  In this Letter, we present a simple model of aqueous interfacial molecular
structure and we use this model to isolate the effects of hydrogen bonding on
the dielectric properties of the liquid water-vapor interface. By comparing
this model to the results of atomistic simulation we show that the anisotropic
distribution of molecular orientations at the interface can be understood by
considering the behavior of a single water molecule interacting with the
average interfacial density field via an empirical hydrogen bonding potential.
We illustrate that the depth dependence of this orientational anisotropy is
determined by the geometric constraints of hydrogen bonding and we show that
the primary features of simulated orientational distributions can be reproduced
by assuming an idealized, perfectly tetrahedral hydrogen bonding geometry. We
also demonstrate that non-ideal hydrogen bond geometries are required to
produce interfacial variations in the average orientational polarization and
polarizability. We find that these interfacial properties contain significant
contributions from a specific type of geometrically distorted three-body
hydrogen bond defect that is preferentially stabilized at the interface. Our
findings thus reveal that the dielectric properties of the liquid water-vapor
interface are determined by collective molecular interactions that are unique
to the interfacial environment.
