Surface tension of water droplets: A molecular dynamics study of model and size dependencies

The applicability of two frequently used interaction potentials for water, the five-site ST2 model and the four-site TIP4P model, is investigated in computer simulations of water droplets of varying cluster size from $\text{N=64}$ to $\text{N=512.}$ The orientation of the water molecules in the surface region is investigated for the both models. Surface properties, 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, the surface potential and the electric potential profiles are calculated. Surface tension is calculated and its dependence on the cluster size is investigated. It is found that surface properties are very sensitive to the used potential models. For example, the water molecules are found to lie differently in the inner region of the surface layer, the ST2 molecules being predominantly perpendicular to the surface, while 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 surface tension, giving a value 54 mN/m which is somewhat lower than the experimental value for the plane surface, while for the ST2 model, larger clusters are needed than those treated in the present study.