Ions Tune Interfacial Water Structure and Modulate Hydrophobic Interactions at Silica Surfaces

Chemistry at Oxide/Water Interfaces: The Role of Interfacial Water

Oxide-water interfaces host many chemical reactions in nature and industry. There, reaction free energies markedly differ from those of the bulk. While we can experimentally and theoretically measure these changes, we are often unable to address the fundamental question: what catalyzes these reactions? Recent studies suggest that surface and electrostatic contributions are an insufficient answer. The interface modulates chemistry in subtle ways. Revealing them is essential to understanding interfacial reactions, hence improving industrial processes. Here, we introduce a thermodynamic approach combined with cavitation free energy analysis to disentangle the driving forces at play. We find that water dictates chemistry via large variations of cavitation free energies across the interface. The resulting driving forces are both large enough to determine reaction output and highly tunable by adjusting interface composition, as showcased for silica-water interfaces. These findings shift the focus from common interpretations based on surface and electrostatics and open exciting perspectives for regulating interfacial chemistry.

Wetting of a Dynamically Patterned Surface Is a Time-Dependent Matter

In nature and many technological applications, aqueous solutions are in contact with patterned surfaces, which are dynamic over time scales spanning from ps to μs. For instance, in biology, exposed polar and apolar residues of biomolecules form a pattern, which fluctuates in time due to side chain and conformational motions. At metal/and oxide/water interfaces, the pattern is formed by surface topmost atoms, and fluctuations are due to, e.g., local surface polarization and rearrangements in the adsorbed water layer. All these dynamics have the potential to influence key processes such as wetting, energy relaxation, and biological function. Yet, their impact on the water H-bond network remains often elusive. Here, we leverage molecular dynamics to address this fundamental question at a self-assembled monolayer (SAM)/water interface, where ns dynamics is induced by frustrating SAM-water interactions via methylation of the terminal -OH groups of poly(ethylene glycol) (PEG) chains. We find that surface dynamics couples to the water H-bond network, inducing a response on the same ns time scale. This leads to time fluctuations of local wetting, oscillating from hydrophobic to hydrophilic environments. Our results suggest that rather than average properties, it is the local─ both in time and space─ solvation that determines the chemical-physical properties of dynamically patterned surfaces in water.

Structure, Properties, and Applications of Silica Nanoparticles: Recent Theoretical Modeling Advances, Challenges, and Future Directions

Silica nanoparticles (SNPs), one of the most widely researched materials in modern science, are now commonly exploited in surface coatings, biomedicine, catalysis, and engineering of novel self-assembling materials. Theoretical approaches are invaluable to enhancing fundamental understanding of SNP properties and behavior. Tremendous research attention is dedicated to modeling silica structure, the silica-water interface, and functionalization of silica surfaces for tailored applications. In this review, the range of theoretical methodologies are discussed that have been employed to model bare silica and functionalized silica. The evolution of silica modeling approaches is detailed, including classical, quantum mechanical, and hybrid methods and highlight in particular the last decade of theoretical simulation advances. It is started with discussing investigations of bare silica systems, focusing on the fundamental interactions at the silica-water interface, following with a comprehensively review of the modeling studies that examine the interaction of silica with functional ligands, peptides, ions, surfactants, polymers, and carbonaceous species. The review is concluded with the perspective on existing challenges in the field and promising future directions that will further enhance the utility and importance of the theoretical approaches in guiding the rational design of SNPs for applications in engineering and biomedicine.

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.

A simplified method for theoretical sum frequency generation spectroscopy calculation and interpretation: The "pop model"

Existing methods to compute theoretical spectra are restricted to the use of time-correlation functions evaluated from accurate atomistic molecular dynamics simulations, often at the ab initio level. The molecular interpretation of the computed spectra requires additional steps to deconvolve the spectroscopic contributions from local water and surface structural populations at the interface. The lack of a standard procedure to do this often hampers rationalization. To overcome these challenges, we rewrite the equations for spectra calculation into a sum of partial contributions from interfacial populations, weighted by their abundance at the interface. We show that SFG signatures from each population can be parameterized into a minimum dataset of reference partial spectra. Accurate spectra can then be predicted by just evaluating the statistics of interfacial populations, which can be done even with force field simulations as well as with analytic models. This approach broadens the range of simulation techniques from which theoretical spectra can be calculated, opening toward non-atomistic and Monte Carlo simulation approaches. Most notably, it allows constructing accurate theoretical spectra for interfacial conditions that cannot even be simulated, as we demonstrate for the pH-dependent SFG spectra of silica/water interfaces.

Unconventional structural evolution of an oxide surface in water unveiled by in situ sum-frequency spectroscopy

Oxide-water interfaces host a wide range of important reactions in nature and modern industrial applications; however, accurate knowledge about these interfaces is still lacking at the molecular level owing to difficulties in accessing buried oxide surfaces. Here we report an experimental scheme enabling in situ sum-frequency vibrational spectroscopy of oxide surfaces in liquid water. Application to the silica-water interface revealed the emergence of unexpected surface reaction pathways with water. With ab initio molecular dynamics and metadynamics simulations, we uncovered a surface reconstruction, triggered by deprotonation of surface hydroxylated groups, that led to unconventional five-coordinated silicon species. The results help demystify the multimodal chemistry of aqueous silica discovered decades ago, bringing in fresh information that modifies the current understanding. Our study will provide new opportunities for future in-depth physical and chemical characterizations of other oxide-water interfaces.

Separating Hofmeister Trends in Stern and Diffuse Layers at a Charged Interface

Understanding the role of pH and ions on the electrical double layer (EDL) at charged mineral oxide/aqueous interfaces remains crucial in modeling environmental and industrial processes. Yet the simultaneous contribution of pH and specific ion effects (SIEs) on the different layers of the EDL remains unknown. Here, we utilize zeta potential measurements, vibrational sum frequency generation, and the maximum entropy method to ascertain the detailed structure of the Stern and diffuse regions of the EDL at the silica/water interface with varying pH values for different alkali chlorides. Both at pH 2, when the surface is nearly neutral, and at pH 12, when the surface is highly charged, we observe that Li+ and Na+ disrupt while Cs+ enhances existing water structures within the Stern layer. Moreover, the SIE trends for the diffuse and Stern layers are opposite to one another at pH 2 (in the amount of ordered water) and at pH 12 (in the amount of net oriented water). Finally, we observe an inversion in Hofmeister (SIE) trends at low and high pH in the zeta that impacts the diffuse layer structure. These results indicate that SIEs play critical yet separable roles in governing both the electrostatic and water-structuring capabilities of the EDL.

Adsorption of Staphylococcus aureus biofilm associated compounds on silica probed with molecular dynamics simulations

Staphylococcus aureus (S. aureus) is a potentially pathogenic bacterium that commonly colonizes surfaces through the formation of biofilms. Silica glass is a common material in the built environment, especially in laboratory and medical spaces. The chemical and physical mechanisms by which S. aureus initially adheres to surfaces are unclear. In this study, the adsorption of several S. aureus biofilm associated compounds on silica is probed using molecular dynamics simulations. Model compounds containing a phosphorylated backbone, N-acetylglucosamine (GlcNAc), or D-alanine (D-Ala) were simulated across a range of pH. GlcNAc adsorption is unfavorable and insensitive to pH. D-Ala adsorption is unfavorable across the range of tested pH. Phosphorylated backbone adsorption is unfavorable at low pH but favorable at high pH. Adsorbate titration and solution salt concentration were probed to establish effects of molecular charge and charge screening. Hydrogen bonding between compounds and the silica surface is a key factor for stronger adsorption. The findings of this study are important for the rational design of improved silica surfaces through chemical functionalization or through the application of optimal chemical disinfectants that discourage the initial stages of biofilm growth.

Bridging molecular-scale interfacial science with continuum-scale models

Solid-water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid-water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

Lead (II) ions enable the ion-specific effects of monovalent anions on the molecular structure and interactions at silica/aqueous interfaces

HYPOTHESIS

The adsorption of heavy metal ions such as Pb(II) onto negatively charged minerals such as silica is expected to alter the structure and the interactions at the silica/aqueous interfaces. Besides the solution pH, the inner-sphere sorption of Pb(II) is expected to regulate the surface charge/potential, hypothesized to control the actions of monovalent anions in the aqueous environment. These complex pictures can be probed directly using surface-sensitive sum-frequency generation (SFG) spectroscopy.

EXPERIMENTS

The pH-dependent water structure within the double layer at silica/aqueous interfaces under the influence of different ions was examined using SFG. The recorded SFG spectra were deconvoluted into the Stern layer (SL) and diffuse layer (DL) using the maximum entropy method in conjunction with the electrical double-layer theory.

FINDINGS

Standalone monovalent sodium salts do not exhibit ion-specific effects on the silica/aqueous interfaces. However, the mixture of Pb(II) species and each of these salts display profound ion-specific effects on the structure of silica/aqueous interfaces, indicating the role of Pb(II) as an enabler of the ion-specificity of the investigated monovalent anions. The interesting effect arises from a complex interplay between the physical processes (i.e., electrostatic interactions, screening effects, etc.) and chemical processes such as the hydrolysis of Pb(II) ions, ion complexation, protonation and deprotonation of the surface silanol group.

Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces

At charged aqueous interfaces, the second-order nonlinear optical response originates from water molecules within the diffuse part of the electrical double layer, which are ordered by the surface field and from water that additionally experiences chemical and physical interactions with the surface in the Stern layer. These two environments can either reinforce or diminish the overall signal and can be disentangled by varying the coherence length of their interaction with external laser fields. Here, we demonstrate a method in which the angle of incidence is varied to afford a significant change in the coherence length. When this technique was applied to the silica-water interface, it was observed that water molecules in the Stern and diffuse layers direct their hydrogen atoms toward the mineral surface at a low ionic strength and neutral pH. A decrease in the signal with increasing ionic strength is attributed to hydrated cation adsorption that competes with free water for deprotonated silanol sites.

Water Adsorption Isotherm and Surface Conductivity of Boroaluminosilicate Glasses

The surface resistivity of boroaluminosilicate display glasses, which may affect the downstream display panel manufacturing, varies with the relative humidity (RH) of the environment, but the origin of this RH dependence has not been well understood. We have measured the water adsorption behavior on Corning Eagle XG (Glass-E) and Lotus NXT (Glass-L) glass panels using Brewster angle transmission infrared spectroscopy. The IR spectra of adsorbed water were analyzed to obtain the effective thickness of adsorbed water, the distribution of hydrogen-bonding interactions among the adsorbed water molecules, and the isosteric heat of water adsorption. These characteristics were compared with the electrical conductivity (inverse of resistivity) of these two glasses [Appl. Surf. Sci. 2015, 356, 1189]. This comparison revealed the correlation between the conductivity and the water layer structure, which could explain the surface resistivity difference between Glass-E and Glass-L as a function of RH. This study also disputed the previous hypothesis that the water adsorption isotherm would be governed by the areal density of the surface hydroxyl group; instead, it suggested that the network modifier ions may also play a critical role, especially in the intermediate RH regime.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15β protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.

Direct Experimental Observations of Ion Distributions during Overcharging at the Muscovite-Water Interface by Adsorption of Rb+ and Halides (Cl- , Br- , I- ) at High Salinity

Classical electric double layer (EDL) models have been widely used to describe ion distributions at charged solid-water interfaces in dilute electrolytes. However, the chemistry of EDLs remains poorly constrained at high ionic strength where ion-ion correlations control non-classical behavior such as overcharging, i. e., the accumulation of counter-ions in amounts exceeding the substrate's surface charge. Here, we provide direct experimental observations of correlated cation and anion distributions adsorbed at the muscovite (001)-aqueous electrolyte interface as a function of dissolved RbBr concentration ([RbBr]=0.01-5.8 M) using resonant anomalous X-ray reflectivity. Our results show alternating cation-anion layers in the EDL when [RbBr]≳100 mM, whose spatial extension (i. e., ~20 Å from the surface) far exceeds the dimension of the classical Stern layer. Comparison to RbCl and RbI electrolytes indicates that these behaviors are sensitive to the choice of co-ion. This new in-depth molecular-scale understanding of the EDL structure during transition from classical to non-classical regimes supports the development of realistic EDL models for technologies operating at high salinity such as water purification applications or modern electrochemical storage.

Effects of Interfacial Molecular Structures on Pressure-Driven Brine Flow in Silica Mesopores

In this work, we conduct molecular dynamics simulations to investigate pressure-driven brine flow in silica mesopores under typical reservoir conditions (323 K and 20 MPa). While surface counterions accumulate strongly in the vicinity of fully deprotonated silica surfaces, water forms multilayer structures due to hydrogen bonding, counterion hydration, and excluded-volume effect. Brine flow behaviors exhibit adsorption, transition, and bulk-like regions in fully deprotonated silica mesopores, while the transition region is negligible in fully protonated ones. In the adsorption region, strong surface hydrogen bonding and a high degree of counterion hydration collectively hinder water mobility. Even without surface hydrogen bonding, persistent ion hydration impedes water flow, leading to the transition region in fully deprotonated silica mesopores and higher viscosity of brine (with 10 wt % NaCl) in the bulk region. This work elucidates the collective effects of surface chemistry and interfacial water structures on brine flow behaviors in silica mesopores from molecular perspectives.

Water Self-Dissociation is Insensitive to Nanoscale Environments

Nanoconfinement effects on water dissociation and reactivity remain controversial, despite their importance to understand the aqueous chemistry at interfaces, pores, or aerosols. The pKw in confined environments has been assessed from experiments and simulations in a few specific cases, leading to dissimilar conclusions. Here, with the use of carefully designed ab initio simulations, we demonstrate that the energetics of bulk water dissociation is conserved intact to unexpectedly small length-scales, down to aggregates of only a dozen molecules or pores of widths below 2 nm. The reason is that most of the free-energy involved in water autoionization comes from breaking the O-H covalent bond, which has a comparable barrier in the bulk liquid, in a small droplet of nanometer size, or in a nanopore in the absence of strong interfacial interactions. Thus, dissociation free-energy profiles in nanoscopic aggregates or in 2D slabs of 1 nm width reproduce the behavior corresponding to the bulk liquid, regardless of whether the corresponding nanophase is delimited by a solid or a gas interface. The present work provides a definite and fundamental description of the mechanism and thermodynamics of water dissociation at different scales with broader implications on reactivity and self-ionization at the air-liquid interface.

Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

HYPOTHESIS

Adsorption of divalent heavy metal ions (DHMIs) at the mineral-water interfaces changes interfacial chemical species and charges, interfacial water structure, Stern (SL), and diffuse (DL) layers. These molecular changes can be detected by probing changing orientation and hydrogen-bond network of interfacial water molecules in response to changing local charges and hydrophobicity.

EXPERIMENTS

Sum-frequency generation (SFG) spectroscopy was used to probe changes in vibrational resonances of interfacial OH vs. DHMI concentration and pH. SFG spectra were deconvoluted using the measured surface potential and maximum entropy method in conjunction with the electrical double-layer theory for the SL and DL structures and correlated by hydrophobicity.

FINDINGS

Three surface charge reversals (CRs) were detected at low (CR1), medium (CR2), and high (CR3) pHs. Unlike CR1, SFG signals were minimized at CR2 and CR3 for DHMIs-silica systems highlighting considerable alterations in the structure of interfacial waters due to the inner-sphere sorption of metal hydroxo complexes. SFG results showed "hydrophobic-like" stretching modes at > 3600 cm^-1 for Pb-, Cu-, and Zn-treated silica. However, contact angle measurements revealed the hydrophobization of silica only in the presence of Pb(II), as confirmed by an in-depth SFG analysis of the hydrogen-bond network of the interfacial water molecules in the SL.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ(2) (Imχ(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

Light, Water, and Melatonin: The Synergistic Regulation of Phase Separation in Dementia

The swift rise in acceptance of molecular principles defining phase separation by a broad array of scientific disciplines is shadowed by increasing discoveries linking phase separation to pathological aggregations associated with numerous neurodegenerative disorders, including Alzheimer’s disease, that contribute to dementia. Phase separation is powered by multivalent macromolecular interactions. Importantly, the release of water molecules from protein hydration shells into bulk creates entropic gains that promote phase separation and the subsequent generation of insoluble cytotoxic aggregates that drive healthy brain cells into diseased states. Higher viscosity in interfacial waters and limited hydration in interiors of biomolecular condensates facilitate phase separation. Light, water, and melatonin constitute an ancient synergy that ensures adequate protein hydration to prevent aberrant phase separation. The 670 nm visible red wavelength found in sunlight and employed in photobiomodulation reduces interfacial and mitochondrial matrix viscosity to enhance ATP production via increasing ATP synthase motor efficiency. Melatonin is a potent antioxidant that lowers viscosity to increase ATP by scavenging excess reactive oxygen species and free radicals. Reduced viscosity by light and melatonin elevates the availability of free water molecules that allow melatonin to adopt favorable conformations that enhance intrinsic features, including binding interactions with adenosine that reinforces the adenosine moiety effect of ATP responsible for preventing water removal that causes hydrophobic collapse and aggregation in phase separation. Precise recalibration of interspecies melatonin dosages that account for differences in metabolic rates and bioavailability will ensure the efficacious reinstatement of the once-powerful ancient synergy between light, water, and melatonin in a modern world.

On the Trail of Molecular Hydrophilicity and Hydrophobicity at Aqueous Interfaces

Uncovering microscopic hydrophilicity and hydrophobicity at heterogeneous aqueous interfaces is essential as it dictates physico/chemical properties such as wetting, the electrical double layer, and reactivity. Several molecular and spectroscopic descriptors were proposed, but a major limitation is the lack of connections between them. Here, we combine density functional theory-based MD simulations (DFT-MD) and SFG spectroscopy to explore how interfacial water responds in contact with self-assembled monolayers (SAM) of tunable hydrophilicity. We introduce a microscopic metric to track the transition from hydrophobic to hydrophilic interfaces. This metric combines the H/V descriptor, a structural descriptor based on the preferential orientation within the water network in the topmost binding interfacial layer (BIL) and spectroscopic fingerprints of H-bonded and dangling OH groups of water carried by BIL-resolved SFG spectra. This metric builds a bridge between molecular descriptors of hydrophilicity/hydrophobicity and spectroscopically measured quantities and provides a recipe to quantitatively or qualitatively interpret experimental SFG signals.

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.

A Form of Non-Volatile Solid-like Hexadecane Found in Micron-Scale Silica Microtubule

Anomalous solid-like liquids at the solid-liquid interface have been recently reported. The mechanistic factors contributing to these anomalous liquids and whether they can stably exist at high vacuum are interesting, yet unexplored, questions. In this paper, thin slices of silica tubes soaked in hexadecane were observed under a transmission electron microscope at room temperature. The H-spectrum of hexadecane in the microtubules was measured by nuclear magnetic resonance. On the interior surface of these silica tubes, 0.2-30 μm in inside diameter (ID), a layer (12-400 nm) of a type of non-volatile hexadecane was found with thickness inversely correlated with the tube ID. A sample of this anomalous hexadecane in microtubules 0.4 μm in ID was found to be formable by an ion beam. Compared with the nuclear magnetic resonance H-spectroscopy of conventional hexadecane, the characteristic peaks of this abnormal hexadecane were shifted to the high field with a broader characteristic peak, nuclear magnetic resonance hydrogen spectroscopy spectral features typical of that of solids. The surface density of these abnormal hexadecanes was found to be positively correlated with the silanol groups found on the interior silica microtubular surface. This positive correlation indicates that the high-density aggregation of silanol is an essential factor for forming the abnormal hexadecane reported in this paper.

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Connecting Thermodynamics of Alkali Ion Exchange on the Quartz (101) Surface with Density Functional Theory Calculations

Periodic plane-wave density functional theory (DFT) calculations were performed on the α-quartz (SiO₂) (101) surface to model exchange of adsorbed Li⁺ and either Na⁺, K⁺, or Rb⁺ in inner- and outer-sphere adsorbed, and aqueous configurations, which are charge-balanced with 2 Cl^-. SiO^- or SiOH groups represented the adsorption surface sites. The SiO^- models included 58 H₂O and 2 H₃O⁺ molecules to approximate an aqueous environment, whereas the SiOH models had 59 H₂O and 1 H₃O⁺ molecules. The goal of this work is to calculate the heats of exchange for these alkali ions and to compare the results with those measured by flow microcalorimetry to ascertain the most probable mechanisms for these cations exchanging on the α-quartz (101) surface. Energy minimizations of each alkali ion adsorbed as outer-sphere complexes on SiOH surface sites, and as inner- and outer-sphere complexes on SiO^- surface sites, were used to determine the energy of exchange (ΔE_ex) with Li⁺ for comparison with experimentally determined ΔH_ex values. Here, we present a novel method for calculating ΔE_ex using the difference in energies of geometry-optimized end member models. The aqueous and surface structures produced are similar to those observed experimentally. Although the trend for the calculated ΔE_ex values is consistent with those from the heats of exchange measured experimentally, the magnitude of our modeled ΔE_ex results is significantly larger than select experimental data from the literature [Peng, L. Zeta-Potentials and Enthalpy Changes in the Process of Electrostatic Self-Assembly of Cations on Silica Surface. Powder Technol. 2009, 193(1), 46-49]; we discuss the reasons for this discrepancy herein. The relative energy differences of the various configurations modeled have implications for the measurements of the surface charge via potentiometric titrations due to the more active role of alkali cations in quartz surface chemistry that have been previously considered as inert background electrolytes.

Are cyclic plant and animal behaviours driven by gravimetric mechanical forces?

The celestial mechanics of the Sun, Moon, and Earth dominate the variations in gravitational force that all matter, live or inert, experiences on Earth. Expressed as gravimetric tides, these variations are pervasive and have forever been part of the physical ecology with which organisms evolved. Here, we first offer a brief review of previously proposed explanations that gravimetric tides constitute a tangible and potent force shaping the rhythmic activities of organisms. Through meta-analysis, we then interrogate data from three study cases and show the close association between the omnipresent gravimetric tides and cyclic activity. As exemplified by free-running cyclic locomotor activity in isopods, reproductive effort in coral, and modulation of growth in seedlings, biological rhythms coincide with temporal patterns of the local gravimetric tide. These data reveal that, in the presumed absence of rhythmic cues such as light and temperature, local gravimetric tide is sufficient to entrain cyclic behaviour. The present evidence thus questions the phenomenological significance of so-called free-run experiments.

Correlation between Electrostatic and Hydration Forces on Silica and Gibbsite Surfaces: An Atomic Force Microscopy Study

The balance between hydration and Derjaguin-Landau-Verwey-Overbeek (DLVO) forces at solid-liquid interfaces controls many processes, such as colloidal stability, wetting, electrochemistry, biomolecular self-assembly, and ion adsorption. Yet, the origin of molecular scale hydration forces and their relation to the surface charge density that controls the continuum scale electrostatic forces is poorly understood. We argue that these two types of forces are largely independent of each other. To support this hypothesis, we performed atomic force microscopy experiments using intermediate-sized tips that enable the simultaneous detection of DLVO and molecular scale oscillatory hydration forces at the interface between composite gibbsite:silica-aqueous electrolyte interfaces. We extract surface charge densities from forces measured at tip-sample separations of 1.5 nm and beyond using DLVO theory in combination with charge regulation boundary conditions for various pH values and salt concentrations. We simultaneously observe both colloidal scale DLVO forces and oscillatory hydration forces for an individual crystalline gibbsite particle and the underlying amorphous silica substrate for all fluid compositions investigated. While the diffuse layer charge varies with pH as expected, the oscillatory hydration forces are found to be largely independent of pH and salt concentration, supporting our hypothesis that both forces indeed have a very different origin. Oscillatory hydration forces are found to be distinctly more pronounced on gibbsite than on silica. We rationalize this observation based on the distribution of hydroxyl groups available for H bonding on the two distinct surfaces.

Second Harmonic Scattering Reveals Ion-Specific Effects at the SiO2 and TiO2 Nanoparticle/Aqueous Interface

Ion-specific effects play a crucial role in controlling the stability of colloidal systems and regulating interfacial processes. Although mechanistic pictures have been developed to explain the electrostatic structure of solid/water colloidal interfaces, ion-specific effects remain poorly understood. Here we quantify the average interfacial water orientation and the electrostatic surface potential around 100 nm SiO2 and TiO2 colloidal particles in the presence of NaCl, RbCl, and CaCl2 using polarimetric angle-resolved second harmonic scattering. We show that these two parameters can be used to establish the ion adsorption mechanism in a low ionic strength regime (<1 mM added salt). The relative differences between salts as a function of the ionic strength demonstrate cation- and surface-specific preferences for inner- vs outer-sphere adsorption. Compared to monovalent Rb+ and Na+, Ca2+ is found to be preferentially adsorbed as outer-sphere on SiO2 surfaces, while a dominant inner-sphere adsorption is observed for Ca2+ on TiO2. Molecular dynamics simulations performed on crystalline SiO2 and TiO2 surfaces support the experimental conclusions. This work contributes to the understanding of the electrostatic environment around colloidal nanoparticles on a molecular level by providing insight into ion-specific effects with micromolar sensitivity.

Cation enrichment in the ion atmosphere is promoted by local hydration of DNA

Electrostatic interactions are central to the structure and function of nucleic acids, including their folding, condensation, and interaction with proteins and other charged molecules. These interactions are profoundly affected by ions surrounding nucleic acids, the constituents of the so-called ion atmosphere. Here, we report precise Fourier Transform-Terahertz/Far-Infrared (FT-THz/FIR) measurements in the frequency range 30-500 cm^-1 for a 24-bp DNA solvated in a series of alkali halide (NaCl, NaF, KCl, CsCl, and CsF) electrolyte solutions which are sensitive to changes in the ion atmosphere. Cation excess in the ion atmosphere is detected experimentally by observation of cation modes of Na⁺, K⁺, and Cs⁺ in the frequency range between 70-90 cm^-1. Based on MD simulations, we propose that the magnitude of cation excess (which is salt specific) depends on the ability of the electrolyte to perturb the water network at the DNA interface: In the NaF atmosphere, the ions reduce the strength of interactions between water and the DNA more than in case of a NaCl electrolyte. Here, we explicitly take into account the solvent contribution to the chemical potential in the ion atmosphere: A decrease in the number of bound water molecules in the hydration layer of DNA is correlated with enhanced density fluctuations, which decrease the free energy cost of ion-hydration, thus promoting further ion accumulation within the DNA atmosphere. We propose that taking into account the local solvation is crucial for understanding the ion atmosphere.

Strong Surface Hydration and Salt Resistant Mechanism of a New Nonfouling Zwitterionic Polymer Based on Protein Stabilizer TMAO

Zwitterionic polymers exhibit excellent nonfouling performance due to their strong surface hydrations. However, salt molecules may severely reduce the surface hydrations of typical zwitterionic polymers, making the application of these polymers in real biological and marine environments challenging. Recently, a new zwitterionic polymer brush based on the protein stabilizer trimethylamine N-oxide (TMAO) was developed as an outstanding nonfouling material. Using surface-sensitive sum frequency generation (SFG) vibrational spectroscopy, we investigated the surface hydration of TMAO polymer brushes (pTMAO) and the effects of salts and proteins on such surface hydration. It was discovered that exposure to highly concentrated salt solutions such as seawater only moderately reduced surface hydration. This superior resistance to salt effects compared to other zwitterionic polymers is due to the shorter distance between the positively and negatively charged groups, thus a smaller dipole in pTMAO and strong hydration around TMAO zwitterion. This results in strong bonding interactions between the O- in pTMAO and water, and weaker interaction between O- and metal cations due to the strong repulsion from the N+ and hydration water. Computer simulations at quantum and atomistic scales were performed to support SFG analyses. In addition to the salt effect, it was discovered that exposure to proteins in seawater exerted minimal influence on the pTMAO surface hydration, indicating complete exclusion of protein attachment. The excellent nonfouling performance of pTMAO originates from its extremely strong surface hydration that exhibits effective resistance to disruptions induced by salts and proteins.

The manner and extent to which the hydration shell impacts interactions between hydrated species

The hydration shell (HS) has a critical impact on every contact between hydrated species, which is a prerequisite for a great many physical and chemical processes, such as ion adsorption at the solution-solid interface. This paper reveals the extent and manner to which the HS interferes with ion adsorption utilizing molecular dynamics. The single-layer HS is the smallest unit that maintains the ionic hydration structure and the force on it. The energy penalty incurred by partial dehydration upon adsorption is one of the approaches through which HS influences ion adsorption, yet the collision of water molecules in HS may be the critical one. The repulsive force during dehydration is, to great extent, neutralized by HS collision. The index for estimating the extent of the influence of the HS is not the hydration energy, but the quantification of the contest between HS' collision and the binding of adsorption sites. The hydration energy is larger for charged functional groups, but the HS' impact is much smaller, as compared with electroneutral group cases. As a result, the order of the adsorption capacity for different ionic species may be quite different between charged and electroneutral cases.

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.

Local Mutations Can Serve as a Game Changer for Global Protein Solvent Interaction

Although it is well-known that limited local mutations of enzymes, such as matrix metalloproteinases (MMPs), may change enzyme activity by orders of magnitude as well as its stability, the completely rational design of proteins is still challenging. These local changes alter the electrostatic potential and thus local electrostatic fields, which impacts the dynamics of water molecules close the protein surface. Here we show by a combined computational design, experimental, and molecular dynamics (MD) study that local mutations have not only a local but also a global effect on the solvent: In the specific case of the matrix metalloprotease MMP14, we found that the nature of local mutations, coupled with surface morphology, have the ability to influence large patches of the water hydrogen-bonding network at the protein surface, which is correlated with stability. The solvent contribution can be experimentally probed via terahertz (THz) spectroscopy, thus opening the door to the exciting perspective of rational protein design in which a systematic tuning of hydration water properties allows manipulation of protein stability and enzymatic activity.

Probing the Mineral-Water Interface with Nonlinear Optical Spectroscopy

The interaction between minerals and water is manifold and complex: the mineral surface can be (de)protonated by water, thereby changing its charge; mineral ions dissolved into the aqueous phase screen the surface charges. Both factors affect the interaction with water. Intrinsically molecular-level processes and interactions govern macroscopic phenomena, such as flow-induced dissolution, wetting, and charging. This realization is increasingly prompting molecular-level studies of mineral-water interfaces. Here, we provide an overview of recent developments in surface-specific nonlinear spectroscopy techniques such as sum frequency and second harmonic generation (SFG/SHG), which can provide information about the molecular arrangement of the first few layers of water molecules at the mineral surface. The results illustrate the subtleties of both chemical and physical interactions between water and the mineral as well as the critical role of mineral dissolution and other ions in solution for determining those interactions.

Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies

Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via density functional theory-molecular dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

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.

Structure and sum-frequency generation spectra of water on neutral hydroxylated silica surfaces

Structural organization and vibrational sum-frequency generation (VSFG) spectra of water on crystalline and amorphous neutral silica surfaces were investigated by classical molecular dynamics simulations. The liquid phase represented with neat water and 1 M NaCl solution was analysed in terms of bonded interfacial layer (BIL), diffuse layer (DL) and bulk region. The simulations show that the structure of BIL depends on the surface morphology and density of surface OH groups. The water-silanol H-bond network and BIL structure are mainly insensitive to the presence of ions in the liquid phase. Molecules in DL of SiO₂/neat water interfaces preferentially orient their OH bonds towards the surfaces. This effect is directly related to an effective negative charge of formally neutral surfaces. Ions of the electrolyte solution affect the intermolecular structure in DL by screening the surface electric field and by the chaotropic effect. Calculated phase-sensitive VSFG (Im[χ⁽²⁾]) spectrum of BIL features low-frequency negative and high-frequency positive bands. Characteristics of the positive band reflect the strength of water-surface interactions and surface crystallinity, while the position and shape of the negative band are common to all interfaces. The Im[χ⁽²⁾] spectrum of DL is dominated by a contribution from the third-order χ⁽³⁾ susceptibility with the sign of the contribution directly related to the sign of electrostatic potential in the interfacial region. The DL spectrum is strongly affected by the presence of solvated ions. The computed intensity and Im[χ⁽²⁾] spectra of the amorphous silica/NaCl solution interface are in a good agreement with the conventional and phase-sensitive experimental VSFG spectra of fused SiO₂/water system at low pH, in contrast to the spectra of the amorphous silica/neat water interface. Origins of the discrepancy are discussed.

Effect of Oxidation Level on the Interfacial Water at the Graphene Oxide-Water Interface: From Spectroscopic Signatures to Hydrogen-Bonding Environment

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.