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.

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.

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.