The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its
crucial importance in many biological, technological, and atmospheric processes. A long-term controversy
pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior
indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present
a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing
monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively
charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or
hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The
behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface.
Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these
acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported
by surface selective nonlinear vibrational spectroscopy(VSFG), which reveals among other things that the hydronium
cations are present at the air/solution interface. The ion specific propensities for the air/solution interface
have important implications for a whole range of heterogeneous physical and chemical processes, including
atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence. 表面和频光谱分析系统(SFG)
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