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Beyond well-documented confinement and surface effects arising from the largeinternal surface and severely confining porosity of nanoporous hosts, thetransport of nanoconfined fluids remains puzzling by many aspects. Withstriking examples such as memory, \textit{i.e.} non-viscous, effects,intermittent dynamics and surface barriers, the dynamics of fluids innanoconfinement challenges classical formalisms (\textit{e.g.} random walk,viscous/advective transport) -- especially for molecular pore sizes. In thiscontext, while molecular frameworks such as intermittent brownian motion, freevolume theory and surface diffusion are available to describe theself-diffusion of a molecularly confined fluid, a microscopic theory for thecollective diffusion (\textit{i.e.} permeability) -- which characterizes theflow induced by a thermodynamic gradient -- is lacking. Here, to fill thisknowledge gap, we invoke the concept of `De Gennes narrowing' which relates thewavevector-dependent collective diffusivity $D_0(q)$ to the fluid structurefactor $S(q)$. First, using molecular simulation for a simple yetrepresentative fluid confined in a prototypical solid (zeolite), we unravel anessential coupling between the wavevector-dependent collective diffusivity andthe structural ordering imposed on the fluid by the crystalline nanoporoushost. Second, despite this complex interplay with marked Bragg peaks in thefluid structure, the fluid collective dynamics is shown to be accuratelydescribed through De Gennes narrowing. Moreover, in contrast to the bulk fluid,departure from De Gennes narrowing for the confined fluid in the macroscopiclimit remains small as the fluid/solid interactions in severe confinementscreen collective effects and, hence, weaken the wavevector dependence ofcollective transport.

On De Gennes Narrowing of Fluids Confined at the Molecular Scale in Nanoporous Materials