Flow dynamics and theoretical modeling of monolayer ionic solutions confined within Ångstrom-scale channels

Comprehending the flow dynamics of ionic solutions within nanoconfined spaces is imperative for diverse applications encompassing desalination, nanofiltration, energy storage, and electrochemical devices. When the confinement space is further reduced to 1 nm (Ångstrom scale), monolayer ionic solutions will emerge. In this regime, ions not only have the ability to influence water properties such as viscosity but also primarily modify the interactions and corresponding slip length (or friction coefficient) between the solution and wall. Notably, ion effects on water flow dynamics at Ångstrom scale exhibit unique characteristics compared to those at nanoscale and macroscale levels. In this study, we investigate the pressure-driven transport of monolayer ionic solution confined within two-dimensional graphene channels and explore the influences of ionic type, concentration, and valency on the flow rate of water via molecular dynamic simulations. Our findings reveal that divalent ions (e.g., Mg2+ and Ca2+) considerably reduce water flow rates due to enhanced viscosity and fluid-solid interface interaction compared to monovalent ions (e.g., Na+ and K+). Subsequently, we develop a theoretical model based on the Hagen-Poiseuille (HP) equation that incorporates modifications for ion-specific viscosity and slip length at the Ångstrom-scale level. By incorporating self-calculated values for water viscosity and friction coefficient/slip length at the graphene-water interface into our modified HP equation, water flow rate is basically predicted while emphasizing the critical role of ion-water interactions in Ångstrom-scale fluid transport.