  The influence of spatial confinement on the thermally excited stochastic
cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium
bis-((trifluoromethyl)sulfonyl)imide ([BuPy][Tf_2N]) inside porous
carbide-derived carbons with various pore sizes in the sub- to a few nanometer
range are investigated by quasi-elastic neutron spectroscopy. Using the
potential of fixed window scans, i.e. scanning a sample parameter, while
observing solely one specific energy transfer value, an overview of the dynamic
landscape within a wide temperature range is obtained. It is shown that already
these data provide a quite comprehensive understanding of the
confinement-induced alteration of the molecular mobility in comparison to the
bulk. A complementary, more detailed analysis of full energy transfer spectra
at selected temperatures reveals two translational diffusive processes on
different time scales. Both are considerably slower than in the bulk liquid and
show a decrease of the respective self-diffusion coefficients with decreasing
nanopore size. Different thermal activation energies for molecular
self-diffusion in nanoporous carbons with similar pore size indicate the
importance of pore morphology on the molecular mobility, beyond the pure degree
of confinement. In spite of the dynamic slowing down we can show that the
temperature range of the liquid state upon nanoconfinement is remarkably
extended to much lower temperatures, which is beneficial for potential
technical applications of such systems.
