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 CaCl 2, NaCl, Na 2 SO 4, (NH 4 ) 2 SO 4, and Na 2 CO 3 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 (NH 4 ) 2 SO 4 > Na 2 SO 4 > Na 2 CO 3 ≥ CaCl 2 > NaCl; the electric field is opposite in direction for the sulfate and carbonate salts relative to the chloride salts.
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Hua et al. (2011) studied this question.
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