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 adsorbed and accommodated into atmospheric aqueous-phase aerosols, and in some cases MSA is thought to be produced via aerosol surface chemistry. The studies described herein probe the surface molecular structure of MSA at aqueous solution surfaces using surface vibrational sum frequency spectroscopy (VSFS). In the studies presented here, it is shown that MSA partitions to the surface and that the surface MSA has a preferred orientation in which the methyl group points away from the liquid surface. The surrounding surface water structure is significantly affected by the adsorption of MSA. Small amounts of MSA at the surface of water cause the water molecule vibrational stretching modes to shift to lower energies. This shift is indicative of stronger coupling between the water molecules caused by the presence of MSA. Additional VSF studies show that MSA is effectively displaced by sulfuric acid at an aqueous surface. The structural details presented here may have implications for understanding atmospheric aerosol growth properties.