Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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)   Here we report the polarization dependent non-resonant second harmonic
generation (SHG) measurement of the interfacial water molecules at the aqueous
solution of the following salts: NaF, NaCl, NaBr, KF, KCl, and KBr. Through
quantitative polarization analysis of the SHG data,the orientational parameter
D value and the relative surface density of the interfacial water molecules at
these aqueous solution surfaces were determined. From these results we found
that addition of each of the six salts caused increase of the thickness of the
interfacial water layer at the surfaces to a certain extent. Noticeably, both
the cations and the anions contributed to the changes, and the abilities to
increase the thickness of the interfacial water layer were in the following
order: KBr > NaBr > KCl > NaCl ~ NaF > KF. Since these changes can not be
factorized into individual anion and cation contributions, there are possible
ion pairing or association effects, especially for the NaF case. We also found
that the orientational parameter D values of the interfacial water molecules
changed to opposite directions for the aqueous solutions of the three sodium
salts versus the aqueous solutions of the three potassium salts. These findings
clearly indicated unexpected specific Na+ and K+ cation effects at the aqueous
solution surface. These effects were not anticipated from the recent molecular
dynamics (MD) simulation results, which concluded that the Na+ and K+ cations
can be treated as small non-polarizable hard ions and they are repelled from
the aqueous interfaces. These results suggest that the electrolyte aqueous
solution surfaces are more complex than the currently prevalent theoretical and
experimental understandings.

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The categories of this paper are: gr-qc
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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: eess
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The categories of this paper are: astro-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: solv-int
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The categories of this paper are: q-bio
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The categories of this paper are: math
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The categories of this paper are: hep-ex
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: funct-an
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The categories of this paper are: econ