COMPUTER-SIMULATION OF LIQUID LIQUID INTERFACES .1. THEORY AND APPLICATION TO OCTANE/WATER/

Statistical ensembles for simulating liquid interfaces at constant pre ssure and/or surface tension are examined, and equations of motion for molecular dynamics are obtained by various extensions of the Andersen extended system approach. Valid ensembles include: constant normal pr essure and surface area; constant tangential pressure and length norma l to the interface; constant volume and surface tension; and constant normal pressure and surface tension. Simulations at 293 K and 1 atm no rmal pressure show consistent results with each other and with a simul ation carried out at constant volume and energy. Calculated surface te nsions for octane/water (61.5 dyn/cm), octane/vacuum (20.4 dyn/cm) and water/vacuum (70.2 dyn/cm) are in very good agreement with experiment (51.6, 21.7, and 72.8 dyn/cm, respectively). The practical consequenc es of simulating with two other approaches commonly used for isotropic systems are demonstrated on octane/water: applying equal normal and t angential pressures leads to an instability; and applying a constant i sotropic pressure of 1 atm leads to a large positive normal pressure. Both results are expected for a system of nonzero surface tension. Mas s density and water polarization profiles in the liquid/liquid and liq uid/vapor interfaces are also compared.