Monte Carlo simulation of the liquid–vapor interface of water using an ab initio potential

Abstract

Monte Carlo calculations have been carried out to study the interfacial properties of liquid water, using the Matsuoka–Clementi–Yoshimine (MCY) potential for the water–water interaction. The surface tension calculated at 298 K is 23.7±3.4 dyn/cm, to be compared with the experimental value of 72 dyn/cm. The interfacial 10–90 thickness is 4.70 Å, with the dipoles of the water molecules near the liquid phase pointing slightly towards the liquid phase and those near the gas phase pointing towards the gas phase. The interfacial water molecules are found to be more restricted in their rotation, as evidenced by the smaller root-mean-squared fluctuations of the dipole directions. The Volta potential difference across the interface arising from the permanent dipoles is estimated to be 0.024 V. A new and efficient method is proposed to calculated the surface excess energy. The excess energy calculated for the MCY water is 119 erg/cm2, to be compared with the experimental value of 120 erg/cm2. From the calculated surface excess energy, the temperature variation of the surface tension, or the surface entropy, for the MCY water is estimated to be −0.32 dyn/(cm2 K). This temperature variation is confirmed by another Monte Carlo study at 310 K to within the calculated uncertainty.