Surface structural studies of methanesulfonic acid at air /aqueous solution interfaces using vibrational sum frequency spectroscopy

Atmospheric gas phase species such as methanesulfonic acid (MSA) are adsorb ed and accommodated into atmospheric aqueous-phase aerosols, and in some ca ses MSA is thought to be produced via aerosol surface chemistry. The studie s described herein probe the surface molecular structure of MSA at aqueous solution surfaces using surface vibrational sum frequency spectroscopy (VSF S). In the studies presented here, it is shown that MSA partitions to the s urface and that the surface MSA has a preferred orientation in which the me thyl group points away from the liquid surface. The surrounding surface wat er structure is significantly affected by the adsorption of MSA. Small amou nts of MSA at the surface of water cause the water molecule vibrational str etching modes to shift to lower energies. This shift is indicative of stron ger coupling between the water molecules caused by the presence of MSA. Add itional VSF studies show that MSA is effectively displaced by sulfuric acid at an aqueous surface. The structural details presented here may have impl ications for understanding atmospheric aerosol growth properties.