On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface-Active Oligomer Species at the Air–Water Interface

Infrared Reflection Absorption Spectroscopy (IRRAS) of Water-Soluble Surfactants: Is it Surface-Specific?

Infrared reflection absorption spectroscopy (IRRAS) is commonly used to study the structure and chemistry of molecules residing at surfaces, including water surfaces, which has far-reaching applications including atmospheric chemistry and food science. However, there is some debate regarding the surface-specificity of IRRAS when examining soluble surfactants on aqueous solutions, and there is some evidence that the surface-specificity may differ between IRRAS of ionic surfactants and soluble organic acids. This paper presents infrared reflection absorption (IRRA) spectra of soluble organic acids underneath monolayers of insoluble surfactants, where the contributions from the insoluble surfactants are subtracted from the spectra to capture "subsurface effects". These "subsurface" spectra demonstrate that IRRA spectra of soluble organic acids are surface specific, and this observation is supported by a simplified model for reflections from "subsurface" layers. Finally, the observations presented here are compared to literature observations regarding the surface-specificity of IRRAS when studying ionic surfactants. Overall, this work demonstrates the utility of IRRAS for studying the structures and chemistry of soluble organic acids at aqueous surfaces.

Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation at the Air-Water Interface in Aqueous Microdroplets

The chemistry of pyruvic acid (PA) under thermal dark conditions is limited in bulk solutions, but in microdroplets it is shown to readily occur. Utilizing in situ micro-Raman spectroscopy as a probe, we investigated the chemistry of PA within aqueous microdroplets in a relative humidity- and temperature-controlled environmental cell. We found that PA undergoes a condensation reaction to yield mostly zymonic acid. Interestingly, the reaction follows a size-dependent sigmoidal kinetic profile, i.e., an induction period followed by reaction and then completion. The induction time is linearly proportional to the surface area (R2), and the maximum apparent reaction rate is proportional to the surface-to-volume ratio (1/R), showing that both the induction and reaction occur at the air-water interface. Furthermore, the droplet size is shown to be dynamic due to changes in droplet composition and re-equilibration with the relative humidity within the environmental cell as the reaction proceeds. Overall, the size-dependent sigmoidal kinetics, shown for the first time in microdroplets, demonstrates the complexity of the reaction mechanism and the importance of the air-water interface in the pyruvic acid condensation reaction.

"Nonlinear" pursuit of understanding pollutant accumulation and chemistry at environmental and biological interfaces

Over the past few decades, the public recognition of the prevalence of certain classes of pollutants, such as perfluoroalkyl substances and nanoplastics, within the environment, has sparked growing concerns over their potential impact on environmental and human health. Within both environmental and biological systems, the adsorption and structural organization of pollutants at aqueous interfaces can greatly impact the chemical reactivity and transformation. Experimentally probing chemical behavior at interfaces can often pose a problem due to bulk solvated molecules convoluting molecular signatures from interfacial molecules. To solve this problem, there exist interface-specific nonlinear spectroscopy techniques that can directly probe both macroscopic planar interfaces and nanoplastic interfaces in aqueous environments. These techniques can provide essential information such as chemical adsorption, structure, and reactivity at interfaces. In this perspective, these techniques are presented with obvious advantages for studying the chemical properties of pollutants adsorbed to environmental and biological interfaces.

Infrared Reflection-Absorption Spectroscopy of α-Keto Acids at the Air-Water Interface: Effects of Chain Length and Headgroup on Environmentally Relevant Surfactant Films

A variety of organic surfactants are found at air-water interfaces in natural environments, including on the surfaces of aqueous aerosols. The structure and morphology of these organic films can have profound impacts on material transfer between the gas and condensed phases, the optical properties of atmospheric aerosol, and chemical processing at air-water interfaces. Combined, these effects can have significant impacts on climate via radiative forcing, but our understanding of organic films at air-water interfaces is incomplete. Here, we examine the impact of the polar headgroup and alkyl tail length on the structure and morphology of organic monolayers at the air-water interfaces. First, we focus on the substituted carboxylic acids, α-keto acids, using Langmuir isotherms and infrared reflection absorption spectroscopy (IR-RAS) to elucidate key structures and phase behaviors of α-keto acids with a range of surface activities. We show that the structure of α-keto acids, both soluble and insoluble, at water surfaces is a compromise between van der Waals interactions of the hydrocarbon tail and hydrogen bonding interactions involving the polar headgroup. Then, we use this new data set regarding α-keto acid films at water surfaces to examine the role of the polar headgroup on organic films using a similar substituted carboxylic acid (α-hydroxystearic acid), an unsubstituted carboxylic acid (stearic acid), and an alcohol (stearyl alcohol). We show that the polar headgroup and its hydrogen bonding interactions can significantly affect the orientation of amphiphiles at air-water interfaces. Here, we provide side-by-side comparisons of Langmuir isotherms and IR-RA spectra for a set of environmentally relevant organic amphiphiles with a range of alkyl tail lengths and polar headgroup structures.

A DFT Study of the Hydrogen Bonded Structures of Pyruvic Acid–Water Complexes

The molecular geometries of the possible conformations of pyruvic acid–water complexes (PA-(H2O)n = 1–4) have been fully optimized at DFT/B3LYP/6-311G++ (d, p) levels of calculation. Among several optimized molecular clusters, we present here the most stable molecular arrangements obtained when one, two, three, and four water molecules are hydrogen-bonded to a central pyruvic acid molecule. Appropriate topological and geometrical parameters are considered primary indicators of H-bond strength. Atoms in molecules analysis shows that pyruvic acid can form a ring structure with water, and the molecular structures are stabilized by both strong O–H⋅⋅⋅O and C–H⋅⋅⋅O hydrogen bonds. In large clusters, classical O–H⋅⋅⋅O hydrogen bonds still exist between water molecules, and a cage-like structure is built around some parts of the central molecule of pyruvic acid.

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Infrared spectroscopy of 2-oxo-octanoic acid in multiple phases

Alpha-keto acids are environmentally and biologically relevant species whose chemistry has been shown to be influenced by their local environment. Vibrational spectroscopy provides useful ways to probe the potential inter- and intramolecular interactions available to them in several phases. We measure and compare the IR spectra of 2-oxo-octanoic acid (2OOA) in the gas phase, solid phase, and at the air-water interface. With theoretical support, we assign many of the vibrational modes in each of the spectra. In the gas phase, two types of conformers are identified and distinguished, with the intramolecularly H-bonded form being the dominant type, while the second conformer type identified does not have an intramolecular hydrogen bond. The van der Waals interactions between molecules in solid 2OOA manifest C-H and CO vibrations lower in energy than in the gas phase and we propose an intermolecular hydrogen bonding scheme for the solid phase. At the air-water interface the hydrocarbon tails of 2OOA do interact with each other while the carbonyls appear to interact with water in the subphase, but not with neighboring 2OOA as might be expected of a closely packed surfactant film.

Theoretical Chemistry and the Calculation of the Atmospheric State

Theoretical chemists have been actively engaged for some time in processes such as ozone photodissociation, overtone photodissociation in nitric acid, pernitric acid, sulphuric acid, clusters and in small organic acids. The last of these have shown very different behaviours in the gas phase, liquid phase and importantly at the air–water interface in aqueous aerosols. The founder of molecular dynamics, B J Alder, pointed out long ago that hydrodynamic behaviour emerged when the symmetry of a random, thermalised population of hard spheres—billiard balls—was broken by a flux of energetic molecules. Despite this, efforts over two centuries to solve turbulence by finding top-down solutions to the Navier–Stokes equation have failed. It is time for theoretical chemistry to try a bottom-up solution. Gibbs free energy that drives the circulation arises from the entropy difference between the incoming low-entropy beam of visible and ultraviolet photons and the outgoing higher-entropy flux of infrared photons over the whole 4π solid angle. The role of the most energetic molecules with the highest velocities will affect the rovibrational line shapes of water, carbon dioxide and ozone in the far wings, where there is the largest effect on radiative transfer and hence on calculations of atmospheric temperature. The atmospheric state is determined by the interaction of radiation, chemistry and fluid dynamics on the microscopic scale, with propagation through the mesoscale to the macroscale. It will take theoretical chemistry to simulate that accurately. A challenging programme of research for theoretical chemistry is proposed, involving ab initio simulation by molecular dynamics of an air volume, starting in the upper stratosphere. The aim is to obtain scaling exponents for turbulence, providing a physical method for upscaling in numerical models. Turbulence affects chemistry, radiation and fluid dynamics at a fundamental, molecular level and is thus of basic concern to theoretical chemistry as it applies to the atmosphere, which consists of molecules in motion.

Water-Air Interfaces as Environments to Address the Water Paradox in Prebiotic Chemistry: A Physical Chemistry Perspective

The asymmetric water-air interface provides a dynamic aqueous environment with properties that are often very different than bulk aqueous or gaseous phases and promotes reactions that are thermodynamically, kinetically, or otherwise unfavorable in bulk water. Prebiotic chemistry faces a key challenge: water is necessary for life yet reduces the efficiency of many biomolecular synthesis reactions. This perspective considers water-air interfaces as auspicious reaction environments for abiotic synthesis. We discuss recent evidence that (1) water-air interfaces promote condensation reactions including peptide synthesis, phosphorylation, and oligomerization; (2) photochemistry at water-air interfaces may have been a significant source of prebiotic molecular complexity, given the lack of oxygen and increased availability of near-ultraviolet radiation on early Earth; and (3) water-air interfaces can promote spontaneous reduction and oxidation reactions, potentially providing protometabolic pathways. Life likely began within a relatively short time frame, and water-air interfaces offer promising environments for simultaneous and efficient biomolecule production.

Assessing the Impact of Solvent Selection on Vibrational Sum-Frequency Scattering Spectroscopy Experiments

The development of vibrational sum-frequency scattering (S-VSF) spectroscopy has opened the door to directly probing nanoparticle surfaces with an interfacial and chemical specificity that was previously reserved for planar interfacial systems. Despite its potential, challenges remain in the application of S-VSF spectroscopy beyond simplified chemical systems. One such challenge includes infrared absorption by an absorptive continuous phase, which will alter the spectral lineshapes within S-VSF spectra. In this study, we investigate how solvent vibrational modes manifest in S-VSF spectra of surfactant stabilized nanoemulsions and demonstrate how corrections for infrared absorption can recover the spectral features of interfacial solvent molecules. We also investigate infrared absorption for systems with the absorptive phase dispersed in a nonabsorptive continuous phase to show that infrared absorption, while reduced, will still impact the S-VSF spectra. These studies are then used to provide practical recommendations for anyone wishing to use S-VSF to study nanoparticle surfaces where absorptive solvents are present.

Photoinduced Oxidation Reactions at the Air-Water Interface

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.