Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions

Abstract

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, 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.

Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide

“…Recent molecular simulations showed enhanced surface affinity of a single hydronium cation to the aqueous surface, compared, e.g., to alkali cations. 57,58 This prediction has recently been supported by a combined molecular dynamics and SFG investigation 59 and a separate SHG study 53 of concentrated acids (such as HCl, HBr, or HI) which explained why the surface tension of monovalent acid solutions is lower than that of water. 55 The force that brings hydronium to the interface is different from that operating for the anions.…”

“…55 The force that brings hydronium to the interface is different from that operating for the anions. 57,59 Hydronium is only capable of forming three hydrogen bonds The yellow, black, and red curves correspond to the S, C, and N atoms of the thiocyanate anion. The densities are normalized so that the area under them is equal, with the bulk water density being one.…”

Interfacial Structures of Acidic and Basic Aqueous Solutions

“…Molecular dynamics (MD) simulations predicted that protons (H + ) could appear at the interface in the form of hydronium(H 3 O + ), but OH -should be repelled from the water surface. 3,[9][10][11][12] The details, such as the depth profile of ion concentrations, depend very much on the molecular model and interaction potentials assumed. [9][10][11][12] Experiments carried out with scanning tunneling microscopy (STM) 13 , X-ray spectroscopy 14,15 , and attenuated total reflection (ATR) 8 have been used to verify the theoretical prediction.…”

“…16 When the acid concentration in water was sufficiently high (pH ≤ 2 with HCl), strong enhancement of both ice-like and liquid-like bands were observed. 7,8,11,18 Appearance of hydronium ions at the surface was used to explain the enhancement. 7,8,11 In basic solution, only the liquid-like band displayed slight changes when pH is sufficiently high (pH≥13).…”

“…7,8,11,18 Appearance of hydronium ions at the surface was used to explain the enhancement. 7,8,11 In basic solution, only the liquid-like band displayed slight changes when pH is sufficiently high (pH≥13). 7 It was believed that at high pH, OH -anions might have emerged at the interface with enough concentration to significantly distort the interfacial hydrogen-bonding network of water.…”