Radical Diffusion Crossover Phenomenon in Glass-Forming Liquids

: We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering. The crossover phenomenon was analyzed by a novel, simple diffusion model, combining collective and single-molecule diffusion processes, and it was compared to the Arrhenius crossover phenomenon. The obtained results suggest that future studies of tracer diffusion could contribute to a better understanding of diffusion mechanisms in glass-forming liquids. The proposed diffusion model could be used to study the crossover phenomena of tracer diffusion measured by other techniques, and it could serve as a base for developing more advanced models.

Automated summary

By studying the diffusion properties of a nitroxide radical in various glass-forming molecular liquids at different temperatures, we found that the transition from single-molecule diffusion to collective diffusion occurs with decreasing temperature. The proposed diffusion model can be used to study the crossover phenomenon of tracer diffusion and serve as a foundation for developing more advanced models.