Local ice-like structure at the liquid water surface

Experiments and computer simulations have established that liquid water's surfaces can deviate in important ways from familiar bulk behavior. Even in the simplest case of an air-water interface, distinctive layering, orientational biases, and hydrogen bond arrangements have been reported, but an overarching picture of their origins and relationships has been incomplete. Here we show that a broad set of such observations can be understood through an analogy with the basal face of crystalline ice. Using simulations, we specifically demonstrate that water and ice surfaces share a set of structural features suggesting the presence of nanometer-scale ice-like domains at the air-water interface. Most prominent is a shared characteristic layering of molecular density and orientation perpendicular to the interface. Similarities in two-point correlations of hydrogen bond network geometry point to shared ice-like intermolecular structure in the parallel direction as well. Our results bolster and significantly extend previous conceptions of ice-like structure at the liquid's boundary, and suggest that the much-discussed quasi-liquid layer on ice evolves subtly above the melting point into a quasi-ice layer at the surface of liquid water.