Surface order in cold liquids: X-ray reflectivity studies of dielectric liquids and comparison to liquid metals

Oscillatory surface-density profiles (layers) have previously been reported in several metallic liquids, one dielectric liquid, and in computer simulations of dielectric liquids. We have now seen surface layers in two other dielectric liquids, pentaphenyl trimethyl trisiloxane, and pentavinyl pentamethyl cyclopentasiloxane. These layers appear below T∼285 K and T∼130 K, respectively; both thresholds correspond to T/ T_c ∼0.2 where Tc is the liquid-gas critical temperature. All metallic and dielectric liquid surfaces previously studied are also consistent with the existence of this T/ T_c threshold, first indicated by the simulations of Chacón [Phys. Rev. Lett. 87, 166101 (2001)]. The layer width parameters, determined using a distorted-crystal fitting model, follow common trends as functions of T_c for both metallic and dielectric liquids.