When graphene oxide (GO) membranes are immersed in water the interlayer spacing between GO flakes increases and the membrane swells. In this swollen state, water and ions are thought to permeate through the membrane via a network of interconnected unoxidised 2D pores.1 Careful control of the swelling has now been realised, enabling ion permeation selectivity through the membrane.2 This breakthrough means that GO membranes could be used for the water purification by nanofiltration in the future.
In this work, the ion-rejection properties of GO membranes were investigated using molecular dynamics simulations and the umbrella sampling technique.2,3 The simulations have demonstrated, using simple 2D pore models, that relative permeation rates are determined by the free energy associated with ion dehydration upon entering the pore. The implications of this finding are discussed in the context of desalination and the removal of problematic radioactive contaminants, such as 99Tc.
In order to resolve quantitative differences in permeation energy barriers between experiment and simulation, we propose alternatives to the widely assumed permeation pathway of ions through GO membranes.
References
1) RK Joshi, P Carbone, FC Wang, VG Kravets, Y Su, IV Grigorieva, HA Wu, AK Geim and RR Nair, Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes, Science, 2014, 343, 752-754.
2) J Abraham, KS Vasu, CD Williams, K Gopinadhan, Y Su, C Cherian, J Dix, E Prestat, SJ Haigh, IV Grigorieva, P Carbone, AK Geim and RR Nair, Tunable Sieving of Ions Using Graphene Oxide Membranes, Nat. Nanotechnol., accepted for publication.
3) CD Williams and P Carbone, Selective Removal of Technetium from Water Using Graphene Oxide Membranes, Environ. Sci. Technol., 2016, 50, 3875-3881.
Advances in molecular simulation , Interfacial and confined phenomena
