Diffusivity and hydration of hydrazine in liquid and supercritical water through molecular dynamics simulations and...

The diffusion properties and hydration structure of hydrazine in an aqueous solution are investigated through molecular dynamics simulations and split-flow pulse injection experiments. The simulations are performed from ambient conditions along the liquid side of the liquid-vapor coexistence curve, up to the critical point, and in the supercritical region at temperatures of 673, 773, 873, and 973 K and at densities ranging from 0.1 to 0.8 g cm(-3). The spatial distributions functions for hydrated water are presented. At ambient conditions, hydrazine is hydrated by 24 water molecules with about 1.6 H-bonds being donated to each nitrogen atom. The hydration number decreases with temperature along the coexistence curve and is seen to increase with system density in the supercritical region. At low density supercritical conditions, hydrazine has no appreciable hydration structure and is surrounded by only 2 water molecules at 873 K and 0.1 g cm(-3). The diffusion coefficients for hydrazine at subcritical state conditions are found to be in agreement with Stokes-Einstein and Wilke-Chang predictions. The diffusion coefficients in the supercritical region are found to correlate more closely with the overall fit to the Dymond equation.