Response to “Comment on ‘Phase-sensitive sum frequency vibrational spectroscopic study of air/water interfaces: H2O, D2O, and diluted isotopic mixtures’” [J. Chem. Phys. 152, 237101 (2020)]

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.

Origin of Strong Hydrogen Bonding and Preferred Orientation of Water at Uncharged Polyethylene Glycol Polymer/Water Interface

Polyethylene glycol (PEG), a water-soluble non-ionic polymer, finds diverse applications from Li-ion batteries to drug delivery. The effectiveness of PEG in these contexts hinges on water's behavior at PEG/water interfaces. Employing heterodyne-detected vibrational sum frequency generation and Raman spectroscopy along with a novel analytical approach, termed difference spectroscopy with simultaneous curve-fitting analysis, we observed that water exhibits both "hydrogen-up" and "hydrogen-down" orientations at PEG(≥400u)/water interfaces. As the molar mass of PEG increases, the contribution of the strongly hydrogen-bonded and H-up-oriented water rises. We propose that the PEG-affected interfacial water originates from the asymmetrical hydration of the surface-adsorbed PEG, as evidenced by the resemblance between the water spectra in the hydration shell of PEG and those at the PEG/water interface. These findings elucidate the molecular mechanism underlying PEG's catalytic role in water splitting at membrane interfaces.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T₁ = 0.13 ps, which is slightly larger than that in the bulk (T₁ = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T₁ = 0.74 ps in the bulk decreases to T₁ = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Progress in phase-sensitive sum frequency generation spectroscopy

Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the molecular level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aqueous interfaces that were once left open by conventional SFG. These problems and their solutions are good examples to demonstrate why PS-SFG is essential. In addition, we briefly note a few terminology issues related with PS-SFG to avoid confusion.

Kosmotropic Electrolyte (Na2CO3, NaF) Perturbs the Air/Water Interface through Anion Hydration Shell without Forming a Well-Defined Electric Double Layer

The ion-driven electric double layer (EDL) and the structural transformation of interfacial water are implicated in unusual reaction kinetics at the air/water interface. By combining heterodyne-detected vibrational sum frequency generation (HD-VSFG) with differential spectroscopy involving simultaneous curve fitting (DS-SCF) analysis, we retrieve electrolyte (Na₂CO₃ and NaF)-correlated OH-stretch spectra of water at the air/water interface. Vibrational mapping of the perturbed interfacial water with the hydration shell spectra (obtained by DS-SCF analysis of Raman spectra) of the corresponding anion discloses that the kosmotropic electrolytes do not form well-defined EDL at the air/water interface. Instead, the interfacial water forms a stronger hydrogen-bond with the surface-expelled anions (CO₃^2- and F^-) and becomes more inhomogeneous than the pristine air/water interface. Together, the results reveal that the perturbation of interfacial water by the kosmotropic electrolyte is a "local phenomenon" confined within the hydration shell of the surface-expelled anion.

Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the maximum entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. Deconvolution reveals that at very low ionic strength, the surface water structure is similar to that of a neutral silica surface near the point-of-zero-charge with waters in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addition of salt, significant restructuring of water is observed, leading to a net decrease in order at the surface.

"Breaking" and "Making" of Water Structure at the Air/Water-Electrolyte (NaXO3; X = Cl, Br, I) Interface

The prevalence of ions at the aqueous interface has been widely recognized, but their effect on the structure of interfacial water (e.g., hydrogen (H)-bonding) remains enigmatic. Using heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman difference spectroscopy with simultaneous curve fitting (DS-SCF) analysis, we show that the ion-induced perturbations of H-bonding at the air/water interface and in the bulk water are strongly correlated. Specifically, the structure-breaking anions such as ClO₃^- decrease the average H-bonding of water at the air/water interface, as it does to the water in its hydration shell in the bulk. The structure-making anion of the same series (IO₃^-) does exactly the reverse. None of the electrolytes (NaXO₃; X = Cl, Br, I) form well-defined electric double layers that significantly increase or reverse the hydrogen-down (H-down) orientation of water at the air/water interface. These results provide a unified picture of specific anion effect at the air/water interface and in the bulk water.

Interaction of α-Synuclein with Phospholipids and the Associated Restructuring of Interfacial Lipid Water: An Interface-Selective Vibrational Spectroscopic Study

Interaction of α-Synuclein (αS) with biological lipids is crucial for the onset of its fibrillation at the cell membrane/water interface. Probed herein is the interaction of αS with membrane-mimicking lipid monolayer/water interfaces. The results depict that αS interacts negligibly with zwitterionic lipids, but strongly affects the pristine air/water and charged lipid/water interfaces by perturbing the structure and orientation of the interfacial water. The net negative αS (-9 in bulk water; pH 7.4) reorients the water as hydrogen-up (H-up) at the air/water interface, and electrostatically interacts with positively charged lipids, making the interface nearly net neutral. αS also interacts with negatively charged lipids: the net H-up orientation of the interfacial water decreases at the anionic lipid/water interface, revealing a domain-specific interaction of net negative αS with the negatively charged lipids at the membrane surface.

Adsorption of Iodine Species (I3-, I-, and IO3-) at the Nuclear Paint Monolayer-Water Interface and Its Relevance to a Nuclear Accident Scenario

Following a nuclear accident, radioactive iodine causes great concern to public health and safety. Organic iodide, because of its ability to escape reactor containment building and high environmental mobility, constitutes a predominant fraction of airborne radioiodine at places far away from the accident site. As the iodine released from a reactor core is inorganic iodine, it is vital to understand the mechanism of organic iodide formation inside reactor containment. In this context, we investigated the surface prevalence and adsorption of various inorganic iodines, I^-, I₃^-, and IO₃^-, at a nuclear paint (used in nuclear installations) monolayer-water interface, mimicking the painted inner walls of an accident-affected containment building that are exposed to the iodine-containing condensed water layer. Vibrational sum frequency generation (VSFG) measurements in the OH and CH stretch regions reveal that the paint-water interface changes its charge characteristics with the pH of the water that affects the degree of interaction with the iodine species. At the acidic condition (bulk pH < 7), the paint becomes positively charged and strongly adsorbs the negatively charged iodine species dissolved in the aqueous phase, whereas at the alkaline condition (bulk pH > 9.5), the paint becomes net neutral and weakly interacts with the iodine species. These interactions change the conformation of the paint such that its hydrophobic alkyl groups orient increasingly away from the aqueous phase. The order of adsorption increases as IO₃^- < I^- < I₃^- for the different iodine species studied.