  Beyond well-documented confinement and surface effects arising from the large
internal surface and severely confining porosity of nanoporous hosts, the
transport of nanoconfined fluids remains puzzling by many aspects. With
striking examples such as memory, \textit{i.e.} non-viscous, effects,
intermittent dynamics and surface barriers, the dynamics of fluids in
nanoconfinement challenges classical formalisms (\textit{e.g.} random walk,
viscous/advective transport) -- especially for molecular pore sizes. In this
context, while molecular frameworks such as intermittent brownian motion, free
volume theory and surface diffusion are available to describe the
self-diffusion of a molecularly confined fluid, a microscopic theory for the
collective diffusion (\textit{i.e.} permeability) -- which characterizes the
flow induced by a thermodynamic gradient -- is lacking. Here, to fill this
knowledge gap, we invoke the concept of `De Gennes narrowing' which relates the
wavevector-dependent collective diffusivity $D_0(q)$ to the fluid structure
factor $S(q)$. First, using molecular simulation for a simple yet
representative fluid confined in a prototypical solid (zeolite), we unravel an
essential coupling between the wavevector-dependent collective diffusivity and
the structural ordering imposed on the fluid by the crystalline nanoporous
host. Second, despite this complex interplay with marked Bragg peaks in the
fluid structure, the fluid collective dynamics is shown to be accurately
described through De Gennes narrowing. Moreover, in contrast to the bulk fluid,
departure from De Gennes narrowing for the confined fluid in the macroscopic
limit remains small as the fluid/solid interactions in severe confinement
screen collective effects and, hence, weaken the wavevector dependence of
collective transport.
