Molecular Dynamics Study of High Temperature Phase-Separation in a H2O/N2 Mixture with Exp-6 Interactions

Abstract

Equimolar H₂O/N₂ fluid mixture was studied by molecular dynamics simulations for NVT ensemble. Calculations were performed with the modified Buckingham (exp-6) potentials at T = 2000 K. Particular attention was given to the phase separation at very high pressures relevant to a detonation environment. Calculations of pair correlation functions and local mole fractions clearly indicated the occurrence of the fluid separation into N₂-rich and H₂O-rich phase. The density at the phase boundary between homogeneous and inhomogeneous phase-separated state was determined to be p = 1.35 g/cm³ on the basis of the static cross correlation factor which is defined by the sum of the local mole fractions. The ratio of the self-diffusion coefficients of N₂ and H₂O at p < 1.35 g/cm³ was found to be approximately equal to the value predicted by the kinetic theory of the ideal gas, whereas the ratio was close to unity at the phase-separated state (p > 1.35 g/cm³). In addition, two distinctive behaviors of the system could be observed for the relaxation from the initial uniform mixture to the phase-separated fluid: at lower densities (1.35 < p < 2.0 g/cm³) the fluid mixture began to relax into the phase-separated system without obvious incubation time, while clear incubation period was associated for the separation at higher densities. During this incubation period, discontinuous jumps in the mean square displacements were found.