Effects of TEMPOL Radical on the Longitudinal NMR Relaxation in Ethylene Glycol: A Simple Controlled Model System

The presence of radicals in fluids can significantly influence the ¹H longitudinal nuclear magnetic resonance relaxation processes. The field-cycling nuclear magnetic relaxometry technique provides a unique approach to study these interactions within a broad Larmor frequency range. The situation is quite common in fluid systems subjected to oxidative stress, such as lubricants in internal combustion engines. Our previous studies in lubricant degradation did not consider in detail specific effects of radicals on the ¹H longitudinal relaxation. In the present work, we focus on a simplified system in which TEMPOL radicals are introduced in controlled concentrations into ethylene glycol samples. The system's behavior is evaluated at different radical concentrations to elucidate the influence of the paramagnetic species on the relaxation dispersion profile. We propose the inclusion of an additional relaxation term to account for intermolecular proton-electron interactions. We observe that, for radical concentrations exceeding approximately 2 × 10¹⁷ radicals/cm³, paramagnetic interactions dominate the relaxation dispersion at Larmor frequencies below 1 MHz. Moreover, we show that both the diffusion constants of ethylene glycol and TEMPOL molecules can be estimated from a single experiment. The consistency of our results with existing literature suggests an in-depth analysis of the paramagnetic contribution in the relaxometric characterization of degraded lubricants.