Stability and disintegration of ultrathin heptane films in water: molecular dynamics simulations

Molecular dynamics simulations of ultrathin heptane films (less than 5 nm in thickness) in water were conducted to study their stability and disintegration behavior. The density distributions of heptane and water molecules across the film were determined for different equilibrium film thicknesses ranging from 1.5 to 4 nm. The potential energy of the system was computed as a function of the heptane number fraction, and the results were employed to determine the excess energy of mixing of heptane in water. The diffusion coefficients of heptane and water obtained from the MD simulations were also compared with experimental data. A good agreement was found between the heptane self-diffusivity obtained from the MD simulations and its literature reported value. Following an analysis of the equilibrium properties of the heptane films and associated structures, we performed simulations where the shapes of the heptane films were initially perturbed. Different perturbations of these ultrathin films led to formation of various associated structures, including cylindrical rodlike heptane droplets, films with holes, and intact films. The different shapes are formed in systems with the same heptane/water composition. An analysis of this behavior is presented showing the possibility of multiple associated structures with similar total energy in these highly confined systems.