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Here we report the polarization dependent non-resonant second harmonicgeneration (SHG) measurement of the interfacial water molecules at the aqueoussolution of the following salts: NaF, NaCl, NaBr, KF, KCl, and KBr. Throughquantitative polarization analysis of the SHG data,the orientational parameterD value and the relative surface density of the interfacial water molecules atthese aqueous solution surfaces were determined. From these results we foundthat addition of each of the six salts caused increase of the thickness of theinterfacial water layer at the surfaces to a certain extent. Noticeably, boththe cations and the anions contributed to the changes, and the abilities toincrease the thickness of the interfacial water layer were in the followingorder: KBr > NaBr > KCl > NaCl ~ NaF > KF. Since these changes can not befactorized into individual anion and cation contributions, there are possibleion pairing or association effects, especially for the NaF case. We also foundthat the orientational parameter D values of the interfacial water moleculeschanged to opposite directions for the aqueous solutions of the three sodiumsalts versus the aqueous solutions of the three potassium salts. These findingsclearly indicated unexpected specific Na+ and K+ cation effects at the aqueoussolution surface. These effects were not anticipated from the recent moleculardynamics (MD) simulation results, which concluded that the Na+ and K+ cationscan be treated as small non-polarizable hard ions and they are repelled fromthe aqueous interfaces. These results suggest that the electrolyte aqueoussolution surfaces are more complex than the currently prevalent theoretical andexperimental understandings.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Specific Na+ and K+ Cation Effects on the Interfacial Water Molecules at  the Air/Aqueous Salt Solution Interfaces Probed with Non-resonant Second  Harmonic Generation (SHG)