Molecular Simulation Studies of Aqueous Interfacial Fluids

Abstract

Aqueous heterogeneous fluids with interfaces are ubiquitous and play an important role in many natural and industrial processes. In this work, two types of these systems: water/oil liquid-liquid and bubbly water liquid-vapor systems are investigated using molecular simulations. First, Monte Carlo simulations are performed to predict the interfacial tension (IFT) for water/n-dodecane, water/toluene, and water/(50 wt% n-dodecane + 50 wt% toluene) mixtures at elevated temperatures and pressures. In order to control the bulk composition of the organic phase for ternary mixture simulations, separate reservoirs for toluene and n-dodecane molecules were utilized. Calculations of the IFT for water/n-decane and water/benzene mixtures were used to benchmark the force fields and to develop mixing rules. With the modified Lennard-Jones cross-interaction parameters, the simulated IFTs agree well with the experimental data. For the bubbly water systems, molecular dynamics simulations in the canonical ensemble were performed to probe a variety of thermophysical properties of both homogeneously stretched and bubbly water systems using both the coarse-grained single-site mW and atomistic TIP4P/2005 water model. Following two simulation protocols starting either from a homogeneous configuration or from a heterogeneous configuration with a single spherical cavity, spinodal cavitation and bubble collapse points were located separately. This behavior of a fluid in a box of fixed volume is analogous to the hysteresis observed for adsorption–desorption isotherms of a subcritical fluid in a mesoporous adsorbent. In addition to equilibrium properties, molecular dynamic simulations for the collapse of a bubble in both neat water and water/nitrogen mixture are performed, with the largest system reaching half a micron in linear dimension using the mW water model. A significant system size dependence is observed. Supersonic wall speeds preceding the initial bubble collapse with extreme local heating at the center of the collapsing bubble are only observed for large bubbles. In contrast to previous equilibrium studies, two water models predict qualitatively different collapse behaviors, which is explained with the help of Rayleigh-Plesset equation. The presence of nitrogen gas inside the bubble leads to less violent collapse and stronger oscillations in bubble size after initial collapse.