Selective Trans-Membrane Transport of Alkali and Alkaline Earth Cations through Graphene Oxide Membranes Based on Cation-pi Interactions

Graphene and graphene oxide (G-0) have been demonstrated to be excellent filters for various gases and liquids, showing potential applications in areas such as molecular sieving and water desalination. In this paper, the selective trans-membrane transport properties of alkali and alkaline earth cations through a membrane composed of stacked and overlapped G-0 sheets (G-0 membrane) are investigated. The thermodynamics of the ion transport process reveal that the competition between the generated thermal motions and the interactions of cations with the G-0 sheets results in the different penetration behaviors to temperature variations for the considered cations (K+ Mg2+, Ca2+ and Ba2+). The interactions between the studied metal atoms and graphene are quantified by first-principles calculations based on the plane-wave-basis-set density functional theory (DFT) approach. The mechanism of the selective ion trans-membrane transportation is discussed further and found to be consistent with the concept of cation-pi interactions involved in biological systems. The balance between cation-pi interactions of the cations considered with the sp(2) clusters of G-0 membranes and the desolvation effect of the ions is responsible for the selectivity of G-0 membranes toward the penetration of different ions. These results help us better understand the ion transport process through G-0 membranes, from which the possibility of modeling the ion transport behavior of cellular membrane using G-0 can be discussed further. The selectivity toward different ions also makes G-0 membrane a promising candidate in areas of membrane separations.