  Experimental evidences have shown that graphene oxide (GO) can be impermeable
to liquids, vapors and gases, while it allows a fast permeation of water
molecules. The understanding of filtration mechanisms came mostly from studies
dedicated to water desalination, while very few works have been dedicated to
distilling alcohols. In this work, we have investigated the molecular level
mechanism underlying the alcohol/water separation inside GO membranes. A series
of molecular dynamics and Grand-Canonical Monte Carlo simulations were carried
out to probe the ethanol/water and methanol/water separation through GO
membranes composed of multiple layered graphene-based sheets with different
interlayer distance values and number of oxygen-containing functional groups.
Our results show that the size exclusion and membrane affinities are not
sufficient to explain the selectivity. Besides that, the favorable water
molecular arrangement inside GO 2D-channels forming a robust H-bond network and
the fast water diffusion are crucial for an effective separation mechanism. In
other words, the separation phenomenon is not only governed by affinities with
the membrane (enthalpic mechanisms) but mainly by the geometry and size factors
(entropic mechanisms). We verified that the 2D geometry channel with optimal
interlayer distance are key factors for designing more efficient alcohol-water
separation membranes. Our findings are consistent with the available
experimental data and contribute to clarify important aspects of the separation
behavior of confined alcohol/water in GO membranes.
