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

摘要

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 (rho), temperature (T), and water mole fraction (x(w)) are studied in the ranges 0.62-6.35 M (M = mol dm(-3)), 250-400 degrees C, and 0.109-0.994, respectively. A polynomial fitting function was developed for a scaled value of Xi = rho DT-1/2 with rho, 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 rho, T, and x(w) are characterized by the activation energy E-a and the activation volume Delta V-Xi(double dagger) 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. Published under license by AIP Publishing.