Specific cation effects at aqueous solution-vapor interfaces: Surfactant-like behavior of Li+ revealed by experiments and simulations

Probing the interfacial structure of aqueous surfactants through helium atom evaporation

Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are "squeezed" out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.

Liquid-jet photoemission spectroscopy as a structural tool: site-specific acid-base chemistry of vitamin C

Liquid-jet photoemission spectroscopy (LJ-PES) directly probes the electronic structure of solutes and solvents. It also emerges as a novel tool to explore chemical structure in aqueous solutions, yet the scope of the approach has to be examined. Here, we present a pH-dependent liquid-jet photoelectron spectroscopic investigation of ascorbic acid (vitamin C). We combine core-level photoelectron spectroscopy and ab initio calculations, allowing us to site-specifically explore the acid-base chemistry of the biomolecule. For the first time, we demonstrate the capability of the method to simultaneously assign two deprotonation sites within the molecule. We show that a large change in chemical shift appears even for atoms distant several bonds from the chemically modified group. Furthermore, we present a highly efficient and accurate computational protocol based on a single structure using the maximum-overlap method for modeling core-level photoelectron spectra in aqueous environments. This work poses a broader question: to what extent can LJ-PES complement established structural techniques such as nuclear magnetic resonance? Answering this question is highly relevant in view of the large number of incorrect molecular structures published.

Molecular Insights into the Influence of Ions on the Water Structure. I. Alkali Metal Ions in Solution

In this study, we explore the impact of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) on the hydration structure of water using molecular dynamics simulations carried out with MB-nrg potential energy functions (PEFs). Our analyses include radial distribution functions, coordination numbers, dipole moments, and infrared spectra of water molecules, calculated as a function of solvation shells. The results collectively indicate a highly local influence of all of the alkali metal ions on the hydrogen-bond network established by the surrounding water molecules, with the smallest and most densely charged Li+ ion exerting the most pronounced effect. Remarkably, the MB-nrg PEFs demonstrate excellent agreement with available experimental data for the position and size of the first solvation shells, underscoring their potential as predictive models for realistic simulations of ionic aqueous solutions across various thermodynamic conditions and environments.

Graphene Nanopores Enhance Water Evaporation from Salt Solutions: Exploring the Effects of Ions and Concentration

With increased water stress, the development of clean water technologies is an active area of research. Evaporation-based solutions offer the advantage of low energy consumption, and recently a 10-30 fold enhancement in water evaporation flux has been observed through Å-scale graphene nanopores (Lee, W.-C., et al., ACS Nano 2022, 16(9), 15382). Herein, using molecular dynamics simulations, we examine the suitability of Å-scale graphene nanopores in enhancing water evaporation from salt solutions (LiCl, NaCl, and KCl). Cation-π interactions between ions and the surface of nanoporous graphene are found to significantly influence ion populations in the nanopore vicinity, leading to varied water evaporation fluxes from different salt solutions. The highest water evaporation flux was observed for KCl solutions, followed by NaCl and LiCl solutions, with the differences reducing at lower concentrations. Relative to the bare liquid-vapor interface, 4.54 Å nanopores exhibit the highest evaporation flux enhancements ranging from 7 to 11, with an enhancement of 10.8 obtained for 0.6 M NaCl solution, which closely resembles seawater compositions. Functionalized nanopores induce short-lived water-water hydrogen bonds and reduce surface tension at the liquid-vapor interface, thereby lowering the free energy barrier for water evaporation with a negligible effect on the ion hydration dynamics. These findings can aid in developing green technologies for desalination and separation processes with low thermal energy input.

Long-Range Electrification of an Air/Electrolyte Interface and Probing Potential of Zero Charge by Conductive Amplitude-Modulated Atomic Force Microscopy

The structure of an electrical double layer (EDL) at the interface of electrode/electrolyte or air/electrode/electrolyte is a fundamental aspect, however not fully understood. The potential of zero charge (PZC) is one of the clues to dictate the EDL, where the excess charge on the electrode surface is zero. Here, a nanoscale configuration of immersion method was proposed by integrating an electrochemical system into conductive atomic force spectroscopy under the amplitude modulation (AM) mode and agarose gel as the solid-liquid electrolyte. The PZC of boron-doped diamond was determined to be at 0.2 V (vs Ag/AgCl). By AM spectroscopy, the capacitive force shows remote electrification without direct electrode/electrolyte contact, which is dependent on the population of ions at the air/electrolyte interface. The surface potential by alignment of water is also evaluated. Prospectively, our study could benefit applications such as PZC measurement and non-electrode electrochemical processes at the air/electrolyte interface.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

Conventional wisdom suggests that cations play a minimal role in the assembly of cationic amphiphiles. Here, we show that at liquid/liquid (L/L) interfaces, specific cation effects can modulate the assemblies of hydrophobic tails in an oil phase despite being attached to cationic headgroups in the aqueous phase. We used oligo-dimethylsiloxane (ODMS) methyl imidazolium amphiphiles to identify these specific interactions at hexadecane/aqueous interfaces. Small cations, such as Li⁺, bind to the O atoms in the ODMS tail and pin it to the interface, thereby imposing a kinked conformation─as evidenced by vibrational sum frequency generation spectroscopy and molecular dynamics simulations. While larger Cs⁺ ions more readily partition to the interface, they do not form analogous complexes. Our data not only point to ways for controlling amphiphile structure at L/L interfaces but also suggest a means for the separation of Li⁺, or related applications, in soft-matter electronics.

Understanding specific ion effects and the Hofmeister series

Specific ion effects (SIE), encompassing the Hofmeister Series, have been known for more than 130 years since Hofmeister and Lewith's foundational work. SIEs are ubiquitous and are observed across the medical, biological, chemical and industrial sciences. Nevertheless, no general predictive theory has yet been able to explain ion specificity across these fields; it remains impossible to predict when, how, and to what magnitude, a SIE will be observed. In part, this is due to the complexity of real systems in which ions, counterions, solvents and cosolutes all play varying roles, which give rise to anomalies and reversals in anticipated SIEs. Herein we review the historical explanations for SIE in water and the key ion properties that have been attributed to them. Systems where the Hofmeister series is perturbed or reversed are explored, as is the behaviour of ions at the liquid-vapour interface. We discuss SIEs in mixed electrolytes, nonaqueous solvents, and in highly concentrated electrolyte solutions - exciting frontiers in this field with particular relevance to biological and electrochemical applications. We conclude the perspective by summarising the challenges and opportunities facing this SIE research that highlight potential pathways towards a general predictive theory of SIE.

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA⁺) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA⁺ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA⁺ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA⁺ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Exploring the Solvation of Acetic Acid in Water Using Liquid Jet X-ray Photoelectron Spectroscopy and Core Level Electron Binding Energy Calculations

Liquid jet X-ray photoelectron spectroscopy was used to investigate changes in the local electronic structure of acetic acid in the bulk of aqueous solutions induced by solvation effects. These effects manifest themselves as shifts in the difference in the carbon 1s binding energy (ΔBE) between the methyl and carboxyl carbons of acetic acid. Furthermore, molecular dynamics simulations, coupled with correlated electronic structure calculations of the first solvation sphere, provide insight into the number of water molecules directly interacting with the carboxyl group that are required to match the ΔBE from the photoelectron spectroscopy experiments. This comparison shows that a single water molecule in the first solvation shell describes the photoelectron ΔBE of acetic acid while at least 20 water molecules are required for the conjugate base, acetate, in aqueous solutions.

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.

Image-charge effects on ion adsorption near aqueous interfaces

Electrostatic interactions near surfaces and interfaces are ubiquitous in many fields of science. Continuum electrostatics predicts that ions will be attracted to conducting electrodes but repelled by surfaces with lower dielectric constant than the solvent. However, several recent studies found that certain "chaotropic" ions have similar adsorption behavior at air/water and graphene/water interfaces. Here we systematically study the effect of polarization of the surface, the solvent, and solutes on the adsorption of ions onto the electrode surfaces using molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two parallel conducting surfaces by exploiting the mirror-expanded symmetry of the exact image-charge solution. With neutral surfaces, the image interactions induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the surface polarity. Under an external electric field, the adsorption of ions is strongly affected by the surface polarization, such that the charge separation across the electrolyte and the capacitance of the cell is greatly enhanced with a conducting surface over a low-dielectric-constant surface. While the extent of ion adsorption is highly dependent on the electrolyte model (the polarizability of solvent and solutes, as well as the van der Waals radii), we find the effect of surface polarization on ion adsorption is consistent throughout different electrolyte models.

Core level photoelectron spectroscopy of heterogeneous reactions at liquid-vapor interfaces: Current status, challenges, and prospects

Liquid-vapor interfaces, particularly those between aqueous solutions and air, drive numerous important chemical and physical processes in the atmosphere and in the environment. X-ray photoelectron spectroscopy is an excellent method for the investigation of these interfaces due to its surface sensitivity, elemental and chemical specificity, and the possibility to obtain information on the depth distribution of solute and solvent species in the interfacial region. In this Perspective, we review the progress that was made in this field over the past decades and discuss the challenges that need to be overcome for investigations of heterogeneous reactions at liquid-vapor interfaces under close-to-realistic environmental conditions. We close with an outlook on where some of the most exciting and promising developments might lie in this field.

Single-Particle Dynamics at the Intrinsic Surface of Aqueous Alkali Halide Solutions

The distribution of ions in the proximity of the liquid-vapor interface of their aqueous solution has been the subject of an intense debate during the last decade. The effects of ionic polarizability have been one of its salient aspects. Much less has been said about the corresponding dynamical properties, which are substantially unexplored. Here, we investigate the single-particle dynamics at the liquid-vapor interface of several alkali halide solutions, using molecular dynamics simulations with polarizable and nonpolarizable force fields and intrinsic surface analysis. We analyze the diffusion coefficient, residence time, and velocity autocorrelation function of water and ions and investigate how these properties depend on the molecular layer where they reside. While anions are found in the first molecular layer for relatively long times, cations are only making quick excursions into it, thanks to thermal fluctuations. The in-layer residence time of ions and their molar fraction in the layer turned out to be linearly dependent on each other. We interpret this unexpected result using a simple two-state model. In addition, we found that, unlike water and other neat molecular liquids that show a different diffusion mechanism at the surface than in the bulk of their liquid phase, ions do not enjoy enhanced mobility in the surface layer of their aqueous solution. This result indicates that ions in the surface layer are shielded by their nearest water neighbors from being exposed to the vapor phase as much as possible. Such positions are available for the ions at the negatively curved troughs of the molecularly rugged liquid surface.

The role of anions in light-driven conductivity in diarylethene-containing polymeric ionic liquids

The role of anion character in the photostationary state, magnitude of conductivity, and light-responsive properties of diarylethene-containing polymeric ionic liquids was investigated.

Competing Segregation of Br- and Cl- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles

Ion-surface scattering experiments can be used to measure elemental depth profiles on the angstrom scale in complex liquid mixtures. We employ NICISS (neutral impact collision ion scattering spectroscopy) to measure depth profiles of dissolved ions and solvent in liquid glycerol containing the cationic surfactant tetrahexylammonium bromide (THA⁺/Br^-) at 0.013 M and mixtures of NaBr + NaCl at 0.4 M total concentration. The experiments reveal that Br^- outcompetes Cl^- in its attraction to surface THA⁺, and that THA⁺ segregates more extensively when more Br^- ions are present. Intriguingly, the depths spanned by THA⁺, Br^-, and Cl^- ions generally increase with Br^- bulk concentration, expanding from ∼10 to ∼25 Å for both Br^- and Cl^- depth profiles. This broadening likely occurs because of an increasing pileup of THA⁺ ions in a multilayer region that spreads the halide ions over a wider depth. The experiments indicate that cationic surfactants enhance Br^- and Cl^- concentrations in the surface region far beyond their bulk-phase values, making solutions coated with these surfactants potentially more reactive toward gases that can oxidize the halide ions.

Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis

The surface tension of all aqueous alkali halide solutions is higher than that of pure water. According to the Gibbs adsorption equation, this indicates a net depletion of these ions in the interfacial region. However, simulations and experiments show that large, soft ions, such as I^-, can accumulate at the liquid/vapor interface. The presence of a loose hydration shell is usually considered to be the reason for this behavior. In this work, we perform computer simulations to characterize the liquid-vapor interface of aqueous alkali chloride and sodium halide solutions systematically, considering all ions from Li⁺ to Cs⁺ and from F^- to I^-. Using computational methods for the removal of surface fluctuations, we analyze the structure of the interface at a dramatically enhanced resolution, showing that the positive excess originates in the very first molecular layer and that the next 3-4 layers account for the net negative excess. With the help of a fictitious system with charge-inverted ion pairs, we also show that it is not possible to rationalize the surface affinity of ions in solutions in terms of the properties of anions and cations separately. Moreover, the surface excess is generally dominated by the smaller of the two ions.

Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out

Neutral impact ion scattering spectroscopy (NICISS) is used to measure the depth profiles of ionic surfactants, counterions, and solvent molecules on the angstrom scale. The chosen surfactants are 0.010 m tetrahexylammonium bromide (THA⁺/Br^-) and 0.0050 m sodium dodecyl sulfate (Na⁺/DS^-) in the absence and presence of 0.30 m NaBr in liquid glycerol. NICISS determines the depth profiles of the elements C, O, Na, S, and Br through the loss in energy of 5 keV He atoms that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA⁺ and its Br^- counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and 150 times more concentrated than in the bulk. With the addition of NaBr, THA⁺ is "salted out" to the surface, increasing the interfacial Br^- concentration by 3-fold and spreading the anions over a ∼30 Å depth. Added NaBr similarly increases the interfacial concentration of DS^- ions and broadens their positions. Conversely, the dissolved Br^- ions are significantly depleted over a depth of 0-40 Å from the surface because of charge repulsion from DS^- ions within the interfacial region. These different interfacial Br^- propensities correlate with previously measured gas-liquid reactivities: gaseous Cl₂ readily reacts with Br^- ions in the presence of THA⁺ but drops 70-fold in the presence of DS^-, demonstrating that surfactant headgroup charge controls the reactivity of Br^- through changes in its depth profile.

Ion at Air-Water Interface Enhances Capillary Wave Fluctuations: Energetics of Ion Adsorption

Recent simulations provide the energetics of ion adsorption at the air-water (a/w) interface: The presence of the ion at the interface suppresses the fluctuations of the capillary waves (CWs) reducing the entropy and displaces the weakly interacting water molecules to the bulk causing a reduction in the enthalpy. Here, we provide atomistic simulation-based evidence that the inferences of the existing studies stem from considering a small simulation volume that pins the CWs. For an appropriate size of the simulation system, an ion at the a/w interface enhances the CW fluctuations. Furthermore, we discover that the characteristics of the waves governing these enhanced CW fluctuations ensure a significant decrease in the pressure-volume work causing the enthalpy decrease, while the same wave characteristics of the CWs become responsible for an entropy decrease. Overall, the paper revisits the free energy picture of this fundamental problem of ion adsorption at the a/w interface.

Enrichment of Surface-Active Compounds in Bursting Bubble Aerosols

The pronounced enrichment of surface-active compounds in the aerosols produced by bubble bursting plays a central role in the chemical transfer from the ocean into the atmosphere and has an important impact on the global Earth's climate. However, the mechanism of chemical enrichment in bursting bubble aerosols remains poorly understood and controversial due to the high complexity and diversity of experimental behaviors. Contrary to the common belief, here we show that the major share of surfactants in the jet droplets produced by individually bursting bubbles at a calm solution surface is released directly from the bubble surface rather than from the solution surface or subsurface microlayer. We reveal that surfactants are accumulated at the surface of a rising bubble in solution following three successive stages with strongly distinct adsorption profiles: linear kinetic, mixed kinetic, and equilibrium. The magnitude of surfactant enrichment in the aerosol is directly determined by which adsorption mode is in control by the moment of the bubble bursting at solution surface. Our mechanistic description explains the diversity of experimental observations regarding the surfactant enrichment in aerosol droplets and lays the ground for understanding the more complex behaviors associated with collective effects at the solution surface (e.g., breaking waves).