Self-diffusion coefficients for water and organic solvents in extremely low-density supercritical states

Self-diffusion coefficients D for benzene and cyclohexane are determined by using the proton pulsed-field-gradient spin echo (PGSE) method at high temperatures of 150-400 °C and in the low-density (0.007-0.21 and 0.005-0.15 g cm− 3 for benzene and cyclohexane, respectively), one-phase region. For benzene, the density-diffusivity product in the zero-density limit divided by the square root of the absolute temperature, (ρD)0/T is smaller than that of the hard sphere model at 150-400 °C and decreases with lowering temperature as a reflection of the attractive interaction effect. The (ρD)0/T for cyclohexane is comparable to the hard-sphere value and its temperature dependence is smaller than that for benzene, showing that the effect of the attractive interaction is stronger for benzene than for cyclohexane. The dynamic effect of the hydrogen bonding is discussed through comparison to our previous results on water [J. Chem. Phys., 125, 074307 (2006); J. Chem. Phys. 126, 089901 (2007)].