Aqueous ion-containing interfaces are ubiquitous and play a key role in a plethora of physical, chemical, atmospheric, and biological processes, from which we mention just a few illustrative examples: (i) Ions at the air/water interface are important for atmospheric chemistry involving ocean surfaces and seawater aerosols, 1-5 as well as that of the Arctic snowpack covered by sea spray. 6,7 (ii) Many salts (such as NaCl) tend to inhibit bubble coalescence, 8-12 which is one of the reasons why foam is formed when waves break in the ocean but not in freshwater lakes. (iii) Brine rejection occurring at the seawater/ice interface has profound climatic effects in polar regions. 13 (iv) The aqueous electrolyte/metal interface is involved in electrode and corrosion processes. 14,15

Specific ion effects at the air/water interface.

A powerful approach to the study of interfaces has been developing rapidly in the past decade. It is based on the spectroscopic methods of second-harmonic (SHG) and sum-frequency generation (SFG). These nonlinear optical techniques, being spectroscopic, provide information at the most fundamental level. A microscopic description of equilibrium and dynamic interface processes requires knowledge of the molecules at the interface, their orientational structure, the energetics that drive chemical and physical processes, and the time scale of molecular motions and relaxation processes. The techniques of second-harmonic and sum-frequency generation have made it possible to selectively probe the chemistry, physics, and biology of gas/liquid, liquid/liquid, liquid/solid, gas/solid, and solid/solid interfaces at the molecular level. In this abbreviated article only liquid interfaces will be considered; gas/solid and solid/solid interfaces are not included. This restriction is necessary because of the enormous increase in SH and SF studies in recent years, which makes it extremely difficult to properly discuss the range of work being carried out around the world. Unfortunately not all of the fine work even in the area of liquid interfaces has been included because of both space and time limitations. A number of review articles are referred to which cover some ofmore » the research material not covered in this article. 99 refs.« less

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Liquid interfaces probed by second-harmonic and sum-frequency spectroscopy

We present results from theoretical studies of aqueous ionic solvation of alkali halides aimed at developing a microscopic description of structure and dynamics at the interface between air and sal...

Ions at the Air/Water Interface

A qualitatively new understanding of the nature of ions at the liquid water surface is emerging Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging

On the nature of ions at the liquid water surface

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.

Unified molecular picture of the surfaces of aqueous acid, base, and salt solutions.

During the past 20 years or so one of the regions of conspicuous growth in the field of physical chemistry has been the study of the structure and behaviour of water and aqueous solutions. There are practical reasons, for example technical and biological, for this interest, but it is also true that the complexity of water as a liquid provides its own motive to the rcsearch worker. It is unlikely that we would spend so much time in the study of water if it were as simple a liquid as Argon. However, strange though the behaviour of liquid water is, it is probably not as strange as it has sometimes been thought to be. The thermal “anomalies” of water and the abnormal “Poly-water” Seem rather likely to fade out of the scientific scene, as have other stimulating but nonviable scientific myths.

J. E. B. Randles

Papers

Structure at the Free Surface of Water and Aqueous Electrolyte Solutions