Effect of Charge on Spin Probe Interaction and Dynamics in the Nanopores of Cotton

To understand the effect of charge on spin probe interactions and dynamics in the nanopores of cotton, an electron paramagnetic resonance (EPR) study of 4-amino-TEMPO (T−NH2) and 4-carboxy-TEMPO (T−COOH) nitroxide radicals deposited on dry cotton has been conducted. Spectra are recorded as a function of probe loading (2 × 10 - 5 to 10 - 2 mol kg - ), temperature (293−333 K), and relative humidity (RH, 6 and 75%) and compared to results from the neutral probe 4-hydroxy TEMPO (T−OH). The effective internal pH of the cotton was measured using codeposited indicator dyes, which showed that both probes were charged, T−COO - and T−NH3 +, at loadings up to and including 10 - 3 mol kg - 1. Three components contribute to the spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous regions, (b) slowly moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. Independent of loading, the mobile T−NH3 + experience a highly polar aqueous-like environment at 75% RH, which became much less polar at 6% RH as water was removed from the cotton. An analogous but much smaller drop in polarity was found for T−COO - , ascribed to the higher hydrogen-bonding ability of the probe, binding water more effectively. The rotational correlation time of mobile T−NH3 + and T−COO - radicals had much larger activation energies than T−OH and different loading dependencies. This was ascribed to interactions with the 5.3 mmol kg - 1 of carboxylic acid groups in cotton, so that the charged probes monitor different aspects of the cellulose chain relaxation. The enthalpy and entropy of the adsorption equilibrium between mobile and adsorbed radicals was extracted at 10 - 3 mol kg - 1. Values were independent of humidity except for T−COO - , because of its stronger interactions with water. Both the T−NH3 + and the T−COO - probes preferentially adsorb on less constricted binding sites than T−OH. At 10 - mol kg - 1, the carboxy probe becomes predominately protonated, T−COOH, radically changing the spectra observed, with a much greater fraction of mobile radicals. For the amino probe, the spectra at 10 - 2 mol kg - 1 are dominated by aggregated radicals.