SURFACE-TENSION OF WATER DROPLETS - A MOLECULAR-DYNAMICS STUDY OF MODEL AND SIZE DEPENDENCIES

The applicability of two frequently used interaction potentials for wa ter, the five-site ST2 model and the four-site TIP4P model, is investi gated in computer simulations of water droplets of varying cluster siz e from N = 64 to N = 512. The orientation of the water molecules in th e surface region is investigated for the both models. Surface properti es, such as work of cluster formation, local density profiles, kinetic and total energy profiles, and pressure profiles as a function of the droplet size, obtained using the two models are compared. Moreover, t he surface potential and the electric potential profiles are calculate d. Surface tension is calculated and its dependence on the cluster siz e is investigated. It is found that surface properties are very sensit ive to the used potential models. For example, the water molecules are found to lie differently in the inner region of the surface layer, th e ST2 molecules being predominantly perpendicular to the surface, whil e the TIP4P molecules lie mainly parallel to the surface. In the outer surface region, the molecular planes are perpendicular to the surface for the both models. The TIP4P model allows a calculation of the surf ace tension, giving a value 54 mN/m which is somewhat lower than the e xperimental value for the plane surface, while for the ST2 model, larg er clusters are needed than those treated in the present study. (C) 19 97 American Institute of Physics.