Affinity of Hydrated Protons at Intrinsic Water/Vapor Interface Revealed by Ion-Induced Water Alignment

Molecular Dynamics Simulation of Complex Reactivity with the Rapid Approach for Proton Transport and Other Reactions (RAPTOR) Software Package

Simulating chemically reactive phenomena such as proton transport on nanosecond to microsecond and beyond time scales is a challenging task. Ab initio methods are unable to currently access these time scales routinely, and traditional molecular dynamics methods feature fixed bonding arrangements that cannot account for changes in the system's bonding topology. The Multiscale Reactive Molecular Dynamics (MS-RMD) method, as implemented in the Rapid Approach for Proton Transport and Other Reactions (RAPTOR) software package for the LAMMPS molecular dynamics code, offers a method to routinely sample longer time scale reactive simulation data with statistical precision. RAPTOR may also be interfaced with enhanced sampling methods to drive simulations toward the analysis of reactive rare events, and a number of collective variables (CVs) have been developed to facilitate this. Key advances to this methodology, including GPU acceleration efforts and novel CVs to model water wire formation are reviewed, along with recent applications of the method which demonstrate its versatility and robustness.

Surface stratification determines the interfacial water structure of simple electrolyte solutions

The distribution of ions at the air/water interface plays a decisive role in many natural processes. Several studies have reported that larger ions tend to be surface-active, implying ions are located on top of the water surface, thereby inducing electric fields that determine the interfacial water structure. Here we challenge this view by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results show that ions in typical electrolyte solutions are, in fact, located in a subsurface region, leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the subsurface layer is ion-enriched. This surface stratification is a key element in explaining the ion-induced water reorganization at the outermost air/water interface.

Art, fact and artifact: reflections on the cross-talk between theory and experiment

With the increasing sophistication of each, theory and experiment have become highly specialized endeavors conducted by separate research groups. A result has been a weakening of the coupling between them and occasional hostility. Examples are given and suggestions are offered for strengthening the traditional synergy between theory and experiment.

Neural Network-Based Sum-Frequency Generation Spectra of Pure and Acidified Water Interfaces with Air

The affinity of hydronium ions (H3O+) for the air-water interface is a crucial question in environmental chemistry. While sum-frequency generation (SFG) spectroscopy has been instrumental in indicating the preference of H3O+ for the interface, key questions persist regarding the molecular origin of the SFG spectral changes in acidified water. Here we combine nanosecond long neural network (NN) reactive simulations of pure and acidified water slabs with NN predictions of molecular dipoles and polarizabilities to calculate SFG spectra of long reactive trajectories including proton transfer events. Our simulations show that H3O+ ions cause two distinct changes in phase-resolved SFG spectra: first, a low-frequency tail due to the vibrations of H3O+ and its first hydration shell, analogous to the bulk proton continuum, and second, an enhanced hydrogen-bonded band due to the ion-induced static field polarizing molecules in deeper layers. Our calculations confirm that changes in the SFG spectra of acidic solutions are caused by hydronium ions preferentially residing at the interface.

Concluding remarks for Faraday Discussion on Water at Interfaces

Water at interfaces is a fascinating and multifaceted topic that has garnered significant attention in various scientific fields due to its relevance and implications. This Faraday Discussion explored the complexity of water at different interfaces. Many of the reports highlight the need for a molecular-level understanding. The Discussion was lively and constructive. In these summarizing remarks, I do not aim to be complete, but will rather try to sketch the status of the field, highlight the progress that we as a community have made, and present eclectic examples of where more work needs to be done.

How the Acidity of Water Droplets and Films Is Controlled by the Air-Water Interface

Acidity is a key determinant of chemical reactivity in atmospheric aqueous aerosols and water microdroplets used for catalysis. However, many fundamental questions about these systems have remained elusive, including how their acidity differs from that of bulk solutions, the degree of heterogeneity between their core and surface, and how the acid-base properties are affected by their size. Here, we perform hybrid density functional theory (DFT)-quality neural network-based molecular simulations with explicit nuclear quantum effects and combine them with an analytic model to describe the pH and self-ion concentrations of droplets and films for sizes ranging from nm to μm. We determine how the acidity of water droplets and thin films is controlled by the properties of the air-water interface and by their surface-to-volume ratio. We show that while the pH is uniform in each system, hydronium and hydroxide ions exhibit concentration gradients that span the two outermost molecular layers, enriching the interface with hydronium cations and depleting it with hydroxide anions. Acidity depends strongly on the surface-to-volume ratio for system sizes below a few tens of nanometers, where the core becomes enriched in hydroxide ions and the pH increases as a result of hydronium stabilization at the interface. These results obtained for pure water systems have important implications for our understanding of chemical reactivity in atmospheric aerosols and for catalysis in aqueous microdroplets.

Ions Speciation at the Water-Air Interface

In typical aqueous systems, including naturally occurring sweet and salt water and tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect the chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific heterodyne-detected sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in solution. We find that more hydrophobic ions are speciated to the interface due to the hydrophilic ions. Quantitative analysis shows that the interfacial hydrophobic ion population increases with decreasing interfacial hydrophilic ion population at the interface. Simulations show that the solvation energy difference between the ions and the intrinsic surface propensity of ions determine the extent of an ion's speciation by other ions. This mechanism provides a unified view of the speciation of monatomic and polyatomic ions at electrolyte solution interfaces.

Acidic Conditions Impact Hydrophobe Transfer across the Oil-Water Interface in Unusual Ways

Molecular dynamics simulation and enhanced free energy sampling are used to study hydrophobic solute transfer across the water-oil interface with explicit consideration of the effect of different electrolytes: hydronium cation (hydrated excess proton) and sodium cation, both with chloride counterions (i.e., dissociated acid and salt, HCl and NaCl). With the Multistate Empirical Valence Bond (MS-EVB) methodology, we find that, surprisingly, hydronium can to a certain degree stabilize the hydrophobic solute, neopentane, in the aqueous phase and including at the oil-water interface. At the same time, the sodium cation tends to "salt out" the hydrophobic solute in the expected fashion. When it comes to the solvation structure of the hydrophobic solute in the acidic conditions, hydronium shows an affinity to the hydrophobic solute, as suggested by the radial distribution functions (RDFs). Upon consideration of this interfacial effect, we find that the solvation structure of the hydrophobic solute varies at different distances from the oil-liquid interface due to a competition between the bulk oil phase and the hydrophobic solute phase. Together with an observed orientational preference of the hydroniums and the lifetime of water molecules in the first solvation shell of neopentane, we conclude that hydronium stabilizes to a certain degree the dispersal of neopentane in the aqueous phase and eliminates any salting out effect in the acid solution; i.e., the hydronium acts like a surfactant. The present molecular dynamics study provides new insight into the hydrophobic solute transfer across the water-oil interface process, including for acid and salt solutions.

Review of High-Frequency Ultrasounds Emulsification Methods and Oil/Water Interfacial Organization in Absence of any Kind of Stabilizer

Emulsions are multiphasic systems composed of at least two immiscible phases. Emulsion formulation can be made by numerous processes such as low-frequency ultrasounds, high-pressure homogenization, microfluidization, as well as membrane emulsification. These processes often need emulsifiers’ presence to help formulate emulsions and to stabilize them over time. However, certain emulsifiers, especially chemical stabilizers, are less and less desired in products because of their negative environment and health impacts. Thus, to avoid them, promising processes using high-frequency ultrasounds were developed to formulate and stabilize emulsifier-free emulsions. High-frequency ultrasounds are ultrasounds having frequency greater than 100 kHz. Until now, emulsifier-free emulsions’ stability is not fully understood. Some authors suppose that stability is obtained through hydroxide ions’ organization at the hydrophobic/water interfaces, which have been mainly demonstrated by macroscopic studies. Whereas other authors, using microscopic studies, or simulation studies, suppose that the hydrophobic/water interfaces would be rather stabilized thanks to hydronium ions. These theories are discussed in this review.

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

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.

Development of interface-/surface-specific two-dimensional electronic spectroscopy

Structures, kinetics, and chemical reactivities at interfaces and surfaces are key to understanding many of the fundamental scientific problems related to chemical, material, biological, and physical systems. These steady-state and dynamical properties at interfaces and surfaces require even-order techniques with time-resolution and spectral-resolution. Here, we develop fourth-order interface-/surface-specific two-dimensional electronic spectroscopy, including both two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy and two-dimensional electronic second harmonic generation (2D-ESHG) spectroscopy, for structural and dynamics studies of interfaces and surfaces. The 2D-ESFG and 2D-ESHG techniques were based on a unique laser source of broadband short-wave IR from 1200 nm to 2200 nm from a home-built optical parametric amplifier. With the broadband short-wave IR source, surface spectra cover most of the visible light region from 480 nm to 760 nm. A translating wedge-based identical pulses encoding system (TWINs) was introduced to generate a phase-locked pulse pair for coherent excitation in the 2D-ESFG and 2D-ESHG. As an example, we demonstrated surface dark states and their interactions of the surface states at p-type GaAs (001) surfaces with the 2D-ESFG and 2D-ESHG techniques. These newly developed time-resolved and interface-/surface-specific 2D spectroscopies would bring new information for structure and dynamics at interfaces and surfaces in the fields of the environment, materials, catalysis, and biology.

A Unified Mathematical Formalism for First to Third Order Dielectric Response of Matter: Application to Surface-Specific Two-Colour Vibrational Optical Spectroscopy

To take advantage of the singular properties of matter, as well as to characterize it, we need to interact with it. The role of optical spectroscopies is to enable us to demonstrate the existence of physical objects by observing their response to light excitation. The ability of spectroscopy to reveal the structure and properties of matter then relies on mathematical functions called optical (or dielectric) response functions. Technically, these are tensor Green’s functions, and not scalar functions. The complexity of this tensor formalism sometimes leads to confusion within some articles and books. Here, we do clarify this formalism by introducing the physical foundations of linear and non-linear spectroscopies as simple and rigorous as possible. We dwell on both the mathematical and experimental aspects, examining extinction, infrared, Raman and sum-frequency generation spectroscopies. In this review, we thus give a personal presentation with the aim of offering the reader a coherent vision of linear and non-linear optics, and to remove the ambiguities that we have encountered in reference books and articles.

What Coordinate Best Describes the Affinity of the Hydrated Excess Proton for the Air-Water Interface?

Molecular dynamics simulations and free energy sampling are employed in this work to investigate the surface affinity of the hydrated excess proton with two definitions of the interface: The Gibbs dividing interface (GDI) and the Willard-Chandler interface (WCI). Both the multistate empirical valence bond (MS-EVB) reactive molecular dynamics method and the density functional theory-based ab initio molecular dynamics (AIMD) were used to describe the hydrated excess proton species, including "vehicular" (standard diffusion) transport and (Grotthuss) proton hopping transport and associated structures of the hydrated excess proton net positive charge defect. The excess proton is found to exhibit a similar trend and quantitative free energy behavior in terms of its surface affinity as a function of the GDI or WCI. Importantly, the definitions of the two interfaces in terms of the excess proton charge defect are highly correlated and far from independent of one another, thus undermining the argument that one interface is superior to the other when describing the proton interface affinity. Moreover, the hydrated excess proton and its solvation shell significantly influence the location and local curvature of the WCI, making it difficult to disentangle the interfacial thermodynamics of the excess proton from the influence of that species on the instantaneous surface curvature.

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

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