Free-Radical and Singlet-Oxygen Mobility in Cotton Probed by EPR Spectroscopy

Abstract

Spin probe EPR experiments using TEMPO and TEMPOL are reported on dry cotton. The spectra are in the slow tumbling region with τrot ∼ 1 ns, the line width is found to be affected by Heisenberg spin exchange and aggregation of the radical. To probe the entire amorphous zone of cotton, ethanol and water are used to deposit the radicals; to probe the fiber surface specifically toluene was used. Analysis shows that in both regions the viscosity is 30 cP and that the fiber surface is an amorphous layer with a depth of approximately 2 nm. The radical motion follows normal liquid theory with a diffusion rate constant of 2 × 108 mol-1 L s-1. A small proportion of the radicals deposit to an area of much higher viscosity. The classical spin probe method is extended via a simple electron spin polarization experiment. This involved irradiation of the sample and analysis of the radical−triplet mechanism polarization of TEMPOL/1O2, which indicates 1O2 diffuses with a similar rate constant.