Solvent-Shared Ion Pairs at the Air-Solution Interface of Magnesium Chloride and Sulfate Solutions Revealed by Sum...

The ion distribution and ion pairing properties of Mg²⁺, SO₄^2-, NO₃^-, and Cl^- in the interfacial region of MgSO₄, Mg(NO₃)₂, and MgCl₂ solutions were investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulations. An electric field reversal relative to Mg(NO₃)₂ and MgCl₂ solutions is observed at the interface of a MgSO₄ 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 Mg²⁺ 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.