. Specifically, we conducted a comparison between two derivatives, one with an octyl terminal group (O-AAP) and the other without this group (H-AAP), focusing on their respective E and Z isomers. From the simulations of these four systems, we see that those with a stronger cluster formation, likely resulting from higher intermolecular attractive interactions, display higher surface tensions for the intermediate surface excess. In some cases, even a small but noticeable maximum in the surface tension isotherm is observed for systems with strong cluster formation. Such a maximum is not observed in the experiments, although such an observation would be compatible with the general properties of the Frumkin isotherm. We exclude that the peak is due to the finite width of the simulation box. Apart from this effect, the general features of the surface tension are consistent between the experiment and simulation. Evidence is also provided that it is primarily the interaction of the aromatic rings that determines the strength of the surfactant interactions.

Measuring complex SFG: Characterizing a phase reference.

Reactions and interactions at interfaces play pivotal roles in processes ranging from atmospheric aerosols influencing climate to battery electrodes determining charge-discharge rates to defects in catalysts controlling the fate of reactants to the outcome of biological processes at membrane interfaces. Tools to probe these surfaces at the atomic-molecular level are thus critical. Chief among non-invasive probes is the vibrational spectroscopy sum frequency generation (SFG). The complex signal amplitude generated by SFG requires techniques to interfere the unknown amplitude with a well-characterized one. An interferometric method is described to characterize the signal from any nonresonant reference material. The technique is demonstrated by measuring the phase of polycrystalline GaAs, chosen due to the strong signal and insensitivity to surface contamination. With a 515nm visible field, the phase of GaAs is 54.5° ± 0.5°. The capability of choosing a reference based solely on its signal intensity enables probing a wide range of interfaces.

The bonded interfacial layer structure of α-Al2O3 (0001)/water at different pH values studied by sum frequency vibrational spectroscopy.

Oxide/water interfaces are ubiquitous, with alumina/water drawing particular interest due to its environmental and industrial applications. Understanding the interfacial structure at the molecular level is crucial for many physical and chemical processes occurring there. However, the exact structure of interfacial H-bonded network at different pH values remains unclear. Here, sum-frequency vibrational spectroscopy in the OH stretch region was employed to study α-Al2O3 (0001)/water interface at different pH values, while suppressing the contribution of the diffusion layer by adding salts. The experimental results revealed although the variation of pH can charge the surface, it has little impact on the structure of the bonded interfacial layer (BIL). The interaction between alumina and water is mainly governed by weak hydrogen bonds. Furthermore, the templating effect of α-Al2O3 (0001) on the interfacial H-bonded network was observed, with the O-H stretch mode of ∼3430cm-1 exhibiting anisotropy consistent with the (0001) surface symmetry. These findings indicate that the BIL structure on Al2O3 (0001) is predominantly influenced by the surface atom configuration, and the effect of charge changes induced by pH on the BIL structure is negligible.

Coherent IR-hyper-Raman four wave mixing spectroscopy.

Nonlinear, four-wave mixing vibrational spectroscopies are commonly used to probe electron-vibration coupling in isotropic media. Most of these methods rely on infrared and/or Raman transitions, but methods involving hyper-Raman transitions are also possible. Hyper difference frequency generation (HDFG) spectroscopy is an underdeveloped four-wave mixing vibrational spectroscopy based upon both infrared absorption and hyper-Raman scattering transitions. Despite several experimental reports on HDFG, its spectroscopic properties have not been fully explored. To this end, we investigate the selection rules and behavior of HDFG spectroscopy as an upconverted infrared spectroscopy and as a probe of vibronic coupling in molecular systems. We discuss the similarities between HDFG, a four-wave mixing technique, and vibrational sum frequency generation (vSFG) spectroscopy, a three-wave mixing technique. vSFG and HDFG appear to provide similar output intensities, making HDFG feasible for vSFG practitioners. HDFG is shown to be a sensitive probe of vibronic coupling in bulk systems and provides an alternative method to investigate electronic-nuclear coordinate correlations.

Sum-Frequency Generation Spectroscopy of Aqueous Interfaces: The Role of Depth and Its Impact on Spectral Interpretation

Sum-Frequency Generation Spectroscopy of Aqueous Interfaces: The Role of Depth and Its Impact on Spectral Interpretation

Effect of para-substituents on NC bonding of aryl isocyanide molecules adsorbed on metal surfaces studied by sum frequency generation (SFG) spectroscopy.

The effect of para-substituents on the NC bond of aryl isocyanide molecules adsorbed on Au, Ag, Pt, and Pd surfaces was examined by vibrational sum frequency generation (VSFG) spectroscopy. The frequency of NC stretching vibration, υNC, depended on the electronic property of the substituents. When the substituents were more electron-withdrawing and more electron-donating, υNC shifted to lower and higher frequencies, respectively. However, υNC shifted to lower frequency again when the substituents were too electron-donating. The strong dependence of υNC on metal substrates was also observed, reflecting the difference in metal d-band energies, which leads to the difference in the contributions of σ-donation and π back-donation of the NC-metal coupling.

Charge Inversion by Monovalent Hydroxide Ions.

The interaction between the hydroxide ion (OH-) and the headgroup of a model cationic lipid (DPTAP, 1,2-dipalmitoyl-3-trimethylammonium-propane chloride) was investigated for different concentrations of NaOH solutions by using sum-frequency vibrational spectroscopy. The OH signal (3000-3700 cm-1) of the interfacial water under the Langmuir monolayer of DPTAP decreased with increasing NaOH concentration, due to screening of the surface charge by OH- counterions. Surprisingly, after reaching a minimum at 5 mM NaOH, the OH signal steadily increased. The phase-sensitive spectra revealed a sign change in the OH stretch band, indicating the overcompensation of the surface charge by OH-. By contrast, for a DODAB (didodecyldimethylammonium bromide) monolayer (a cationic surfactant without ester groups), the OH stretch signal decreased monotonically with NaOH addition. This charge inversion behavior is driven by the specific interaction between the ester moiety of DPTAP and the OH- that overcomes the Coulomb repulsion between the adsorbed ions.

Vibrational sum frequency generation (VSFG) spectroscopy of water adsorption on surfaces of yttria-stabilized cubic zirconia (YSZ).

Yttria-stabilized zirconia (YSZ) is found in a wide range of applications, from solid-oxide fuel cells to medical devices and implants. A molecular-level understanding of the hydration of YSZ surfaces is essential for optimizing its performance and durability in these applications. Nevertheless, only a limited amount of literature is available about the surface hydration of YSZ single crystals. In this study, we employ surface-sensitive non-linear vibrational sum frequency generation spectroscopy to investigate the hydration of YSZ(100), (110), and (111) single crystal substrates under ambient laboratory conditions. Three types of hydroxyl groups were identified at all three YSZ-D2O interfaces: (i) hydroxyls on the metal sites of Zr or Y resulting from the dissociative chemisorption of water, (ii) hydroxyls from proton adsorption to O sites formed from water dissociation, and (iii) hydroxyl groups as part of the physisorbed water at the interface.

Orientational Disorder of Alcohol Molecules at Their Solution Surfaces in Low Concentration Range: A Molecular Dynamics Simulation Study.

Molecular dynamics (MD) simulations of short-chain alcohols (methanol, ethanol, and 1-propanol) in solution were carried out to examine the orientational disordering (randomizing) of alcohol molecules at the surface under diluted conditions. Recent vibrational sum frequency generation (VSFG) spectroscopy, combined with photoelectron spectroscopy, has successfully measured the disordering structure at low concentrations. The present MD simulations accurately reproduce this observation for the first time. To ensure reliable results through MD simulations, several widely used force field models for water and alcohol, including polarizable models, were examined. This examination involved a comparison of structural and thermodynamic quantities, such as surface density times orientation and surface excess concentration, which were obtained through surface-specific measurements like VSFG spectroscopy and surface tension measurements. It is found that the width of the density profile for alcohol molecules at the surface, along the surface normal, increases as the concentration decreases in the diluted condition, which is consistent with the results obtained from the previous neutron and X-ray grazing incidence reflection experiments. A molecular mechanism explaining the disordering of alcohol molecules with decreasing concentration is also discussed.

Specific and high-affinity adsorption of volatile organic compounds on titanium dioxide surface.

The interaction between metal oxides and volatile organic compounds (VOCs) from the ambient atmosphere plays an important role in environmental and catalytic applications. Previous scanning probe microscopy and x-ray spectroscopy studies revealed surprisingly that the TiO2 [rutile (110)] surface selectively adsorbed atmospheric carboxylic acids, which typically exist in only parts-per-billion concentrations. In this work, we used in situ sum-frequency vibrational spectroscopy to study the interaction between rutile (110) and typical VOC molecules, including formic acid, acetic acid, and formaldehyde. Spectra from all three adsorbed molecules on rutile (110) were similar to the rutile surface spectrum in the ambient atmosphere, showing a broad resonance near 2950cm-1 that can be attributed to the bridging bidentate adsorption of corresponding compounds. In contrast, on a fused silica surface, a molecular monodentate adsorption configuration was observed for all the molecules, with aliphatic carbons appearing to be the dominant adventitious species.