Molecular dynamics simulation of the liquid-vapor interface of dipolar fluids under different electrostatic boundary conditions

Molecular dynamics simulations are performed for a liquid film of strongly and moderate dipolar fluids in equilibrium with its vapor in both, three- and two-dimensional (2-D) periodic systems. In the three-dimensional periodic system, the long-range dipolar interactions are treated by the conventional Ewald summation, where the shape of the basic simulation cell is modified in order to study the influence of periodicity on the interfacial properties. In the two-dimensional periodic system, an alternative method is used that is based on dividing the 2-D-lattice sum into a lattice sum of the position vectors of the molecules and additional terms describing the dipole orientations independently of the positions. The results from both techniques agree very well in case that in the 3-D treatment the distance between the liquid films is sufficiently large. The phase equilibrium data obtained from the simulations are verified by a comparison with a recent equation of state. The surface tension results are compared to experimental values using the corresponding state principle. Moreover, the orientation of strongly dipolar elongated molecules and Stockmayer molecules at the interface is found to be parallel to the interface on the liquid as well as on the vapor side. (C) 2001 American Institute of Physics.