Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Sum rule comparison of narrowband and broadband sum frequency generation spectra and comparison with theory

Earlier sum frequency generation (SFG) experiments involve one infrared and one visible laser, and a measurement of the intensity of the response, yielding data on the surface sensitive properties of the sample. Recently, both the real and imaginary components of the susceptibility were measured in two different sets of experiments. In one set, a broadband infrared laser was used, permitting observations at very short times, while in another set the infrared laser was narrowband, permitting higher spectral resolution. The differences in the spectrum obtained by the two will be most evident in studying narrow absorption bands, e.g., the band due to dangling OH bonds at a water interface. The direct comparisons in the integrated amplitude (sum rule) of the imaginary part of the dangling OH bond region differ by a factor of 3. Due to variations in experimental setup and data processing, corrections were made for the quartz reference, Fresnel factors, and the incident visible laser wavelength. After the corrections, the agreement differs now by the factors of 1.1 within broadband and narrowband groups and the two groups now differ by a factor of 1.5. The 1.5 factor may arise from the extra heating of the more powerful broadband laser system on the water surface. The convolution from the narrowband SFG spectrum to the broadband SFG spectrum is also investigated and it does not affect the sum rule. Theory and narrowband experiments are compared using the sum rule and agree to a factor of 1.3 with no adjustable parameters.

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation

Ions containing iodine atoms at the vapor-aqueous solution interfaces critically affect aerosol growth and atmospheric chemistry due to their complex chemical nature and multivalency. While the surface propensity of iodide ions has been intensely discussed in the context of the Hofmeister series, the stability of various ions containing iodine atoms at the vapor-water interface has been debated. Here, we combine surface-specific sum-frequency generation (SFG) vibrational spectroscopy with ab initio molecular dynamics simulations to examine the extent to which iodide ions cover the aqueous surface. The SFG probe of the free O-D stretch mode of heavy water indicates that the free O-D group density decreases drastically at the interface when the bulk NaI concentration exceeds ∼2 M. The decrease in the free O-D group density is attributed to the spontaneous appearance of triiodide that covers the topmost interface rather than to the surface adsorption of iodide. This finding demonstrates that iodide is not surface-active, yet the highly surface-active triiodide is generated spontaneously at the water-air interface, even under dark and oxygen-free conditions. Our study provides an important first step toward clarifying iodine chemistry and pathways for aerosol formation.

Interfacial molecular structure of phosphazene-based polymer electrolyte at the air-aqueous interface using sum frequency generation vibrational spectroscopy

The change induced in the physicochemical properties of polymer while hosting ions provides a platform for studying its potential applications in electrochemical devices, water treatment plants, and materials engineering science. The ability to host ions is limited in very few polymers, which lack a detailed molecular-level understanding for showcasing the polymer-ion linkage behavior at the interfacial region. In the present manuscript, we have employed sum frequency generation (SFG) vibrational spectroscopy to investigate the interfacial structure of a new class phosphazene-based methoxyethoxyethoxyphosphazene (MEEP) polymer in the presence of lithium chloride salt at the air-aqueous interface. The interfacial aspects of the molecular system collected through SFG spectral signatures reveal enhanced water ordering and relative hydrogen bonding strength at the air-aqueous interface. The careful observation of the study finds a synchronous contribution of van der Waals and electrostatic forces in facilitating changes in the interfacial water structure that are susceptible to MEEP concentration in the presence of ions. The observation indicates that dilute MEEP concentrations support the role of electrostatic interaction, leading to an ordered water structure in proximity to diffused ions at the interfacial region. Conversely, higher MEEP concentrations promote the dominance of van der Waals interactions at the air-aqueous interface. Our study highlights the establishment of polymer electrolyte (PE) characteristics mediated by intermolecular interactions, as observed through the spectral signatures witnessed at the air-aqueous interface. The investigation illustrates the polymer-ion linkage adsorption effects at the interfacial region, which explains the macroscopic changes observed from the cyclic voltammetry studies. The fundamental findings from our studies can be helpful in the design and fine-tuning of better PE systems that can offer improved hydrophobic membranes and interface stability for use in electrochemical-based power sources.

DFT-Based Calculation of Molecular Hyperpolarizability and SFG Intensity of Symmetric and Asymmetric Stretch Modes of Alkyl Groups

Vibrational sum frequency generation (SFG) spectroscopy has been extensively used for obtaining structural information of molecular functional groups at two-dimensional (2D) interfaces buried in the gas or liquid medium. Although the SFG experiment can be done elegantly, interpreting the measured intensity in terms of molecular orientation with respect to the lab coordinate is quite complicated. One of the main reasons is the difficulty of determining the hyperpolarizability tensors of even simple molecules that govern their SFG responses. The single-bond polarizability derivative model has been proposed to estimate the relative magnitude of SFG-active hyperpolarizability by assuming that the perturbation associated to each vibration is negligible. In this study, density functional theory was used to calculate the polarizability and dipole derivative tensors of the CH3 stretch mode of CH3I, CH3CH2I, CH3OH, and CH3CH2OH. Then, the hyperpolarizability tensors of symmetric and asymmetric vibration modes were calculated considering the Boltzmann distribution of representative conformers, which allowed us to theoretically calculate their SFG intensities at all polarization combinations as a function of the tilt angle of the CH3 group with respect to the surface normal direction. Then, the ratios of the calculated SFG intensities for the CH3 symmetric and asymmetric stretch peaks used in experimental studies for the CH3 tilt angle determination were compared. This comparison clearly showed that the effect of vibrational coupling among neighboring functional groups is significant and cannot be assumed to be negligible. This study presents new parameters that can be used in determining the average tilt angle of the CH3 group at the 2D interface with SFG measurements as well as limitations of the method.

Fully First-Principles Surface Spectroscopy with Machine Learning

Our current understanding of the structure and dynamics of aqueous interfaces at the molecular level has grown substantially due to the continuous development of surface-specific spectroscopies, such as vibrational sum-frequency generation (VSFG). As in other vibrational spectroscopies, we must turn to atomistic simulations to extract all of the information encoded in the VSFG spectra. The high computational cost associated with existing methods means that they have limitations in representing systems with complex electronic structure or in achieving statistical convergence. In this work, we combine high-dimensional neural network interatomic potentials and symmetry-adapted Gaussian process regression to overcome these constraints. We show that it is possible to model VSFG signals with fully ab initio accuracy using machine learning and illustrate the versatility of our approach on the water/air interface. Our strategy allows us to identify the main sources of theoretical inaccuracy and establish a clear pathway toward the modeling of surface-sensitive spectroscopy of complex interfaces.

Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source during hydrogen evolution

The electrochemical conversion of CO2 represents a promising way to simultaneously reduce CO2 emissions and store chemical energy. However, the competition between CO2 reduction (CO2R) and the H2 evolution reaction (HER) hinders the efficient conversion of CO2 in aqueous solution. In water, CO2 is in dynamic equilibrium with H2CO3, HCO3 -, and CO3 2-. While CO2 and its associated carbonate species represent carbon sources for CO2R, recent studies by Koper and co-workers indicate that H2CO3 and HCO3 - also act as proton sources during HER (J. Am. Chem. Soc. 2020, 142, 4154-4161, ACS Catal. 2021, 11, 4936-4945, J. Catal. 2022, 405, 346-354), which can favorably compete with water at certain potentials. However, accurately distinguishing between competing reaction mechanisms as a function of potential requires direct observation of the non-equilibrium product distribution present at the electrode/electrolyte interface. In this study, we employ vibrational sum frequency generation (VSFG) spectroscopy to directly probe the interfacial species produced during competing HER/CO2R on Au electrodes. The vibrational spectra at the Ar-purged Na2SO4 solution/Au interface, where only HER occurs, show a strong peak around 3650 cm-1, which appears at the HER onset potential and is assigned to OH-. Notably, this species is absent for the CO2-purged Na2SO4 solution/gold interface; instead, a peak around 3400 cm-1 appears at catalytic potential, which is assigned to CO3 2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH- product) and HCO3 - reduction (CO3 2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO3 electrolyte reveals that the proton donor switches from HCO3 - at low overpotential to H2O at higher overpotential. This work represents the first direct detection of OH- on a metal electrode produced during HER and provides important insights into the surface reactions that mediate selectivity between HER and CO2R in aqueous solution.

Ion-Specific Water-Macromolecule Interactions at the Air/Aqueous Interface: An Insight into Hofmeister Effect

The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH₂-groups, which follows the Hofmeister series: SO₄^2- > Cl^- > NO₃^- > Br^- > ClO₄^- > SCN^-. The minimal orientation angle of CH₂-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO₄^-, and SCN^-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH₂-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.

Molecular Orientation of Carboxylate Anions at the Water-Air Interface Studied with Heterodyne-Detected Vibrational Sum-Frequency Generation

The carboxylate anion group plays an important role in many (bio)chemical systems and polymeric materials. In this work, we study the orientation of carboxylate anions with various aliphatic and aromatic substituents at the water-air interface by probing the carboxylate stretch vibrations with heterodyne-detected vibrational sum-frequency generation spectroscopy in different polarization configurations. We find that carboxylate groups with small aliphatic substituents show a large tilt angle with respect to the surface normal and that this angle decreases with increasing size of the substituent. We further use the information about the orientation of the carboxylate group to determine the hyperpolarizability components of this group.

Probing the Gold/Water Interface with Surface-Specific Spectroscopy

Water is an integral component in electrochemistry, in the generation of the electric double layer, and in the propagation of the interfacial electric fields into the solution; however, probing the molecular-level structure of interfacial water near functioning electrode surfaces remains challenging. Due to the surface-specificity, sum-frequency-generation (SFG) spectroscopy offers an opportunity to investigate the structure of water near working electrochemical interfaces but probing the hydrogen-bonded structure of water at this buried electrode-electrolyte interface was thought to be impossible. Propagating the laser beams through the solvent leads to a large attenuation of the infrared light due to the absorption of water, and interrogating the interface by sending the laser beams through the electrode normally obscures the SFG spectra due to the large nonlinear response of conduction band electrons. Here, we show that the latter limitation is removed when the gold layer is thin. To demonstrate this, we prepared Au gradient films on CaF₂ with a thickness between 0 and 8 nm. SFG spectra of the Au gradient films in contact with H₂O and D₂O demonstrate that resonant water SFG spectra can be obtained using Au films with a thickness of ∼2 nm or less. The measured spectra are distinctively different from the frequency-dependent Fresnel factors of the interface, suggesting that the features we observe in the OH stretching region indeed do not arise from the nonresonant response of the Au films. With the newfound ability to probe interfacial solvent structure at electrode/aqueous interfaces, we hope to provide insights into more efficient electrolyte composition and electrode design.

The effect of thermodynamic changes in the cooling of saline soils on the corrosion system of carbon steels

In this experiment, Q235 and X80 carbon steels, which are widely used in oil and gas pipelines and ancillary facilities, were selected to study the changes in the corrosion behaviour and mechanism of carbon steels in the process of natural saline soil cooling to a freezing state through electrochemical testing. The equivalent circuit model of carbon steel before and after the freezing phase transformation in the soil was determined. Based on the corrosion kinetic parameters and soil thermodynamic changes, the influencing factors of steel corrosion during the cooling process were systematically analysed. It was found that temperature mainly affected carbon steel corrosion by changing the properties of the solution. The main factors affecting the corrosion behaviour of the carbon steel were the thermal motion of molecules, ions, and electrons in solution, oxygen dissolution and diffusion, ion adsorption, diffusion mass transfer, and unfrozen water content change during the cooling process.

The corrosion behavior and mechanism of carbon steels during cooling to a freezing state in natural saline soil were studied by an electrochemical test.

Investigation of Water Evaporation Process at Air/Water Interface using Hofmeister Ions

Evaporation is an interfacial phenomenon in which a water molecule breaks the intermolecular hydrogen (H-) bonds and enters the vapor phase. However, a detailed demonstration of the role of interfacial water structure in the evaporation process is still lacking. Here, we purposefully perturb the H-bonding environment at the air/water interface by introducing kosmotropic (HPO₄^-2, SO₄^-2, and CO₃^-2) and chaotropic ions (NO₃^- and I^-) to determine their influence on the evaporation process. Using time-resolved interferometry on aqueous salt droplets, we found that kosmotropes reduce evaporation, whereas chaotropes accelerate the evaporation process, following the Hofmeister series: HPO₄^-2 < SO₄^-2 < CO₃^-2 < Cl^- < NO₃^- < I^-. To extract deeper molecular-level insights into the observed Hofmeister trend in the evaporation rates, we investigated the air/water interface in the presence of ions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The SFG vibrational spectra reveal the significant impact of ions on the strength of the H-bonding environment and the orientation of free OH oscillators from ∼36.2 to 48.4° at the air/water interface, where both the effects follow the Hofmeister series. It is established that the slow evaporating water molecules experience a strong H-bonding environment with free OH oscillators tilted away from the surface normal in the presence of kosmotropes. In contrast, the fast evaporating water molecules experience a weak H-bonding environment with free OH oscillators tilted toward the surface normal in the presence of chaotropes at the air/water interface. Our experimental outcomes showcase the complex bonding environment of interfacial water molecules and their decisive role in the evaporation process.

The dielectric function profile across the water interface through surface-specific vibrational spectroscopy and simulations

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Proton Configuration in Water Chain on Pt(533)

In this paper, a wetting behavior of Pt(533) is studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under an ultrahigh-vacuum condition at 145 K. The imaginary parts of the surface nonlinear susceptibility (Imχ⁽²⁾) of the H-bonded OH stretching region are successfully obtained for submonolayer water coverage that show negative bands indicating H-down (proton pointing to the substrate) configurations both for the water at the step and at the terrace. The growth manner of the Imχ⁽²⁾ signal with coverage and the results of an isotopic dilution are consistent with a model in which a one-dimensional (1D) chain at the step forms a "zigzag" structure that contains H-down orientations. This finding resolves the previous controversy in the literature concerning the proton configuration in the 1D water chain at the step.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

Sum-frequency generation (SFG) spectroscopy provides a unique optical probe for interfacial molecules with interface-specificity and molecular specificity. SFG measurements can be further carried out at different polarization combinations, but the target of the polarization-dependent SFG is conventionally limited to investigating the molecular orientation. Here, we explore the possibility of polarization-dependent SFG (PD-SFG) measurements with heterodyne detection (HD-PD-SFG). We stress that HD-PD-SFG enables accurate determination of the peak amplitude, a key factor of the PD-SFG data. Subsequently, we outline that HD-PD-SFG can be used not only for estimating the molecular orientation but also for investigating the interfacial dielectric profile and studying the depth profile of molecules. We further illustrate the variety of combined simulation and PD-SFG studies.

Temporal and Chirp Effects of Laser Pulses on the Spectral Lineshape in Sum-Frequency Generation Vibrational Spectroscopy

Assignment and interpretation of the sum-frequency generation vibrational spectra (SFG-VS) depend on the ability to measure and understand the factors affecting the SFG-VS spectral lineshape accurately and reliably. In the past, the formulation of the polarization selection rules for SFG-VS and the development of the sub-wavenumber high-resolution broadband SFG-VS (HR-BB-SFG-VS) have provided solutions for many of these needs. However, despite these advantages, HR-BB-SFG-VS has not been widely adopted. The majority of SFG measurements so far still relies on the picosecond scanning SFG-VS (ps-SFG-VS) or the conventional broadband SFG-VS (BB-SFG-VS) with the spectral resolution around (mostly above) 10 cm-1, which also results in less ideal spectral lineshape in the SFG spectra due to the temporal and chirp effects of the laser pulses used in experiment. In this report, the temporal and the chirp effects of laser pulses with different profiles in the SFG experiment on the measured SFG-VS spectral lineshape are examined through spectral simulation. In addition, the experimental data of a classical model system, i.e., OTS (octadecyltrichlorosilane) monolayer on glass, obtained from the ps-SFG-VS, the BB-SFG-VS, and the HR-BB-SFG-VS measurements, are directly compared and examined. These results show that temporal and chirp effects are often significant in the conventional BB-SFG-VS, resulting lineshape distortions and peak position shifts besides spectral broadening. Such temporal and chirp effects are less significant in the ps scanning SFG-VS. For the HR-BB-SFG-VS, spectral broadening, and temporal and chirp effects are insignificant, making HR-BB-SFG-VS the choice for accurate and reliable measurement and analysis of SFG-VS spectra.

Accurate molecular orientation at interfaces determined by multimode polarization-dependent heterodyne-detected sum-frequency generation spectroscopy via multidimensional orientational distribution function

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation (SFG) spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG datasets obtained from the different vibrational modes, allows us to determine the molecular orientation. We apply this new approach to formic acid molecules at the air–water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multidimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation.

The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO2 Reduction on Gold

The selectivity and activity of the carbon dioxide reduction (CO₂R) reaction are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in situ-generated CO on Au in the presence of alkali cations, we quantify the total electric field present at catalytic active sites and deconvolute this field into contributions from (1) the electrochemical Stern layer and (2) the Onsager (or solvation-induced) reaction field. Contrary to recent theoretical reports, the CO₂R kinetics does not depend on the Stern field but instead is closely correlated with the strength of the Onsager reaction field. These results show that in the presence of adsorbed (bent) CO₂, the Onsager field greatly exceeds the Stern field and is primarily responsible for CO₂ activation. Additional measurements of the cation-dependent water spectra using vibrational sum frequency generation spectroscopy show that interfacial solvation strongly influences the CO₂R activity. These combined results confirm that the cation-dependent interfacial water structure and its associated electric field must be explicitly considered for accurate understanding of CO₂R reaction kinetics.

Charge transfer across C-H⋅⋅⋅O hydrogen bonds stabilizes oil droplets in water

[Figure: see text].

First hyperpolarizability of water at the air-vapor interface: a QM/MM study questions standard experimental approximations

Surface Second-Harmonic Generation (S-SHG) experiments provide a unique approach to probe interfaces. One important issue for S-SHG is how to interpret the S-SHG intensities at the molecular level. Established frameworks commonly assume that each molecule emits light according to an average molecular hyperpolarizability tensor β(-2ω,ω,ω). However, for water molecules, this first hyperpolarizability is known to be extremely sensitive to their environment. We have investigated the molecular first hyperpolarizability of water molecules within the liquid-vapor interface, using a quantum description with explicit, inhomogeneous electrostatic embedding. The resulting average molecular first hyperpolarizability tensor depends on the distance relative to the interface, and it practically respects the Kleinman symmetry everywhere in the liquid. Within this numerical approach, based on the dipolar approximation, the water layer contributing to the Surface Second Harmonic Generation (S-SHG) intensity is less than a nanometer. The results reported here question standard interpretations based on a single, averaged hyperpolarizability for all molecules at the interface. Not only the molecular first hyperpolarizability tensor significantly depends on the distance relative to the interface, but it is also correlated to the molecular orientation. Such hyperpolarizability fluctuations may impact the S-SHG intensity emitted by an aqueous interface.

Efficient Quantum Vibrational Spectroscopy of Water with High-Order Path Integrals: From Bulk to Interfaces

Vibrational spectroscopy is key in probing the interplay between the structure and dynamics of aqueous systems. To map different regions of experimental spectra to the microscopic structure of a system, it is important to combine them with first-principles atomistic simulations that incorporate the quantum nature of nuclei. Here we show that the large cost of calculating the quantum vibrational spectra of aqueous systems can be dramatically reduced compared with standard path integral methods by using approximate quantum dynamics based on high-order path integrals. Together with state-of-the-art machine-learned electronic properties, our approach gives an excellent description not only of the infrared and Raman spectra of bulk water but also of the 2D correlation and the more challenging sum-frequency generation spectra of the water-air interface. This paves the way for understanding complex interfaces such as water encapsulated between or in contact with hydrophobic and hydrophilic materials through robust and inexpensive surface-sensitive and multidimensional spectra with first-principles accuracy.

Ordered Water Layer on the Macroscopically Hydrophobic Fluorinated Polymer Surface and Its Ultrafast Vibrational Dynamics

Hydrophobic-like water monolayers have been predicted at the metal and some polar surfaces by theoretical simulations. However, direct experimental evidence for the presence of this water layer at surfaces, particularly at biomolecule and polymer surfaces, is yet to be validated at room temperature. Here we observe experimentally that an ordered molecular water layer is present at the hydrophobic fluorinated polymer such as polytetrafluoroethylene (PTFE) surface by using sum frequency generation vibrational spectroscopy. The macroscopic hydrophobicity of PTFE surface is actually hydrophilic at the molecular level. The macroscopically hydrophobic character of PTFE is indeed resulting from the hydrophobicity of the ordered two-dimension (2D) water layer, in which cyclic water tetramer structure is found. The water layer at humidity of ≤40% has a vibrational relaxation time of 550 ± 60 fs. The vibrational relaxation time in the frequency range of 3200-3400 cm^-1 shows remarkable difference from the interfacial water at the air/H₂O interface and the lipid/H₂O interface. No discernible frequency dependence of the vibrational relaxation time is observed, indicating the homogeneous dynamics of OH groups in the water layer. These insights into the water layer at the macroscopically hydrophobic surface may contribute to a better understanding of the hydrophobic interaction and interfacial water dynamics.

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decomposition (SVD) analysis, the vibrational relaxation (T₁) times of the free OH at the neat H₂O and isotopically-diluted water interfaces are determined to be 0.87 ± 0.06 ps (neat H₂O), 0.84 ± 0.09 ps (H₂O/HOD/D₂O = 1/2/1), and 0.88 ± 0.16 ps (H₂O/HOD/D₂O = 1/8/16). The absence of the isotope effect on the T₁ time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water molecules. The determined sub-picosecond T₁ time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water molecule.

Water’s hydrogen-bond network is truncated at hydrophobic interfaces and the dynamics of the resulting free OH groups is not well understood. The authors experimentally show that the main vibrational relaxation mechanism for free OH at the air-water interface is a diffusive molecular reorientation, rather than intramolecular energy transfer.

Chemical Imaging of Surfaces with Sum Frequency Generation Vibrational Spectroscopy

Surface chemistry is a key area of study in the chemical sciences, and many system properties are dominated by the chemistry at the interface between two bulk media. While the interface may have a large influence on the system behavior, there are relatively few molecules at the interface compared to the bulk; thus, probing their unique properties has become a specialized field in physical chemistry. In addition to the heterogeneous phase chemistry, surfaces also present spatial heterogeneity (Chemistry in Two Dimensions). This 2D chemistry affects the properties as much as the heterogeneous phases. If we consider the Cartesian z-axis as defining the dimension across the interface between the two bulk phases, then the x-y plane is the 2D region of the surface. We might even consider that the majority of surface chemistry has been concerned with this z-dimension, i.e., surface structure, partition excess, thermodynamics, etc. relative to the bulk, where the 2D distribution was only considered on average. This treatment is understandable since few techniques provide the spatial and chemical resolution needed to deduce the effects of 2D heterogeneity on the surface properties. It is desirable to use an all-optical technique for interface studies because the optical methods provide the chemical specificity through spectroscopy. Also, the use of second-order spectroscopy is typically surface-sensitive without background subtractions or enhancement mechanisms that could limit the range of systems to be investigated.In this Account, the development and selected results of sum frequency generation microscopy and its contributions to the surface chemistry are presented. Sum frequency generation (SFG) provides a unique probe for studying surface chemistry in ambient conditions with surface specificity. SFG provides image contrast based on multiple-chemically important-mechanisms such as chemical functional groups, molecular orientation, surface concentration, molecular conformation, local electric fields, among others. To understand the spatial distribution of heterogeneous chemistry, multiple microscopy methods have been developed which utilize the SFG process to yield spatial information with chemical sensitivity. These spectroscopic-microscopies come with unique advantages as well as challenges. Multiple solutions have been developed in this field to overcome the challenges and improve the advantages. In this Account, some of the leading SFG surface microscopies for surface studies are introduced. Initially, direct imaging of the SFG signal onto a CCD camera provided spatially and spectrally resolved imaging of monolayers on surfaces. However, to speed up the imaging process, the technique of compressive sensing was applied to SFG imaging. Most recently the use of machine learning methods and target factor analysis have improved the quality and acquisition speed of SFG images.

Liquid/Vapor Interface of Dimethyl Carbonate-Methanol Binary Mixtures Investigated by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulation

In the present work, the dimethyl carbonate (DMC)-methanol binary mixture was used as a benchmark system to study the molecular structures of the liquid/vapor interface of organic-organic mixtures by sum frequency generation vibrational spectroscopy (SFG-VS) and molecular dynamics (MD) simulations. It was discovered that both the methanol and DMC molecules are anisotropically oriented at the surface, yielding strong SFG-VS signals in the C-H stretching frequency range for both molecules. The detailed analyses of the spectroscopic and MD data reveal that the increase of the methanol bulk concentrations reduces the orientational order of the methyl groups for both the interfacial DMC and methanol molecules but does not significantly affect the orientations of the carbonyl group in DMC. Moreover, no obvious correlations were found between the room-temperature orientations of the surface molecules and the azeotropic mole fraction. The present work paves the road for future investigations on the molecular structures of the liquid/vapor interfaces of other organic-organic mixtures, especially those that are important in industrial separations.

Specific counter-cation effect on the molecular orientation of thiocyanate anions at the aqueous solution interface

Understanding the interfacial structure of aqueous electrolyte solutions is important and relevant to a wide range of systems, ranging from atmospheric aerosols to electrochemistry, and biological environments. Though significant efforts have been made to unravel the interfacial structure of water molecules, the structure and dynamics of ions at the interface have not yet been fully elucidated. Here, the interfacial structure of the aqueous solution was investigated directly by monitoring the thiocyanate (SCN-) anions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The molecular orientation of the SCN- anions and their adsorption behavior at the air/water interface were systematically determined by quantitative polarization analysis. The transition dipole of the CN stretching of the SCN- anion is oriented around 44° from the surface normal of the NaSCN aqueous solution surface and remained unchanged with the bulk concentration varying from 1 mol kg-1 to 13 mol kg-1. The free energy of adsorption of SCN- anions at the air/water interface was determined to be -1.53 ± 0.04 kcal mol-1. Furthermore, a new SFG peak positioned at 2080 cm-1 in the ppp polarization combination was observed at the air/15.0 mol kg-1 NaSCN aqueous solution interface for the first time. Concentration-dependent SFG analysis and density functional theory (DFT) calculation further revealed that the SCN- anions form an ion clustering structure at the air/water interface. The subtle and specific Na+ and K+ counter-cation effects on the interfacial structure of the SCN- anions at the aqueous solution interface were also observed, which showed that ion cooperativity plays an important role in affecting the interfacial structure of ions at the air/water interface. The results are expected to yield significant insights into the understanding of the structure of aqueous solution surfaces and the molecular level mechanism of the cationic Hofmeister effect.

Molecular Structure and Modeling of Water-Air and Ice-Air Interfaces Monitored by Sum-Frequency Generation

From a glass of water to glaciers in Antarctica, water-air and ice-air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water-air and ice-air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces.

Determination of Surface Preference Using Heterospectral Surface-Bulk Correlation Spectroscopy

We outline a method by which the surface preference of a species in a multicomponent mixture may be obtained using surface-specific visible-infrared sum frequency generation (SFG) spectroscopy combined with bulk infrared absorption and/or Raman data. In general, the problem is complicated by the fact that the SFG signal is a function of both the surface coverage and the structure of the molecules. Two-dimensional correlation analysis can be used to reveal which spectral features are changing synchronously, that is, in phase with each other, and which ones are evolving in a manner that is phase-shifted by 90° (asynchronous correlation) as a function of the bulk composition. We provide a framework for determining the surface preference from the correlations between the vibrational modes in the SFG spectra and between the modes from SFG and bulk infrared and/or Raman spectra. When compared to the equivalent analysis performed using the SFG spectra alone, this method can be used with the data obtained using a single-beam polarization and in congested spectral regions where fitting to isolate the behavior of individual vibrational modes is not robust.

Sum frequency generation, calculation of absolute intensities, comparison with experiments, and two-field relaxation-based derivation

The experimental sum frequency generation (SFG) spectrum is the response to an infrared pulse and a visible pulse and is a highly surface-sensitive technique. We treat the surface dangling OH bonds at the air/water interface and focus on the absolute SFG intensities for the resonant terms, a focus that permits insight into the consequences of some approximations. For the polarization combinations, the calculated linewidths for the water interface dangling OH SFG band at 3,700 [Formula: see text] are, as usual, too large, because of the customary neglect of motional narrowing. The integrated spectrum is used to circumvent this problem and justified here using a Kubo-like formalism and theoretical integrated band intensities rather than peak intensities. Only relative SFG intensities are usually reported. The absolute integrated SFG intensities for three polarization combinations for sum frequency, visible, and infrared beams are computed. We use molecular dynamics and the dipole and the polarizability matrix elements obtained from infrared and Raman studies of [Formula: see text]O vapor. The theoretical expressions for two of the absolute susceptibilities contain only a single term and agree with experiment to about a factor of 1.3, with no adjustable parameters. The Fresnel factors are included in that comparison. One of the susceptibilities contains instead four positive and negative terms and agrees less well. The expression for the SFG correlation function is normally derived from a statistical mechanical formulation using a time-evolving density matrix. We show how a derivation based on a two-field relaxation leads to the same final result.

Nature of Excess Hydrated Proton at the Water-Air Interface

Understanding the interfacial molecular structure of acidic aqueous solutions is important in the context of, e.g., atmospheric chemistry, biophysics, and electrochemistry. The hydration of the interfacial proton is necessarily different from that in the bulk, given the lower effective density of water at the interface, but has not yet been elucidated. Here, using surface-specific vibrational spectroscopy, we probe the response of interfacial protons at the water-air interface and reveal the interfacial proton continuum. Combined with spectral calculations based on ab initio molecular dynamics simulations, the proton at the water-air interface is shown to be well-hydrated, despite the limited availability of hydration water, with both Eigen and Zundel structures coexisting at the interface. Notwithstanding the interfacial hydrated proton exhibiting bulk-like structures, a substantial interfacial stabilization by -1.3 ± 0.2 kcal/mol is observed experimentally, in good agreement with our free energy calculations. The surface propensity of the proton can be attributed to the interaction between the hydrated proton and its counterion.

Hydrogen bonding and molecular orientations across thin water films on sapphire

HYPOTHESIS

Water vapor binding to metal oxide surfaces produces thin water films with properties controlled by interactions with surface hydroxo sites. Hydrogen bonding populations vary across films and induce different molecular orientations than at the surface of liquid water. Identifying these differences can open possibilities for tailoring film-mediated catalytic reactions by choice of the supporting metal oxide substrate.

EXPERIMENTS

The (0001) face of a single sapphire (α-Al₂O₃) sample exposed to water vapor and the surface of liquid water were probed by polarization dependent Sum Frequency Generation-Vibration Spectroscopy (SFG-VS). Molecular dynamics (MD) provided insight into the hydrogen bond populations and molecular orientations across films and liquid water.

FINDINGS

SFG-VS revealed a submonolayer film on sapphire exposed to 43% relative humidity (R.H.), and a multilayer film at 78% R.H. Polarization dependent SFG-VS spectra showed that median tilt angles of free OH bonds on the top of films are at ∼43° from the normal of the (0001) face but at 38° on neat liquid water. These values align with MD simulations, which also show that up to 36% of all OH bonds on films are free. This offers new means for understanding how interfacial reactions on sapphire-supported water films could contrast with those involving liquid water.

Structure and Dynamics of Water at the Water-Air Interface Using First-Principles Molecular Dynamics Simulations. II. NonLocal vs Empirical van der Waals Corrections

van der Waals (vdW) correction schemes have been recognized to be essential for an accurate description of liquid water in first-principles molecular dynamics simulation. The description of the structure and dynamics of water is governed by the type of the vdW corrections. So far, two vdW correction schemes have been often used: empirical vdW corrections and nonlocal vdW corrections. In this paper, we assess the influence of the empirical vs nonlocal vdW correction schemes on the structure and dynamics of water at the water–air interface. Since the structure of water at the water–air interface is established by a delicate balance of hydrogen bond formation and breaking, the simulation at the water–air interface provides a unique platform to testify as to the heterogeneous interaction of water. We used the metrics [Ohto et al. J. Chem. Theory Comput., 2019, 15, 595−60230468702] which are directly connected with the sum-frequency generation spectroscopic measurement. We find that the overall performance of nonlocal vdW methods is either similar or worse compared to the empirical vdW methods. We also investigated the performance of the optB88-DRSLL functional, which showed slightly less accuracy than the revPBE-D3 method. We conclude that the revPBE-D3 method shows the best performance for describing the interfacial water.

Direct Observation of the Orientational Anisotropy of Buried Hydroxyl Groups inside Muscovite Mica

Muscovite mica (001) is a widely used model surface for controlling molecular assembly and a common substrate for environmental adsorption processes. The mica (001) surface displays near-trigonal symmetry, but many molecular adsorbates-including water-exhibit unequal probabilities of alignment along its three nominally equivalent lattice directions. Buried hydroxyl groups within the muscovite structure are speculated to be responsible, but direct evidence is lacking. Here, we utilize vibrational sum frequency generation spectroscopy (vSFG) to characterize the orientation and hydrogen-bonding environment of near-surface hydroxyls inside mica. Multiple distinct peaks are detected in the O-H stretch region, which we attribute to Si/Al substitution in the SiO₄ tetrahedron and K⁺ ion adsorption above the hydroxyls based on density functional theory simulations. Our findings demonstrate that vSFG can identify the absolute orientation of -OH groups and, hence, the surface termination at a mica surface, providing a means to investigate how -OH groups influence molecular adsorption and better understand mica stacking-sequences and physical behavior.

Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential

The orientational distribution of free O-H (O-D) groups at the H_{2}O- (D_{2}O-)air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ∼63°, substantially larger than previous estimates of 30°-40°. This discrepancy can be traced to erroneously assumed Gaussian or stepwise orientational distributions of free O-H groups. Instead, the MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface.

Neat Water-Vapor Interface: Proton Continuum and the Nonresonant Background

Whether the surface of neat water is "acidic" or "basic" remains an active and controversial field of research. Most of the experimental evidence supporting the preferential adsorption of H₃O⁺ ions stems from nonlinear optical spectroscopy methods typically carried out at extreme pH conditions (pH < 1). Here, we use vibrational sum frequency spectroscopy (VSFS) to target the "proton continuum", an unexplored frequency range characteristic of hydrated protons and hydroxide ions. The VSFS spectra of neat water show a broad and nonzero signal intensity between 1700 and 3000 cm^-1 in the three different polarization combinations examined. By comparing the SF response of water with that from dilute HCl and NaOH aqueous solutions, we conclude the intensity does not originate from either adsorbed H₃O⁺ or OH^- ions. Contributions from the nonresonant background are then critically considered by comparing the experimental results with many-body molecular dynamics (MB-MD) simulated spectra.

Vibrational spectroscopic study of pH dependent solvation at a Ge(100)-water interface during an electrode potential triggered surface termination transition

The charge-dependent structure of interfacial water at the n-Ge(100)-aqueous perchlorate interface was studied by controlling the electrode potential. Specifically, a joint attenuated total reflection infrared spectroscopy and electrochemical experiment was used in 0.1M NaClO₄ at pH ≈ 1-10. The germanium surface transformation to an H-terminated surface followed the thermodynamic Nernstian pH dependence and was observed throughout the entire pH range. A singular value decomposition-based spectra deconvolution technique coupled to a sigmoidal transition model for the potential dependence of the main components in the spectra shows the surface transformation to be a two-stage process. The first stage was observed together with the first appearance of Ge-H stretching modes in the spectra and is attributed to the formation of a mixed surface termination. This transition was reversible. The second stage occurs at potentials ≈0.1-0.3 V negative of the first one, shows a hysteresis in potential, and is attributed to the formation of a surface with maximum Ge-H coverage. During the surface transformation, the surface becomes hydrophobic, and an effective desolvation layer, a "hydrophobic gap," developed with a thickness ≈1-3 Å. The largest thickness was observed near neutral pH. Interfacial water IR spectra show a loss of strongly hydrogen-bound water molecules compared to bulk water after the surface transformation, and the appearance of "free," non-hydrogen bound OH groups, throughout the entire pH range. Near neutral pH at negative electrode potentials, large changes at wavenumbers below 1000 cm^-1 were observed. Librational modes of water contribute to the observed changes, indicating large changes in the water structure.

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique.

Hydrogen bonding and vibrational energy relaxation of interfacial water: A full DFT molecular dynamics simulation

The air-water interface has been a subject of extensive theoretical and experimental studies due to its ubiquity in nature and its importance as a model system for aqueous hydrophobic interfaces. We report on the structure and vibrational energy transfer dynamics of this interfacial water system studied with equilibrium and non-equilibrium molecular dynamics simulations employing a density functional theory -based description of the system and the kinetic energy spectral density analysis. The interfacial water molecules are found to make fewer and weaker hydrogen (H)-bonds on average compared to those in the bulk. We also find that (i) the H-bonded OH groups conjugate to the free OH exhibit rather low vibrational frequencies (3000-3500 cm^-1); (ii) the presence of a significant fraction (>10%) of free and randomly oriented water molecules at the interface ("labile water"), neither of whose OH groups are strong H-bond donors; (iii) the inertial rotation of free OH groups, especially from the labile water, contribute to the population decay of excited free OH groups with comparable rate and magnitude as intramolecular energy transfer between the OH groups. These results suggest that the labile water, which might not be easily detectable by the conventional vibrational sum frequency generation method, plays an important role in the surface water dynamics.

Cooperative Effects of Zwitterionic-Ionic Surfactant Mixtures on the Interfacial Water Structure Revealed by Sum Frequency Generation Vibrational Spectroscopy

Cooperative effects of a series of equimolar binary zwitterionic-ionic surfactant mixtures on the interfacial water structure at the air-water interfaces have been studied by sum frequency generation vibrational spectroscopy (SFG-VS). For zwitterionic surfactant palmityl sulfobetaine (SNC₁₆), anionic surfactant sodium hexadecyl sulfate (SHS), and cationic surfactant cetyltrimethylammonium bromide (CTAB) with the same length of alkyl chain, significantly enhanced ordering of interfacial water molecules was observed for the zwitterionic-anionic surfactant mixtures SNC₁₆-SHS, indicating that SNC₁₆ interacts more strongly with SHS than with CTAB because of the strong headgroup-headgroup electrostatic attraction for SNC₁₆-SHS. Meanwhile, the SFG amplitude ratio of methyl and methylene symmetric stretching modes was used to verify the stronger interaction between SNC₁₆ and SHS. The conformational order indicator increased from 0.64 for SNC₁₆ to 7.17 for SNC₁₆-SHS but only 0.94 for SNC₁₆-CTAB. In addition, another anionic surfactant sodium dodecyl sulfate (SDS) was introduced to study the influence of chain-chain interaction. Decreased SFG amplitude of interfacial water molecules for SNC₁₆-SDS was observed. Therefore, both the headgroup-headgroup electrostatic interaction and chain-chain van der Waals attractive interaction of the surfactants play an important role in enhancing the ordering of interfacial water molecules. The results provided experimental and theoretical bases for practical applications of the surfactants.

The structure of a lanthanide complex at an extractant/water interface studied using heterodyne-detected vibrational sum frequency generation

Eu³⁺ at an extractant/water interface is bound to extractants from the upper side and to water molecules from the lower side, and forms a unique interfacial complex.