A multiscale framework for structure of confined water and electrolytes

Nanoconfined water and aqueous solutions play a prominent role in nature and technological applications. Understanding molecular origins of the properties of aqueous interfaces is critical to devising novel nanofluidic applications related to energy, water, and health. Near an interface, the inhomogeneous and anisotropic arrangement of water molecules gives rise to pronounced variations in the structural, thermodynamic, dynamic, and electrochemical properties of the nanofluidic systems. Classical continuum theories fail to accurately describe such atomic level variations in the properties of nanoconfined fluids. Moreover, accurate modeling of molecular-level details of water is still a long-standing challenge for liquid state theories. In this work, we present an empirical potential based quasi-continuum theory (EQT) to accurately predict the molecular-level properties of interfacial water and aqueous electrolytes. In EQT, we employ rigorous statistical mechanics tools to incorporate interatomic interactions, long-range electrostatics, molecular correlations, and polarization effects at a continuum-level. Explicit consideration of atomic interactions of water molecules is both theoretically and numerically challenging. We develop a systematic coarse-graining approach to oarse-grain interactions of water molecules and electrolyte ions from a high-resolution atomistic scale to the continuum scale. We show that EQT can predict the density profiles, i.e., molecular arrangement, of confined water as accurately as fully atomistic simulations. To demonstrate the ability of EQT to incorporate the water polarization, ion hydration, and electrostatic correlations effects, we simulate electric double layers and show that EQT can accurately predict the distribution of ions in a thin EDL and also reproduce the complex phenomenon of charge inversion. Furthermore, we show that EQT can also be combined with the classical density functional theory to model grand potential function for inhomogeneous fluids and accurately predict thermodynamic properties. Since EQT systematically and accurately links details of the atomic scales to the macroscopic continuum scales, it is inherently a multiscale approach