Nitric acid plays a role in many important chemical processes that happen in our environment, often at surfaces where less is known about its reactive behavior. Recent studies have shown that undissociated nitric acid is present on the surface of a nitric acid solution. Using ab initio molecular dynamics simulations, we show that a nitric acid molecule present on an aqueous solution surface structures and orients in a way that significantly reduces its ability to be the strong dissociating acid that it is in aqueous solution. Hydrogen bonding to surface solvating water molecules plays a key role in this altered molecular behavior.
The presence of organic materials adsorbed to the surfaces of aerosol particles has been demonstrated to be a determining factor in relevant atmospheric processes. Malonic acid is a small, water-soluble organic acid that is common in aerosols and is surface-active. A comprehensive investigation of the adsorption of malonic acid to the air/water interface was accomplished using vibrational sum frequency spectroscopy (VSFS) and surface tension measurements as functions of concentration and pH. Malonic acid was found to be weakly solvated at the air/water interface, and its orientation as a function of concentration was explored through different VSFS polarization schemes. pH-dependent experiments revealed that the surface-active species is the fully protonated species. Computational analyses were used to obtain depth-specific geometries of malonic acid at the air/water interface that confirm and enrich the experimental results.
The interface formed between an aqueous salt solution and a hydrophobic liquid has been investigated using molecular dynamics simulations. The salt solutions of NaCl, NaNO 3 , and Na 2 SO 4 have been studied to determine their presence and distribution in the interfacial region and their effect on interfacial water molecules. Density and orientation profiles reveal the formation of ionic double layers with widths that vary with the respective anions' surface affinities and effects on the geometry of interfacial water molecules. The NO 3 anion shows enhanced surface concentration above that of the bulk aqueous phase, whereas the Cland SO 42anions exhibit similar characteristics as are found for corresponding air-water interfaces. Sum frequency spectra were calculated for the OH vibrational modes of water to show the effect of the various ions on the hydrogen-bonding network strength of interfacial water. These calculated spectra show good agreement with the conclusions and observations of our recent spectroscopic experimental study, while providing important new detailed insights into interfacial behavior to augment that study.
One might expect the high surface tension of water to be a barrier to absorption of a gas into the liquid phase, but we know that gaseous adsorption onto and subsequent absorption into a water surface is a common phenomenon on this planet. What is not commonly known is how an atmospheric gas such as SO2 and molecules at the water surface can overcome the barrier created by strong water–water surface bonding interactions. What this interplay looks like, the distances from the water surface at which these attractive interactions begin, and how they influence the orientational nature of both SO2 and surface water molecules is the focus of this computational study. The results fill a void in the information about this system existing from previous experimental studies by providing information about the dimensional nature of the gas–surface interactions, and the details of how the two species twist and turn orientationally with increased surface interactions. Classical molecular dynamics have been employed in both equilibrium and steered molecular dynamics (SMD) simulations for SO2 at a neat-water surface and at a surface with high interfacial SO2 concentrations. The results provide new molecular insights for understanding the interaction of this prevalent gas on aerosols and other aqueous surfaces in the environment.
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