UC Berkeley

The surface of liquid water is characterized by large fluctuations that are too complex to describe exactly but, like with other aspects of molecular liquids, can be described in a useful way when approached statistically. The water liquid/vapor interface is composed of a sharp drop in density and reverberations from that drop that extend about 1 nm from the surface. Transient but characteristic structure forms as water molecules realign to maximize hydrogen bonding without ready neighbors in vapor. Much of the structure of water’s surface can be accounted for as a broadening of the bilayer structure of the basal plane of the ice crystal, particularly the layering in density and the orientational trends.The mean field surface model predicts the average orientational behavior of structured molecular liquids, which is demonstrated in two dimensions with the water-like Mercedes-Benz (MB) model and in three dimensions with comparisons to the SPC/E and TIP5P models of water. Liquids with strong directional bonding tend to orient predictably when potential bonding neighbors are removed. The mean field surface model assumes that bonds are most likely to form in regions of high density, then uses a self-referencing iterative approach to determine whether neighbors are also likely to be oriented in a way that is favorable to bonding. Physically motivated changes to the reference density, bonding rules, and molecular structure clarify which aspects of molecular water are actually essential to the orientational structure at the interface. It is shown that moderate changes to bonding rules, like flexibility, strength, and asymmetry, have little impact on the main surface patterns. The layering in the density profile of water is by contrast necessary to accurately predict water’s surface structure. All of these observations can be rationalized through liquid water’s relationship to the structure of the basal plane of ice. The dielectric response of water is also affected by the introduction of an interface. The Gaussian field model known as dielectric continuum theory[1–3] (DCT) makes predictions for the effect of an interface on the dielectric behavior of water. Using the method of images[4] and correcting for the dielectric saturation of a rigid water model, it is shown that polarization fluctuations near the air-water interface largely follow these predictions at scales as small as 5 ̊A despite the macroscopic character of DCT.