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/ρ_c⩽2.48 and the temperature range 1.2⩽T/T_c⩽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, τ_J. Interpretation of the experimental τ_J 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⩾2ρ_c the rough hard sphere model proposed by Chandler is valid and the calculated parameter a (T) =τ_E/τ_J, where τ_E 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[C_L(σ)], where a (T) is the limiting value of τ_E/τ_J for ρ≳2ρ_c, and C_L(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.