On the design of graphene oxide nanosheets membranes for water desalination

According to current researches, graphene oxide (GO) membranes show promising desalination properties due to ease of synthesis, low production cost, and high efficiency. There are several experimental works to study ionic sieving properties of GO membranes. However, it is difficult to characterize atomistic mechanism of water permeation and ion rejection by experimental approaches. On the other hand, there exist a few reports in which the atomistic picture of water permeation across GO membranes is investigated by means of molecular dynamics (MD) simulation. In the present work, in addition to water desalination, the atomic scale mechanism of ion rejection is studied using large scale MD simulation. For this purpose, surface color maps based on the potential of mean force (PMF) are computed between GO nanosheets to indicate interaction between functional groups and existing species in saline water. The radial distribution function (RDF) between water molecules and functional groups are measured to study the disordering of water molecules between GO nanosheets. Furthermore, the effect of different layers separation value and applied pressure are examined to explore the optimal design of GO membranes. According to our simulation results, the oxygen atoms in hydroxyl and epoxide groups play an important role in rejection of the Cl ions and attraction of the Na ions. The hydroxyl groups have the most impact on disordering of the water molecules between GO membranes. In addition, our designed GO membrane, showed a water permeability of one to three orders of magnitude higher than commercial reverse osmosis membranes.