Electric Field Reversal of Na2SO4, (NH4)2SO4, and Na2CO3 Relative to CaCl2 and NaCl at the Air/Aqueous Interface Revealed by Heterodyne Detected Phase-Sensitive Sum Frequency

Phase-sensitive sum frequency generation (PS-SFG) spectroscopy, heterodyne detected, was used to investigate the average direction of the transition dipole moment of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ(2). The organization of water at air/aqueous inorganic salts interfaces of CaCl2, NaCl, Na2SO4, (NH4)2SO4, and Na2CO3 was inferred. We attribute our findings to the net charge separation arising from the ion distributions at the air/water interface assuming similar ion distribution widths for all systems studied. This is most evident for the aqueous ammonium sulfate solution where the electric field has a greater magnitude relative to the other salt solutions studied. The magnitude of the electric field in the interfacial region decreases in the order (NH4)2SO4 > Na2SO4 > Na2CO3 ≥ CaCl2 > NaCl; the electric field is opposite in direction for the sulfate and carbonate salts relative to the chloride salts.