Ion behavior at the air/water interface
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-running controversy exists between the standard picture of an ion-free surface layer and the strongly ion-specific behavior that indicates, in many cases, a tendency for simple inorganic ions to be found at the interface. PNNL researcher Liem X. Dang and an international team of collaborators developed 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 attraction to the surface, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant attraction of hydronium cations to 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, 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.
The illustrations represent the water-air interface of various ions in water. The graphs show the relative vertical abundance of the components; colors of the lines match the colors of the components in the drawings. For example, in the top, the acid and hydronium ions are more abundant at the water-air interface. In the middle, the water is more abundant at the interface while the base and hydroxyl ions become abundant below the surface.
Adapted from: M Mucha, T Frigato, LM Levering, HC Allen, DJ Tobias, LX Dang, and P Jungwirth, 2005 Feature Article, "Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions." Journal of Physical Chemistry B 109(16):7617-7623. DOI:10.1021/jp0445730.