Quantitative electron paramagnetic resonance study of the motion, adsorption, and aggregation of TEMPOL radicals in the nanopores of dry cotton

To understand the factors controlling the motion of molecules contained within the nanopores of dry cotton, an electron paramagnetic resonance spin probe study of TEMPOL (4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy) nitroxide radicals in dry cotton was conducted. Spectra were recorded between 10 and 70 degreesC with TEMPOL loadings from 6 x 10(-5) to 5 x 10(-2) mol kg(-1). At low loadings (<3 x 10(-3) mol kg(-1)), the radicals are in an equilibrium between a mobile state in the amorphous pores and adsorbed onto the crystallites. The temperature dependence of the equilibrium constant, K, was measured, and from this, the enthalpy and entropy of the adsorption sites were obtained. These show a remarkable loading dependence, DeltaH(0) dropping from 75 to 50 kJ mol(-1) and DeltaS(0) from 0.25 to 0.20 kJ mol(-1) K-1 between loadings of 2 x 10(-4) and 3 x 10(-3) mol kg-1. The viscosity of the amorphous region, obtained from the rotational correlation time of the mobile radicals, shows an analogous dependence on loading rising from 20 to 35 cP between 2 x 10(-4) and 3 x 10(-3) mol kg(-1) at 298 K. The effects are indicative of microdomains of different character with the preferred microdomains being those with pores of equivalent size to TEMPOL (d approximate to 1 nm). There are approximately 4 x 10(-4) mol kg(-1) of these pores. The absolute values of the viscosity, DeltaH(0) and DeltaS(0), are discussed in terms of the structure and interactions in cotton. At high loadings (> 3 x 10(-3) mol kg(-1)), aggregation of the radicals occurs, which is well-modeled using the Poisson distribution and the assumption that aggregation occurs when the number of radicals in a microdomain is double the number of pores.