  Molecular Dynamics simulations of glycerol confined in $\gamma$-Al$_2$O$_3$
slit nanopores are used to explain controversial and inconsistent observations
reported in the literature regarding the dynamics of viscous fluids in confined
geometries. Analysing the effects of the degree of confinement and pore
saturation in this system, we found that the presence of the solid/liquid
interface and the liquid/gas interface in partially saturated pores are the
main contributors for the disruption of the hydrogen bond network of glycerol.
Despite the reduction of hydrogen bonds between glycerol molecules caused by
the presence of the solid, glycerol molecules near the solid surface can
establish hydrogen bonds with the hydroxyl groups of $\gamma$-Al$_2$O$_3$ that
significantly slow-down the dynamics of the confined fluid compared to the bulk
liquid. On the other hand, the disruption of the hydrogen bond network caused
by the liquid/gas interface in unsaturated pores reduces significantly the
number of hydrogen bonds between glycerol molecules and results in a faster
dynamics than in the bulk liquid. Therefore, we suggest that the discrepancies
reported in the literature are a consequence of measurements carried out under
different pore saturation conditions.
