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

Abstract

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-NH 2 ) 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 -1 ), 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-NH 3 + , 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-NH 3 + 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-NH 3 + 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-NH 3 + and the T-COO -probes preferentially adsorb on less constricted binding sites than T-OH. At 10 -2 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.

New insights into dye chemistry and physics

“…This points to the aggregation in cotton being determined by the cotton microstructure and not by the identity of the chemical, a perhaps surprising feature. Some significant changes in behaviour were found for the two charged probes, amino-and carboxy-TEMPO [23], notably that activation energy for rotational motion increased over TEMPOL. This was ascribed to interaction with the measured 5 9 10 À3 mol kg À1 of carboxylic acid groups in cottons.…”

“…Electron paramagnetic resonance (EPR) spin probing [19][20][21][22][23] has proved to be particularly useful. Electron paramagnetic resonance (EPR) spin probing [19][20][21][22][23] has proved to be particularly useful.…”

“…To probe the complex supramolecular structure of cotton, a variety of modern techniques have been applied to study the properties of reporter molecules inside the pores. Electron paramagnetic resonance (EPR) spin probing has proved to be particularly useful. This involves doping the fibre with chemically stable nitroxide radicals and then recording their EPR spectrum.…”

“…The nitroxide EPR spectra were surprisingly complex and dynamic , with the radicals found in at least three different environments, as depicted in Figure . For TEMPOL, at low loadings of up to ~5 × 10 −3 mol kg −1 , the radicals existed in an equilibrium between being adsorbed on the surface of the crystallite or freely mobile within the amorphous regions .…”

B4C‐Nanowires/Carbon‐Microfiber Hybrid Structures and Composites from Cotton T‐shirts

“…Cotton has been used by mankind for at least 7000 years 26, 27. It is the oldest and most commonly used nanoporous material, which is constructed from polysaccharide chains arranged into amorphous and crystalline regions 26, 28. Today, the world production of cotton is approximately 20 million tons per annum, mainly for clothing, paper, and medical uses.…”