Solvent-Shared Ion Pairs at the Air–Solution Interface of Magnesium Chloride and Sulfate Solutions Revealed by Sum Frequency Spectroscopy and Molecular Dynamics Simulations

Abstract

The ion distribution and ion pairing properties of Mg 2+, SO 4 2–, NO 3 –, and Cl – in the interfacial region of MgSO 4, Mg(NO 3 ) 2, and MgCl 2 solutions were investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulations. An electric field reversal relative to Mg(NO 3 ) 2 and MgCl 2 solutions is observed at the interface of a MgSO 4 solution. We show that, although magnesium cations are expected to have preference for bulk solvation, solvent-shared ion pairs (SIPs) exist in the interfacial region in which Mg 2+ cations are closer to the solution surface than sulfate anions. While interfacial SIPs are few, they dominate the electric field effect observed. Thus, SIPs play a significant role in determining the electric field direction and magnitude at the air–aqueous interface. In addition to impact on the fundamental understanding of aqueous surfaces and interfacial ion–ion interactions, these findings have implications for atmospheric aerosol chemistry and thundercloud electrification.