Dense liquids. I. The effect of density and temperature on self-diffusion of tetramethylsilane and benzene-d6

Abstract

Using the NMR spin-echo technique the self-diffusion coefficients of liquid tetramethylsilane and benzene-d6 have been measured as a function of temperature and pressure over the density range 2.5⩽ρ/ρc⩽3.5 and the temperature range 0.5⩽T/Tc⩽0.8. Densities have also been determined under the same experimental conditions. The experimental self-diffusion coefficients are successfully interpreted in terms of the rough hard sphere model of liquids based on the modified Enskog theory. In order to obtain a good agreement between the experimental and predicted values of D, the temperature dependence of the effective hard core diameter is assumed. The deviation of the experimental diffusion constants from those predicted from the hard sphere model at high densities (ρσ3≳0.93) is of particular interest. This discrepancy suggests a possible density dependence of the effective hard-core diameter for highly compressed liquids; or alternatively, the rough hard sphere theory begins to break down at densities for which the hard sphere fluid is metastable.