  Atomically thin molecular carbon nanomembranes (CNMs) with intrinsic
sub-nanometer porosity are considered as promising candidates for next
generation filtration and gas separation applications due to their extremely
low thickness, energy efficiency and selectivity. CNMs are intrinsically porous
which is advantageous over other 2D materials such as graphene and transition
metal dichalcogenides where defects and pores need to be introduced after
synthesis. It was already discovered that water and helium permeate through
4,4-terphenylthiol (TPT) CNM above the limit of detection. Additionally, the
permeation of water vapour was nonlinear against its pressure and 1000 stronger
than permeation of helium despite their similar kinetic diameters. However,
there was no clear permeation mechanism which could explain permeation of both
species. Here, we demonstrate that permeation of all gas species is defined by
their adsorption. We performed gas permeation measurements through TPT CNM at
different temperatures and found that all measured gases experienced an
activation energy barrier which correlated with their kinetic diameters.
Furthermore, we identified that entropy loss during adsorption and permeation
is the fundamental reason of strong nonlinear permeation of water. Our results
also demonstrated that adsorption plays a major role in permeation of all
gases, not just water.
