Electronic structure computations of Newton Black Films

Abstract

We investigate using quantum density functional theory (DFT) the electronic structure of sodium dodecyl sulfate Newton Black Films (NBF), consisting of water sandwiched between surfactants surrounded by vacuum. We generate a classical trajectory of the film using full-atom empirical potentials. DFT theory is employed to investigate the electron density and electrostatic properties of the film. The electrostatic potential derived directly from the electron and nuclear densities shows an important drop towards the vacuum side, similar to recent findings for the water–vacuum interface. We discuss the physical significance of this potential drop. We also consider as an alternative definition of the electrostatic potential, the deformation charge potential (DCP), to quantify the electrostatic properties of the interfaces. The analysis of the DCP potential indicates there is charge separation at the surfactant alkyl tails, showing that the surfactant–vacuum surface has an excess of positive charge. This result is consistent with molecular dynamics simulations of classical force-fields that incorporate partial charges to model the surfactant alkyl chain. The DCP potential also provides evidence for the existence of electronic polarization in the hydrophobic region of the NBF, whose origin is connected to charge transfer between the surfactant tails and the head group. We suggest that the deformation charge potential provides a route to compare the electrostatic potentials obtained from DFT and emprirical computations. This idea is illustrated by computing the electrostatic potential of the water-vapor interface. We discuss the relevance of these results in the context of DFT computations of soft interfaces.