Heterodyne detected phase-sensitive sum frequency generation (PS-SFG) spectroscopy has become an important second order optical spectroscopy to understand molecular organization at air/aqueous interfaces. PS-SFG, a variant of SFG, is used in our laboratory to investigate the average direction of the transition dipole of interfacial water molecules. The orientation of water at air/aqueous inorganic salts interfaces of CaCl2, NaCl, Na2SO4, (NH4)2SO4, and Na2CO3 is inferred from the direct measurement of the transition dipole moment. We find that charge separation at the air/water interface is most obvious for the aqueous ammonium sulfate solution where the local electric field has a greater magnitude at this interface relative to the other salt solutions. The magnitude of the electric field in the surface extending to the subsurface regions decreases in the order: (NH4)2SO4 > Na2SO4 > Na2CO3 = CaCl2 > NaCl; the electric field is opposite in direction for the sulfates and carbonate relative to the chloride salts. Additionally, other ion distributions will also be presented. Application to thundercloud electrification is discussed.

Ion organization and inherent electric fields at aqueous surfaces

244th National Meeting (2012)

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