Influence of a neutral component on the liquid–vapor coexistence and the surface tension of an ionic fluid

Abstract

We investigate the effect that a neutral component has on the liquid–vapor coexistence curve and the surface tension of an ionic fluid. Molecular dynamics simulations of a ternary mixture containing ions and neutral particles are performed. We found that the vapor branch is dominated by neutral particles and the liquid phase by ions. A comparison of our results and those of the soft primitive model (SPM) of a pure ionic fluid is made. For a fraction of neutral particles x0=0.2, the density of the liquid phase is the same as that of the SPM, except for temperatures close to the critical point. As compared to the SPM, the surface tension takes the same values for low temperatures. For high temperatures, deviations of about 30% are observed. The estimated critical temperature and density of this ternary mixture are higher than those of the pure ionic fluid. In a preliminary study our results indicate that as x0 increases the vapor density shifts to higher values, basically due to the concentration of neutral particles in that region. The liquid branch changes slowly until x0=0.5, from that concentration of neutral particles, the liquid density decreases considerably. For high concentrations of the neutral component, the repulsive interactions dominate the system and the coexistence region in strongly reduced. Concerning the surface tension, we observed an increase with the fraction of neutral particles until x0=0.5, from that value it reduces significantly, also as an indication of the strong repulsion in the system.