NMR study of angular momentum relaxation in fluids. I. Compressed CF4

Abstract

Angular momentum relaxation has been studied in the spherical top molecule of CF4 from the dense to dilute fluid region. For this purpose the NMR 19F spin-lattice relaxation times, T1, have been measured in CF4 as a function of pressure and temperature over the wide density range 0.28⩽ρ/ρc⩽2.48 and the temperature range 1.2⩽T/Tc⩽1.64. Since spin-rotation interactions provide the dominant relaxation mechanism for the fluorine nuclei, the analysis of the T1 data yields the angular momentum correlation time, τJ. Interpretation of the experimental τJ data in terms of the rough hard sphere model of liquids gives the hard core diameters for CF4 and its temperature dependence. In the dense fluid region for ρ⩾2ρc the rough hard sphere model proposed by Chandler is valid and the calculated parameter a (T) =τE/τJ, where τE is the Enskog relaxation time is weakly temperature dependent and independent of density. This parameter a (T) is a measure of the roughness of the hard spheres indicating how efficiently angular momentum is transferred during a collision. At intermediate and low densities the parameter a (T) increases with decreasing density as a result of less effective screening of the attractive forces due to harsh short-ranged repulsive forces. To account for the attractive forces we propose a modified hard sphere model based on optimized cluster theory with the parameter b (T,ρ) =a (T) exp[CL(σ)], where a (T) is the limiting value of τE/τJ for ρ≳2ρc, and CL(r) is the renormalized intermolecular potential based on the Lennard-Jones potential for CF4. By including the effect of the attractive forces the modified rough hard sphere model accounts well for the experimental a (T) behavior over the entire density range.