Graphene Interfacial Magnesium Salt Solution Structure

Abstract

The properties of aqueous solutions may undergo alteration when situated in proximity to interfaces or confined environments. This includes aspects such as the thermodynamics of ion selectivity at these interfaces, the transition states and pathways of chemical reactions, and the processes of nucleation and phase growth. Inspired by the abnormal crystallization of alkali metal and alkaline earth metal chloride solutions at the graphene interface, this study uses X-ray diffraction (XRD) experiments to investigate the behavior of unsaturated MgCl2 and Mg(NO3)2 solutions at the reduced graphene oxide (rGO) interface. The findings show that MgCl2 and Mg(NO3)2 solutions hardly crystalize at the graphene interface. Only magnesium nitrate hexahydrate crystals form in high-concentration magnesium nitrate solutions, with the (011) crystal surface—rich in Mg2+—being preferentially precipitated. We analyze the molecular mechanisms leading to the different interfacial behaviors observed between magnesium salt and calcium salt solutions through theoretical calculations. The highly stable hydration layer surrounding Mg2+ weakens the cation-π interactions with graphene. In contrast, the hydration layer of Ca2+ is more easily disrupted, allowing for stronger cation-π interactions. Molecular dynamics simulations reveal that Mg2+ has a strong affinity for water molecules, which hinders the effective transfer of π electrons to cations in graphene. As a result, there is a low tendency for interfacial crystallization. This study clarifies the differing crystallization behaviors of magnesium and calcium ions at interfaces, providing valuable insights and theoretical guidance for the design of nano-interface materials, ion sieving, and electrolyte applications.

Keywords: graphene, magnesium, calcium, interface, cation-π interaction