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.

Ultrafast fragmentation of highly-excited doubly-ionized deoxyribose: role of the liquid water environment.

Ab initio molecular dynamics simulations are used to investigate the fragmentation dynamics following the double ionization of 2-deoxy-D-ribose (DR), a major component in the DNA chain. Different ionization scenarios are considered to provide a complete picture. First focusing on isolated DR2+, fragmentation patterns are determined for the ground electronic state, adding randomly distributed excitation energy to the nuclei. These patterns differ for the two isomers studied. To compare thermal and electronic excitation effects, Ehrenfest dynamics are also performed, allowing to remove the two electrons from selected molecular orbitals. Two intermediate-energy orbitals, localized on the carbon chain, were selected. The dissociation pattern corresponds to the most frequent pattern obtained when adding thermal excitation. On the contrary, targeting the four deepest orbitals, localized on the oxygen atoms, leads to selective ultrafast C-O and/or O-H bond dissociation. To probe the role of environment, a system consisting of a DR molecule embedded in liquid water is then studied. The two electrons are removed from either the DR or the water molecules directly linked to the sugar through hydrogen bonds. Although the dynamics onset is similar to that of isolated DR when removing the same deep orbitals localized on the sugar oxygen atoms, the subsequent fragmentation patterns differ. Sugar damage also occurs following the Coulomb explosion of neighboring H2O2+ molecules due to interaction with the emitted O or H atoms.

Stripping away ion hydration shells in electrical double-layer formation: Water networks matter.

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl- hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

HiREX: High-Throughput Reactivity Exploration for Extended Databases of Transition-Metal Catalysts.

A method is introduced for the automated analysis of reactivity exploration for extended in silico databases of transition-metal catalysts. The proposed workflow is designed to tackle two key challenges for bias-free mechanistic explorations on large databases of catalysts: (1) automated exploration of the chemical space around each catalyst with unique structural and chemical features and (2) automated analysis of the resulting large chemical data sets. To address these challenges, we have extended the application of our previously developed ReNeGate method for bias-free reactivity exploration and implemented an automated analysis procedure to identify the classes of reactivity patterns within specific catalyst groups. Our procedure applied to an extended series of representative Mn(I) pincer complexes revealed correlations between structural and reactive features, pointing to new channels for catalyst transformation under the reaction conditions. Such an automated high-throughput virtual screening of systematically generated hypothetical catalyst data sets opens new opportunities for the design of high-performance catalysts as well as an accelerated method for expert bias-free high-throughput in silico reactivity exploration.

Salt Bridge Structure of Microhydrated Arginine Kinetically Trapped in the Gas Phase by Evaporative Cooling.

Amino acids and peptides generally exhibit zwitterionic forms with salt bridge (SB) structures in solution but charge-solvated (CS) motifs in the gas phase. Here, we report a study of non-covalent complexes of the protonated amino acid arginine, ArgH+(H2O)n (n = 1-5), produced in the gas phase from an aqueous solution with a controlled number of retained water molecules. These complexes were probed by cold ion spectroscopy and treated by quantum chemistry. The spectroscopic changes induced upon gradual dehydration of arginine were assigned by structural calculations to the transition from SB to CS geometries. SB conformers appear to be present for the complexes with as few as 3 retained water molecules, although energetically CS structures should become prevailing already for ArgH+ with 7-8 water molecules. We attribute the revealed kinetic trapping of arginine in native-like zwitterionic forms to evaporative cooling of the hydrated complexes to as low as below 200 K.

Algorithmic Graph Theory, Reinforcement Learning and Game Theory in MD Simulations: From 3D Structures to Topological 2D-Molecular Graphs (2D-MolGraphs) and Vice Versa.

This paper reviews graph-theory-based methods that were recently developed in our group for post-processing molecular dynamics trajectories. We show that the use of algorithmic graph theory not only provides a direct and fast methodology to identify conformers sampled over time but also allows to follow the interconversions between the conformers through graphs of transitions in time. Examples of gas phase molecules and inhomogeneous aqueous solid interfaces are presented to demonstrate the power of topological 2D graphs and their versatility for post-processing molecular dynamics trajectories. An even more complex challenge is to predict 3D structures from topological 2D graphs. Our first attempts to tackle such a challenge are presented with the development of game theory and reinforcement learning methods for predicting the 3D structure of a gas-phase peptide.

Gas-Phase Infrared Spectroscopy of Neutral Peptides: Insights from the Far-IR and THz Domain.

Gas-phase, double resonance IR spectroscopy has proven to be an excellent approach to obtain structural information on peptides ranging from single amino acids to large peptides and peptide clusters. In this review, we discuss the state-of-the-art of infrared action spectroscopy of peptides in the far-IR and THz regime. An introduction to the field of far-IR spectroscopy is given, thereby highlighting the opportunities that are provided for gas-phase research on neutral peptides. Current experimental methods, including spectroscopic schemes, have been reviewed. Structural information from the experimental far-IR spectra can be obtained with the help of suitable theoretical approaches such as dynamical DFT techniques and the recently developed Graph Theory. The aim of this review is to underline how the synergy between far-IR spectroscopy and theory can provide an unprecedented picture of the structure of neutral biomolecules in the gas phase. The far-IR signatures of the discussed studies are summarized in a far-IR map, in order to gain insight into the origin of the far-IR localized and delocalized motions present in peptides and where they can be found in the electromagnetic spectrum.

Molecular hydrophobicity at a macroscopically hydrophilic surface.

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle.

Unexpectedly High Capacitance of the Metal Nanoparticle/Water Interface: Molecular-Level Insights into the Electrical Double Layer.

The electrical double-layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico-chemical information on the capacitance and structure of the electrical double-layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order-of-magnitude than predicted by the traditional mean-field models of the double-layer such as the Gouy-Chapman-Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid-solvent and solvent-solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability.

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

The structure and ultrafast dynamics of the electric double layer (EDL) are central to chemical reactivity and physical properties at solid/aqueous interfaces. While the Gouy-Chapman-Stern model is widely used to describe EDLs, it is solely based on the macroscopic electrostatic attraction of electrolytes for the charged surfaces. Structure and dynamics in the Stern layer are, however, more complex because of competing effects due to the localized surface charge distribution, surface-solvent-ion correlations, and the interfacial hydrogen bonding environment. Here, we report combined time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD) to get direct access to the molecular-level understanding of how ions change the structure and dynamics of the EDL. We show that innersphere adsorbed ions tune the hydrophobicity of the silica-aqueous interface by shifting the structural makeup in the Stern layer from dominant water-surface interactions to water-water interactions. This drives an initially inhomogeneous interfacial water coordination landscape observed at the neat interface toward a homogeneous, highly interconnected in-plane 2D hydrogen bonding (2D-HB) network at the ionic interface, reminiscent of the canonical, hydrophobic air-water interface. This ion-induced transformation results in a characteristic decrease of the vibrational lifetime (T1) of excited interfacial O-H stretching modes from T1 ∼ 600 fs to T1 ∼ 250 fs. Hence, we propose that the T1 determined by TR-vSFG in combination with DFT-MD simulations can be widely used for a quantitative spectroscopic probe of the ion kosmotropic/chaotropic effect at aqueous interfaces as well as of the ion-induced surface hydrophobicity.

Enhanced conductivity of water at the electrified air-water interface: a DFT-MD characterization.

DFT-based molecular dynamics simulations of the electrified air-liquid water interface are presented, where a homogeneous field is applied parallel to the surface plane. We unveil the field intensity for the onset of proton transfer and molecular dissociation; the protonic current/proton conductivity is measured as a function of the field intensity/voltage. The air-water interface is shown to exhibit a proton conductivity twice the one in the liquid water for field intensities below 0.40 V Å-1. We show that this difference arises from the very specific organization of water in the binding interfacial layer (BIL, i.e. the air-water interface region) into a 2D-HBond-network that is maintained and enforced at the electrified interface. Beyond fields of 0.40 V Å-1, water in the BIL and in the bulk liquid are aligned in the same way by the rather intense fields, hence leading to the same proton conductivity in both BIL and bulk water.

Conformational assignment of gas phase peptides and their H-bonded complexes using far-IR/THz: IR-UV ion dip experiment, DFT-MD spectroscopy, and graph theory for mode assignment.

The combined approach of gas phase IR-UV ion dip spectroscopy experiments and DFT-based molecular dynamics simulations for theoretical spectroscopy reveals the 3D structures of (Ac-Phe-OMe)1,2 peptides using their far-IR/THz signatures. Both experimental and simulated IR spectra are well-resolved in the 100-800 cm-1 domain, allowing an unambiguous assignment of the conformers, that could not be achieved in other more congested spectral domains. We also present and make proofs-of-principles for our newly developed theoretical method for the assignment of (anharmonic) vibrational modes from MD simulations based on graph theory coupled to APT-weighted internal coordinates velocities DOS spectra. The principles of the method are reviewed, applications to the simple gas phase water and NMA (N-methyl-acetamide) molecules are presented, and application to the more complex (Ac-Phe-OMe)1,2 peptidic systems shows that the complexity in assigning vibrational modes from MD simulations is reduced with the graphs. Our newly developed graph-based methodology is furthermore shown to allow an easy comparison between the vibrational modes of isolated monomer(s) and their complexes, as illustrated by the (Ac-Phe-OMe)1,2 peptides.

Interactions of aggregating peptides probed by IR-UV action spectroscopy.

Peptide aggregation, the self-assembly of peptides into structured beta-sheet fibril structures, is driven by a combination of intra- and intermolecular interactions. Here, the interplay between intramolecular and formed inter-sheet hydrogen bonds and the effect of dispersion interactions on the formation of neutral, isolated, peptide dimers is studied using infrared action spectroscopy. Therefore, four different homo- and heterogenous dimers resulting from three different alanine-based model peptides have been formed under controlled and isolated conditions. The peptides differ from one another by the presence and location of a UV chromophore containing end cap. The conformations of the monomers of the peptides direct the final dimer structure: strongly bonded or folded structures result in weakly bound dimers. Here, intramolecular hydrogen bonds are favored over new intermolecular hydrogen bond interactions. In contrast, linear monomers are the ideal template to form parallel beta-sheet type structures. The weak intramolecular hydrogen bonds present in the linear monomers are replaced by the stronger inter-sheet hydrogen bond interactions. The influence of π-π dispersion interactions on the structure of the dimers is minimal, and the phenyl rings have a tendency to fold away from the peptide backbone to favour intermolecular hydrogen bond interactions over dispersion interactions. Quantum chemical calculations confirm our experimental observations.

Deconvolution of BIL-SFG and DL-SFG spectroscopic signals reveals order/disorder of water at the elusive aqueous silica interface.

Through the prism of the rather controversial and elusive silica/water interface, ab initio DFT-based molecular dynamics simulations of the structure and non-linear SFG spectroscopy of the interface are analysed. Following our recent work [Phys. Chem. Chem. Phys., 2018, 20, 5190-5199], we show that once the interfacial water is decomposed into BIL (Binding Interfacial Layer) and DL (Diffuse Layer) interfacial regions, the SFG signals can be deconvolved and unambiguously interpreted, and a global microscopic understanding on silica/water interfaces can be obtained. By comparing crystalline quartz/water and amorphous (fused) silica/water interfaces, the dependence of interfacial structural and spectroscopic properties on the degree of surface crystallinity is established, while by adding KCl electrolytes at the quartz/water interface, the chaotropic effect of ions on the interfacial molecular arrangement is unveiled. The evolution of structure and SFG spectra of silica/water interfaces with respect to increasing surface deprotonation, i.e., with respect to pH conditions, is also evaluated. Spectroscopic BIL-SFG markers that experimentally allow one detect the water order/disorder in the BIL as a function of surface hydroxylation and ion concentration are revealed, while the pH-induced modulations in the experimentally recorded SFG spectra are rationalized in terms of changes in both BIL and DL SFG signatures.

Heterogeneous Interactions between Gas-Phase Pyruvic Acid and Hydroxylated Silica Surfaces: A Combined Experimental and Theoretical Study.

The adsorption of gas-phase pyruvic acid (CH3COCOOH) on hydroxylated silica particles has been investigated at 296 K using transmission Fourier transform infrared (FTIR) spectroscopy and theoretical simulations. Under dry conditions (<1% relative humidity, RH), both the trans-cis (Tc) and trans-trans (Tt) pyruvic acid conformers are observed on the surface as well as the (hydrogen bonded) pyruvic acid dimer. The detailed surface interactions were further understood through ab initio molecular dynamics simulations. Under higher relative humidity conditions (above 10% RH), adsorbed water competes for surface adsorption sites. Adsorbed water is also observed to change the relative populations of the different adsorbed pyruvic acid configurations. Overall, this study provides valuable insights into the interaction of pyruvic acid with hydroxylated silica surfaces on the molecular level from both experimental and theoretical analyses. Furthermore, these results highlight the importance of the environment (relative humidity and coadsorbed water) in the adsorption, partitioning, and configurations of pyruvic acid at the surface.

DFT-MD of the (110)-Co3O4 cobalt oxide semiconductor in contact with liquid water, preliminary chemical and physical insights into the electrochemical environment.

Within the general context of the electrochemical oxygen evolution reaction of the water oxidation/electrolysis, we focus on one essential aspect of electrochemical interfaces, i.e., the comprehension of the interaction and organisation of liquid water at the (semiconductor) (110)-Co3O4 surface using density functional theory-molecular dynamics simulations. A detailed characterization of the chemical and physical properties of the aqueous interface is provided in terms of structure, dynamics, electric field, work function, and spectroscopy, as a preliminary step into the modelling of the (110)-Co3O4 aqueous surface in more relevant electrochemical conditions. The water at the aqueous B-termination is, in particular, shown more dynamical than that at the A-termination and more "undisciplined": the water is indeed mostly an HB-acceptor with the solid, with an orientation of their dipole moments found opposite the field generated by the negative surface charge. At both aqueous interfaces, the work function is twice lower than that at the bare (non-hydroxylated) surfaces. The SFG (Sum Frequency Generation) spectroscopy is shown dominated by the water in the diffuse layer, while the SFG signal from the binding interfacial layer reflects the single orientation of water at the aqueous A-termination and the two orientations of water at the aqueous B-termination.

Structural definition of the BIL and DL: a new universal methodology to rationalize non-linear χ(2)(ω) SFG signals at charged interfaces, including χ(3)(ω) contributions.

This work provides unambiguous definitions from theoretical simulations of the two interfacial regions named the BIL (binding interfacial layer) and DL (diffuse layer) at charged solid/water and air/water interfaces. The BIL and DL nomenclature follows the pioneering work of Wen et al. [Phys. Rev. Lett. 2016, 116, 016101]. Our definitions are based on the intrinsic structural properties of water only. Knowing the BIL and DL interfacial regions, one is then able to deconvolve the χ(2)(ω) non-linear SFG (sum frequency generation) response into χ(ω) and χ(ω) contributions, thus providing a detailed molecular interpretation of these signals and of the measured total SFG. We furthermore show that the χ(ω) spectrum arises from the χ(3)(ω) non-linear third order contribution of bulk liquid water, here calculated for several charged interfaces and shown to be universal. The χ(ω) contribution therefore has the same origin in terms of molecular normal modes at any charged interface. The molecular interpretation of χ(ω) is hence at the heart of the unambiguous molecular comprehension and interpretation of the measured total SFG signal at any charged interface.

Influence of argon and D2 tagging on the hydrogen bond network in Cs+(H2O)3; kinetic trapping below 40 K.

The influence of enthalpic and entropic effects as well as of kinetic trapping processes on the structure of Ar/D2-tagged Cs+(H2O)3 clusters is studied by temperature-dependent infrared photodissociation spectroscopy combined with harmonic vibrational spectra calculations and anharmonic free energy profiles from finite temperature metadynamics molecular dynamics simulations. Each tag favors a different hydrogen bond network of water molecules, with Ar-tagging (vs. D2-tagging) of Cs+(H2O)3 leading to the lower energy conformation. The relative population of these conformers can be tuned over a temperature range of 12 to 21 K. The formation mechanisms of these tagged clusters can be deduced from the free energy profiles. This investigation demonstrates that a variety of factors, both thermodynamic and kinetic, play a role in the structure of flexible molecular species, even at cryogenic temperatures.

Proton Collision on Deoxyribose Originating from Doubly Ionized Water Molecule Dissociation.

In this work, we studied the fragmentation dynamics of 2-deoxy-d-ribose (DR) in solution that arises from the double ionization of a water molecule in its primary hydration shell. This process was modeled in the framework of ab initio molecular dynamics. The charge unbalanced in the solvent molecules produces a Coulomb explosion with the consequent release of protons with kinetic energy in the few electronvolts range, which collide with the surrounding molecules in solution inducing further chemical reactions. In particular, we observe proton collisions with the solute molecule DR, which leads to a complete ring opening. In DNA, damage to the DR moiety may lead to DNA strand breaking. This mechanism can be understood as one of the possible steps in the radiation-induced fragmentation of DNA chains.

2D-HB-Network at the air-water interface: A structural and dynamical characterization by means of ab initio and classical molecular dynamics simulations.

Following our previous work where the existence of a special 2-Dimensional H-Bond (2D-HB)-Network was revealed at the air-water interface [S. Pezzotti et al., J. Phys. Chem. Lett. 8, 3133 (2017)], we provide here a full structural and dynamical characterization of this specific arrangement by means of both Density Functional Theory based and Force Field based molecular dynamics simulations. We show in particular that water at the interface with air reconstructs to maximize H-Bonds formed between interfacial molecules, which leads to the formation of an extended and non-interrupted 2-Dimensional H-Bond structure involving on average ∼90% of water molecules at the interface. We also show that the existence of such an extended structure, composed of H-Bonds all oriented parallel to the surface, constrains the reorientional dynamics of water that is hence slower at the interface than in the bulk. The structure and dynamics of the 2D-HB-Network provide new elements to possibly rationalize several specific properties of the air-water interface, such as water surface tension, anisotropic reorientation of interfacial water under an external field, and proton hopping.