Molecular dynamics simulation of the rupture of nanometer-sized oil/water interface

Molecular dynamics simulations are carried out to study formation and stability of nanometer thick films of heptane in water. It is considered as a representative model of oil films in aqueous media during deemulsification of water droplets in diluted oil. The simulations are performed using the Gromacs freeware employing the Gromos96 force field. The simulations are conducted assuming simple point charge (SPC) model of water. Dependent on the system potential energy, the thickness of heptane layer is obtained. The effective Hamaker constant of heptane-water systems calculated from our simulations. Molecular dynamics simulations allow determination of the diffusion coefficients of heptane and water. A good agreement is found between the simulated heptane self-diffusivity and its experimental values. The curvature of the heptane/water interface along with the heptane volume fraction were found to cause rupture of the heptane layer resulting in the formation of cylindrical-shape micelles. The simulations are capable of providing the disjoining pressure isotherms of heptane films in water. It was observed that although the continuum models are sufficiently accurate for such films as long the film thickness is above 3 nm, these models are generally inadequate for thinner films. In such thin films (thickness ∼ 1 nm), the heptane layer tends to become "permeable" and water molecules tend to diffuse across the heptane layer causing a rupture of the film.