Spectroscopic Studies of Atmospheric Relevant Air-Aqueous and Air-Silica Interfaces

Marine and mineral oxides aerosols are the most abundant naturally emitted aerosols. In marine aerosols, Mg²⁺ and Cl^- play an active role in atmospheric chemistry. The study of the interactions of these ions with water molecules at the interface was conducted to help elucidate the role of these ions in atmospheric processes. In the case of mineral particles, not only do they affect atmospheric chemistry but they also have been related to human respiratory problems. SiO₂ particles are some of the most notoriously toxic particles in pulmonary diseases. In an attempt to elucidate the molecular effect of the adhesion of silica particles to cell membranes, the surface organization of dipalmitoylphosphatidylcholine (DPPC), as a model of membranes, at air-water and air-silica surfaces was examined. Additionally, the adsorption of gas-phase species onto the surface of solid and liquid particles is a common process in the atmosphere. As models of these phenomena, interfacial water vapor uptake by silica after the exposure to mid RH values and adsorption of alkyl halides and alcohols to the air-water interface were examined.

Sum frequency generation (SFG), was employed as the main technique to investigate the interfaces and surfaces of the systems discussed here.

Aqueous solvation of Mg²⁺ as a function of concentration in the bulk was investigated. The analysis supports the absence of ion pairing in MgCl₂ through at least 3.1 M. At a higher concentration, an increased number of solvent-share ion pairs are formed. At the air-aqueous interface, at concentrations <1 M the hydrogen bonding environment is highly perturbed. The 2.1 M solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region.

The air-silica interface, before and after adsorption of water in the gas phase was examined. Free silanol OH groups are observed after 72 h of exposure to mid RH conditions. Hydration and orientation of the phosphate group of DPPC monolayers in the presence of Na⁺ and Ca²⁺ ions at air-aqueous and air-silica interfaces were investigated. Na⁺ affects the phosphate hydration subtly, while Ca²⁺ causes a marked dehydration. Silica-supported DPPC monolayers reveal similar hydration behavior relative to that observed in the corresponding liquid subphase for the case of water and in the presence of Na⁺. However, in the presence of Ca²⁺ the phosphate group dehydration is greater than that from the corresponding liquid subphase. The calculated average tilt angles from the surface normal of the PO₂^- group of DPPC monolayers on the water surface and on the silica substrate are found to be 63° and 74°, respectively.

The adsorption of gas-phase alkyl halides and alcohols to the air-water interface was studied. Methanol and butanol are adsorbed into the aqueous solution. Alkyl halides are not detected at the air-water interface, although methyl chloride is observed in the bulk. Orientation and low number density have been invoked to explain the absence of the methyl chloride signal at the interface.