Nanoroughness, intrinsic density profile, and rigidity of the air-water interface.

From large-scale molecular dynamics simulations of the free water surface the capillary wave spectrum is obtained and analyzed in terms of a wave vector dependent surface tension. The bending rigidity is positive but suggested to change sign in the presence of long-range dispersive interactions. Nanoroughness and capillary waves give rise to a scale-dependent density profile that displays minimal broadening at a nanometer scale which defines the intrinsic density profile.