Spectroscopic studies of atmospheric relevant air-aqueous and air-silica interfaces

In marine aerosols, Mg2+ and Cl- play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface was conducted to help elucidate the role of inorganic cations and anions in atmospheric processes.Aqueous solvation of Mg2+ as a function of MgCl2 concentration in the bulk was investigated with Raman spectroscopy. The analysis of the Mg---Owater hexaaquo stretch supports the absence of ion pairing in MgCl2 through at least 0.06x (mole fraction). At a higher concentration, a change in the hydration environment of Mg 2+ is observed and assigned to an increased number of solvent-share ion pairs. No contact ion pairs are clearly observed in aqueous solutions of MgCl2. At the air-aqueous interface, the disturbance of the hydrogen bonding environment is dependent on the MgCl2 concentration. At concentration lower than < 1 M minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration 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 and more strongly oriented water molecules relative to other concentrations. SFG studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M).Interfacial studies of aqueous solutions of NaCl and a model seawater solution (composed of NaCl and MgCl2) were also explored. The interfacial hydrogen bonding environment of the model seawater is highly perturbed. The presence of MgCl2 in this model solution has a strong effect on water structure and on its electrostatic environment.In a study related to solid-phase aerosols, the air-silica interface, before and after adsorption of water in the gas phase and in the liquid phase, was examined. Free silanol OH groups are observed after 72 h of exposure to mid RH conditions. The free silanol average orientation determined is 26 +/- 2° from the surface normal.In order to study the interaction of mineral aerosols with cell membranes, interfacial hydration and orientation of the phosphate group of DPPC monolayers in the presence of sodium ions and calcium ions at air--aqueous and air-silica interfaces were investigated. Sodium ions affect the phosphate hydration subtly, while calcium ions cause 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 sodium ions. However, in the presence of calcium ions 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 63º +/- 3º and 74º +/- 3º, respectively. The hydration environment and orientation of the phosphate moiety of DPPC monolayers are affected considerably at the silica surface compared to that with the aqueous subphase.Finally, the adsorption of gas-phase alkyl halides and alcohols to the air-water interface was studied as a model of gas-phase uptake by liquid aerosols. Methanol and butanol are adsorbed into the aqueous solution as revealed by surface (SFG) and bulk (Raman) spectroscopies. Alkyl halides are not detected at the air-water interface, although methyl chloride is readily observed in the bulk. Orientation (disordered molecules) and low number density have been invoked to explain the absence of the methyl chloride signal in the SFG spectrum. (Abstract shortened by UMI.)...