Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy.

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Šdeep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.

Interfacial Insights into a Carbon Capture System: CO2 Uptake to an Aqueous Monoethanolamine Surface.

Monoethanolamine (MEA) is a benchmark scrubber for CO2 gas emissions reductions. Preliminary studies have indicated surface monoethanolamine could influence the greater chemistry of CO2 uptake. MEA is known to be surface active and orients at aqueous surfaces such that its nitrogen lone pair electrons are pointing toward the gas phase. This MEA orientation has the potential to facilitate CO2 surface chemistry; however, a thorough description of the chemistry at play during this important carbon capture reaction is lacking. These studies investigate the surface behavior of MEA during CO2 gas flow, monitoring product formation and species migration. A combination of experimental vibrational sum frequency spectroscopy (VSFS), surface tensiometry, molecular dynamics simulations, and density functional calculations are used to investigate this complex chemistry. CO2 is shown to react with MEA, perturbing the interface and leading to the presence of carbamic acid in the surface region. However, throughout this chemistry the surface remains largely populated by unreacted MEA. These studies provide insight into this important carbon capture reaction and provide a framework for future work examining interfacial reactions and dynamics.

Correction: In solution SERS sensing using mesoporous silica-coated gold nanorods.

Correction for 'In solution SERS sensing using mesoporous silica-coated gold nanorods' by Zhe Gao, et al., Analyst, 2016, 141, 5088-5095.

In solution SERS sensing using mesoporous silica-coated gold nanorods.

Mesoporous silica-coated gold nanorods (AuNR@MS) act as a colloidally stable Raman sensing platform with a built-in analyte size cutoff. Herein, these core-shell plasmonic nanostructures were presented with a range of thiolated Raman-active molecules to probe the limits of this platform for SERS sensing. The experimental results show generally, that the transport of molecules through the mesopores is highly dependent on the size of the molecule and specifically, that AuNR@MS with pores of ∼4 nm diameter are able to sense analytes with molecular dimensions smaller than 1.5 nm. This sensing platform will likely find broad use, performing well even in complex media based on the high colloidal stability imbued by the mesoporous silica shell.

Solvation station: microsolvation for modeling vibrational sum-frequency spectra of acids at aqueous interfaces.

Vibrational sum-frequency spectra of a pair of poly(methacrylic acid) isomers at an oil/water interface and glutaric acid at an air/water interface were calculated in the carbonyl stretching region. Orientational, conformational, and solvation information was determined using classical molecular dynamics (MD), while second-order susceptibility vibrational response tensors were determined for a set of density functional theory (DFT) structures. The DFT structures were microsolvated with water molecules corresponding to the major solvation states present in the MD calculations. The inclusion of the microsolvating waters incorporates solvation effects important to the carboxylic acid stretching modes in the studied spectral region. The calculated spectra strongly agree with experimental spectra when a cutoff of 1.975 Å is used to define a hydrogen bond in the MD trajectories. With the chosen cutoff, the most common solvation state of the carboxylic acid moieties involves a single hydrogen bond to the carbonyl oxygen and a single hydrogen bond to the carboxylic acid hydrogen. The sensitivity of the spectra to the hydrogen bond cutoff definition and the included DFT structures was investigated. Moderate changes in the relative intensities of the contributing peaks were found in both cases. Shortening the hydrogen bond cutoff definition predictably leads to a decrease in the relative intensity of peaks corresponding to well-solvated structures, while altering the set of DFT solvation structures results in more complex behavior that is dependent on the specific structures included.

A means to an interface: investigating monoethanolamine behavior at an aqueous surface.

The use of amine scrubbers to trap carbon dioxide from flue gas streams is one of the most promising avenues for atmospheric carbon dioxide reduction. However, modifications are necessary to efficiently scale these scrubbers for use in fossil fuel plants. Current advances in tailoring amines for CO2 capture involve improvements of bulk kinetic and thermodynamic parameters, with little consideration to surface chemistry and behavior. Aqueous alkanolamine solutions, such as monoethanolamine (MEA), are currently highly favored sorbents in CO2 post-combustion capture. Although numerous studies have explored MEA-CO2 chemistry at the macroscopic scale, few have investigated the role of the interface in the gas adsorption process. Additionally, as these amines become more industrially ubiquitous, their presence on and the need to understand their behavior at atmospheric and environmental surfaces will increase. This study investigates the surface behavior of monoethanolamine at the vapor/water interface, with particular focus on MEA's surface orientation and footprint. Using vibrational sum frequency spectroscopy, surface tensiometry, and computational techniques, MEA is found to adopt a constrained gauche interfacial conformation with its methylene backbone oriented toward the vapor phase and its functional groups solvated in the bulk solution. Computational and experimental analysis agree well, giving a complete picture with vibrational mode assignments and surface orientation of MEA. These findings can assist in the tailoring of amine structures or to facilitate improvements in engineering design to exploit favorable surface chemistry, as well as to serve as a starting point toward understanding aqueous amine surface behavior relevant to environmental systems.

Hydration, Orientation, and Conformation of Methylglyoxal at the Air-Water Interface.

Aqueous-phase processing of methylglyoxal (MG) has been suggested to constitute an important source of secondary organic aerosol (SOA). The uptake of MG to aqueous particles is higher than expected because its carbonyl moieties can hydrate to form geminal diols, as well as because MG and its hydration products can undergo aldol condensation reactions to form larger oligomers in solution. MG is known to be surface active, but an improved description of its surface behavior is crucial to understanding MG-SOA formation. These studies investigate MG adsorption, focusing on its hydration state at the air-water interface, using a combined experimental and theoretical approach that involves vibrational sum frequency spectroscopy, molecular dynamics simulations, and density functional theory calculations. Together, the experimental and theoretical data show that MG exists predominantly in a singly hydrated state (diol) at the interface, with a diol-tetrol ratio at the surface higher than that for the bulk. In addition to exhibiting a strong surface activity, we find that MG significantly perturbs the water structure at the interface. The results have implications for understanding the atmospheric fate of methylglyoxal.

Twist and turn: effect of stereoconfiguration on the interfacial assembly of polyelectrolytes.

Understanding the conditions that promote the adsorption, assembly, and accumulation of charged macromolecules at the interface between aqueous and hydrophobic liquids is important to a multitude of biological, environmental, and industrial processes. Here, the oil-water interfacial behavior of stereoisomers of polymethacrylic acid (PMA), a model system for both naturally occurring and synthetic polyelectrolytes, is investigated with a combination of vibrational sum-frequency (VSF) spectroscopy, surface tension, and computations. Syndiotactic and isotactic isomers both show rapid adsorption to the oil-water interface with a net orientation indicative of a high degree of ordering. The stereoconfiguration is found to affect whether only a single layer or multiple layers assemble at the interface. Surface tension measurements show additional adsorption for syndiotactic PMA over time. The additional layers do not contribute to the VSF spectrum indicating disorder in all but the initial layer. The isotactic isomer shows no evidence of accumulation at the interface beyond the single ordered layer. Molecular dynamics calculations show marked differences between the two isomers in the orientation of their substituent groups at the interface. The hydrophilic and hydrophobic moieties in the isotactic isomer are easily partitioned to the water and oil phases, respectively, whereas a fair portion of hydrophobic groups remain in the water phase for the syndiotactic PMA. The available hydrophobic contacts in the water phase at the interface are credited with allowing further adsorption.

Doubling down: delving into the details of diacid adsorption at aqueous surfaces.

The behavior of complex interfacial systems is central to an ever-increasing number of applications. Vibrational sum frequency (VSF) spectroscopy is a powerful technique for obtaining surface specific structural information. The coherent nature of VSF that provides surface specificity, however, also creates difficulty in spectral interpretation especially as the system complexity increases. Computations of VSF spectra shed light on the molecular level source of the experimental VSF signal, allowing for the analysis of more complicated systems. Unfortunately, the majority of calculations of VSF spectra look at the response of the solvent or of rigid molecules and therefore often poorly reflect the experimental environment of most VSF spectroscopic measurements. In this work, flexible solute molecules at interfaces are investigated by doubling down, obtaining and comparing experimental and theoretical spectra, to determine a more accurate computational treatment. The surface behavior and VSF spectra of glutaric acid and adipic acid at the air/water interface are determined experimentally and calculated using a combination of classical molecular dynamics and density functional theory. Both diacids are found to be surface active. At high concentrations, glutaric acid forms dimers altering its VSF response and acidic properties. Calculated VSF spectra are found to be sensitive to vibrational mode frequencies, with ordering and spacing affecting relative intensities, as well as molecular conformation. A proper description requires consideration of multiple conformers and anharmonic effects on the molecular vibrational energies.

Ion-induced reorientation and distribution of pentanone in the air-water boundary layer.

Organic material at the surface of atmospheric aerosols is ubiquitous and plays an important role in Earth's atmosphere. Small ketones, such as 3-pentanone, are found in aerosols and as surface-active species on aerosols. This study uses 3-pentanone as a model ketone to understand how such molecules adsorb at the vapor-water interface on aqueous solutions containing sulfate, carbonate, or chloride ions. By combining surface spectroscopic experiments with computational methods, very detailed information about the molecular bonding, geometries, and surface orientation of 3-pentanone as a function of depth has been obtained. The results show that, for pure water, 3-pentanone resides at the topmost surface of water with the carbonyl pointing into the aqueous phase where it is weakly solvated. For Na2SO4-containing solutions, we found that sulfate ions in the boundary layer provoke changes in the geometry and interfacial position of 3-pentanone that are not seen in solutions containing sodium chloride or sodium carbonate. The results provide important insight into the behavior of ketones in the presence of salts at the surface of aerosols in the atmosphere.

Sink or surf: atmospheric implications for succinic acid at aqueous surfaces.

Small organic compounds are increasingly being invoked as important players in atmospheric processes that occur on aerosol surfaces. The diacid succinic acid is one such constituent that is prevalent in the troposphere, surface active, and also water-soluble. This article presents a thorough examination of the surface characteristics of succinic acid at the vapor/water interface using a combination of theoretical simulation and experiments using vibrational sum frequency spectroscopy and surface tension. The adsorption and orientation of succinic acid at the water surface is characterized for a series of aqueous solution compositions relevant to atmospheric conditions. Fully protonated succinic acid is found to be particularly surface active. A new computational technique is introduced that provides a detailed picture of the different surface species that are contributing to the experimentally derived spectroscopic measurements. Additional results are presented for how SO2, a copollutant of succinic acid in the atmosphere, behaves at a water surface in the presence of surface adsorbed succinic acid.

Staying hydrated: the molecular journey of gaseous sulfur dioxide to a water surface.

A water surface is a dynamic and constantly evolving terrain producing a vast array of unique molecular properties and interactions with chemical species in the environment. The complex dynamics of water surfaces permit life on earth to continue, but also complicate the development of a complete microscopic picture of the specific behaviors that take place within interfacial aqueous environments. This computational study examines a piece of the water puzzle by elucidating the bonding, dynamic interactions, and hydrate structures of sulfur dioxide gas adsorbing to a water cluster. Results described herein address the specific ways in which sulfur dioxide gas molecules bind to a water cluster, and paint a more complete picture of the adsorption pathway than was previously developed from experimental and computational studies. Ab initio molecular dynamics have been employed to study sulfur dioxide and water interactions at two environmentally relevant temperatures on a water cluster. The results of this study on a common environmental and industrially important gas provide molecular insight to aid our understanding of interactions on aqueous surfaces, and gaseous adsorption processes.

Near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) for the identification of eosin Y: theoretical calculations and evaluation of two different nanoplasmonic substrates.

This work demonstrates the development of near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) for the identification of eosin Y, an important historical dye. NIR-SERS benefits from the absence of some common sources of SERS signal loss including photobleaching and plasmonic heating, as well as an advantageous reduction in fluorescence, which is beneficial for art applications. This work also represents the first rigorous comparison of the enhancement factors and the relative merits of two plasmonic substrates utilized in art applications; namely, citrate-reduced silver colloids and metal film over nanosphere (FON) substrates. Experimental spectra are correlated in detail with theoretical absorption and Raman spectra calculated using time-dependent density functional theory (TDDFT) in order to elucidate molecular structural information and avoid relying on pigment spectral libraries for dye identification.

Methyl substituent effects in radical cation Diels-Alder reactions.

The effect of methyl substitution on the Diels-Alder radical cation reaction was studied using B3LYP with a 6-31G* basis set. Five separate pathways, one concerted and four stepwise, were examined for each possible position of a methyl substituent. None of the concerted transition structures could be located without symmetry constraints, and all but one of the structures obtained under these conditions were destabilized by a second-order Jahn-Teller distortion. A concerted pathway with simultaneous bond formation at C1 and C4 is therefore excluded. Stepwise pathways that had the methyl group either on a carbon involved in the initial bond formation or in a position where it could not stabilize the radical/cation were a few kcal/mol above alternate pathways. High transition state energies for the formation of vinylcyclobutane derivatives cause it to be a minor product in general. The pathway that proceeds through an anti-intermediate is the most favored, while the pathways forming the gauche-out intermediate that converts to the anti-intermediate is also strongly represented. Both of the major pathways lead directly to the formation of the methylcyclohexene product.