Ab Initio Molecular Dynamics of Covalent Organic Frameworks in an Aqueous Solution of NaCl

3 billion people cannot take access to clean water for granted while over 1 billion do not even have access to clean water. The state-of-the-art method for providing fresh water is from membrane reverse osmosis (RO) which utilizes polyamide membranes that – while having a high salt rejection and water flux – require high operating pressures and are prone to biofouling from chlorine treatments. Other nanostructured materials – such as carbon nanotubes, zeolites, and single-layered graphene – have been studied, but these materials do not show the high salt rejection, ordered pore systems, and stability required to outperform conventional materials A relatively new category of materials, covalent organic frameworks (COFs), can have highly ordered and tunable pores, and a high thermal and chemical stability. Computational studies are an excellent tool for studying the vast space of theoretical COFs and their possible applications at a much faster rate than experimental methods currently allow, which is key to improve desalination technology before access to fresh water is no longer a reversible issue. In this work we present a study of the permeability and salt rejection of water-stable COFs to elucidate trends in performance when evaluated against conventional membrane materials, and the effects of solvation in the solubility and diffusion of ions through highly confined environments. Large-scale<i> ab-initio </i>molecular dynamics simulations were conducted using the tight-binding density functional (DFTB) package DFTB+. DFTB calculations have shown increased speed and minimal loss of accuracy relative to traditional density functional techniques, resulting in efficient methods for geometry optimization and structure calculations. The uncertainty of the DFTB dynamics calculations is estimated based on static modeling of the same COF geometries with standard density functional theory.