Imidazole-doped
nanocrystalline cellulose (CNC-Im) is a new proton conductor based
on imidazole-functionalized nanocrystalline cellulose with a conductivity
of approximately 10–1 S/m at 160 °C. Its conductivity
is possible due to the transport of protons from imidazoles. The dynamics
of local processes were studied by 15N and 13C nuclear magnetic resonance (NMR) spectroscopy under the conditions
of 1H–15N and 1H–13C cross-polarization (CP) and magic angle spinning (MAS)
and by heteronuclear correlation (HETCOR) spectroscopy. The 15N and 13C NMR spectra showed the coexistence of two fractions
of imidazole molecules: slowly reorienting and exchanging protons
and fast reorienting and fast exchanging protons. Analysis based on
the two-phase model enabled the determination of the energy distribution
of imidazole tautomerization, whose maximum value is 38 kJ/mol. The
HETCOR experiment allowed determination of the binding of nitrogen
protons from imidazoles to cellulose hydroxyl groups and possibly
residual water. NMR studies conducted on the 13C isotope
confirmed the reorientation of imidazoles. The proton transport in
CNC-Im was shown to consist in the exchange of protons between imidazoles
via the OH groups of cellulose and residual water conditioned by the
reorientation of imidazole rings. The described proton transport leads
to the observed conductivity in CNC-Im, assuming the dissociation
of imidazole into anion and cation additionally.
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