  Separation of salt ions from seawater using graphene oxide (GO) membrane is
an emerging desalination method. Here, we investigate the dynamic behavior of
water, sodium, and chloride ions inside GO membrane using a computational
approach. First, we developed four different models of GO membrane with
different distances between GO sheets (0.7, 0.9, 1.1, and 1.3 nm), and
performed molecular dynamics (MD) simulations of water inside each membrane for
20 ns. With the analysis of the mean squared displacement of water and the
Einstein relation equation for diffusion, we measured the diffusion coefficient
of water in each membrane. We found that the diffusion coefficient of water
inside the GO membrane decreased as the distance between sheets (d) decreased.
The measured diffusion coefficients (in unit of 0.00001 (cm*cm)/sec) of water
are 0.03 (d = 0.7 nm), 0.91 (d = 0.9 nm), 1.18 (d = 1.1 nm), and 1.49 (d = 1.3
nm), whereas it is 2.49 for bulk water. Second, for analyzing the desalination
performance of each membrane, we also calculated the free energy profile of
penetration of water, sodium, and chloride ions via the interlayer of the
membrane. The free energy profile of penetration revealed that the optimum
interlayer distance between GO sheets for desalination is 0.9 ~ 1.1 nm. With
0.9 and 1.1 nm interlayer distance between GO sheets, the free energy barrier
of membrane penetration of water is negligible, where it is 2 ~ 4 kcal/mol for
sodium or chloride ions. We believe that our simulation results would be a
significant contribution to designing a new GO-based membrane for desalination.
