Effect of Dissolved Oxygen on the Spin-Lattice Relaxation Time of Free Radicals in Petroleum Oils

Abstract
Electron paramagnetic resonance studies of free radicals in petroleum oils reveal that the presence of dissolved oxygen significantly affects the free radical spin‐lattice relaxation time,T 1. Changes in T 1 can occur upon: (a) dilution with solvents, (b) exposure to light, (c) bubbling with oxygen, and (d) stripping of dissolved oxygen. In this investigation it is shown that these changes all result from the influence of dissolved oxygen. Dilution with ordinary cp solvents results in a decrease of T 1. If the solvent is first stripped of dissolved oxygen, there will be no changes in T 1. The effect is reversible, i.e., the value of T 1 for free radicals in a petroleum oil may be arbitrarily decreased or increased by adding or removing dissolved oxygen, respectively. It was observed that free radicals in oil, or in oil in solution, experienced an increase in spin‐lattice relaxation time when allowed to remain sealed in a quartz tube. This change was traced to the effect of light and appears to occur for wavelengths of the order of 5400 A and lower. It is concluded that this phenomenon is due to the photochemical takeup of dissolved oxygen whose presence decreases T 1. This is based on the fact that T 1 may be lowered to its original value merely by adding molecular oxygen to the petroleum oil. Depending upon the degree of saturation being effected, changes in T 1 will be reflected as changes in ``apparent'' free radical content. The true free radical content was unchanged by the processes studied.