Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with Thermodynamics

Solvation of Nanoions in Aqueous Solutions

In recent years it has been increasingly recognized that different classes of large ions with multiple valency have effects conceptually similar to weakly solvated ions in the Hofmeister series, also labeled by the term chaotropic. The term "superchaotropic effect" has been coined because these effects are much more strongly pronounced for nanometer-sized ions, whose adsorption properties often resemble typical surfactants. Despite this growing interest in these nanometer-sized ions, a simple conceptual extension of the Hofmeister series toward nanoions has not been achieved because an extrapolation of the one-dimensional surface charge density scale does not lead to the superchaotropic regime. In this work, we discuss a generic model that is broadly applicable to ions of nearly spherical shape and thus includes polyoxometalates and boron clusters. We present a qualitative classification scheme in which the ion size appears as a second dimension. Ions of different sizes but the same charge density differ in their bulk solvation free energy. As the ions grow bigger at constant surface charge density, they become more stable in solution, but the adsorption behavior is still governed by the surface charge density. A detailed molecular dynamics simulation study of large ions that is based on a shifted Lennard-Jones potential is presented that supports the presented classification scheme.

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.

A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces

Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.

Bursting the bubble: A molecular understanding of surfactant-water interfaces

Surfactant science has historically emphasized bulk, thermodynamic measurements to understand the microemulsion properties of greatest industrial significance, such as interfacial tensions, phase behavior, and thermal stability. Recently, interest in the molecular properties of surfactants has grown among the physical chemistry community. This has led to the application of cutting-edge spectroscopic methods and advanced simulations to understand the specific interactions that give rise to the previously studied bulk characteristics. In this Perspective, we catalog key findings that describe the surfactant-oil and surfactant-water interfaces in molecular detail. We emphasize the role of ultrafast spectroscopic methods, including two-dimensional infrared spectroscopy and sum-frequency-generation spectroscopy, in conjunction with molecular dynamics simulations, and the role these techniques have played in advancing our understanding of interfacial properties in surfactant microemulsions.

Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate-ion (TPB^- )/water and tetraphenylarsonium-ion (TPA⁺ )/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB^- interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA⁺ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.

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.

Hydration-Shell Vibrational Spectroscopy

Hydration-shell vibrational spectroscopy provides an experimental window into solute-induced water structure changes that mediate aqueous folding, binding, and self-assembly. Decomposition of measured Raman and infrared (IR) spectra of aqueous solutions using multivariate curve resolution (MCR) and related methods may be used to obtain solute-correlated spectra revealing solute-induced perturbations of water structure, such as changes in water hydrogen-bond strength, tetrahedral order, and the presence of dangling (non-hydrogen-bonded) OH groups. More generally, vibrational-MCR may be applied to both aqueous and nonaqueous solutions, including multicomponent mixtures, to quantify solvent-mediated interactions between oily, polar, and ionic solutes, in both dilute and crowded fluids. Combining vibrational-MCR with emerging theoretical modeling strategies promises synergetic advances in the predictive understanding of multiscale self-assembly processes of both biological and technological interest.

The chemical reactions in electrosprays of water do not always correspond to those at the pristine air-water interface

The recent application of electrosprays to characterize the air-water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene-water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, ab initio calculations and molecular dynamics simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air-water interfaces and oil-water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air-water interface.

Counteracting Interfacial Energetics for Wetting of Hydrophobic Surfaces in the Presence of Surfactants

Surface active agents (surfactants) are commonly used to improve the wetting of aqueous solutions on hydrophobic surfaces. The improved wettability is usually quantified as a decrease of the contact angle θ of a droplet on the surface, where the contact angle θ is given by the three surface tensions involved. Surfactants are known to lower the liquid-vapor surface tension, but what they do to the two other surface tensions is less clear. We propose an improved Zisman method for quantifying the wetting behavior of surfactants at the solid surface. This allows us to show that a number of very common surfactants do not change the wettability of the solid: they give the same contact angle as a simple liquid with the same liquid-vapor surface tension. Surface-specific sum-frequency generation spectroscopy shows that nonetheless surfactants are present at the solid surface. The surfactants therefore change the solid-liquid and solid-vapor surface tensions by the same amount, leading to an unchanged contact angle.

Alkyl Chain Length Dependent Structural and Orientational Transformations of Water at Alcohol-Water Interfaces and Its Relevance to Atmospheric Aerosols

Although the hydrophobic size of an amphiphile plays a key role in various chemical, biological, and atmospheric processes, its effect at macroscopic aqueous interfaces (e.g., air-water, oil-water, cell membrane-water, etc.), which are ubiquitous in nature, is not well understood. Here we report the hydrophobic alkyl chain length dependent structural and orientational transformations of water at alcohol (C_nH_2n+1OH, n = 1-12)-water interfaces using interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman multivariate curve resolution (Raman-MCR) spectroscopic techniques. The HD-VSFG results reveal that short-chain alcohols (C_nH_2n+1OH, n < 4, i.e., up to 1-propanol) do not affect the structure (H-bonding) and orientation of water at the air-water interface; the OH stretch band maximum appears at ∼3470 cm^-1, and the water H atoms are pointed toward the bulk water, that is, "H-down" oriented. In contrast, long-chain alcohols (C_nH_2n+1OH, n > 4, i.e., beyond 1-butanol) make the interfacial water more strongly H-bonded and reversely orientated; the OH stretch band maximum appears at ∼3200 cm^-1, and the H atoms are pointed away from the bulk water, that is, "H-up" oriented. Interestingly, for the alcohol of intermediate chain length (C_nH_2n+1OH, n = 4, i.e, 1-butanol), the interface is quite unstable even after hours of its formation and the time-averaged result is qualitatively similar to that of the long-chain alcohols, indicating a structural/orientational crossover of interfacial water at the 1-butanol-water interface. pH-dependent HD-VSFG measurements (with H₂O as well as isotopically diluted water, HOD) suggest that the structural/orientational transformation of water at the long-chain alcohol-water interface is associated with the adsorption of OH^- anion at the interface. Vibrational mapping of the water structure in the hydration shell of OH^- anion (obtained by Raman-MCR spectroscopy of NaOH in HOD) clearly shows that the water becomes strongly H-bonded (OH stretch max. ≈ 3200 cm^-1) while hydrating the OH^- anion. Altogether, it is conceivable that alcohols of different hydrophobic chain lengths that are present in the troposphere will differently affect the interfacial electrostatics and associated chemical processes of aerosol droplets, which are critical for cloud formation, global radiation budget, and climate change.