Self-diffusion of water-cyclohexane mixtures in supercritical conditions as studied by NMR and molecular dynamics...

The self-diffusion coefficients of water (D_w) and cyclohexane (D_ch) in their binary mixtures were determined using the proton pulsed field gradient spin-echo method from medium to low densities in subcritical and supercritical conditions. The density (ρ), temperature (T), and water mole fraction (x_w) are studied in the ranges 0.62-6.35 M (M = mol dm^-3), 250-400 °C, and 0.109-0.994, respectively. A polynomial fitting function was developed for a scaled value of Ξ = ρDT^-1/2 with ρ, T, and x_w as variables in combination with a comprehensive molecular dynamics (MD) simulation. The NMR and MD results agree within 5% for water and 6% for cyclohexane, on average. The differences between D_w and D_ch in the dependence on ρ, T, and x_w are characterized by the activation energy E_a and the activation volume ΔV_Ξ ^‡ expressed by the scaled fitting function. The decrease in the ratio D_w/D_ch and the increase in the E_a of water with increasing x_w are related to the increase in the number of hydrogen bonds (HBs). The D_w value for a solitary water molecule at a low x_w is controlled by the solvation shell, most of which is occupied by nonpolar cyclohexane molecules that provide less friction as a result of weaker interactions with water. A microscopic diffusion mechanism is discussed based on an analysis of the HB number as well as the first-peak height of the radial distribution functions that are taken as measures of the potential of the mean field controlling self-diffusion.